• Organisms have more than one gene !
• we can consider more than one gene at a time

Mendel’s second law - The Law of Independent Assortment - If genes are on different chromosomes, they will assort independently during meiosis

• Imagine two genes, each with two alleles.
• Genes affecting fur in cats:
• L = long hair ; l = short hair
• S = white spotted ; s=not white spotted
• A double heterozygote: L/l S/s

• Cross two double heterozygotes and predict the genotypes and phenotypes of the offspring
• List the gametes for a Punnett Square:
• L/l S/s will produce:
• LS, Ls, lS, ls

If gametes with either of the "L" alleles are produced independently of gametes with either of the "S" alleles, then the presence of one allele does not affect the chance that there will be another.

• Autosomes and sex chromosomes
• Sex chromosomes are the only chromosomes that have the potential to be unpaired under normal circumstances
• Sex chromosomes -- big ones = "X" ; little ones = "Y"
• Punnett Square for sex determination?

• In mammals: XX = female XY = male
• In some insects, some fish, and birds: XX=male and XY = female
• In some other insects XX=female and XO = male
• In plants, crocodilians, some lizards, and some fish, there are no sex chromosomes at all!

• Chicken feather color (in text)
• B = black, B’ = white speckled
• B/B’ heterozygote = blue

• A,B,O blood groups
• Possible genotypes:
• A
• AB
• B
• O

• Sex linkage can occur when genes are carried on the "X" chromosome (mammals)
• often thought of in terms of genetic disease and recessive traits
• heterozygous females (X/X*) are termed carriers, males are hemizygous

• Ploidy levels:
• haploid, diploid, triploid, tetraploid, etc.
• Euploidy
• Aneuploidy
• non-disjunction, monosomy, trisomy
• inversion, translocation, deletion, duplication