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PRINCIPLES OF INHERITANCE AND VARIATION

INTRODUCTION

  • Term genetics was given by W. Bateson (Father of Modern Genetics).
  • Genetics is a branch of biology that deals with the collective study of heredity & variations.
  • Heredity is the transmission of genetic characters from parent to offspring.
  • Individuals of same species have some differences, these are called variation.
Flow Chart:Variation

  • Muller proposed the term "cytogenetics" and is the father of actinobiology.
  • Morgan is the father of experimental genetics. He experimented on Drosophila & proposed various concepts like- 
    • Gene theory, according to which genes are linearly located on chromosome.
    • Linkage theory. 
    • Crossing over and linkage term.
    • Criss -cross inheritance.
    • Linkage map on Drosophila.
  • A. Garrod is father of human genetics and biochemical genetics. Garrod discovered first human metabolic genetic disorder which is called alkaptonuria (black urine disease). 
He also gave the concept ‘One mutant gene one metabolic block’.

BASIC TERMS USED IN INHERITANCE STUDIES

  • Factors : Factor is the unit of heredity which is responsible for inheritance and appearance of characters.
These factors were referred as genes by Johannsen (1909). Mendel used term “element” for factor. Morgan first used symbol to represent the factor. Dominant factors are represented by capital letter while recessive factor by small letter.
  • A pure line is a plant or animal that is genetically pure for particular character and will give rise to same character after self fertilization or inter breeding.
  • Allele : It is an alternative form of a gene which are located on same position (loci) on the homologous chromosome. Term allele was coined by Bateson.
  • Homozygous :  A zygote is formed by fusion of two gametes having identical factors is called homozygote and organism developed from this zygote is called homozygous. 
E.g., TT, RR, tt.
  • Heterozygous : A zygote is formed by fusion of two different types of gamete carrying different factors is called heterozygote (Tt, Rr) and individual developed from such zygote is called heterozygous.

Table : Examples of some autosomal characters in human
  • The term homozygous and heterozygous are coined by Bateson.
  • Hemizygous : If individual contains only one gene of a pair then individual is said to be hemizygous. Male individual is always hemizygous for sex linked gene.
  • Phenotype : It is the external and morphological appearances of an organism for a particular character.
  • Genotype : It is the genetic constitution or genetic make-up of an organism for a particular character.
  • Genotype and phenotype terms were coined by Johannsen.
  • Phenocopy : If different genotypes are placed in different environmental conditions then they produce same phenotype. Then these genotypes are said to be phenocopy of each other.
  • Back cross : A back cross is a cross in which F1 individuals are crossed with any of their parents.
  • When F1 individual is crossed with dominant parent then it is termed out cross. The generations obtained from this cross, all possess dominant character, so no analysis is possible in F1 generation.
  • Test cross : When F1 progeny is crossed with recessive parent then it is called test cross. The total generations obtained from this cross are 50% having dominant character and 50% having recessive character (monohybrid test cross). Test cross helps to find out the genotype of dominant individual.
  • Monohybrid test cross : The progeny obtained from the monohybrid test cross are in equal proportion, means 50% is dominant phenotypes and 50% is recessive phenotypes. It can be represented in symbolic forms as follows.
  • Dihybrid test cross : The progeny is obtained from dihybrid test cross are of four types and each of them is 25%.
Conclusion : In test cross, phenotype and genotype ratio are same.
  • Reciprocal cross : When two parents are used in two experiments in such a way that in one experiment “A” is used as the female parent and “B” is used as the male parent, in the other experiment “A” will be used as the male parent and “B” as the female parent, such type of a set of two experiments is called reciprocal cross.
Characters which are controlled by karyogene are not affected by reciprocal cross. In case of cytoplasmic inheritance, result changes by reciprocal cross. Reciprocal crosses are useful in knowing sex-linked traits. Sex linked characters show a difference in reciprocal cross.
  • Checker Board Method : It was firstly used by Reginald. C. Punnett (1875-1967) in (1927). This method is the representation of generations to analyse in the form of symbols of squares. Male gametes lie horizontally and female gamete lie vertically. It depicts both genotypes and phenotypes of the progeny.

MENDEL’S LAW OF INHERITANCE

  • Mendelism means experiments performed by Mendel on genetics. Description of mechanisms of hereditary processes and formulation of principles are known as Mendelism.
Mendel postulated various experimental laws in relation to genetics.
  • Gregor Johann Mendel (1822 - 1884) started his historical experiments of heredity on pea (Pisum sativum) plant.
  • The results of his experiments were published in the science journal “Nature for schender varein” in 1866. This journal was in German language. Title was “Verschue uber Pflangen Hybridan”.
  • Mendel’s experiment involved 4 steps as selection, hybridization, selfing and calculations. His results led to the formation of laws of genetics later.
  • Mendel performed monohybrid & dihybrid crosses and gave three principles of inheritance.
  • Mendel’s three principles of inheritance are-
    • law of dominance
    • law of segregation or law of purity of gametes.
    • law of independent assortment. 
  • He proposed that the ‘factors’ (later named as genes) regulating the characters are found in pairs known as alleles. He observed that the expression of the characters in the offspring follow a definite pattern in different–first generations (F1), second (F2) and so on.
  • The dominant characters are expressed when factors are in heterozygous condition (Law of Dominance). 
  • The recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous condition. A recessive character that was not expressed in heterozygous condition may be expressed again when it becomes homozygous. 
  • Law/Principle of segregation states that when a pair of contrasting factor or gene are brought together in a hybrid, these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation.
  • The above law is also known as law of purity of gametes because each gamete is pure in itself i.e. having either T (i.e. gene for tallness) or t (i.e. gene for dwarfness). Mendel formulated this law with the help of monohybrid cross.
  • Principle of independent assortment states that genes of different characters located in different pairs of chromosomes are independent of one another in this segregation during gamete formation.
  • Principles of segregation and independent assortment can be explained on the basis of chromosomal theory of inheritance.
  • Importance of mendelism are
    • on the basis of mendelism, different breeds in animals and varieties of plants have been produced.
    • Science of eugenics (development of superior progeny) is based on mendelism.
    • On the basis of mendelism, heterosis has been utilised in different organisms.

