Download nondisjunction

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 X R
chromosome and half containing the Y chromosome. Recall that during meiosis the X r 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 X rXr 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.