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Beyond Dominant & Recessive Alleles

Incomplete Dominance & Codominance

Learning Goals

1. Describe incomplete dominance. Describe codominance.

2. Compare and contrast incomplete dominance and codominance. Give 2 similarities and 1 difference.

3. Explain how letters are chosen when doing punnett square problems for incomplete dominant and codominant traits

4. Explain what multiple alleles are and give an example

5. Describe what a polygenic trait is and give an example.

Beyond Dominant and Recessive Alleles

Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance 

Incomplete dominance - when one allele is not completely dominant over another. Heterozygous genotype = BLEND

Since one allele is not completely dominant over the other allele, we use two different capital letters to represent the alleles

Example: A cross between red (RR) and white (WW) snapdragons produces pink flowers (RW).

Homozygous

RR(Red)

HeterozygousRW

(Pink)

HomozygousWW

(White)

Codominance 

Codominance - both alleles contribute to the phenotype. Heterozygous genotype = both traits appear

Since one allele is not completely dominant over the other allele, and both traits appear together, we use two different capital letters to represent the alleles

Example: A cross between black (BB) and white (WW) chickens produces chickens speckled with black and white feathers (BW).

Multiple Alleles 

Multiple alleles -genes that are controlled by more than two alleles. An individual can’t have more than

two alleles (Remember you get one copy from each parent). However, more than two possible alleles can exist in a population.

•Example: A human’s blood type is determined by a single gene that has three different alleles.

Polygenic Traits  

Polygenic traits -traits controlled by two or more genes. Skin color in humans is a polygenic

trait controlled by more than four different genes.

Learning Goals

1. Describe incomplete dominance. Describe Codominance.

2. Compare and contrast incomplete dominance and codominance. Give 2 similarities and 1 differences

3. Explain how letters are chosen when doing punnett square problems for incomplete dominant and codominant traits

4. Explain what multiple alleles are and give an example

5. Describe what a polygenic trait is and give an example.

Whiteboard Practice

Step 1: Incomplete Dominance or Codominance??????

Step 2: You will be given the 3 phenotypes, Write the corresponding genotypes. 2 alleles (1 from mom, 1 from dad) Use the 1st letter for each trait Both letters are capitol

Step 3: Set up and fill-in a punnett square. Calculate percent probability for ALL 3 TRAITS!!!

Let’s do the 1st one together

Birds can be blue, white, or white with blue-tipped feathers

1. Incomplete Dominance or Codominance? 2. Genotypes:

blue-white – blue-tipped –

3. Show a cross between a white and blue-tipped bird

Practice Identifying

A cat can be black, tan, or tabby (black & tan)

1. Incomplete Dominance or Codominance? 2. Genotypes:

black-tan – tabby –

3. Show a cross between a heterozygous tabby and tan cat

Practice Identifying

A Who can have curly hair, spiked hair, or wavy

1. Incomplete Dominance or Codominance? 2. Genotypes:

curly-spiked – wavy-

3. Show a cross between a curly and wavy Who

Practice Identifying

A sneech can be tall, short, or medium 1. Incomplete Dominance or Codominance? 2. Genotypes:

tall-short – medium –

3. Show a cross between two heterozygous sneeches

Practice Identifying

A horse can be black, white, or roan (black & white)

1. Incomplete Dominance or Codominance? 2. Genotypes:

black – white –roan -

3. Show a cross between a heterozygous roan and black horse

Practice Identifying

Flowers can be white, pink, or red 1. Incomplete Dominance or Codominance? 2. Genotypes:

white-pink – red-

3. Show a cross between a white and red flower