MONOHYBRID CROSS

  • When we consider the inheritance of one character at a time in a cross, this is called monohybrid cross. 
  • First of all, Mendel selected tall and dwarf plants.
CONCLUSIONS OF MONOHYBRID CROSS
  • Ist Conclusion : According to Mendel, each genetic character is controlled by a pair of unit factor. It is known as conclusion of paired factor or unit factor.
  • IInd Conclusion : This conclusion is based on F1-generation. When two different unit factors are present in a single individual, only one unit factor is able to express itself and known as dominant unit factor. Another unit factor fails to express is the recessive factor. In the presence of dominant unit factor, recessive unit factor can not express and it is known as conclusion of dominance.
  • IIIrd Conclusion :  During gamete formation, the unit factors of a pair segregate randomly and transfer inside a different gamete. Each gamete receives only one factor of a pair; so gametes are pure for a particular trait. It is known as conclusion of purity of gametes or segregation.

DIHYBRID CROSS

  • A cross made to study the inheritance of two pairs of contrasting traits.
  • Mendel selected traits for dihybrid cross for his experiment as follows :
    • Seed form → Round (R) and Wrinkled (r)
    • Colour of cotyledons → Yellow (Y) and Green (y)
When F1 plants were self pollinated to produce four kinds of plants in F2 generation such as yellow round, yellow-wrinkled, green round and green wrinkled, in the ratio of 9:3:3:1, then this ratio is known as dihybrid ratio.
  
CONCLUSION 
  • The F2 generation plant produce two new phenotypes, so inheritance of seed coat colour is independent from the inheritance of shape of seed. Otherwise it is not possible to obtain yellow wrinkled and green round type of seeds.
  • This observation lead to  Mendel’s conclusion that different type of characters present in plants assort independently during inheritance.
This is known as conclusion of Independent Assortment. 

FORK LINE METHOD 
  • To find out the composition of factors inside the gamete, we use fork line method.

DEMONSTRATION OF DIHYBRID CROSS BY CHECKERBOARD METHOD

Phenotype :
Yellow Round = 9/16
Yellow Wrinkled = 3/16
Green Round = 3/16
Green Wrinkled = 1/16
Thus, phenotypic ratio = 9 : 3 : 3 : 1

Genotype:
Homozygous yellow & Homozygous round - YY  RR = 1
Homozygous yellow & Heterozygous round - YY  Rr = 2
Heterozygous yellow & Homozygous round - Yy  RR = 2
Heterozygous yellow & Heterozygous round - Yy  Rr = 4
Homozygous yellow & Homozygous wrinkled - Yy  rr = 1
Heterozygous yellow & Homozygous wrinkled -Yy  rr = 2
Homozygous green & Homozygous round - yyRR = 1
Homozygous green & Heterozygous round - yyRr = 2
Homozygous green & Homozygous wrinkled -  yyrr = 1
Thus, genotypic ratio = 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

CHROMOSOMAL THEORY OF INHERITANCE

  • Chromosome theory of inheritance was first proposed by W.S. Sutton & Boveri in 1902.
  • This theory states that chromosomes are vehicle of hereditary information & expression as genes are present over them.
  • Salient features of this theory are-
    • Gametes serve as the bridge between two successive generations.
    • Male and female gametes play an equal role in contributing hereditary components of future generation.
    • Only the nucleus of sperm combines with ovum. Thus, the hereditary information is contained in the nucleus. 
    • Chromatin in the nucleus is associated with the cell division in the form of chromosomes.
    • Any type of deletion or addition in the chromosomes can cause structural and functional changes in living beings.
    • A sort of parallelism is observed between Mendelian factors and chromosomes.
    • A number of genes or Mendelian factors are found in each chromosome. 
    • Determination of sex in most of the animals and plants is affected by specific chromosomes. These chromosomes are called sex chromosomes.

PARALLELISM BETWEEN GENE AND CHROMOSOMES

  • Chromosomes are also transferred from one generation to the next as in the case of genes (Mendelian factors).
  • The number of chromosomes is fixed in each living species. These are found as homologous pairs in diploid cells. One chromosome from father and the other contributed by the mother constitute a homologous pair.
  • Before cell division, each chromosome as a whole and the alleles of genes get replicated and are separated during mitotic division.
  • Meiosis takes place during gamete formation. Homologous chromosomes form synapsis during prophase-I stage which in later course get separated and transferred to daughter cells. Each gamete or a haploid cell has only one allele of each gene present in the chromosome.
  • A characteristic diploid number is again established by the union of the two haploid gametes.
  • Both chromosomes and the alleles (Mendelian factors) behave in accordance to Mendel’s law of segregation.
In the homologous chromosomes of a pure tall plant, allele (T) is found for tallness in each chromosome. Likewise, in a pure dwarf plant (tt), allele (t) is present in each chromosome. These homologous chromosomes gets separated during meiotic division. Hence, each gamete possesses only one chromosome of each pair. Accordingly, all the gametes of tall plants possess a chromosome with an allele of tallness (T), while the gametes of dwarf plants possess a chromosome with an allele for dwarfness (t). Their cross to produce F1 generation will yield tall hybrid plants with homologous chromosomal pair containing (Tt) allelic pair. In this generation, two kinds of gametes will be formed during gametogenesis, 50% with the allele (T) for tallness and 50% with the allele for dwarfness (t). Random combination of these gametes will produce offsprings in F2 generation in the ratio of 25% pure tall (TT), 50% hybrid tall (Tt) and 25% dwarf (tt).

