1. GENE LINKAGE AND GENDER CHROMOSOMES

2. Gene linkage Chromosomes are independently assorted They are NOT attached – move independently during meiosis Genes – may/may not be independently assorted If far apart, Independently assorted (NOT linked) If closer together More tightly “linked” (NOT independently assorted) Ex) 2 genes on 2 separate chromosomes? Completely independently assorted Ex) 2 genes next to each other on 1 chromosome? Very “tightly linked” Ex) 2 genes very far apart on 1 chromosome? Very “loosely linked” See board behind screen for visual… What are more likely to separate during crossing over: 2 tightly- or 2 loosely-linked genes? 2 loosely-linked genes

3. Gene linkage, continued… Example: Reebop genes Antenna (A or a) and head coloration genes (B or R) Two scenarios: tightly- and loosely-linked (see board) What are possible gametes? Assume crossing over occurs just in one place for this example, but tightly-linked genes would cross over together. Example: human genes Color blindness and gender; red hair and freckles (more tightly linked than other traits) Summary Tightly-linked alleles: more likely to be inherited together as a “package” Gene linkage reduces variation Independent assortment = more variability Gene linkage disrupts Punnett square predictions Punnett squares assume independent assortment Questions?

4. Gender chromosomes and gene linkage What do you know about human biological gender and chromosomes? 23 pairs of chromosomes (46 total) 22 pairs are “autosomal” – not gender-related (44 total) Chromosome #23: gender chromosomes (X and Y) X chromosome Larger, many non-gender genes (1,000+ X-linked genes) Ex) Color vision, blood clotting Y chromosome Very small, most genes related to male characteristics Only a few non-gender genes (50-60 Y-linked genes) Most of the non-gender genes (all?) are also on the X chromosome Punnett square for gender Board…

5. Gender chromosomes and gene linkage, continued… Predicting inheritance patterns for X- or Y-linked genes (“sex-linked genes”) More complicated than autosomal gene predictions Example: red/green colorblindness Recessive, X-linked (trouble seeing differences between red/green) See board, record phenotypes for all possible genotypes (shown) Ex) Show Punnett square for a woman who sees red/green but is heterozygous and a man who can see red/green. What are possible phenotypes? Why is it more common in men? Example: hemophilia Recessive, X-linked (blood won’t clot well) Ex) Show Punnett square for a homozygous normal woman and a man with hemophilia What are possible phenotypes?