Sex-linked means that the gene is carried on a sex chromosome, usually the X chromosome. As you know sex in human s is determined by the sex chromosomes. If you have two copies of the X chromosome, you're female. If you have one copy of X and one of Y, you're a male. It works the same way in flies. Females are XX and males are XY. But when a gene is contained only on the X chromosome, it means that the males will only have one copy of that gene and you will see the recessive trait even if only one copy of the allele is present because there is no allele on the other chromosome to mask it. So, for sex-linked genes we get strange results in some crosses and we also need to have a way of indicating sex in the Punnett Square so that we can correctly interpret the results. The Punnett Square above does just that . Starting at the very top of the diagram, you see that we are crossing a red eyed female who has two copies of the X-chromosome and therefore two copies of the gene for eye color. Her X-chromosomes appear as an orangey color and the orange color indicates the presence of the dominant red allele. She's being crossed to a white eyed male. Here, his single X chromosome is represented by a green chromosome shaped like the female X chromosome but the green color is meant to signify the presence of the recessive white eye color allele in this male. The Y chromosome in the male is a sort of extended cigar shaped rod, also in green. The point is that the male parent only has one copy of the eye color gene on the X chromosome and it will be expressed even though it is recessive. The female fly is true-breeding so she can only produce the dominant red allele and for that reason, only one chromosome and one allele appear to represent the female parent on the Y-axis or left hand set of boxes. It just saves space but it doesn't change the results. And those results are that half of the progeny will be female, half male. All of the females will receive an orange chromosome from their mother with the dominant allele and a green X chromosome from their father with the recessive allele. The females will, thus, all be heterozygotes and all will be red eyed. Among the male progeny, each will inherit an orange X chromosome their mother and a green Y chromosome from their fathers. This means that all of the males will have red eyes because there is no gene for eye color on the Y chromosome.
Now move to the next cross on the graphic above. Here, we're taking a female parent who is heterozygous for the eye color trait and crossing her with a normal red eyed male. Based on our experience thus far, what do you expect the eye color of the progeny to be? Write the answer on line 1 of the response sheet below. For female progeny in this cross, each female will get a orange X chromosome with the red eye allele from her father and each female can get EITHER a green chromosome with a recessive allele or an orange X chromosome with a red allele from her mother because mom is a heterozygote. All females will be red because all will have at least one copy of the red allele but half of the females will be heterozygotes while the other half will be homozygous dominant. The males of this cross can inherit either the green X-chromosome with the white recessive allele OR the orange X chromosomes with the red dominant allele from mom. Their Y chromosome without the eye color gene will come from dad. At this point, write the prediction for phenotypes and genotypes for both males and females in slot 2 of the response sheet below.
Hopefully, you predicted that there would be a strange sex-link difference among males and females with respect to eye colors. Specifically, although all the females were red-eyed half would be heterozygotes. Among the males, have would be red eyed while the other half of the males are white eyed. Think back to the pea experiments with Mendel. Did you see this sort of sex-based change in the genotypic or phenotypic ratios in pea plant traits? Put the answer on line 3 of the response sheet.
Here's here you get to have fun. I know, I am a bit twisted. But, here's the problem. Please draw a Punnett Square indicating a cross between a heterozygous female with red eyes and a white eyed male. Start by sorting out the X and Y chromosomes, then figure out the eye color genes. Once you've done that, you should be able to do the cross. Write the genotypic and phenotypic ratios for male and female progeny in slot 4 of the response sheet. Please include Punnett Square.
1. What ratio of red to white eyed progeny do you predict? Is there a difference for the two sexes? Explain.
2. What are the phenotypic and genotypic ratios of white and red eyed males and females in second cross?
3. Did you see sex-based differences in genotypes and phenotypes in Mendel's pea experiments? Why or why not?
4. What are the genotypic and phenotypic ratios of flies in cross between red eyed heterozygous female and white eyed male?
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