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Sex Linkage in Humans A little back ground information It all started with Thomas Morgan in 1866. He was studying fruit flies. Fruit flies have only 8 chromosomes and reproduce very quickly. In males the last 2 chromosomes are different. We call them X and Y chromosomes. In females the last 2 chromosomes are the same. We call them X and X chromosomes. These hormones determine the sex of the individual. Human Male 46 chromosomes, 23 pairs, one XY chromosomes the Y chromosome is very small Human Female XX chromosomes All of the chromosomes produced in the human body will have an X chromosome. Only ½ of the males sperm will contain an X, the other half contain a Y chromosome Therefore the sex of the offspring depends on which chromosome it gets from the father! Not the mother Some traits are only past on my male or female sex chromosomes. This is called sex-linkage. For example red/green colour blindness. The gene for this blindness is located on the X chromosome. Females have 2 copies of the X chromosomes and males only have 1 copy. Here is how it works: Regular vision is dominant, red/green colour blindness is recessive. For the female to be colour blind she needs to have the gene on BOTH of her X chromosomes. If only 1 X chromosome has the gene for colour blindness, then the other gene has the normal colour vision and it will be expressed because it is dominant. If the male carries he WILL BE colour blind because his Y chromosome does not carry the corresponding full color vision gene to give him normal vision Example 1; Red eye color (R) is dominant, and white (r) is recessive. The alleles are XR, Xr, x. Draw a Punnett square for mating a female with white eyes and a male with red eyes. Possible gametes from male Parent XR Possible gametes from Female Parent Y Xr XRXr XrY Xr XRXr XrY So the female will have Red eyes and the males will have white eyes. Example 2; Draw a Punnett square for mating a female with 2 alleles for red eyes and a male with white eyes Possible gametes from male Parent XR Possible gametes from Female Parent XR Y XRXR XRY XR XRXR XRY 1. What percent of the offspring will have: White eyes ________ Red eyes _______ 1. a) 0% will have white eyes. b) 100% will have red eyes. 2. Are the male offspring able to pass on the trait for white eyes on to the next generation? 2. None of the male offspring have the white-eye allele so they cannot pass on the white-eye trait. 3. Are the female offspring able to pass on the trait for white eyes on to the next generation? 3. All of the female offspring have the white-eye allele so they can pass on the white eye trait. 4a. If having white eyes resulted in reduced vision in a fly, would any of the offspring also have reduced vision? 4. a) None of the offspring have the white-eye trait so none have reduced vision. 4b. Would any of the offspring be carriers of the reduced-vision trait? b) The female offspring all carry one white-eye allele so they are all carriers. 4c. When any of the offspring be genetically free of the reduced vision trait? c) The male offspring do not carry the white-eye allele so they are free of the trait. Sex Linkage in Humans A little back ground information It all started with Thomas Morgan in 1866. He was studying fruit flies. Fruit flies have only 8 chromosomes and reproduce very quickly. In males the last 2 chromosomes are different. We call them ____ and ____ chromosomes. In females the last 2 chromosomes are the same. We call them ___ and ____ chromosomes. These hormones determine the ________ of the individual. Human Male 46 chromosomes, 23 pairs, one ______ ______ chromosomes the Y chromosome is very small Human Female _____ _____ chromosomes All of the chromosomes produced in the human body will have an X chromosome. Only ½ of the males sperm will contain an X, the other half contain a Y chromosome Therefore the sex of the offspring depends on which chromosome it gets from the ___________! Not the mother Some traits are only past on my male or female sex chromosomes. This is called _________________. For example red/green colour blindness. The gene for this blindness is located on the X chromosome. Females have 2 copies of the X chromosomes and males only have 1 copy. Here is how it works: Regular vision is dominant, red/green colour blindness is recessive. For the female to be colour blind she needs to have the gene on ___________ ___________________________ chromosomes. If only 1 X chromosome has the gene for colour blindness, then the other gene has the normal colour vision and it will be expressed because it is dominant. If the male carries he ________________ colour blind because his Y chromosome does not carry the corresponding full color vision gene to give him normal vision Example 1; Red eye color (R) is dominant, and white (r) is recessive. The alleles are XR, Xr, x. Draw a Punnett square for mating a female with white eyes and a male with red eyes. Possible gametes from male Parent Possible gametes from Female Parent So the female will have Red eyes and the males will have white eyes. Example 2; Draw a Punnett square for mating a female with 2 alleles for red eyes and a male with white eyes Possible gametes from male Parent Possible gametes from Female Parent 1. What percent of the offspring will have: White eyes ________ Red eyes _______ 2. Are the male offspring able to pass on the trait for white eyes on to the next generation? 3. Are the female offspring able to pass on the trait for white eyes on to the next generation? 4a. If having white eyes resulted in reduced vision in a fly, would any of the offspring also have reduced vision? 4b. Would any of the offspring be carriers of the reduced-vision trait? 4c. When any of the offspring be genetically free of the reduced vision trait?
