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Chromosome Numbers Humans have 46 chromosomes: 2 are the sex chromosomes XX or XY The other 44 are 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? Is this individual male or female?? Some genes are carried only on the X chromosome “Sex-Linked” Traits Examples: Red/Green Colorblindness, Hemophilia **Since males have only one copy of the X chromosome, any traits found on the X (even recessive ones) are automatically expressed.** Wait. Why’s that?? Think about it… How many alleles would a female have for the gene that determines 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. NN Nn nn 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. • Pairs of genes that tend to be inherited together • Why? • Found on the same chromosome Linked Genes Fruit Fly Black Body Purple Eyes Shriveled Wings http://ws4.niai.affrc.go.jp/dbsearch2/pmap/chrom13.gif Crossing Over • Exchange of pieces of DNA between pairs of chromosomes • The further apart genes are on a chromosome, the more likely they are to recombine during crossing over. Chromosome Map Shows the order of genes on a chromosome 1 map unit = frequency of recombining during crossing over 1 percent of the time Black Body (48.5) Purple Eyes (54.5) Shriveled Wings (67.0) http://ws4.niai.affrc.go.jp/dbsearch2/pmap/chrom13.gif Example Chromosome Map (Human X Chromosome) http://embryology.med.unsw.edu.au/Notes/images/urogen/Xchr.gif