question_answer

(i) Discuss and work out Mendel's monohybrid cross upto \[{{F}_{2}}-\] generation by giving an example of seed colour as a trait in Pisum sativum.

(ii) State the law of inheritance which can be derived from such a cross.

(iii) How is the phenotypic ratio of \[{{F}_{2}}-\]generation of monohybrid cross different from that of dihybrid cross?

Or

The pattern of inheritance of ABO blood group is known to exhibit dominance, codominance and multiple allelism. Explain each concept with the help of blood group genotypes.

Answer:

(i) In a monohybrid cross, when homozygous dominant and homozygous recessive parents are crossed in generation all hybrids are obtained.

These are heterozygous for that trait and express only dominant allele. This can be explained by crossing plants of Pisum sativum having yellow seeds (YY) and green seeds (yy). Both are homozygous for their characters. In \[{{F}_{1}}-\]generation, all plants having yellow seeds are obtained. These are hybrids. On selfing in \[{{F}_{2}}-\]generation, yellow seed and green seed plants are Obtained in 3 : 1 ratio.                                      (2)

(ii) All plants of the \[{{F}_{1}}-\]generation are heterozygous containing both alleles of seed colour (i.e. Y and y) but only one trait, i.e. yellow colour appeared in the first generation. This shows that yellow seed colour is dominant over the green seed colour. This explains Mendel's law of dominance. This law states that characters are controlled by discrete units called factors, which occur in pairs. In a dissimilar pair of factors, one member of the pair which expresses itself in \[{{F}_{1}}-\]generation is dominant over the other.

(iii) Phenotypic ratio in the \[{{F}_{2}}-\]generation of monohybrid cross is different from that of dihybrid cross in the following way In monohybrid cross 3 : 1 In dihybrid cross 9 : 3 : 3 : 1

Or

In humans, ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene I has three alleles \[{{I}^{A}},\,\,{{I}^{B}}\] and i. The alleles \[{{I}^{A}}\] and \[{{I}^{B}}\] produce a slightly different form of the sugar while, allele i does not produce any sugar. In humans, each person possesses any two of the three l gene alleles. ABO blood group exhibits dominance, codominance and multiple allelism.

Dominance   In this phenomenon, one allele expresses itself in the presence of other allele. The alleles\[{{I}^{A}}\]and \[{{I}^{B}}\]are dominant over allele i. The former two form antigen-A and antigen-B, respectively, whereas i do not form any antigen.

Codominance In this phenomenon, \[{{F}_{1}}-\]generation resembles both the parents whose characters are expressed simultaneously. Alleles \[{{I}^{A}}\] and \[{{I}^{B}}\]are codominant as both of them are expressed in blood group AB by forming antigen - A and B. Multiple Allelism It is the phenomenon of occurrence of a gene is more than two allelic forms on the same locus. The ABO blood groups in humans are determined by three different allelic forms \[{{I}^{A}}\] and \[{{I}^{B}}\] and i.