Genetics II

Mendelian Genetics

• Complete dominance

Non-mendelian Genetics

• Codominance• Incomplete dominance• Multiple alleles• Pleiotropy• Epistasis• Polygenic inheritance• Sex-related inheritance

Pleiotropy (pleion, “more”)

• Ability of a gene to affect an organism in multiple ways

• Examples:– 40% of cats with white fur and blue

eyes are deaf– Phenylketonuria in humans (PKU)

• Lack of an enzyme that converts Phe Tyr

• Mental retardation, reduced hair and skin pigmentation, eczema

– Sickle-cell anemia• Problem with hemoglobin molecules

misshapen RBC’s at low O2 concs kidney and bone damage

EpistasisOne gene affects expression of another

• Effects of one gene are modified by one or several other genes called modifier genes

• Gene whose phenotype is expressed epistatic

• Examples:– Coat color in mammals– Color of wheat grains– Fruit color in squash– Bombay phenotype in

ABO blood groups in man

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 (ABC) and non-contributory 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

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

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

3. What proportion of plants have 14 mm petals?

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

5. How many contributing alleles does a plant with 4mm 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

(a) The X-Y system

44 +XY

44 +XX

Parents

22 +X

22 +Y

22 +XY

Sperm Ova

44 +XX

44 +XY

Zygotes(offspring)

22 +XX

22 +X

76 +ZZ

76 +ZW

16(Haploid)

32(Diploid)

(b) The X–0 system

(c) The Z–W system

(d) The haplo-diploid system

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

(1) Father affected with the disorder Xa, Mother is homozygous dominant

(2) Carrier mates with a male of normal phenotype

(3) Carrier mates with a male with the disorder

Examples of X-linked allelesRecessive alleles• Color-blindness• Duchenne muscular

dystrophy• Hemophilia• Testicular feminization

Dominant traits• Hypophosphatemia

Mammalian females• One of the two X chromosomes is randomly inactivated• If female is heterozygous for a gene located on the X chromosome, she is a

mosaic

Two cell populationsin adult cat:

Active X

Orangefur

Inactive X

Early embryo:X chromosomes

Allele forblack fur

Cell divisionand X

chromosomeinactivation

Active X

Blackfur

Inactive X

Examples of Y-linked (Holandric) Inheritance• Transmission of genes from father to son– Testis-determining factor (TDF/SRY gene)– hypertrichosis

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

• e.g. milk productioncryptorchidismfeathers in domestic fowl

Genotype FemaleMale

HH hen-featheredhen-

featheredHh hen-feathered

hen-featheredhh 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 unaffected and express the trait regardless of the hormone produced

•e.g. pattern baldness•gene for hair growth pattern has 2 alleles: one that produces hair all over the head and another that causes pattern baldness

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

Genotype Male (dominant)Female

(recessive)b1b1 bald

baldb1b2 bald

not baldb2b2 not bald

not bald