Lecture G5 (Extensions of Mendelian Genetics II)

What is epistasis?

A type of gene interaction in which the phenotypic expression of one gene alters or masks the expression of another gene

In Labrador retrievers, which gene controls pigment color?

The B/b gene - B (black) is dominant to b (brown)

In Labrador retrievers, which gene controls pigment deposition?

The E/e gene — E allows pigment to be deposited; ee prevents pigment deposition

Which gene is epistatic in Labrador retrievers?

The E/e gene is epistatic to the B/b gene

What is the result when the epistatic gene (E/e) prevents pigment deposition?

The dog appears yellow regardless of its B/b genotype

How does epistasis affect phenotypic ratios?

It modifies expected Mendelian ration (ex. 9:3:4 instead of 9:3:3:1)

What does a dilution gene do?

It lightens coat color when two recessive dilute alleles are present

What inheritance pattern does a dilution gene follow?

Autosomal recessive

What is polygenic inheritance?

An additive effect of two or more genes on a single character

What types of traits are controlled by polygenic inheritance?

Quantitative traits that vary continuously in the population

Examples of polygenic traits

Height, skin color, weight, intelligence

Why are polygenic traits quantitative?

Because each contributing allele adds a small, cumulative effect on the phenotype

What type of dominance do polygenic traits often display?

Incomplete dominance — alleles have cumulative, additive effects

What are multifactorial traits?

Traits influenced by both multiple genes and environmental factors

Example of a multifactorial trait in animals

Birth weight in beef cattle — influenced by over 100 genes and environmental conditions

What is non-mendelian inheritance?

Inheritance patterns that do not follow Mendel’s simple dominant/recessive laws

Some examples of non-mendelian inheritance

• epistasis

• Polygenic inheritance

• Genomic imprinting

• Organelle inheritance

• Environmental effects

How can the environment affect phenotype?

It can influence gene expression and alter the phenotype

What are multifactorial characters?

Traits whose phenotypic expression depends on multiple genes and environmental influences

How does the environment impact phenotype in plants?

Tree leaves → sunlight affects size and greenness

Soil composition → affects flower color

How does the environment affect phenotype in animals?

Nutrition → affects height and weight

Temperature → affects coat color in siamese cats

Why do siamese cats have dark extremities?

Their pigment enzyme is temperature-sensitive — active in cooler areas, inactive in warmer areas

How does soil pH affect hydrangea flower color?

Acidic soil with free aluminum → blue or violet flowers

Basic soil → pink flowers

What is genomic imprinting?

When the expression of an allele depends on which parent it was inherited from, due to silencing of one allele during gamete formation

When does genomic imprinting occur?

During gamete formation (in sperm or egg)

How common is genomic imprinting?

It affects only a small fraction of mammalian genes, most of which are important for embryonic development

What is an example of an imprinted gene?

The IGF2 gene — only the paternal allele is expressed

What happens if both copies of an imprinted gene are mutated or silenced?

Developmental abnormalities or embryonic death can occur

What are organelle (extranuclear) genes?

Genes located in organelles such as mitochondria and chloroplasts, rather than in the nucleus

How are organelle genes inherited?

Maternally — because the egg provides nearly all of the zygote’s cytoplasm and organelles

Why aren’t organelle genes inherited from the father?

Sperm contribute almost no cytoplasm or organelles to the zygote

What human diseases are caused by defective mitochondrial genes?

Mitochondrial myopathy and Leber’s hereditary optic neuropathy

Why do mitochondrial diseases mainly affect muscles and the nervous system?

These tissues require large amounts of ATP, and mitochondrial defects impair energy production