Biology_2007_Virtual_Lab_Answer_Key-1

Describe how you determined the percentage one

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Describe how you determined the percentage. One hundred percent of the male offspring will have white eyes. This was determined by either crossing a red-eyed male fruit fly with a white eyed female fruit fly in the Drosophila laboratory and observing the results, or by performing a genetic cross using a Punnett square and crossing a red-eyed male (XRY) and a white-eyed female (XrXr) to determine that the genotypes that resulted were: 1 XRXr: 1 XrY. One out of one, or 100 percent, of the males are white- eyed. 6. Hemophilia, a blood disorder in humans, results from a sex linked recessive allele. Suppose that a daughter of a mother without the allele and a father with the allele marries a man with hemophilia. What is the probability that the daughter’s children will develop the disease? Describe how you determined the probability. There is a 50:50 chance that her offspring will have the disease. In other words, 50 percent of the children are expected to have hemophilia. This was determined by performing genetic crosses using two Punnett squares. In the first Punnett square, a female without the allele (XHXH) was crossed with a male with the allele (XhY). The genotypes of the offspring that resulted were: 1 XHXh:1XHY. One out of one, or 100 percent, of the female offspring are carriers (XHXh). If one of these females marries a man with hemophilia, the probability of their children developing the disease can be determined by drawing a second Punnett square. A female carrier (XHXh) is crossed with a male with hemophilia (XhY). The genotypes of the offspring that result are: 1 XhXh:1 XHXh:1 XHY:1 XhY. Two out of four, or 50 percent, of the offspring are expected to develop the disease.
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7. Color blindness results from a sex-linked recessive allele. Determine the genotypes of the offspring that result from a cross between a color-blind male and a homozygous female who has normal vision. Describe how you determined the genotypes of the offspring. All daughters will have normal vision, but they will be carriers of the allele. All males will have normal vision. This was determined by performing a genetic cross using a Punnett square and crossing a color-blind male (XbY) and a homozygous normal female (XBXB). The genotypes of the offspring that resulted were: 1 XBXb:1 XBY. Two out of two, or 100 percent, of the offspring have normal vision. 8. Explain why sex-linked traits appear more often in males than in females. In order for a sex-linked trait to appear in females, there must be recessive alleles on both of their X chromosomes. If there is a recessive allele on just one of their X chromosomes, then the females will be carriers and they will not display the sex-linked trait. Males, however, have only one X chromosome. If the recessive allele for a sex- linked trait appears on the male’s single X chromosome, the male will have the trait.
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