Exceptions to Mendelian Inheritance

Mendelian Inheritance means… one gene = one trait 2 different alleles (versions) for a gene one dominant and one recessive allele (heterozygous = dominant phenotype) segregation independent assortment genotype determines phenotype

Exceptions to Mendel Linkage ✓ Incomplete dominance Codominance Multiple Alleles Polygenic Traits X-linked traits ✓ Environmental Influences

Incomplete Dominance Neither allele is dominant Heterozygous phenotype is intermediate Example: Flower color in snapdragons

CWCW=white CRCR=red CRCW=pink

Codominance Both alleles are dominant Heterozygous phenotype displays both traits Example: Coat color in horses

HRHW=roan (red and white) HRHR=red HWHW=white HRHW=roan (red and white)

Multiple Alleles More than 2 alleles (versions) for a gene Example: Human Blood Types

Multiple Alleles – Blood Types Alleles: IA (dominant), IB (dominant), i (O, recessive) Genotypes: IA IA, IA i, IB IB, IB i, IAIB, ii What phenotype is produced by each of these genotypes?

X-Linked Traits Genes on X chromosome produce a different inheritance pattern in males vs. females Example: Eye color in fruit flies

X-Linked Traits Alleles: XRXR – red eye female XRXr – red eye female XR – red eyes (dominant) Xr – white eyes (recessive) Genotypes: XRXR – red eye female XRXr – red eye female XrXr – white eye female XRY – red eye male XrY – white eye male

Polygenic Traits Traits that are determined by multiple genes produce a range of phenotypes on a bell-shaped continuum. Example: Human Skin Color

Modes of Selection

Environmental Influences Environmental factors can influence an individual’s phenotype (without changing the genotype).

IQ Score Correlations: Genes vs. Environment Genetic similarity: 100% 100% 50% 50% 0%

1. In humans, straight and curly hair textures demonstrate incomplete dominance. The heterozygous individuals have wavy hair. The alleles are HC and HS. A woman and her husband who both have wavy hair desire a child with curly hair. What are the chances that their first child will have curly hair? Show the Punnett square. In cats, the trait for calico (multicolored) cats demonstrates codominance and X-linkage. Females that receive a XB and an XR gene have black and orange splotches on white coats. Males can only be black or orange, but never calico. A female black cat is crossed with a male orange cat. What percentage off the offspring will be calico? What color would the males be? Show the Punnett square. 4. A man is suing his wife for divorce on grounds of infidelity (he thinks she is having an affair). Their 1st and 2nd children, whom they both claim, have blood types O and AB, respectively. The 3rd child, whom the father disclaims, is blood type B. Could he be the father of the third child? Show the Punnett square.