Genetics. Genetics is the study and understanding of the phenomena of heredity and variation. Bateson first coined the term «genetics» in 1906. In Latin,

Genetics

Genetics is the study and understanding of the phenomena of heredity and variation.

Bateson first coined the term «genetics» in 1906. In Latin, it means genesis or origination of organisms.

Heredity is the transmission of characters from one generation to the next i.e. from parents to their offspring. Because of heredity, the offspring resemble their parents. Heredity is the essence of self-reproduction. It is owing to heredity and self-reproduction that we commonly observe the phenomenon of “like”.

Variations are the visible differences between the parents and the offspring or between two offspring’s of the same parents.

(1865) (1911)

I • Law of dominance III• Law of independent

II. • Law of segregation assortment

Laws of inheritance

Mendel’s Laws Morgan’s works

Monohybrid cross Dihybrid cross

Linkage

Mendel (1822-1884) Gregor Johann is an Austrian monk who performed experiments on garden peas to understand inheritance patterns.

Mendel selected garden peas (Pisum sativum) for his experiments after considering many other plants. His choice of garden pea was sound because

1) several pairs of contrasting traits have been noticed;

2) they are easily cultivated in large numbers;

3) the reproductive organs are enclosed within petals, hence naturally always self – pollinated;

4) artificial cross pollination is possible;

5) the hybrids obtained are completely fertile.

The breeding experiments were conducted over a period of eight (8) years. He presented the results of his work before the Society for the Study of natural Science in Brunn in 1866. Mendel’s discoveries did not receive attention then. In 1900 three eminent biologists

  - Holland

- Germany

- Austria

Independently rediscovered Mendel’s work. In his honour, all his discoveries were described as Mendelism and he is considered as “Father of Genetics”.

Hugo De Vries

Karl Correns

Von Tschermark

1 trait all traits of an individual Phenotype

A a

Two alleles

A - dominant allele (capital letter)

a - recessive allele (small letter)

Genotype genic constitutionof an individual

AA – dominant homozygous

aa – recessive homozygous

Aa – heterozygous or hybrid

Mendel coined the new experiment method what is named hybridologic method. Peculiarities of

hybridologic method are: 

1) using “pure – breeding” or “true – breeding” parents;

2) considering one or two (less quantity) character (s) at a time during the breeding experiments;

3) always keeping the generations separate;

4) maintaining separate record of the progeny of each individual ;

5) using statistical and mathematical principles to compute the obtained results;

Mendel conducted his experiments in three stages:

• I. Selection of a pair of parents showing contrasting forms of the chosen character. Mendel ensured that each of the 14 – fourteen varieties (seven pairs of contrasting traits) were “true – breeding”. Such “pure, true – breeding” parents were obtained by Mendel through repeated self – fertilization, generation after generation.

• II.The second stage is to hybridize or cross – pollinate such parent plants with the alternate forms of the trait. The cross where two alleles of a single trait are brought together or hybridized is called MONOHYBRID – CROSS. The hybrid off spring constitutes the next generation and is termed “First Filial” or F1 generation.

• III.The third stage was self – pollination of hybrid (F1) offspring plant to produce “Second Filial” or F2 generation. Mendel counted the number of each type of contrasting forms in every generation.

Law of DOMINANCEThe law of dominance is often described as “Mendel’s first law of inheritance”

“In a hybrid union (heterozygous condition), the allele which expresses itself phenotypically, is the dominant allele while the other allele which fails to express itself phenotypically is the recessive allele”.

The hybrid individual shows phenotypically only the dominant character.

  pure round seeds pure wrinkled seeds

P: RR x rr(genotypes) (homozygous dominant) (homozygous recessive)

G: (tupes of gametes all round seeds by parents)

  F1: Rr (hybrid) (heterozydous)

R r

Law of segregationIt also called Mendel’s second law of heredity

When a pair of alleles is brought together in hybrid union i.e. in heterozygous condition, the members of the allelic pair remain together without mixing, diluting or altering each other, and separate or segregate from each other when the hybrid forms

gametes.

P (F1): ♀ Rr x ♂ Rr Punnet’s square

G:

F2: RR; Rr; Rr; rr

round wrinkled

Phenotypic ratio 3 : 1 Genotypic ratio: 1 : 2 : 1

R R ♂

♀R r

R RR Rr

r Rr rr

r r

The results of MONOHYBRID CROSS are summarized as follows:

1. When a cross is made between plants having contrasting forms of a trait only one form the dominant trait is expressed in F1 – generation, while the other remains latent and is called recessive.

2. In F1, one form of the trait is hidden while in F2 both the forms are expressed in the ratio of 3:1 in the offspring.

3. Thus, there are definite hereditary units responsible for the transmission of characters. Mendel was the first to put forward clearly the idea that the hereditary units are present as definite particles and referred them as factors. These were later named as genes.

4. There are two alleles for each trait, called allelomorphic pair.

5. These factors segregate into the gametes of F1 hybrid so that each gamete carries only one factor either dominant or recessive, this is called purity of gametes.

 Purity of gametes: Factors responsible for the transmission of characters

segregate into gametes on factor either dominant or recessive, this is called purity of gametes.

A cross between two pure, true – breeding parents in which the inheritance pattern of two allelomorphic pairs is considered (studied) simultaneously is called a dihybrid cross.

Law of independent assortmentIt is also referred to as Mendel’s third law of heredity.

