Non-Mendelian

Genetics

Mendelian Genetics

•Complete dominance

•Law of segregation

•Law of independent

assortment

•One gene one trait

Non-mendelian Genetics

• Codominance

• Incomplete dominance

• Multiple alleles

• Pleiotropy

• Epistasis

• Polygenic inheritance

• Sex-related inheritance

Pleiotropy (pleion, “more”)

40% of cats with white fur and blue

eyes are deaf

Phenylketonuria (lack of enzyme that breaks

down Phe)

Sickle cell anemia (misshapen RBCs at

low O2 concs)

• One gene, many

phenotypic effects

http://3.bp.blogspot.com/-

bUvXE8h_wuE/TycwHFuFqrI/AAAAAAAAA4

E/fWfyDR0ETNU/s1600/Phenylketonuria+(P

KU).jpg

Epistasis One gene affects expression of another

• Effects of one gene are

modified by one or

several other genes

• Examples: – Coat color in mammals

– Color of wheat grains

– Fruit color in squash

– Bombay phenotype in ABO blood

groups in man

Two genes affect coat color

B black, b brown, E pigment deposited, e no pigment

Bombay phenotype

Polygenic inheritance Additive effects of two or more genes on a phenotypic character

• Quantitative characters

vary in a population along a

continuum or gradation

• Due to the presence of

contributory or

dominant (ABC) and

non-contributory or

recessive alleles (abc)

• Expression can be affected

by environmental factors

http://tadoscoppernicus.blogspot.com/2010/05/bizarre-birth-identical-twins-with.html

Example:

– Skin pigmentation controlled by at least three separately inherited genes

– Each dark skin allele contributes a unit of ‘darkness’ to the individual

– AABBCC very dark

– aabbcc very light

(very light) (very dark)

(medium brown)

Aa Bb Cc x Aa Bb Cc

brown

Solving polygenic inheritance problems

• Parents are usually heterozygous for all

traits (F2 generation)

• Interested in any of the following:

• # of genes affecting the character

• # of phenotypes present for the character

• probabilities for each phenotype

• sample genotypes

Example 1 A man is 5 feet 7 inches tall.

1. According to the chart on the

right, how many dominant alleles

(heads) and recessive alleles

(tails) will he have? .

2. How many inches does each

contributory allele add to the

height?

3. List all possible combinations of

alleles he can give to his child.

4. If he marries a woman who is 5

feet and 3 inches tall, what are

the possible values for the height

of their offspring?

Coin Situation Height

O Tails and 6 Heads 6 feet 1 inch

1 Tail and 5 Heads 5 feet 11 inches

2 Tails and 4 Heads 5 feet 9 inches

3 Tails and 3 Heads 5 feet 7 inches

4 Tails and 2 Heads 5 feet 5 inches

5 Tails and 1 Head 5 feet 3 inches

6 Tails and 0 Heads 5 feet 1 inch

Example 1

5. Given parents that are both

heterozygous (AaBbCc) for all, give

the probability that the offspring

would have the heights specified in

the height table.

Coin Situation Height

O Tails and 6 Heads 6 feet 1 inch

1 Tail and 5 Heads 5 feet 11 inches

2 Tails and 4 Heads 5 feet 9 inches

3 Tails and 3 Heads 5 feet 7 inches

4 Tails and 2 Heads 5 feet 5 inches

5 Tails and 1 Head 5 feet 3 inches

6 Tails and 0 Heads 5 feet 1 inch

Listing or Forked Line Method

• number of possible combinations of r objects from a set of n objects

• r is specified # of dominant alleles

• n is the total number of alleles

Using the Combination Formula

Using Pascal’s Triangle

Example 2

Petal length of a plant ranges from 4mm to 12mm to 20mm. Out of 770 plants, only 3 of them have 4mm petals.

1. How many genes affect petal length?

2. How many phenotypes are present for petal length?

3. Give one genotype for a plant with 12 mm petals.

4. Give two possible genotypes for plants with 6 mm petals

5. What proportion of plants have 14 mm petals?

6. What is the phenotype of plants with 7 contributory alleles?

7. How many contributing alleles does a plant with 8 mm petals have?

Sex-related inheritance expression of traits is affected by the sex of the individual

• sex is an inherited phenotypic

character determined by the

presence/absence of certain

chromosomes

• SRY (sex-determining region of

Y) in humans

1. Sex-linked inheritance

2. Sex-limited inheritance

3. Sex-influenced inheritance

Sex-linked inheritance

• Gene linkage – genes located on the same chromosome are

inherited together

• Sex-linkage

• Sex chromosomes contain genes for many characters unrelated to sex

• X-linked/Y-linked gene

Examples of X-linked traits

Recessive alleles

• Color-blindness

• Duchenne muscular

dystrophy

• Hemophilia

• Complete androgen

insensitivity syndrome

Dominant traits

• Hypophosphatemia

Red-green colorblindness is a recessive x-

linked trait. If a man and a woman with

normal color vision, but whose fathers are

affected with the trait marry, what is the

probability that they will give rise to

a. A color-blind girl?

b. A color-blind boy?

c. A child with normal vision?

Examples of Y-linked (Holandric) Inheritance

Transmission of genes from father to son

• Testis-determining factor (TDF/SRY gene)

• Hypertrichosis of the ears

Sex-limited inheritance

• involves autosomal genes that are expressed only in either males or females

• resulting in a part or function of the body that is present in one sex but not the other

• Examples:

• milk production

• cryptorchidism

• feathers in domestic fowl

Genotype Female Male

HH hen-feathered hen-feathered

Hh hen-feathered hen-feathered

hh hen-feathered rooster-feathered

Rooster feathering in chickens is a trait limited in

expression only in males and determined by the

autosomal recessive genotype hh. The dominant

allele (H) produces hen-feathered males. All

females are hen-feathered regardless of genotype.

A rooster-feathered male is mated to a hen and

they have twelve offspring: six males and six

females. Three of the six males have rooster

feathers. Determine the possible genotypes of the

six female offspring.

Sex-influenced inheritance

• dominant in one sex but recessive in the other

• autosomal

• difference in expression due to the hormonal difference between the sexes

• in heterozygotes, the expression of the trait is affected by sex hormones

• homozygotes are unaffected and express the trait regardless of the hormone produced

Sex-influenced

inheritance

•pattern baldness • baldness allele is dominant in males but recessive in females • a heterozygous male is bald, but a heterozygous female is not

Genotype Female

recessive

Male

dominant

b1b1 bald bald

b1b2 not bald bald

b2b2 not bald not bald

Pattern baldness is a sex-influenced trait

that is dominant in men and recessive in

women. A heterozygous bald man marries a

bald woman, what is the probability that

they will give rise to

a. An affected girl?

b. An affected boy?

c. A child who is unaffected?