Dominance

Relationships

Recap

• So …

• If you show the dominant phenotype, what could your genotype be?

•What is genotype and phenotype again?

If you are unsure if the organism is a pure-

bred, you can do a Test Cross

• Test Cross: Cross the individual that shows dominant phenotype with a recessive individual

• If offspring 100% dominant

• Original was pure (HD)

• If offspring not 100% dominant

• Original was heterozygous

Predicting a Test Cross

Remember, Punnett Squares are usually good only for single-gene traits.

A a

A

a Aa

AA Aa

aa

Looking at just one trait is called a MONOHYBRID Cross

Looking at TWO traits in one Punnett Square is called a DIHYBRID Cross

TRY A DIHYBRID CROSS!

• Show the probabilities of a cross between two people, both heterozygous Full Lips (Ff), Clefted Chin (Cc)!

• F = Full Lips (dominant), f = Thin Lips (recessive)

• C = Cleft Chin (dominant), c = Non-clefted Chin

• The hardest part is segregating the alleles

• Start with FfCc X FfCc … Go!

FfCc X FfCc

Ff Cc

Ff C

c

FC

FC FFCC(full lips, cleft chin)

Finish Up!

Fc fC fc

Numbers:

• How many Full Lips/Clefted Chins? _____

• How many Thin Lips/Clefted Chins? _____

• How many Full Lips/Non-clefted? _____

• How many Thin Lips/Non-clefted? _____

• So, what’s your HomD:Het:Het:HomR ratio?

___:___:___:___

9:3:3:1 -This is the classic ratio for a heterozygous dihybridcross!

9/16

3/16

3/16

1/16

9 3 3 1

Dominance Relationships

• Complete dominance: one allele shows full dominanceover the other (dominant v. recessive)

• Incomplete dominance: neither allele is fully dominant over the other (mixed phenotype)

• Codominance: Both alleles dominant; one phenotype “wins” (shows) in some cells, the other shows in other cells (either can show)

Which Is This

• Complete Dominance

• Incomplete Dominance

• Codominance

Which Is This

• Complete Dominance

• Incomplete Dominance

• Codominance

Which Is This

• Complete Dominance

• Incomplete Dominance

• Codominance

(Though darker colors tend to show more strongly than lighter colors)

Which Is This

• Complete Dominance

• Incomplete Dominance

• Codominance

Incomplete Dominance

• In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and purple. This trait is controlled by a single gene with incomplete dominance.

• A homozygous (BB) individual is blue, a homozygous (bb) individual is red, and a heterozygous (Bb) individual is purple.

• What would be the genotypes and phenotypes of the offspring if a blue wildcat were crossed with a red one?

• Cross a Purple Cat and a Red cat. Can they have Blue offspring?

Co-Dominance

• Longhorns may be white (CWCW), red (CRCR) or roan (CRCW). Roan longhorns have a mixture of both white hairs and red hairs due to a codominant gene.

• Cross a Roan Bull with a Red Female!• What are the genotypes and Phenotypes?

• Can roan cows produce more roan cows?