complex patterns of inheritance. mendel’s laws law of segregation: organisms inherit two copies of...

Complex Patterns of Inheritance

Mendel’s Laws

Law of Segregation:

• Organisms inherit two copies of each gene, one from each parent

• Organisms donate only one copy of each gene in their gametes. Thus, the two copies of each gene segregate, or separate, during gamete formation.

Law of Independent Assortment: the alleles of different genes separate independently of

one another during gamete formation

I. Sex-linked traits

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

A) Hairy Ears: 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

A person with red-green color blindness sees a number 2

Hemophilia• In this pedigree, only males are affected, and sons

do not share the phenotypes of their fathers.– Thus, hemophilia is linked to a sex chromosome–the X.

• Expression of hemophilia often skips generations.– Thus, it is recessive.

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

Cross a carrier mother with a normal father.

No daughters with hemophilia, ½ of sons with hemophilia

XH Xh

XH XHXH XHXh

Y XHY XhY

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

All daughers are carriers, no sons have hemophilia.

XH XH

Xh XHXh XHXh

Y XHY XHY

II. 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

All offspring are pink, with RW genotype

RW RW

RW

RW

R R

W

W

What happens if you cross two pink snapdragons?

Offspring are 1 red, 2 pink, and 1 white

RR RW

RW WW

R W

R

W

1 red (RR), 2 pink (RW), 1 white (WW)

Incomplete

dominance

in horse

coat color

III. 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

IV. Multiple Alleles• Genes with three or more alleles for a

particular trait.

• Examples are the ABO blood types, fur color in many animals.

Coat color in rabbitsPhenotype 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 gray

3. 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?

Himalayan is chc and chinchilla parent is cchc

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

VI. Epistasis

• One gene that interferes with the expression of other genes.

• Example is with albinism.

• One gene interferes with all other genes for pigment production.

VII. 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