Complex Patterns of Inheritance

I. Incomplete Dominance• An individual displays a trait that is

intermediate between the two parents

• Example: a red snapdragon

crossed with a white

snapdragon produces

pink offspring

Red = RR, White = WW, Pink = RW

Make a Punnett square for a red plant crossed with a white plant

R R

W

W

All offspring are pink, with RW genotype

RW RW

RW

RW

R R

W

W

Show the cross between two pink snapdragons

Offspring are 1 red, 2 pink, and 1 white

RR RW

RW WW

R W

R

W

Incomplete

dominance

in horse

coat color

II. Codominance

• Two dominant alleles are expressed at the same time

• Roan coat color in horses or cows is an example

• A roan horse has both red and white hairs

• Roan has red and white hairs

Parents

Codominancein chicken feather color.

Black and White feathers

III. Multiple Alleles• Traits controlled by more than 2 alleles.

• Examples are pigeon feather colors

and fur color in many animals.

Coat color in rabbits

Phenotype Allele Pattern of Inheritance

Dark gray coat C Dominant to all other alleles

Chinchilla (silver-gray)

cch Dominant to Himalayan and white

Himalayan

(white with dark ears)

ch

Dominant to white

White c recessive

1. List all possible genotypes for a: a) dark gray-coated rabbit CC, Ccch, Cch, Cc b) chinchilla rabbit cchcch, cchch, cchc c) Himalayan rabbit chch, chc d) white rabbit cc

2. Predict the phenotype for a rabbit with a chcch genotype

chinchilla Predict the phenotype for a rabbit with a

Cch genotype dark gray3. Would it be possible to obtain white

rabbits if one parent is white and the other is chinchilla?

Yes, if the chinchilla parent has a white allele

4. Would it be possible to obtain chinchilla rabbits if one parent is Himalayan and the other is white?

No, because chinchilla is dominant to both Himalayan and white, so the allele couldn’t be “hiding”.

5. A chinchilla rabbit is mated with a Himalayan. Some offspring are white. What are the parents’ genotypes?

Chinchilla is cchc and Himalayan is chc .

IV. Sex-linked traits

• A sex-linked trait is a trait whose allele is located on either the X or Y chromosome.

A) Hairy Ears are caused by a gene on the Y Chromosome

B) Most sex-linked traits on X chromosome are recessive

• Examples include: hemophilia, red-green colorblindness, and a form of muscular dystrophy

People with red-green color blindness see the number 35, while people with normal vision see the

number 57

Hemophilia

• A disorder in which the blood does not clot properly

• Sex-linked recessive (on the X chromosome)• Males receive the hemophilia gene from their

mothers

Extensive bruising of

the left forearm

and hand in a patient

with hemophilia.

Genotypes for females

XHXH = normal blood clotting, non-carrier

XHXh = normal blood clotting, carrier of gene

XhXh = female with hemophilia

Queen Victoria of the

United Kingdom was a

carrier of hemophilia

Genotypes for Males

• XHY = normal blood clotting

• XhY = male with hemophilia

• Tsarevich Alexei of Russia

had hemophilia

What is Alix’s genotype?

1. Cross a carrier mother with a normal father.

XH

Xh

XH

Y

What percent of sons have hemophilia?What percent of daughters have

hemophilia?

XH Xh

XH XHXH XHXh

Y XHY XhY

50% of sons have hemophilia, 0% of daughters have hemophilia.

XH Xh

XH XHXH XHXh

Y XHY XhY

2. Cross a hemophiliac father with a normal (non-carrier) mother.

XH

XH

Xh

Y

What percent of sons have hemophilia? What percent of daughters are carriers?

XH XH

Xh XHXh XHXh

Y XHY XHY

0% percent of sons have hemophilia. 100% percent of daughters are carriers.

XH XH

Xh XHXh XHXh

Y XHY XHY

3. Cross a carrier mother with a hemophiliac father.

XH

Xh

Xh

Y

What percent of sons have hemophilia?What percent of daughters have hemophilia?

XH Xh

Xh XHXh XhXh

Y XHY XhY

50% percent of sons have hemophilia. 50% percent of daughters have hemophilia.

XH Xh

Xh XHXh XhXh

Y XHY XhY

4. Cross a hemophiliac mother with a normal father

• What percent of sons have hemophilia?

• What percent of daughters have hemophilia?

100% percent of sons have hemophilia. 0% percent of daughters have hemophilia.

Xh Xh

XH XHXh XHXh

Y XhY XhY

V. Polygenic traits• Traits that are influenced by 2 or more

genes

• Examples include human height, weight, hair color, eye color, and skin color

Coat color in Labrador retrievers

Coat color in Labrador retrievers

Is it possible for two chocolate lab

parents to have black lab offspring?

VI. Traits influenced by the environment.

• Color of hydrangea flowers (blue in acidic soil, pink in basic soil)

• Color of arctic fox

• Human height, skin color, behavior