CYTOPLASMIC INHERITANCE

  • Inheritance of characters which are controlled by cytogene or cytoplasm is called cytoplasmic inheritance. It was discovered by Correns. 
  • Genes which are present in the cytoplasm called ‘cytogene’ or ‘plasmagene’ or ‘extra nuclear gene’.
A gene which is located in the nucleus is called karyogene.
  • Inheritance of cytogene in higher plants occurs only through the female. Because female gamete has karyoplasm, simultaneously it has cytogene because of more cytoplasm.
  • The male gamete of higher plant is called male nucleus. It has very minute (equivalent to nil) cytoplasm, so male gamete only inherits karyogene.
  • Thus, inheritance of cytogene only through female is also called maternal inheritance.
  • If there is a reciprocal cross in this condition, then results may be affected.
  • Cytoplasmic inheritance are of three types : Predetermination, dauermodification and organellar genetics.
  • Predetermination : Maternal effect indirectly depends on nuclear genes and are involved in no known cytoplasmic hereditary unit called as predetermination. In this, maternal effect is determined before fertilization.
  • Example of predetermination :
Shell coiling in snail (Limnaea peregra)
  • Dauermodification : Cytoplasmic inheritance involving dispensable and infective hereditary particle in cytoplasm which may or may not depend on nuclear genes is called as dauermodification.
  • Example of Dauermodification 
    • Sigma particle in Drosophila : These particles are virus like particles which are present in Drosophila and related to CO2 sensitivity. Inheritance of sigma particle takes place through the egg cytoplasm.
  • Cytoplasmic inheritance involving essential organelles like chloroplast, mitochondria are called as organellar genetics.
  • Example of Organellar Genetics (True examples of cytoplasmic inheritance)
    • Plastid inheritance in Mirabilis jalapa 
    • Male sterility in maize plant

EXCEPTIONS OF CONCLUSIONS OF MENDEL

EXCEPTION OF DOMINANCE

There are two exceptions of law of dominance–
  • Incomplete dominance
  • Co-dominance

(A) INCOMPLETE DOMINANCE
  • According to Mendel’s law of dominance, dominant character must be present in Fl generation. But in some organisms, Fl generation is different from the both parents.
Both factors such as dominant and recessive are present in incomplete dominance but dominant factors are unable to express their character completely, resulting in different type of generation which is different from both the  parents.
  • Incomplete dominance was first discovered by Correns in Mirabilis jalapa. This plant is called as ‘4 O’ clock plant’ or ‘Gul-e-Bans’.
  • Examples :
    • Three different types of plant are found in Mirabilis on the basis of flower colour, such as red, white and pink. When plants with red flowers is crossed with white flower plants, pink flower is obtained in Fl generation. The reason of this is that the genes of red colour is incompletely dominant over the genes of white colour.
When Fl generation of pink flower is self pollinated then the phenotypic ratio of F2 generation is red,
pink, white = 1 : 2 : 1 in place of normal monohybrid cross - 3: 1.
The ratio of phenotype and genotype of F2 generation in incomplete dominance is always same.
    • Incomplete dominance is also seen in Antirrhinum majus plant. This plant is also known as ‘'Snapdragon’ or ‘Dog flower’. 

(B) CO-DOMINANCE
  • In co-dominance, both genes are expressed for a particular character in F1 hybrid progeny. There is no blending of characters, whereas both the characters are expressed equally.
  • Examples : Co-dominance is seen in animals for coat colour.
When a black parent is crossed with white parent, a roan colour in F1 progeny is produced.
When we obtain F2 generation from the F1 generation, the ratio of black; black-white  (Roan) ; white of animals is 1 : 2 : 1

NOTES
R1R1 = Black -1; R1R2 = Roan-2; R2R2 = White-1
It is obvious by the above analysis that the ratio of phenotype as well as genotype is 1 : 2 : 1 in co-dominance.
In incomplete dominance, characters are blended phenotypical, while in co-dominace, both the genes of a pair exhibit both the characters side by side and effect of both the character is independent from each other.
  • Other examples of co-dominance are :
    • AB blood group inheritance (IAIB)
    • Carrier of sickle cell anaemia (HbA HbS)

CONCLUSION OF SEGREGATION

  • There is no exception of law of segregation.
  • The segregation is essential during the meiotic division in all sexually reproducing organisms. 
(Nondisjunction may be an exception of this law).
EXCEPTION OF CONCLUSION OF INDEPENDENT ASSORTMENT
  • The law of independent assortment is most criticized. Linkage is an exception to this.

LINKAGE 

  • Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation. This is due to their location on the same chromosomes.
  • Linkage was first seen by Bateson and Punnett in Lathyrus odoratus and gave coupling and repulsion phenomenon. But they did not explain the phenomenon of linkage.
  • Sex linkage was first discovered by Morgan in Drosophila and coined the term linkage. He proposed the theory of linkage.
  • Linkage and independent assortment can be represented in dihybrid plant.
In case of linkage in dihybrid, AaBb
It produces two types of gamete AB : ab
In case of independent dihybrid, AaBb
It produces four types of gametes AB: ab: aB: Ab
  • Linked genes are linearly located on the same chromosome. They get separated if exchange (crossing over) takes place between them.
  • Strength of linkage ∝ 1/ distance between the genes. It means, if the distance between two genes is increased then strength of linkage is reduced and it proves that greater is the distance between genes, the greater the probability of their crossing over. Crossing over obviously disturbs or degenerates linkage. Linked genes can be separated by crossing over.
  • Factors affecting crossing over (C.O.)
    • Distance ↑ = C.O.  ↑
    • Temperature ↑ = C.O.  ↑
    • X-Ray ↑ = C.O.  ↑
    • Age ↑ = C.O.  ↓
    • Sex - Male C.O. ↓ (Crossing over totally absent in male Drosophila.)
  • Arrangement of linked genes on chromosomes :
    • The arrangement of linked genes in any dihybrid plant is of two types cis- and trans-arrangement.
    • Cis - arrangement : When two dominant genes are located on one chromosome and both recessive genes are located on another chromosome, such type of arrangement is termed as cis-arrangement. Cis-arrangement is an original arrangement.
Two types of gamete can be produced in cis arrangement. 
→ (AB) and (ab).
    • Trans-arrangement : When a chromosome bears one dominant and one recessive gene, and another chromosome also possess one dominant and one recessive gene, such type of arrangement is called trans-arrangement. Trans-arrangement is not an original form. It is due to crossing over. Two types of gamete are also formed in trans-arrangement but it is different from cis-arrangement (Ab) and (aB).

