Genetic Basis of Inheritance

Introduction.

Mendels experiments.

Procedure of the experiments.

Monohybrid cross experiments and monohybrid ratio and its explanation.

Mendels laws of inheritance.

Dihybrid cross experiments and dihybrid ratio and its explanation.

Test cross or back cross and monohybrid test cross

Deviations from Mendelian ratios.

introduction

Continuity of life and perpetuation of species is due to the remarkable characteristic of living organisms to reproduce.

All organisms inherit characters (traits) from their parents.

Hence offspring resemble their parents but they may have some dissimilarities.

This similarities and differences are the two aspects of the mechanism of inheritance.

Heredity is defined as transmission of character from one generation to the next or from parents to offspring.

Variation are the differences between parents and offspring (play important role in evolution).

Genetics is the brance of biology that deals with heredity and variations.

Genetics → to grow into → William Bateson in

1960

The first scientific explanation regarding inheritance was given by Gregor Johann Mendel in 1866.

“father of genetics “

He performed experiments on garden pea and give proposed postulates which is popularly now known as Mendels laws of inheritance or Mendelism.

Johann Mendel was the first one to say that the characters are transmitted from one generation to the next through particles “factors”, that are now known as genes.

Mendels experiments :

How are the characteristics transmitted from parent to the offspring?

Why there are similarities as well as differences?

How certain traits reapper in grand children?

Answers to all these question were to be found out through experimentation.

Character Dominant trait Recessive trait

Stem height Tall (T) Drawf (t)

Seed colour Yellow (Y) Green (y)

Seed shape Round (R) Wrinkled (r)

Pod colour Green (R) Yellow (g)

Pod shape Inflated (I) Constricted (i)

Flower position Axial (A) Terminal (a)

Flower colour Coloured (C) White (c)

Selection of material :

Mendel made a very wise choice of experimental material.

He selected garden pea (Pisum Sativum .L) for his expt. Due to following reasons :

Garden pea is an annual plant and completes the life cycle within three to four month due to short life span he could grow three generations in a year.

It is small herbaceous plant that produce many seeds and so he could grow thousands of pea plant in small plot.

It is naturally self pollinating and was available in the form of many varieties with contrasting characters such as tall, dwarf plant etc there were no intermediate characters.

Though flower are naturally self pollinating, they are large enough for easy emasculation required for artificial cross and produce fertile offspring.

SELECTION OF CHARACTERS :

Before learning about mendels expt. Let us clear with genetic term and symbols.

1. Character : It is a feature of the organism eg:height of stem

2. Trait : An inherited character and its detectable variant eg : tall or dwarf

3. Factor : It is a unit of heredity, a particle present in the organism which is responsible for inheritance and expression of character.

4. Gene : It is a particular segment of DNA which is responsible for inheritance and expression of character.

5. Alleles / allelomorphs : The two alternative form of a gene are called alleles of each other. They occupy identical loci an homologous chromosome.

6. Dominant : It is an allele that expresses its trait even in the presence of an alternative allele ie in heterozygous condition.

7. Recessive : It is an alleles which is not expressed in the presence of an alternative alleles ie in heterozygous condition. It express in presence of identical alleles.

8. Phenotype : The external appearance of an individual for any trait is called phenotype for the trait.

9. Genotype : It is representation of the genetic constitution of an individual with respect to single character or set of character.

10. Homozygous : An individual possessing similar alleles for a particular trait is called homozygous or pure for that trait.

11. Heterozygous : An individual possessing dissimilar alleles for a particular trait is called heterozygous or hybrid for that trait.

12. Pure line : An individual or group of individuals that is homozygous or true breeding for one or more trait.

13. Hybrid : It is heterozygous individual produce from any cross involving pure parent having one or more contrasting trait.

14. Monohybrid : It is heterozygous for one trait and produce in a cross between two pure parents differing in single pair of contrasting character

eg : TT x Tt

T t

Tt

15. F1 generation : F = filial (offspring)

The progeny produce from a cross is called first filial or F1 generation.

16. F2 generation : The second generation produce by selfing of F1 generation offspring is called F2 generation.

17. Punnet square / checker board : It is a diagram used to show possibilities of combination in particular cross or breeding experiment. (Reginald C. Punnet)

Phenotypic ratio : tall : dwraf = 3:1

Genotypic ratio : TT : Tt : tt = 1:2:1

Male gamets Female gamets

T t

T TT Tt

t Tt tt

18. Homologous chromosome : the morphologically and structurally similar character present in adiploid cell are called homologous chromosome.

Procedure of experiments He performed monohybrid, dihybrid cross and

trihybrid crosses.

The quantitative and qualitative record was kept for three generations.

He conducted ample of crosses and reciprocal crosses to eliminate chance factor.

He conducted experiments in three steps as follow.

Step I : Selection of parents and obtaining pure lines.

Step II : Artificial cross of the selected parents to raise F1 generation.

Step III : Selfing of F1 hybrids to raise F2 generation.

Monohybrid cross experiments and monohybrid ratio :

Monohybrid cross – A cross between two pure parents in which inheritance pattern of only one pair of contrasting characters is studied is called monohybrid cross.

Step I : He selected pure tall plant as a female and pure dwarf plant as male and he confirm that they are true breeding by selfing for three generation.

Step II : He then performed artificial cross, sowed waited for seeds and waited for results (expectation – 50%will be tall and 50% dwarf). But he found that all were tall.

Step III : He allowed natural selfing of all hybrids, he found dwarfness reappeared.

He thought that it may be due to female dominance. So he performed reciprocal cross i.e now he selected tall plant as a male and dwarf plant as a female and repeated the procedure.

He got similar result i.e all tall in F1 generation and

tall and dwarf in app. 3:1 ratio in F2 generation.

