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 Tt tt (Tall) (Tall) (short)

Incomplete Dominance The dominant allele does not completely cover the recessive allele Results in a blended phenotype Ex: Hair type: HH Hh hh (Curly) (Wavy) (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 ii = Type O IAIA or IAi = Type A IBIB or IBi Genotypes ii IAIA or IAi IBIB or IBi IBIA Phenotypes = Type O = Type A = Type B = Type AB

Blood Type in Humans IA i IA IB IB i IA i i i IB 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: AaBbCc = black hair AAbbCC = 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