TYPES OF LINKAGE

  • There are two types of linkage : complete and incomplete.
  • Complete linkage
    • Linkage in which genes always show parental combination then this is called complete linkage. It never forms new combination.
    • Crossing over is absent in it. Such genes are located very close on the chromosomes. Such type of linkage are very rare in nature, e.g., male Drosophila, female silk moth.
  • Incomplete linkage
    • When new combinations also appear along with parental combination in offsprings, then this type of linkage is called incomplete linkage. 
    • The new combinations are formed due to crossing over. 
    • The percentage of new combination is equal to the percentage of crossing over (< 50%).
    • Distance can be identified by the incomplete linkage. 
    • It’s unit is centimorgan(cM)
    • Strength of linkage 

LINKAGE GROUP 

  • Linkage group is a group of linearly arranged linked gene which are inherited as a single unit due to their being present on a chromosome.
  • All the genes which are located on one pair of homologous chromosome form one linkage group. 
  • Genes which are located on homologous chromosomes are allelic so we consider one linkage group. 
Linkage group = haploid no. of homologous chromosomes.
  • In prokaryotes, only one linkage group is found because it contains only one molecule of DNA. Thats why all the genes present on that DNA form one linkage group. 

GENETIC MAP/LINKAGE MAP/CHROMOSOME MAP

  • In genetic map, different genes are linearly arranged according to percentage of crossing over (∝ distance) between them.
With the help of genetic map, we can find out the position of a particular gene on chromosome. 
  • Genetic map is helpful in the study of genome and for the use of genetic engineering, it can help to separate the genes from the chromosomes.

SEX LINKAGE

  • When the genes of vegetative/somatic characters are present on sex-chromosome, it is termed as sex linked gene and such phenomenon is known as sex-linkage. 
  • Two - types of sex linkage are – X and Y linkage.
  • X-linkage : Genes of somatic characters are found on X-chromosome. The inheritance of X-linked character may be through the males and females, e.g., Haemophilia, Colour blindness.
  • Y-linkage : The genes of somatic characters are located on Y- chromosome. The inheritance of such type of character occur only through the males, such type of character is called holandric character. These characters are found only in male. E.g., Hypertrichosis (excessive hair on ear pinna.)
  • Example of X-Sex linkage 
    • Eye colour in Drosophila : Eye colour in Drosophila is controlled by a X-linked gene.
If a red eyed colour gene is represented as '+' and white eyed colour represented as 'w', then on the basis of this, different type of genotypes are found in Drosophila.
Gene for red eye dominant (+) and white colour of eye is recessive (w)
Homozygous red eyed female =  X+X+
Heterozygous red eyed female =  X+Xw
Homozygous white eyed female =  XwXw
Hemizygous red eyed male =  X+Y
Hemizygous white eyed male =  XwY
It is clear by above different types of genotype that female is either homozygous or heterozygous for eye colour. But, for the male eye colour, it is always hemizygous.
    • Haemophilia : Haemophilia is also called “bleeder’s disease” and first discovered by John Otto (1803). The gene of haemophilia is recessive and X-linked lethal gene.
  On the basis of X-linked, following types of genotype are found–
XhX = Carrier female
XhXh = Affected female
XhY = Affected male.
But, XhXh type of female dies during embryo stage because in homozygous condition, this gene becomes lethal and causes death.
    • Colour Blindness : The inheritance of colour-blindness is alike to haemophilia, but it is not a lethal disease so it is found in male and female (discovered by Horner).
  • Other examples of X-sex linkage are :
    • Diabetes insipidus (recessive).
    • Duchenne muscular dystrophy (recessive). 
    • Fragile X syndrome (recessive).
    • Pseudo Rickets (dominant)
    • Defective enamel of teeth (dominant)

TYPES OF INHERITANCE OF SEX LINKED CHARACTERS

CRISS CROSS INHERITANCE (MORGAN)
  • In criss-cross inheritance, male or female parent transfer a X-linked character to grandson or grand daughter through the offspring of opposite sex.
  • Inheritance in which characters are inherited from father to the daughter and from daughter to grandson are called diagenic.
Father → Daughter → Grand son.
  • Inheritance in which characters are inherited from mother to the son and from son to grand daughter are called diandric. 
Mother → Son → Grand-daughter.

NON CRISS-CROSS INHERITANCE
  • In this inheritance, male or female parent transfer sex linked character to grand son or grand daughter through the offspring of same sex.
  • In Hologenic :
Mother → Daughter → Grand-daughter (female to female)
  • In Holandric : 
Father → Son → Grand-son (male to male).

SEX-LIMITED CHARACTER

  • These characters are present in one sex and absent in another sex. But their genes are present in both the sexes and their expression is dependent on sex hormone.
  • Example : Secondary sexual characters. These genes are located on the autosomes and present in both male and female, but effect of these depend upon presence or absence of sex-hormones.
For example, Genes of beard-moustache express their effects only in the presence of male hormone-testosterone.

SEX INFLUENCED CHARACTERS 

Genes of these characters are also present on autosomes but these are influenced differently in male and female. 
In heterozygous condition, their effect is different in both the sexes.
Example : Baldness : Gene of baldness is dominant (B).
Gene Bb shows partiality in male and female. Baldness is found in male due to effect of this gene, but baldness is absent in female with this genotype.

