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Sex-Linked Traits Worksheet Background Information: Hemophilia and colorblindness are two common sex-linked recessive disorders. In hemophilia, the defective allele prevents the synthesis of a factor needed for blood clotting. In colorblindness, the defective allele prevents a person from seeing certain colors. Use the information below to answer the following questions. XH- X chromosome with normal dominant allele (no hemophilia) Xh - X chromosome with recessive hemophilia allele Y - Y chromosome (does not contain comparable gene) XB - X chromosome with normal dominant allele (not colorblind) Xb - X chromosome with recessive colorblind allele Y -Y chromosome (does not contain comparable gene) 1. Write the genotypes for the following phenotypes of red-green color blindness. a. Normal male ______XBY_______ b. Normal female carrying no colorblind alleles (Homozygous) ___ XBXB_____ c. Colorblind male ___________ XbY______________ d. Normal female carrying the colorblind allele (Heterozygous) ____ XBXb_____ e. Colorblind female ______ XbXb________ 2. XBXB x XB XbY a. What proportion/percent of the male children are colorblind? _____0%_______ Xb b. What proportion/percent of the female children are colorblind?_____0%_______ Y 3. XBXb x XBY XB XBXb XBXb XBY XBY XB Xb a. What proportion of the male children are colorblind? ______50%________ XB XBXB XBXb b. What proportion of the female children are colorblind?________0%_______ Y XBY XbY 4. What is the probability that a colorblind woman who marries a man with normal vision will have a colorblind child? ______50%________ Xb Xb XB XBXb XBXb Y XbY XbY For the following Sex-Linked Punnett Squares: H= normal blood clotting 6. XHXh x h=hemophilia XHY a. What is the probability that any of their offspring will have hemophilia? _______25%________ XH Xh XH XHXH XHXh Y XHY XhY 7. A woman who is a carrier for hemophilia marries a hemophiliac man. XHXh x XhY XH Xh XHXh XhXh Y XHY XhY a. What proportion of the male children are hemophiliacs?_______50%_______ b. What proportion of the female children are hemophiliacs?______ 50%_______ 8. A phenotypically normal man marries a homozygous normal woman. XHXH x XHY a. What is the probability that any of their children will be hemophiliacs?________0%___________ XH Y Xh XH XH XHXH XHXH XHY XHY ANSWER THE FOLLOWING QUESTIONS USING YOUR KNOWLEDGE OF SEXLINKED TRAITS, THE BACKGROUND INFORMATION AND YOUR NOTES. 10. What is a sex-linked trait? Those traits whose genes are found on the X chromosome but NOT on the Y chromosome 11. Why must males inherit colorblindness or hemophilia from their mothers? Because their mother gives them their X chromosome. 12. Why is colorblindness or hemophilia more common in males than in females? Because the trait is recessive and since they only have one allele, there is no other allele expressed. So what they have on their one X chromosome is what is expressed. Females can have one recessive allele but it is masked by the dominant normal allele.
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