Conclusion 1. He coined the term factor.

2. Each factor exists in two contrasting or alternative form called alleles.

3. One of the two forms is dominant and other is recessive.

4. In an organism, inheritance of each character is controlled by pair of factors, one of is contributed by male and other by female parent.

5. From the reappearance of trait in F2 generation he concluded that in the F1 generation factor do not mix with each other but they just remain together.

Mendels law of inheritance

Law of dominance or first law of inheritance : In a cross between two organism pure for any pair of contrasting characters, the character that appear in F1 generation is called dominant and one which is suppressed is called recessive.

This law is significant and true but not universally applicable.

Law of segregation or second law of inheritance : Members of allelic pair in ahybrid remain together without mixing with each other and separate or segregate during gamate formation.

Thus gamets receive only one of the two factors and are pure for given trait (law of purity of gametes).

This law is universally applicable.

Dihybrid cross experiments and dihybrid ratio

Dihybrid cross : A cross between two pure parents in which the inheritance pattern of two pairs of contrasting characters is considered simaltaniously is called dihybrid cross.

Dihybrid : It is heterozygous for two traits and produce in a cross between two pure parents differing in two pairs of contrasting character.

Dihybrid ratio : The phenotypic ratio of two different types of offsprings obtained in F2 generation of di hybrid cross is called dihybrid ratio.

Step I : He selected a variety of pea plant having yellow and round seeds as a female and another having green and wrinkled seeds as male parent. He obtain pure lines by selfing.

Step II : He performed artificial cross (emasculation). He opened pods and and collected the seeds. All seeds of F1 generation were yellow and round as excepted.

Step III : He sowed the seeds and raised the plants of F1 generation and allowed selfing. He open and collected seeds of F2 generation. He was expecting 75% yellow round and 25% green wrinkled but he find four types of seed i.e yellow round, yellow wrinkled, green round, green wrinkled in 9:3:3:1.

.

Conclusion 1. From the apperance of new combinations in the F2

generation, uniform pattern of results of dihybrid cross and dihybrid ratio , he establised principle of independent assortment

2. He performed tri hybrid crosses and then concluded that, when a dihybrid form gametes each gamete receives only one allele from each pair and assortment (distribution) of alleles of different traits is independent of their parental combinations i.e each alleles of any one pair of alleles is free to enter the gamete with any allele from each of the remaining pairs of alleles.

Law of independent assortment

When the two homozygous parents differing in two pairs of contrasting traits are crossed, the inheritance of one pair is independent of the other.

Or when a dihybrid forms gametes assortment of alleles of different traits is independent of their original combinations in the parents.

It is not universally applicable.

Test cross or back cross Test cross is a simple method devised by mendel to

verify the genotype of F1 / F2 /F3 hybrids.

When hybrid expresses recessive trait eg dwarf plant, its genotype is definitely tt but if hybrid tall, it is not possible to know its genotype.

Hence to determine whether the tall plant is homozygous or heterozygous test cross can be performed.

In test cross, F1 hybrid is crossed with the homozygous recessive parent.

Since the offspring is crossed back with one of parents it is also called back cross.

Mono hybrid test cross

In mono hybrid cross of pure tall and pure dwarf plant, all F1 hybrids are tall and with genotype Tt.

To verify its genotype, when they are crossed with recessive parent the progeny will be 50% tall and 50% dwarfs.

This is because recessive parent will produce only one type of gamete.

Therefore half is Tt and half will be tt

Thus dominant and recessive are 1:1 ratio .

This called test cross ratio.

The cross between F1 hybrid and homozygous tall parent is also back cross but this will produce all tall, 50% pure tall and 50% hybrid tall.

Therefore back cross with dominant parent cannot be used to test the genotype.

Thus a test cross is a back cross but back cross is not necessarily a test cross.

Deviations from Mendelian ratios

Post – Mendelian genetics or Neo – Mendelian genetics.

It was observed that the phenotypic expression of a gene can be modified by other gene.

These gene interaction is of two type intergenic and intragenic.

Intragenic : occur between alleles of same gene eg: incomplete dominance, co-dominance and multiple alleles.

Intergenic :occur between alleles of different genes on same or different chromosome eg :pleiotrophy, polygenes, complementary genes, supplementary genes etc.

Incomplete dominance Incomplete dominance both the genes of an

allelomorphic pair express themselves partially.

One gene cannot supress the expression of other completely.

Thus the pair is not as one dominant and other recessive.

In such cases there is intermediate expression in F1 hybrids.

Eg Four o’clock plant.

Co-dominance In co-dominance, both the genes of an allelomorphic

pair express themselves equally in F1 hybrids.

Such alleles which are able to express themselves independently even if present together in hybrids called codominant alleles.

Eg coat colour of cattle.

Multiple alleles More than two alternative form of alleles of a gene in

a population occupying the same locus on a chromosome are known as multiple alleles.

Multiple alleles arises due to mutations of wild type of gene.

Wild type is dominant over all other mutant alleles.

Multiple alleles donot under go crossing over.

Eg Drosophilla wings, ABO blood groups.

Pleiotrophy When a single gene controls two different traits, it is

called pleiotropic gene and this phenomenon is called pleiotrophy.

The ratio 2:1 instead of 3:1.

Eg sickle cell anaemia.

Polygenic inheritance In human population, characters such as height, skin

colour, and intelligence show gradations in expression and not only two contrasting expression.

These two characters are determined by two or more gene pairs and they are additive or cumulative.

These genes are called polygenes or multi factors or cumulative genes.

The ratio is 1:4:6:4:1 if there are 2 genes and if there are 3 genes the ratio is 1:6:15:20:15:6:1