PLEIOTROPIC GENE 

  • Gene which controls more than one character is called pleiotropic gene. This gene shows multiple phenotypic effect.
  • For example :
In pea plant : 
 
  • In Drosophila, recessive gene of vestigial wings also influence some other characters like –
    • Structure of reproductive organs
    • Longevity (Length of Body)
    • Bristles on wings.
    • Reduction in egg production.
  • Examples of pleiotropic gene in human
    • Sickle cell anaemia : Gene  provide a classical example of pleiotropy. It not only causes haemolytic anaemia but also results in increased resistance to one type of malaria that is caused by the parasite Plasmodium  falciparum.
The sickle cell  allele also has pleiotropic effect on the development of many tissues and organs such as bone, lungs, kidney, spleen, heart.
    • Cystic fibrosis : Cystic fibrosis is a hereditary metabolic disorder that is controlled by a single autosomal recessive gene.
The gene specifies an enzyme that produces a unique glycoprotein. This glycoprotein results in the production of mucous.
More mucous interfere with the normal functioning of several exocrine glands including those in the skin, lungs, liver and pancreas.

LETHAL GENE

  • Gene which causes death of individual in early stage when it comes in homozygous condition is called lethal gene. 
  • It may be dominant or recessive both, but mostly recessive for lethality.
  • Lethal gene was discovered by L. Cuenot in coat colour of mice.
  • Gene of yellow body colour of mice is lethal. So, homozygous yellow mice are never obtained in population. It dies in embryonal stage.
  • When yellow mice were crossed among themselves, segregation for yellow and brown body colour was obtained in 2 : 1 ratio.
   
  • YY - death in embryonal stage 
Modified ratio – 2 : 1
  • In plant, lethal gene was first discovered by E. Baur in Snapdra (Antirrhinum majus)
Modified Ratio = 2 : 1
Homozygous golden leaves are never obtained.
  • In human, gene of sickle cell anaemia HbS is the example of lethal gene.
When two carrier individuals of sickle cell anaemia are crossed then offsprings are obtained in 2 : 1 ratio.
It is a sub lethal gene but ratio is 2 : 1

MULTIPLE ALLELE

  • More than 2 alternative forms of the same gene is called as multiple allele.
  • Multiple allele is formed due to mutation and located on the same locus of homologous chromosome.
  • A diploid individual contains two alleles and gamete contains one allele for a character. 
  • Blood group - 3 alleles
Coat colour in rabbit - 4 alleles
If n is the number of allele of a gene then 
number of different possible genotype =

EXAMPLE OF MULTIPLE ALLELE
(1) ABO blood group : ABO blood groups are determined by allele IA, allele IB, allele IO
IA = dominant
IB = dominant
IO = recessive
Possible phenotypes - A, B, AB, O

Possible genotype number =  = 6 genotype

NOTES
  • The position of a gene on the chromosome is called a locus.
  • Dr. Karl Landsteiner got Nobel Prize in 1930, for discovery of blood groups A, B and O.
  • Blood group AB was discovered by De Castelo and Sturli.
  • Blood groups M, N and MN were discovered by K. Landsteiner and A. S. Wiener.
  • Rh factor was discovered by K. Landsteiner and A. S. Wiener. 
  • If one parent has AB blood group (IAIB) and other parent is with O blood group (i i) then none of their offspring can have blood groups of any parent i.e., AB and O.
  • If both parents are with blood group A and any of their offspring has blood group O, it means both parents are heterozygous (IAi). 
  • If one parent has AB blood group (IAIB) then none of their offspring can have O blood group.
  • If one parent has O blood group (i i) then none of their offspring can have AB blood group.

(2) Coat colour in rabbit : There are four alleles for coat colour in rabbit.
Wild type = Full coloured = agouti = C+
Himalayan [white with black tip on extremities (like nose, tail and feet)] = ch 
Chinchilla [mixed coloured and white hairs) = Cch
Albino = Colourless = ca
These alleles show a gradient in dominance C+ > cch > ch > ca

Possible genotypes
Coloured = C+C+, C+cch, C+ch, C+ca
Chinchilla = cchcch, cchch, cch ca
Himalayan = chch, chca
Albino : caca
Possible genotype = = 10 genotypes

(3) Eye colour in Drosophila and self incompatibility genes in plants are also the example of multiple allelism.

GENE INTERACTION

  • Gene interaction is the modification of normal phenotypic expression of genes due to their alleles and non-allelic genes.
  • Gene interaction is a post-mendelian discovery.
  • Gene interaction are of two types- intragenic interaction and intergenic interaction. 
Flow Chart : Types of gene interaction

EPISTASIS

  • When a gene prevents the expression of another non-allelic gene, then it is known as epistatic gene and the phenomenon is known as epistasis.
  • Expression of gene which is suppressed by epistatic gene is called hypostatic gene.
  • Example : Hair colour in dog :
B = Dominant allele for black colour of hair.
b = Recessive allele for brown colour of hair.
I = Epistatic gene.
If the genotype bbii is for brown colour and BBII is for white colour, then the following types of generation will be obtained by following crosses.
Phenotypic ratio of F2- generation in epistasis is = 12 : 3 : 1.

DUPLICATE GENES

  • Two pairs of non-allelic genes is required for a character. If anyone of the gene is dominant, then this character is expressed. Such type of gene is called duplicate gene.
  • Example : Fruit shape in Capsella bursa-pastoris (Shepherd’s Purse)
Two pair of non-allelic genes are present in Capsella for triangular shape of fruits.
If any one gene (out of them) is dominant, the shape of fruit is triangular and no one gene is dominant then fruits will be elongated.
If, TTDD = For triangular shape
ttdd = For elongated shape of fruits; then
Phenotypic ratio of F2 = Triangular shaped -   15 : 1

COMPLIMENTARY GENE

  • Two pairs of non-allelic genes are essential in dominant form to produce a particular character. Such genes that act together to produce an effect that neither can produce its effect separately are called complementary genes.
  • Both types of gene must be present in dominant form.
  • Example : Colour of flowers in Lathyrus odoratus
CC - PP : Purple coloured
CC - pp : Colourless
cc - P : Colourless
cc - pp : Colourless
 