If we consider the inheritance of two or more pairs of genes at

a time, their distribution in the gametes and in the progeny

of subsequent generations is independent of each other.Mendel followed the same three stages of experimentation as he did for

monohybrid cross:

a)    First: he established true – breeding pea lines.

b)    Second step: he crossed a double dominant (Homozygous dominant) parent with double recessive(homozygous recessive) parent.

c)    Third step: Mendel self – pollinated the F1 – offspring and raised the F2-

generation

yellow round green wrinkled

P: ♀ YYRR x ♂ yyrr homozygous homozygous dominant recessive

G:

yellow round

F1: (dihybrid) YyRr double heterozygous,i.e

heterozygous for two pairs of alleles

P(F1): ♀YyRr x ♂YyRr G:

YR yr

YR yR

yrYr

YR yR

Yr yr

♀

♂

YR

Yr yR yr

YR YYRR

YYRr YyRR YyRr

Yr YYRr YYrr YyRr Yyrr

yR

YyRR

YyRr yyRR

yyRr

yr YyRr Yyrr

yyRr yyrr

YyRr

As each gene has two alleles, it can be said that each allelic pair assorts completely independent of the other during gamete formation. Independent assortment of genes does not negate, the law of segregation of alleles. Round and wrinkled seeds occur in the ratio of 3:1 as do yellow and green seeds (3:1) in the dihybrid cross.

Third Mendel’s law is to be realized by the following conditions:

1. nonallelic genes (Y;R) are localized in nonhomologic chromosomes;

2. allelic genes interact by complete dominance type.

Back Cross and Test Cross

A back cross is a cross when the F1 hybrid is crossed with either of the parent.

In the test cross the individual having dominant phenotype is crossed with one having a recessive phenotype that is, of course, homozygous.

In p dominant phenotype had been heterozygous, the result would be the 1:1

P: ♂ Rr x ♀ rr

G: r

F1: Rr ; rr - 1 : 1If P dominant phenotype had been homozygous, all offsprings of F1

generation would be identical

P: ♂ RR x ♀ rr

G: ? r

F1: Rr - all identical 

This test cross is done to identify the genotype of the dominant phenotype.

? r

? r

TERMINOLOGY USED

Factor: A particle or a unit in the organism which is responsible for the inheritance and expression of a particular character.

Gene: Mendel’s factor is now known as gene. Gene is a specific segment of DNA molecule which determines the inheritance and expression of a particular character.

Alleles or Allelomorphs: Two or more alternative forms of a gene are called alleles or allelomorphs. For example: in pea, gene for producing seed shape may occur in two alternative forms; round -R and wrinkled -r. Round and wrinkled forms of gene are alleles of each other. Alleles occupy same locus on homologous chromosomes.

Dominant: Of the two alternative forms (allelomorphs) of a trait, the one which appears in the F1 hybzid is called dominant trait (dominant allele). Recessive: Of the two alternative allelomorphs of a trait, the one which is suppressed (does not appear) in the F1 hybrid is called recessive trait (recessive allele).

Homologues or Homologous chromosomes: The morphologically similar looking chromosomes in a diploid cell (one chromosome coming from male parent and the other from the female parent) are called homologous chromosomes. They have identical gene loci bearing alleles.

Locus: Position or location of a gene on a chromosome. Genotype: The genetic make up or genic constitution of an

individual (which it inherits from the parents) is called the genotype, e.g. genotype of pure round seeded parent will be RR. Hence, when we refer to the type of combination of genes the word genotype is used.

Phenotype: The external (morphological) appearance of an individual for any trait or traits is called the phenotype, e.g. for seeds, round shape or wrinkled shape is the phenotype. Hence, when we refer to the expression of genes we the word phenotype.

• Pure breeding parent: Such pure, true – breeding parents were obtained by Mendel through repeated self – fezti – lization, generation after generation.

Homozygous: An individual possessing (receiving from parents) identical alleles for a trait is said to be homozygous or pure for that trait e.g. plant with RR alleles is homozygous for seed shape. A homozygous always breeds true for that trait.

Heterozygous: An individual receiving dissimilar alleles for a trait is said to be heterozygous or impure for that trait e.g. plant with Rr alleles is heterozygous for seed shape. Heterozygous is also called hybrid.

Parent generation (P): The parents used for the first cross represent the parental or P1 generation.

F1 generation: The progeny produced from a cross between two parents (P1) is called First Filial or F1 generation.

Inbreeding: When the individuals of a progeny (e.g. F1 generation) are allowed to cross with each other, it is called inbreeding

Monohybrid cross: The cross between two parents differing in a single pair of contrasting forms of a character is called monohybrid cross, and the F1 offspring as the hybrid (heterozygous for one trait only). The cross involving the inbreeding between two F1 monohybrid individuals is also called monohybrid cross.

Monohybrid ratio: The phenotype ratio of 3 dominants: 1 recessive obtained in the F2 generation from the monohybrid cross is called monohybrid ratio.

Dihybrid cross: The cross between two parents in which two pairs of contrasting characters are studied simultaneously for inheritance pattern. The F1 offspring is described as dihybrid or double heterozygous (i.e. with dissimilar alleles for two characters). The cross involving the inbreeding between two F1 dihybrid individuals is also called a dihybrid cross.

Dihybrid ratio: The phenotypic ratio obtained in the F2 generation from a dihybrid cross is called dihybrid ratio. In Mendelian experiments, this ratio is 9:3:3:1.

F2 generation: The progeny resulting from selfing or inbreeding of F1 individuals is called Second Filial or F2 generation.