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Name: _____________________________ Note Review: Sex-Linked Traits Use your in-class notes AND p. 307-8 to fill in the blanks and answer the questions. Chromosome Numbers Humans have _____ chromosomes: 2 are the sex chromosomes: XX or XY _____ _____ The other 44 are ________________ ●Remember, however, that gametes (sperm/eggs/pollen) carry half of the DNA. So, in a human, sperm and eggs carry 23 chromosomes (one from each pair of chromosomes). In addition to 22 other chromosomes (autosomes)... •Egg cells carry one X chromosome. •Sperm cells can carry either an X or a Y chromosome. With that in mind, which parent determines the sex of the child? _______________ Some genes are carried only on the X chromosome... “Sex-Linked” Traits Examples: ____________________________ & _________________________ Since males have only one copy of the X chromosome, any traits found on the X (even recessive ones) are automatically expressed.** Wait. What?? Think about it… How many alleles* would a female have for the gene that determines red/green color vision? 2- one on each X chromosome The “normal” gene is dominant and the colorblind gene is recessive. The only way a female could be colorblind is if she receives 2 copies of the defective gene (a pretty rare event). * An allele is a form of the gene, like “T” or “t” However… How many alleles for the same gene does a male receive?? (Remember that this gene is carried ONLY on the X chromosome…) 1! If his X chromosome carries the gene for colorblindness, he is automatically colorblind because he doesn’t have a “back-up” X chromosome like females do… Statistically, inheriting 1 colorblindness gene is MUCH more likely than inheriting 2. So, men are much more likely to develop these recessive sex-linked characteristics than women are. Do you REALLY get that??? Let’s say it another way: Who is more likely to inherit a sex-linked trait, men who get only 1 copy of the X chromosome, or women who get 2 copies of the X chromosome? Men, because they do not have a second X chromosome to protect them if their first X chromosome is faulty. Other info about chromosomes and Inheritance: Linked Genes ●Pairs of genes that tend to be _______________________________ •Why? •Found on the same chromosome Crossing Over •The exchange of pieces of DNA between pairs of chromosomes during ____________________ formation. •The further apart genes are on a chromosome, the more likely they are to recombine during crossing over. •Increases ____________________ Chromosome Map Shows the order of genes on a chromosome 1 map unit = frequency of recombining during crossing over 1 percent of the time