When Morgan was doing his research with fruit flies, one of the lab members noticed that a single male fruit fly had white eyes instead of the red eyes that are normally found in the flies. Morgan crossed this white-eyed male with a normal red-eyed female, and found that all the F1 offspring had red eyes, as shown in Figure 12-3a. This demonstrated that the red-eye trait is dominant to the white-eye trait. Morgan next crossed F1 males with F1 females, as shown in Figure 12-3b. The resulting F2 generation showed the expected ratio of three red-eyed flies to one white-eyed fly. Unexpectedly, however, all of the white-eyed flies were male.
Based on this surprising observation, Morgan hypothesized that the gene for eye color is carried on the X chromosome and that the Y chromosome lacks an allele for the eye-color gene. An X chromosome carries a gene for eye color, either XR (red-eye allele) or Xr (white-eye allele). In a cross of an XRXR female (red-eyed) with an XrY male (white-eyed), all of the F1 females will be XRXr (red-eyed), and all of the F1 males will be XR Y (red-eyed).
In the F2 generation, half of the females will be XRXR, and the other half will be XRXr. Because all have the dominant allele R, all will be red-eyed. In the F2 males, however, half will be XRY (red-eyed), but the other half will be XrY (white-eyed).
The results of these experiments showed Morgan not only that genes reside on chromosomes but also that the red eye-color gene resides on the X chromosome. Morgan called genes located on the X chromosome X-linked genes. He called genes on the Y chromosome, such as SRY in humans, Y-linked genes. The term sex-linked trait refers to a trait that is coded for by an allele on a sex chromosome. The X chromosome is much larger than the Y chromosome, so there are more X-linked than Y-linked traits. Most X-linked alleles have no homologous counterpart on the Y chromosome. Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait.
Materials Two kinds of candy, toothpicks, pencil, paper Procedure Use two kinds of candy, each kind of which has two colors, to represent genes for two traits. Long noses are dominant over short noses. Large ears are dominant over small ears. One color of candy will represent the dominant allele, and a different color candy will represent the recessive allele. Use these materials to determine the outcome of a cross between two individuals, each heterozygous for both traits. Your teacher will tell you if the genes are linked or not.
1. Draw a Punnett square. Use the appropriate alleles to make gametes for each individual. Then place the allele combinations in each square representing the possible zygotes from that cross.
2. If your genes are linked, you must use toothpicks to link the genes together before you arrange gametes on your Punnett square.
Analysis What is the phenotypic ratio in the offspring when the genes are not linked? What is the phenotypic ratio when the genes are linked? Explain the difference.
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