Advanced Genetics

Review Dominant 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 Example: 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 Alleles For 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. Codominance If 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 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 A IbIb or Ibi– Type B IaIb – Type AB ii – Type O

C. Incomplete dominance Sometimes, 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 Traits These 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