Let us take one of the females from the cross in which the original male parent was white eyed (The cross on the far right of the previous
card) and mate her with a normal eyed male. This cross is shown at the right. Again, click on the button which describes the percentage of offspring in that generation displaying the phenotype in question. Make sure you proceed in an orderly fashion from the top of the question list to the bottom. That way the program will run without problems.
If the phenotype is not to be found in the generation, then do not click on any button. After viewing this card and the previous card, what can you say about sex-linked inheritance? Place your answer in the space below and then click on the scroll arrow to bring further information into view after you have run through the exercise.
COMMENT:
Note that the white eye condition only appears in the males. Half of the females are carriers of the trait and half are not. However, no female displays the trait. It appears only in males and hence is sex-linked. The only way a sex-linked trait can appear in a female is for a carrier female to be mated with a male displaying the characteristic. Then 50% of the female F1 from such a cross would show the sex-linked trait. Since the frequency of such mutant genes is so low... the occurrence of females displaying sex-linked traits is very rare. However, an unusual type of inheritance was discovered by Bridges who crossed a white-eyed male fly with a true breeding normal female and obtained a white-eyed male in the F1. Trace back through the previous cards and see if this is possible. The data seemes to contradict the accepted theories of sex-linkage or sex determination in Drosophila. It involves a phenomenon called nondisjunction and can be studied by clicking on the NONDISJUNCTION button at the right.