Thus, phenotypic ratio of complementary genes 
= Coloured : Colourless = 9 : 7

SUPPLEMENTARY GENE OR RECESSIVE EPISTASIS

  • A pair of gene change the effect of another non allelic gene is called supplementary gene.
  • Example : Coat colour in Mice.
If alleles, C = Black coat colour
c = Albino (Colourless coat) or (it has no effect)
A = Supplementary gene
When black coat mice crossed with albino mice, the Fl generation is agouti. It means, here the effect of non-allelic gene is changed. 
Thus, recessive epistasis or supplementary gene ratio in
F2 - agouti, black, albino  is 9 : 3: 4

TYPES OF INHERITANCE

  • There are two types of inheritance - qualitative and quantitative.
  • Qualitative inheritance is a type of inheritance in which a single dominant gene influences a complete trait. The gene controlling inheritance are called monogenes. This type of inheritance produces a sort of discontinuous trait variations in the progeny, e.g., either tallness or dwarfness.
  • The genes involved in quantitative inheritance are called polygenes (also called cumulative genes).
  • Inheritance of characters in which one character is controlled by many genes and intensity of character depends upon the number of dominant allele is called polygenic or quantitative inheritance.
  • Polygenic inheritance first described by Nilsson- Ehle in kernal colour of wheat.
Nilsson-Ehle said that kernal colour of wheat is regulated by two pairs of gene.

SEX DETERMINATION

  • Establishment of sex through differential development in an individual at an early stage of life, is called sex determination. 
  • Sex determination in most plants and animals is concerned with the study of factors which are responsible for making individual male, female or hermaphrodite.
  • Various methods operate in sex determination
    like environmental, non-allosomic genetic determination, allosomic sex determination and haplodiploidy.
  • Sex Determination on the basis of fertilization are of three types-
    • Progamic : Sex is determined before fertilization.
      E.g., - Drone in honey bee.
    • Syngamic : Sex is determined during fertilization.
      E.g., - Most of plants & animals.
    • Epigamic : Sex is determined after fertilization.
      E.g., - Female in honey bee.

MECHANISM OF SEX DETERMINATION

  • Wilson & Stevens proposed chromosomal theory for sex determination.
  • Chromosomes are of two types – autosome and allosomes
  • Autosomes or somatic chromosomes regulate somatic characters.
  • Allosomes or heterosomes or sex chromosomes   are associated with sex determination. 
  • Sex chromosomes first discovered by McClung in grasshopper.
  • X chromosome (called X body) was discovered by Henking when he found that in the testes of male bug one chromosomes has no homologue.
  • There are 5 types of mechanisms of sex determinations-
    • In XX-XY type of sex determination, female is homogametic i.e. produces only one type of gamete.
 
In male, X-chromosome containing gametes is called gynosperm and Y-chromosome containing gamete is called androsperm. E.g., Man, Drosophila and dioecious plants like Coccinea, Melandrium.
    • In ZW-ZZ type of sex determination, female is heterogametic i.e. produces two types of gamete (ZW) and male individual is homogametic i.e., produces one type of gamete (ZZ).
It is found in some insects like butterflies, moths and vertebrates like birds, fishes and reptiles.
In plant kingdom, this type of sex determination is found in Fragaria elatior.
    • In XX-XO type of sex determination, there is deficiency of one chromosome in male. In this type, female is homogametic and male is heterogametic.
It is found in grasshopper, squash bug anasa, cockroach, Ascaris  and in plants like - Dioscorea sinuta & Vallisneria spiralis.
    • In ZO-ZZ type of sex determination, female had odd sex chromosome while the males have two homomorphic sex chromosomes (AA + ZZ). The females are heterogametic, and produce two types of eggs, male forming with one sex chromosome (A + Z) and female forming without the sex chromosome (A + O). The males are heterogametic, forming similar types of sperms. It is found in some butterflies and moths. It is exactly opposite the condition found in cockroaches and grasshoppers.
    • Haplodiploidy is the sex determination method in which one sex is haploid (male) while  other is diploid (female), e.g., honeybees. It also occurs in wasps and ants.
In honey bee, male individual (drone) develops from unfertilized eggs (haploid). Male is always parthenote. Queen and worker bees develop from diploid eggs i.e. fertilized egg.

SEX DETERMINATION IN HUMAN

  • Human beings have 22 pairs of autosomes and one pair of sex chromosomes.
  • There occurs a special gene on differential region of Y-chromosome of human, called Sry-gene (Sex determine region on Y chromosome ). This gene forms a proteinaceous factor called TDF (testes determining factor). 
  • TDF is responsible for the development of male reproductive organs. So, presence and absence of Y-chromosome determines sex.

SEX DETERMINATION IN DROSOPHILA

  • According to Bridges, in Drosophila,  Y-chromosome is heterochromatic, so it is not active in sex determination. In Drosophila, sex determination takes place by sex index ratio.
Sex index ratio =
  • In Drosophila, gene of femaleness (Sxl- gene) (Sxl = Sex lethal gene) is located on X-chromosome and gene of maleness is located on autosome.
  • Gene of male fertility is located on Y-chromosome and in Drosophila, Y-chromosome plays additional role in spermatogenesis and development of male reproductive organ, so Y-chromosome is essential for the production of fertile male.
  • Sex index ratio
  • Body of some Drosophila have some cells with male genotype (XO) and some cells with female genotype (XX). Body of such type of Drosophila has half lateral part of male and half lateral part of female and it is called bilateral gynandromorph. It is formed due to loss of one X-chromosome at metaphase plate during first zygotic division. Formation of gynandromorph is the best evidence that Y-chromosome does not play any role in sex differentiation.

