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

• X-linked traits

• Incomplete dominance

• Codominance

• Multiple Alleles

• Polygenic 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

HWHW=white

HRHR=red

HRHW=roan (red and white)

Multiple Alleles

• More than 2 alleles 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?

Polygenic Traits• Traits that are determined by multiple genes

produce a range of phenotypes on a bell-shaped continuum (applies to most human traits).

Example: Human Skin Color

Modes of Selection

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

1. A cross between a black cat (CB) and a tan cat (CT) produces a tabby pattern (black and tan colors together).

a. What type of inheritance does this illustrate?

b. What is the probability of producing a tabby kitten if a tabby cat is crossed with a tan cat?

2. 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.

3. 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.