Sex is determined by the composition of the sex chromosomes in an organism's genotype [XX = female,
XY = male]. The Y chromosome is generally devoid of genes while the X chromosome contains a fair number. This means that recessive genes on the X chromosome in males can be phenotypically expressed, even when occurring as a single dose, because the Y homologue very likely will have no masking dominant allele. In females, such recessives must be homozygous before they can be expressed. An exercise at the right demonstrates the phenomenon of sex linkage. It involves the inheritance of white eye color in Drosophila... a characteristic that is sex linked. You are asked to click on the buttons that describe the percentage of the final population that displays the phenotype in question. If you think the phenotype is not displayed at all, do not click on any button. These are reciprocal crosses. Run them both and then click on the lower scroll arrow to bring the rest of this material into view. Note that in each case, the female gametes are listed vertically on the left and the male gametes horizontally across the top of the Punnett square. Note also that the Y chromosome for the male is delineated as a /.
You will note that in the cross with the white-eyed female, only the males display the white eye. All of the females appear normal but they are carriers. In the cross with the normal eyed female, all of the F1 appear to have normal eye color. However flip to the next card for a continuation of this exercise.