LINKAGE AND RECOMBINATION IN NEUROSPORA 

  • Detection of linkage and recombination of genes in haploid organisms as in fungi, bacteria etc. is comparatively simple. 
  • Fungus Neurospora is one of the favourite material with geneticists, because :
    • The life cycle of Neurospora is the product of a single meiosis.
    • The life cycle is of a short duration.
    • The meiotic products are linearly arranged in ascus with 8 ascospores as ordered tetrads (i.e., the eight ascospores are arranged in the same order in which chromatids were on the meiotic metaphase plate).
  • Tetrad analysis : In Neurospora, the nuclei from hyphae of opposite mating type (+) and (–) fuse to form a diploid zygote. The zygote is the only diploid stage in the life cycle of Neurospora. The zygote nucleus divides meiotically producing four haploid nuclei, each of which then undergoes mitosis. The eight cells produced this way, form 8 haploid ascospores enclosed in the ascus. The three divisions proceed along the longitudinal axis, so the ascospores are arranged in the line in a specific order that indicates the order of arrangement of chromatids on the meiotic metaphase plate. This is called linear or ordered tetrad. Each of the four products of meiosis can be cultured separately to study their phenotypes and genotypes. This is called tetrad analysis.
  • Single Gene Mapping in Neurospora :
In Neurospora, centromere behaves as a gene for mapping gene pair. In such a case, distance of gene from the centromere is calculated by calculating the percentage of crossovers between centromere and gene. 
E.g. If 10% asci show crossing over in ascocarp, what will be the distance between gene and centromere? 
Sol. If total 100 asci are present in a Neurospora
1 asci is derivative of 4 chromatids 
100 asci are derivative of 400 chromatids = total chromatids
10 asci are derivative of 40 chromatids
(Out of 40 only 20 will be the recombinant type)
% C.O.  =
=centiMorgan

MUTATION

  • Sudden heritable change in genetic material of an organism is called a mutation. 
  • Mutation are discontinuous source of variation.
  • Mutation word was given by Hugo De Vries.
  • De Vries studied mutations in the plant Oenothera lamarckiana (evening primrose). It is a hybrid plant.
  • He observed 834 mutation in 54343 plants of Oenothera lamarckiana.
  • Credit of discovery of mutation is given to Morgan. He observed some white eyed male Drosophila in a population of red eyed Drosophila.
In Drosophila, eye colour is a sex linked character. Gene of eye colour is located on X chromosome. Gene of red eye is dominant over the gene of white eye. 
  • Muller discovered the induced mutations. He induced mutations in Drosophila with the help of X-rays.
  • Mac Farlane Burnitt, Neil Jerne induced mutations in B-lymphocytes of blood to obtain new antibodies.
  • Beadle and Tatum induced mutations in Neurospora to study nutritional mutation with the help of U.V. rays or X-rays.
Wild Neurospora  Mutant Neurospora
(Prototroph) (Auxotroph) 
Normal-Neurospora can be grown in minimal medium (which lacks some nutrients), because it can make all nutrients for it. This is known as prototroph.
Mutant Neurospora doesn't has capability to grow in minimal medium because due to mutation it loses those genes which prepare some special nutrients for it. E.g., Vitamin-B or Thiamine.
When Vit-B or Thiamine was given to mutant Neurospora then the growth of Neurospora was normal. This form is known as auxotroph.
  • M.S. Swaminathan induced mutations in wheat with the help of γ-rays to obtain good varieties, for e.g., Sharbati sonora. 
  • Gene which induce mutation in another gene is called mutator gene and gene in which mutation is induced is called as mutable gene.
  • Muton (unit of mutation) is the smallest part of DNA which undergoes mutation. It is one nucleotide.
  • Mutation are of two types - chromosomal mutation and gene mutation.
  • Chromosomal mutation is the changes that occur in the morphology of chromosomes resulting in change in number or sequence of gene without any change in  ploidy.
  • Types of chromosomal mutation are - Heteroploid/Genomatic mutation. 
  • Heteroploidy / Genomatic mutation : It is change in number of sets or chromosomes in sets. It is of two types : euploidy (Change in number of sets) and aneuploidy (Change in number of chromosomes in set).
  • Chromosomal aberrations : It is change in structure of chromosomes. It is of 4 types-deletion, inversion, duplication and translocation.
  • Deletion is the loss of a part or segment of chromosome which leads to loss of some gene. It is of 2 types : terminal and intercalary.
    •  Terminal deletion is loss of chromosomal segment from one or both ends.
E.g., The cry-du-chat syndrome is an example of terminal deletion in 5th chromosome.
    • Intercalary deletion is the loss of chromosomal part between the ends.
  • Inversion is breakage of chromosomal segment but reunion on same chromosome in reverse order is called inversion.
    It leads to change in distance between genes on chromosome or sequence of genes on chromosome, so crossing over is affected. It is of 2 types : paracentric and pericentric.
    • Paracentric : Inversion occur only in one arm and inverted segment does not include centromere.
    • In pericentric inversion inverted segment include centromere.
  • Duplication : There is occurrence of a chromosomal segment twice on a chromosome. In this segment if any recessive gene is present, then it gives it's expression due to homozygous condition. If in this segment any recessive but lethal gene is present, it leads to death of organism.
Example: In Drosophila, “Barr eye character” is observed due to duplication in X-chromosome. Barr eye is a character where eyes are narrower as compared to normal eye shape.
  • In translocation, a part of the chromosome is broken and may be joined with non-homologous chromosome. This is also known as Illegitimate crossing over (illegal crossing over)

Three types of translocation are :
    • Simple translocation : When a chromosomal segment breaks and attach to the terminal end of a non- homologous chromosome.
    • Interstitial or shift translocation : If a segment of chromosome breaks and gets inserted in interstitial position of a non-homologous chromosome.
    • Reciprocal Translocation : Exchange of segments between two non-homologous chromosome.
E.g., Chronic myeloid leukemia [CML] is a type of blood cancer. This disease is a result of reciprocal translocation between 22 and 9 chromosome.

