Dominance and Multiple Allele Notes

Mendel’s Principles – A Review

• Inheritance of traits is determined by genes.

• Genes are passed from parents to offspring.

• Alleles can be dominant or recessive.• In sexually reproducing organisms –

each adult has two copies of each gene – one from each parent.

Exceptions to Mendel’s Work

• Some alleles are neither dominant or recessive.

• Many traits are controlled by multiple alleles or multiple genes.

Incomplete dominance – When the alleles are blended and the offspring have a mix of their parent traits.

 ex. Snap Dragons

R = red

W = white

Offspring can be pink!

Codominance – in this case both alleles are expressed.

BB= black corn

YY= yellow corn

BY = black and yellow corn

Heterozygous genotype

Incomplete Dominance or Codominance?

Incomplete Dominance or Codominance?

Incomplete or Codominance?

Polygenic Traits• A polygenic trait is determined by multiple

genes. (poly=many, genic=genes)

Example: eye color and height

Skin color is controlled by more than four genes

Homologous chromosomes - Chromosomes occur in

pairs.(homologous means “same”)

 - The different alleles of a

gene occupy the same positions on each chromosome

So far each gene we have discussed has been made of two possible alleles.

 

Ex. B = blue b= yellow

R = red r = white

However, it is possible to have several different allele possibilities for one gene.

 Multiple alleles is when there are more than two allele possibilities for a gene.

Coat color in rabbits is determined by a single gene with 4 possible alleles.

In traits with multiple alleles, each individual can carry any two of the

several possible alleles. 

Ex. BLOOD TYPE 

The gene for blood type has 3 possible alleles.

IA, IB, and i

In this case both A and B are dominant to O (recessive).

A and B are codominant (both expressed) 

So... there are four human blood typesGenotype Phenotype

IAIA, IAi Blood type A

IBIB, IBi Blood type B

IAIB Blood type AB

ii Blood Type O

Alleles for blood type are:-IA (dominant)-IB (dominant)-i (recessive)

Practice Problems:1. A mother is AB and a father is O. Draw the punnett

square.IA IB

i i

 What is the probability their offspring will have A type

blood? ____ Can they have an offspring with O blood? ____

2. A mother is IAi and a father is IB IB. Draw the punnett square.

  What is the probability their offspring will have B

type blood? Can they have an offspring with O blood?

3. If a child has AB blood which of the following could NOT be the parents?

a) ii, IAIB

b) IAi, IBIB

c) IBIB, IAIA

d) IAi, IBi