05: Non-Mendelian Inheritance

Types of non-Mendelian inheritance

• Maternal effects

• Epigenetic inheritance

• Extranuclear in heritance

• Linkage

Maternal affect

The genotype of the mother determines the phenotype of the offspring through nurse cell products that affect developing eggs

When do maternal affects impact offspring phenotype

Oogenesis (before fertilization!!)

Maternal effect example

Water snail shell twist direction

• If the mother is anything but dd, there will be mRNA and proteins translated from the wild type D gene, and the egg will develop into an offspring that has a dextral (right handed) twist

Epigenetic inheritance

Non-permanent changes to nuclear genes

When does epigenetic inheritance occur

During embryonic/offspring development

Dosage compensation

The mechanism where the number of active sex chromosomes (usually X) is reduced to one to prevent overexpression

Barr body

Condensed structure that results when an X chromosome is inactivated

When are Barr bodies formed

Interphase, in early embryonic development

Dosage compensation example

Variegated coat (black and white mice, tortoiseshell, calico)

How does a Barr body inactivate a chromosome

The DNA is supercoiled to the point where it can’t be accessed by transcription machinery

What is responsible for X-recognition and Barr body formation

X inactivation centers

Pseudoautosomal gene

A gene on a Barr body that is not totally inactivated, due possibly to a loosening of chromatin. The gene is present on both the X and Y chromosomes, and so overexpression is not a biological concern

Genomic imprinting

A DNA segment is marked (methylation), and that marking determines activation or inhibition of expression. It is maintained throughout the organism’s life

Another name for genomic imprinting

Monoallelic expression

Genomic imprinting example

Mouse Igf2 gene

Reciprocal cross

Swapping the parental genotype

What happens with a reciprocal cross for imprinted genes

You can have offspring with identical genotypes but different phenotypes

What is responsible for imprinting

Imprinting control region

How is imprinting maintained from parent to offspring

In gametogenesis, imprinting is erased and then re-established sometime before fertilization

Extranuclear inheritance

Inheritance involving genetic material from outside the nucleus

Organelles responsible for most extranuclear inheritance

Mitochondria and chloroplasts

Where is DNA located in the organelles responsible for extranuclear inheritance

Nucleoids

Purpose of mitochondrial DNA (mtDNA) in humans

Codes for rRNA, tRNA, and oxidative phosphorylation

In mammals (heterogamous species), where does most of the mtDNA come form and why

The mother is the source of most of the mtDNA because most of the zygote cytoplasm comes from the egg, so most of the organelles are inherited that way

Paternal leakage, in reference to mtDNA inheritance

The inheritance of mtDNA from the male in mammals, is on the level of a handful per 100,000 maternal mitochondria

Where are mitochondrial diseases inherited from in humans

Mothers

What do most mitochondrial diseases affect and why

Muscle and nerve cells, because they need a lot of ATP and have a lot of mitochondria