NOTES
If exchange of segments takes place in between homologous chromosomes then it is called crossing over.
  • Point mutation are changes that occur in an individual gene. These are due to a structural change in the DNA molecule at a single locus. These alter the message conveyed by genes.
  • Point mutation are limited to a single nucleotide or a single base affecting a single gene.
  • Gene mutation are sudden and distinct changes in the genes which can be detected by the visible changes in the phenotype of an organism.
  • Point mutation could be dominant, recessive, lethal, harmless, X-linked or autosomal.
  • It is of two types - substitution and frameshift mutation.
  • Replacement of one nitrogenous base by another nitrogenous base is called as substitution.
  • It causes a change in one codon in genetic code which leads to change in one amino acid in the structure of protein. E.g., sickle cell anaemia.
  • Frameshift mutation/Gibberish mutation : The chemicals like acridine and proflavin causes loss or addition of one or two nitrogenous bases in structure of DNA leading to complete change in reading of genetic code. It leads to change in all amino acids in structure of protein so a new protein is formed which is completely different from previous protein.
ATG ACG GAC AGA AAC .............
ATG C G G ACA GAA AC...............
  • Frameshift mutations are more harmful as compared to substitution. E.g., Thalassemia (Lethal genetic disorder)
  • Mutagens are those substances which cause mutations. 
  • Mutagens are of two types - physical and chemical. 
  • Physical mutagens includes temperature and high energy radiations.
  • Increase in temperature increases the rate of mutation with Q10 = 5.
  • Radiation are of two types –
    • Ionising :- α, β, γ, X-ray
    • Non-ionising :- U.V. rays.
  • U.V. rays has less penetration power and skin of higher organisms absorb radiations. So, they don't cause any effect in higher animals, but U.V. rays and radiations are effective mutagens in microbes and due to more effect, leads to death of microbes. So, U.V. rays are used to sterilize operation theatre.
  • U.V. rays and HNO2 cause deamination of nitrogenous base which means they remove amino group from nitrogenous base by deamination of
Adenine → Hypoxanthine
Guanine → Xanthine
Cytosine → Uracil
U.V. rays do not cause deamination in thymine.
  • Chemical mutagens are more harmful than radiation because the body is not protected against chemicals. Source of chemical mutagens are food, air and water.
  • Effect of radiation is localised, while chemical mutagens spread in complete body through blood circulation and when they reach in gonads they cause germinal mutation.
  • Chemicals also cause chromosomal mutations.
E.g., Mustard gas (first identified chemical mutagens), Carbon tetra sulphide, Nitrous acid (HNO2), Organic peroxide, Ethyl urethane, Pesticides, DDT (Dichloro Diphenyl Trichloro Ethane) and LSD (Lysergic acid diethylamide)
  • Antibiotics like (neomycin, kanamycin and streptomycin) combine with small subunit of prokaryotic ribosome and cause misreading of genetic code or induce error in translation. Same effect of puromycin antibiotic occurs in eukaryotes.

HUMAN GENETICS

Sir Francis Galton, 1883 proposed the idea of improvement in human species through change in hereditary characters in a scientific manner and named it eugenics. Because of this Sir Francis Galton is known as “Father of Eugenics”.
The study and analysis of human genetic is performed by many methods like pedigree analysis statistical analysis and human karyotyping.

PEDIGREE ANALYSIS

  • Study of ancestral history of man of transmission of genetic characters from one generation to the next, is called pedigree analysis. Dwarfism, albinism, colour blindness, haemophilia etc. are genetically transmitted characters. 
  • To study and analyse them, a pedigree of genetic facts/data and following symbols are used.
  • Symbol used in Pedigree:
  • Pedigree analysis provides valuable information regarding genetic make up of human beings. If any genetic disease is occurring in a family, then pedigree analysis provides guidance to forthcoming parents about their future progenies, for example- polydactyly in humans.

HUMAN KARYOTYPE

  • Humans have 23 pairs (46) of chromosomes. In this method, the chromosomes (autosomes and sex chromosomes) are arranged according to their size and structure.
  • Based on the position of centromere and relative lengths of both arms of chromosome, three types of chromosomes are found in human - metacentric, submetacentric and acrocentric.
  • Karyotype helps to know the relative structures (morphology) of chromosomes. Besides, it helps in chromosomal identification and it's nomenclature. It is also used in studying chromosomal abnormalities.

GENETIC DISORDERS


  • A genetic disorder is a disease that is caused by an abnormality in an individual’s DNA.
  • Genetic disorder may be grouped into two categories - Mendelian disorders and chromosomal disorders.
  • Mendelian disorders are chiefly determined by alteration or mutation in the single gene. E.g., Haemophilia, cystic fibrosis, sickle cell anaemia, thalassemia, colour blindness, phenylketonuria, etc.
  • Haemophilia is an inherited disorder of blood in which an essential clotting factor is either partly or completely missing.
  • In sickle-cell anaemia, glutamic acid (glutamine) is replaced by valine at the sixth position in β chain of haemoglobin. It is a blood disease. Where, the red blood blood cells become sickle shaped as compared to normal one.
  • Thalassemia is due to an autosomal mutant gene. It is a group of genetic disorder which results from defective synthesis of subunits of haemoglobin (α & β-globin chains of haemoglobin).
  • Gaucher's disease is a genetic disorder associated with abnormal fat metabolism.
  • Huntington's chorea is a disease caused by a dominant gene. 
  • The chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s) called aneuploidy. Types of chromosomal disorders are - Down’s syndrome, Klinefelter’s syndrome and Turner’s syndrome.
  • Down’s syndrome is caused by the presence of an additional copy of the chromosome number 21 (trisomy of 21). The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.
  • Klinefelter’s syndrome is caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY. Such an individual has overall masculine development , however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. Such individuals are sterile.
  • Turner’s syndrome is caused due to the absence of one of the X chromosomes, i.e., 45 with XO, Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.

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Best Coaching Institute in Delhi for IIT JEE, NEET, AIIMS, PMT - Clear IIT Medical: Principles of Inheritance and Variation | Biology Notes for NEET/AIIMS/JIPMER
Principles of Inheritance and Variation | Biology Notes for NEET/AIIMS/JIPMER
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