Incomplete Dominance & Codominance

Section 8-4 in Book Pgs. 177-179

Review of Mendel’s Principles

Genes are passed parents offspring; get one allele from each parent

During Meiosis, the alleles for a gene segregate from each other.

During Meiosis, genes independently assort with each other.

Exceptions to Mendel’s principles

Sometimes, there is no dominant or recessive gene, or the trait is controlled by many alleles or genes.

In Incomplete Dominance, every genotype has its own phenotype. (One allele not completely dominant over the other.) Third phenotype that is a blending of the parental traits. (2 alleles produce 3 phenotypes.) Result: Heterozygous phenotype somewhere in

between homozygous phenotype.

Incomplete Dominance

1. Incomplete Dominance

Examples: Trait: Flower Color

Expressions: Red x White Pink

RR= Red; RW= pink; WW= white

straight hair, wavy, curly

Incomplete Dominance

In codominance, neither allele are dominant; both are expressed. A cross between organisms with two different phenotypes produces offspring with both phenotypes of the parental traits shown. 

Codominance

2. Codominance Both alleles contribute to the phenotype.

Example: In come chickens

Black Chicken x White Speckled Chicken

YOU tell me which type of dominance…

Codominance!

Type of Dominance?

Incomplete Dominance!

Type of Dominance?

Incomplete Dominance!

Test Cross (Back Cross)…

Used to determine whether an individual is homozygous dominant or heterozygous.

RULES: Always cross unknown genotype with a homozygous

recessive. Observe (count) large numbers of offspring to insure

accuracy in determining the unknown genotype. Then…

if ANY offspring show the recessive trait, the unknown genotype is heterozygous

if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant

Example:

TT or Tt You would cross both with tt. What would you expect the outcome to

be?