Chapter 15, Problem 15.1CR

What characteristic of the sex chromosomes allowed Morgan to correlate their behavior with that of the alleles of the eye color gene?

Summary Introduction

To review: The characteristics of sex chromosomes that helped Morgan to correlate their behavior with that of the alleles of the eye color gene.

Concept introduction: Sex chromosomes are those chromosomes that play a crucial role in determination of Sex of the individual. They have important function in the development of sexual characteristics in the organisms.

Explanation of Solution

The X and Y chromosomes act as sex chromosomes in most of the organisms. The genes which are present on sex chromosome are called sex-linked genes. There are approximately 1,100 X-linked genes. On the other hand, Y chromosome contains relatively very few genes.

The sex-linked genes display a unique pattern of inheritance. The expression of recessive X-linked trait requires two copies of alleles in females (homozygous) and only one copy of the allele in males (hemizygous).

The probability of a male to receive a single mutated X-linked gene from their mother is high as compared to a female receiving both the X-linked mutated genes from the mother (carrier) and father (carrier).

When Morgan crossed red-eyed female (wild-type) with white-eyed male (mutant), all the offspring were red-eyed in the first generation (F1). Then he again crossed the F1red-eyed female to the red-eyed male of F1. The typical ratio of 3:1 (red-eyed flies: white eyed flies) was observed in the F2, but all white-eyed flies were males.

Morgan concluded that this could be due to hemizygous condition (presence of single X-chromosome) of the males. Females required both the mutated alleles to express the phenotype (white eye color) while male needed only one mutated allele to show the phenotype. The probability to inherit both mutated copy is less. Hence, no females displayed the white coloration of eye.

Conclusion

Sex-linked genes exhibit unique inheritance pattern