Chapter 11

Advanced GeneticsCodominance Multiple Allele Traits Polygenic Inheritance  Lethal Alleles Epistatic Alleles

Incomplete Dominance

Traits appear to "blend" in offspring

RR x WW

      RW  (pink)

Show:

Pink x Red

Pink x Pink

White x White

Figure 11.14

This illustrates another style of "letters" to denote genotypes

R1 and R2

Don’t let letter conventions throw you off on the AP test.

ROAN COW -           What happens when you cross a white and a red cow?

Roan is codominant - both alleles R and W are expressed

What happens when two Roan Cows are Crossed?

Blood Types  - Multiple Alleles

When doing blood type crosses, you will need to know whether at type A or B person is heterozygous or homozygous. Type O's are automatically OO and type AB is automatically AB. Crosses are performed the same as any other.

Show Crosses:

A B   x   A O                        A O  x  B O                         A A  x  A B

Determine the genotypes of these aliens given that the trait is controlled by multiple alleles. 

Polygenic Traits - controlled by many genes

●Each allele intensifies or diminishes the phenotype.●Variation is continuous or quantitative (adding up) - also called quantitative inheritance

●Seed Color in wheat - aabbcc, Aabbcc, AaBbcc, AaBbCc, AABbCc, AABBCC (light, intermediate colors, dark)

●In humans - hair color, height, skin color

Pg 197

Polygenic InheritanceAABBCC x aabbcc (P)

AaBbCc x AaBbCc (F1)

Seven Possible Phenotypes in the F2

Figure 11.16

Figure 11.17

Environment and Phenotype

Temperature, water, food sources can have an affect on how a gene is expressed

Rabbits have a gene that codes for darker pigments - this gene is more active at low temperatures. Parts of the body that are colder will develop the darker pigmentation - ears and feet 

Figure 11.18b

SIAMESE CATS

 EPISTATIC ALLELES

In labrador retriever, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles.

Black is dominant to chocolate B or b

Yellow is recessive and epistatic 

(when present, it blocks the expression of the black and chocolate alleles) E or e

Epistasis

Epistasis is a phenomenon that consists of the effect of one gene being dependent on the presence of one or more 'modifier genes' (genetic background). Similarly, epistatic mutations have different effects in combination than individually. It was originally a concept from genetics but is now used in biochemistry, population genetics,computational biology and evolutionary biology. It arises due to interactions, either between genes, or within them leading to non-additive effects. Epistasis has a large influence on the shape of evolutionary landscapes which leads to profound consequences for evolution and evolvability of traits.

Black Lab

BBEE BbEE BBEe BbEe

Chocolate Lab

bbEEbbEe

Yellow Lab BBeeBbeebbee

1. A black lab (BBEe) x yellow lab (bbee)

2. A chocolate lab (bbEe) x black lab (BbEe)

3. Two black labs (BBEE x BbEe)