Advanced Genetics

ReviewDominant vs. Recessive

If an individual inherits even one dominant allele, that trait will be expressed.

– BB– Bb

Continued…

An individual must inherit two copies of a recessive allele for that trait to be expressed.

– bb

Genes

Each “recipe” on the DNA is called a gene.– Ex. Hair color and hair texture are two

different genes found at different loci (locations) on the

chromosomes.

Alleles

There is often more than one version for a gene.

These are called alleles.

Example:Gene = flower color

– Alleles = purple or white

Beyond Dominant and Recessive Alleles

Not all genes show simple dominant/recessive patterns.

In most organisms, genetics is much more complicated.

The majority of genes have more than two alleles.

A. Multiple AllelesFor some genes, there are more

than two alleles possible. –For example, for eye color there are

alleles for shades of brown, blue, green, and hazel.

B. CodominanceIf more than one allele is

dominant for a trait, they are called codominant.

Both phenotypes will be expressed.

–Both black and white are dominant in this breed of chicken. A cross results in a chicken that has some black feathers and some white feathers.

Red and white fur are both dominant in this breed of cow…

If a cow inherits both (RW), it is described as roan.

In humans, blood type is determined by the presence (or absence) of certain antigens (A and B).

These two antigens are codominant.

Blood type genotypes

IaIa or Iai– Type AIbIb or Ibi– Type BIaIb – Type ABii – Type O

C. Incomplete dominanceSometimes, neither allele can

dominate the other.A blend of the traits will appear.

–Example: Red and white flowers produce pink offspring when crossed.

D. Polygenic TraitsThese traits are controlled by

more than one gene and have a wide range of phenotypes. –Height–Skin color

Define the following vocabulary words…pg 270

Independent assortment Incomplete dominance Codominance Multiple alleles Polygenic traits