Question

1.What is the physical basis of Mendel’s first and
second law?

2.What could be a reason that the first naturally
occurring mutant fly Morgan saw involved a
gene on a sex chromosome?

Answer 1

Ans(1): The physical basis of Mendel’s laws is the process of meiosis. Basis of law of segregation (2nd law) is the first division of meiosis in which the homologous chromosomes which have different versions of genes are segregated into daughter nuclei during formation of gametes. Hence, only one of the two alleles is present in a gamete and two alleles are independently transfered to next generation.

Basis of law of dominance(1st law) is fusion of gametes prodeuced by meiosis from two parents to produce offspring. As gametes are from two different parents, hence, they always have different versions of genes. Out of two different versions one may be dominant over other which form heterozygous (i.e. one form superior protein than other) or may produce similar offspring which produce homozygous (i.e. both form same proteins)

Ans(2): We know that a female fly has XX sex chromosomes and male fly has XY sex chromosomes. Hence, male has only one X chromosomes and for a male, a single copy of mutant X will cause a the change in phenotype. Hence, greater number of male will be affected. However, for other chromosomes where a single mutant chromosome cannot a change in phenotype only if it is dominant which is generally not the case.