A Large Number of Genes Have Been Found to Be Sex-Linked. Each Exhibits the Same Pattern

NONDISJUNCTION

A large number of genes have been found to be sex-linked. Each exhibits the same pattern of inheritance as white eyes. One such gene for vermillion eye color(Xr), which is a recessive allele to the wild-type, red-eye allele. What type of offspring would you expect from the crossing of a vermillion-eyed female and a wild-type male? The following is a diagram of the cross.

P1: XrXr x XRY

F1: all XrY and all XRXr

vermillion (males) red (females)

In making the vermillion-wild cross, C. B. Bridges observed that occasionally, once in two thousand flies, a vermillion-eyed daughter was produced. Also, an occasional wild-type son occurred. Clearly these flies are exceptions and normally impossible. These observations raised doubts about the validity of the chromosome theory of heredity. The red-eyed male is XRY and the vermillion-eyed female is XrXr. Females in the F1 must receive XR from the male and Xr from the female. How could a vermillion-eyed female XrXr result? Yet the condition occurred and must be accounted for in terms of sex chromosomes and sex-linked traits. Bridges examined the chromosomes in the vermillion female and found that it had two X chromosomes (XX) and a Y chromosome. Thus the vermillion condition could be represented as XrXrY. The extra X chromosome produces a female even if a Y chromosome is present. The two X chromosomes must carry the recessive vermillion gene, thus producing the impossible vermillion-eyed female from this mating.

The next question is, how do these flies get this abnormal chromosome number? The answer is called nondisjunction. During this mating the male parent produces normal sperm, half containing the XR chromosome and half containing the Y chromosome. Recall that during meiosis the Xr chromosome produces a copy of itself. Normally the two split apart, separating the two chromatids in different eggs and providing a sex chromosome for each egg. In the female of this cross the XrXr chromatids fail to separate so that one egg is produced with the two X chromosomes (XrXr) and one egg without a sex chromosome. This cross can be illustrated as shown at right.

The finding of Bridges helps support the chromosome theory. Also, this find leaves no doubt that the X chromosome in Drosophila carries genes. Nondisjunction occurs in autosomes as well as sex chromosomes. Nondisjunction has been found in many organisms, including man, and will be discussed more in lesson 11.