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”)• One gene, many

phenotypic effects

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EpistasisOne 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 inheritanceAdditive 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 non-contributory or recessive alleles (abc)recessive alleles (abc)

• Expression can be affected by environmental factors

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

Polygenic Inheritance Problem Solving

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 inheritanceexpression 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 inheritance2. Sex-limited inheritance3. 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• Testicular

feminization

Dominant traits• Hypophosphatem

ia

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 MaleHH hen-feathered hen-feathered

Hh hen-feathered hen-feathered

hh hen-feathered rooster-feathered

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 Femalerecessive

Maledominant

b1b1 bald bald

b1b2 not bald bald

b2b2 not bald not bald