Question

Mendel's Law of Inheritance.​

Answer 1

The three laws of inheritance proposed by Mendel include:

Law of Dominance

Law of Segregation

Law of Independent Assortment

Mendel stated that each individual has two alleles for each trait, one from each parent. Thus, he formed the “first rule”, the Law of Segregation, which states individuals possess two alleles and a parent passes only one allele to his/her offspring. One allele is given by the female parent and the other is given by the male parent. The two factors may or may not contain the same information. If the two alleles are identical, the individual is called homozygous for the trait. If the two alleles are different, the individual is called heterozygous. The presence of an allele does not promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals, the only allele that is expressed is the dominant. The recessive allele is present, but its expression is hidden. The genotype of an individual is made up of the many alleles it possesses. An individual’s physical appearance, or phenotype, is determined by its alleles as well as by its environment.

Mendel also analyzed the pattern of inheritance of seven pairs of contrasting traits in the domestic pea plant. He did this by cross-breeding dihybrids; that is, plants that were heterozygous for the alleles controlling two different traits. Mendel then crossed these dihybrids. If it is inevitable that round seeds must always be yellow and wrinkled seeds must be green, then he would have expected that this would produce a typical monohybrid cross: 75 percent round-yellow; 25 percent wrinkled-green. But, in fact, his mating generated seeds that showed all possible combinations of the color and texture traits. He found 9/16 of the offspring were round-yellow, 3/16 were round-green, 3/16 were wrinkled-yellow, and 1/16 were wrinkled-green. Finding in every case that each of his seven traits was inherited independently of the others, he formed his “second rule”, the Law of Independent Assortment, which states the inheritance of one pair of factors (genes) is independent of the inheritance of the other pair. Today we know that this rule holds only if the genes are on separate chromosomes.

Answer 2

☆ Mendel's Law of Inheritance :

• The laws related to explain similarities and differences in closely related individuals and to explain mechanism of inheritance of characters are called Mendel's Law of Inheritance.

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❥ The Mendel's law of inheritance include :

• ☞ The law of dominance and recessiveness.

• ☞ The law of segregation or purity of gametes.

• ☞ The law of independent assortment.

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⚘ Explanation :

• The law of dominance and recessiveness :

✾ The law of dominance states that out of a pair of contrasting characters present together, only one is able to express itself while the other one remains masked. The one that is expressed is dominant (for example, T out of Tt) and the other one that remains masked is recessive (for example, t out of Tt).

• The law of segregation or purity of gametes :

✾ The law of segregation states that when a pair of contrasting characters is brought together in a heterozygote (hybrid), the two members of the allelic pair remain together without blending. The two alleles separate out forming two types of gametes.

✾ In these F₁ gametes (i.e., T and t), 50 per cent of gametes from each parent have dominant gene (T) and the other 50 per cent have recessive gene (t). Their random fusion to produce F₂ generation results in four different combinations : TT, Tt, Tt and tt. The plants with TT, Tt genotypes are tall and the plants with tt genotype and dwarf. But only one - third tall plants of them are homozygous (TT), while two - thirds are heterozygous with genotype Tt. The tall or dwarf character is their phenotype.

• The law of independent assortment :

✾ When there is simultaneous inheritance of two pairs of contrasting characters, the distribution of factors of each pair in the gametes is independent of the distribution of other pair of characters. It means during gamete formation, segregating pairs of unit factors assort independent of each other.

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