Complex Patterns of Inheritance There are some exceptions to Mendel’s Laws.

Variations on Mendel’s Laws Mendel’s experiments did not explain ALL patterns of inheritance Most alleles are not simply dominant or recessive

1. INCOMPLETE DOMINANCE Neither allele is completely dominant to the other The heterozygous phenotype is a mix between the two homozygous traits. EX: Snapdragons – red flower (RR) crossed with white flower (rr) produces pink flower (Rr) In some case you will see it written as Red = RR White = WW Pink = RW

2. CODOMINANCE Both alleles are expressed equally EX: cross a black chicken with a white chicken, offspring will be black and white checkered. Roan cow (RW) Red cow (RR)White cow (WW)

4. Polygenic inheritance-2 or more genes affect the phenotype. Ex: height, weight, skin color Heterochromia- uneven distribution of pigment resulting from disease or injury

5. SEX-LINKED TRAITS: controlled by genes located on sex chromosomes Usually carried on X chromosome Since females are XX, they are usually carriers of the trait Since males are Xy, they have one big & one small, stumpy chromosome. The small chromosome (y) does not carry an allele so whatever allele is on the X (donated by mom) is what the boy will have. Boys cannot be carriers of a sex linked trait.

X-linked disorders  Recessive  Red-green color blindness  Hemophilia  Duchenne muscular dystrophy  Ichthyosis  Dominant  Hypertrichosis

Males can pass it to all of their daughters, none to sons Females have 50/50 chance of passing it to all of their children Ex: – Hemophilia- can’t clot blood – Colorblindness- can’t see certain colors.

What a red-green colorblind person would see NormalRed-green colorblind

Genes on the Y Chromosome  Few genes are on the Y chromosome  Hairy pinna

Interpreting a Pedigree What can you tell from a pedigree? – Whether a family has an autosomal or sex-linked disease or disorder Autosomal disorder: appears in both sexes equally Sex-linked disorder: allele is located only on the X or Y chromosome. Most sex-linked genes are on the X chromosome and are recessive – Whether a disorder is dominant or recessive

Is this disorder 1) autosomal or sex linked, 2) dominant or recessive? You Parents Aunts, Uncles Grandparents Brother Sex Linked! (in this case allele is recessive and located on the X chromosome)

Is this disorder 1) autosomal or sex linked, 2) dominant or recessive? You Parents Aunts, Uncles Grandparents Brother Autosomal dominant!

Is this disorder 1) autosomal or sex linked, 2) dominant or recessive? You Parents Aunts, Uncles Grandparents Brother Autosomal Recessive