Exam 2: #1 Non-Mendelian Inheritance

Maternal Effect

Phenotype of the offspring is determined not by its own genotype, but by the genotype of the mother 

• Mother’s genotype!

• Due to accumulation of gene products found only in eggs

  • Embryo does NOT make own genes for a long time, relies on mom’s genes

Nurse cells

Oogenesis: nurse cells provide proteins and nutrients to egg

• Variants in mom’s genome can cause a phenotype in egg

You cross a sinistral female (dd) to a dextral male (DD). What are the expected genotypes and phenotypes of the offspring?

A. All sinistral (Dd)

Epigenetic Inheritance

Definition: transmission of gene expression patterns (or traits) from one generation to the next without changes to the underlying DNA sequence

• genes themselves stay the same, but chemical or structural modifications control whether they are “on” or “off,

• Can be transmitted to future generations

• It is reversible

Dosage Compensation (Epigenetic Inheritance)

A process that keeps gene expression balanced between males and females, even thought they have different numbers of sex chromosomes

• In mammals:

  • Females have two X chromosome (XX)

  • Males have one X and one Y chromosome (XY)

  • To compensate: One of the X chromosomes in the females is turned off in each

Drosophila (Dosage Compensation)

The level of genes on the X Chromosome is doubled.

• Male Drosophila “turn up” the activity of their single X chromosome, they double the gene expression so that is matches the total output from the 2 X’s in females

C. Elegans (Dosage Compensation)

The female: level of expression of genes on each X chromosome is decreased to 50%

X Inactivation

Process in female mammals: one of the two X chromosomes in each cell is inactivated so that only on X chromosome is active

• Dosage if X-linked genes is balance

• Barr Body = inactive X chromosome

• Occurs randomly

Disorders with extra X chromosome

Extras are converted to Barr bodies to keep only 1 active X chromosome

• Ex: XXX = 2 barr bodies

Extra Nuclear Inheritance

Inheritance of genes outside of the nucleus (cytoplasmic inheritance)

• Mitochondria and chloroplast have their own DNA

Mitochondria

Egg: contains a lot of cytoplasm with thousands of mitochondria

• In animals: all mitochondria comes from the mother

Heteroplasmy

Cells can have more than 1 organelle variant

• Ex: When a cell has a mix of normal and mutated mitochondrial DNA

  • If most of the mitochondria in a cell have normal (healthy) DNA, then the cell can usually make enough energy to function properly

Human Mitochondrial Diseases

Mitochondria produces energy (ATP): tissues with high energy demand are most affected

• Neurons

• Muscles