Genetics: Incomplete Dominance & Codominance

Biology 12

Joke of the day:

Review of Mendel’s Principles

Genes are passed parents offspring; get one allele from each parent

During Meiosis, the alleles for a gene segregate from each other.

During Meiosis, genes independently assort with each other.

Review: Dominant/Recessive One allele is dominant over the other

(capable of masking the recessive allele)

PP = purple pp = white Pp = purple

Review Problem: In pea plants, purple flowers (P) are dominant over

white flowers (p) show the cross between two heterozygous plants.

P

p

P p

pp

Pp

Pp

PP- PP (1); Pp (2); pp (1)- ratio 1:2:1

- purple (3); white (1)- ratio 3:1

GENOTYPES:

PHENOTYPES:

Exceptions to Mendel’s principles

Some alleles are neither dominant nor recessive.

Many traits are controlled by more than one gene (polygenic traits)

Incomplete dominance: A situation in which neither allele is dominant. When both alleles are present a “new”

phenotype appears that is a blend of each allele. The heterozygous phenotype is

somewhere in between homozygous phenotype.

2 alleles produce 3 phenotypes

Incomplete Dominance:

CRCR = red CrCr = white CRCr = pink

Rr = pink

Problem: Incomplete Dominance

Show the cross between a pink and a white flower.

- Rr (2); rr (2)- ratio 1:1

- pink (2); white (2)- ratio 1:1

R r

r

r

rrRr

rrRr

GENOTYPES:

PHENOTYPES:

Codominance:

Neither allele are dominant; both are expressed.

A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown.

Example: codominance In some chickens:

Black Chicken x White Speckled Chicken

Roan coat color in horses and cattle are codominant for the alleles R and r. Homozygous R individuals are red (or bay)Homozygous r individuals are whiteWhile heterozygous Rr individuals are roan (which means they have both white and red hair, giving them a lightened appearance.

Example:

What about the F2

generation? Phenotype?

1 red, 2 roan and 1 white 1:2:1

Genotype? 1Hr Hr, 2 HrHw and 1Hw Hw

Which type of dominance…

Codominance!

To do:

Page 145: questions 2, 3, 4, 5 Practise sheet: Codominance and

Incomplete dominance Colouring sheet