Special Inheritance Patterns Simple Dominance So far we

Special Inheritance Patterns

Simple Dominance • So far, we have only studied simple dominant/recessive heredity • In simple dominance – dominant allele completely hides the recessive allele TT (Tall) Tt (Tall) tt (short)

Incomplete Dominance • The dominant allele does not completely cover the recessive allele • Results in a blended phenotype • Ex: Hair type: HH (Curly) Hh (Wavy) hh (Straight)

Incomplete Dominance in Carnations RR Rr rr

Codominance • Two alleles can be expressed at the same time • Ex: people with type AB blood (the A and B alleles are expressed at the same time) Roan coat color in horses is another example (both red and white show up)

ABO Blood Typing

Multiple Alleles • There are more than just 2 forms of a gene (alleles) for a trait Example: Human blood type has 3 alleles Alleles: IA , IB, i

Human Blood Types Genotypes Phenotypes ii = Type O IAIA or IAi = Type A IBIB or IBi = Type B IB IA = Type AB

Blood Type in Humans IB i IA IB IB i IA i i i

Polygenic Traits • A trait can be controlled by multiple genes • A certain combination is needed to show a specific phenotype • Example traits: Skin color, eye color, hair color • • Example genotypes and phenotypes: Aa. Bb. Cc = black hair AAbb. CC = light brown hair Aabbcc = blonde hair

Sex-Linked Inheritance • When a trait is on either the X or the Y chromosome • Males are more affected • Females are less affected, but can be carriers • Types of sex-linked inheritance § X-linked recessive § X-linked dominant § Y-linked

Environment • Environment can influence gene expression • Environmental Factors: – Hormones – Age – Temperature