Single Gene Disorders

Huntington's Disease

A dominant gene disorder

• Rare
• Late onset

Causes loss of brain cells

• Intellect, personality, muscle control declines

50% chance of inheritance

Very few dominant gene diseases

• Most kill the carrier when young

This is a cross section of the brain of a person who suffered from Huntington's disease. Notice the enlarged ventricles.

This is an MRI (magnetic resonance imaging) scan of a normal brain. Notice the size of the ventricles (the white, butterfly-shape in the middle of the brain).

These are two MRI scans of the brain of a person who suffers from Huntington's disease. Notice the enlarged ventricles.

PKU (Phenylketonuria)

A recessive gene disorder

• Fairly rare
• Early onset

Causes reduced brain size & poor motor control

• Mental retardation, inability to use language

Caused by:

• Gene controlling synthesis of an enzyme
• Enzyme controls body's use of phenylalanine
• Phenylalanine builds up in the body
  • Poisonous to the brain

Genetic/environment interaction

• Put affected infants on low phenylalanine diet
• Must be on diet soon after birth
• Defective gene has no effect if not exposed to phenylalanine

PKU disorder is recessive. So:

• Only seen in phenotype if both parents are PKU carriers

Two Punnett Squares showing the possible outcomes from parents with the PKU recessive genemating.

Chromosomal Disorders

Sometimes there is a mistake during meiosis

Both chromosome copies in same egg/sperm

Piece of a chromosome is lost

Usually creates non-viable zygote; not always

Sex Chromosomes

• Females: XX
• Males: XY

Survivable errors:

• XXX ... female
• XYY ... male
• X0 ... female
  • Turner's syndrome
• XXY ... male
  • Klinefelter's syndrome

Typically, does not affect mental abilities much

Down Syndrome

Usually an extra chromosome 21

Effects physical appearance and mental abilities

Abnormal protein coats on nerve cells in brain

• Amyloid plaques, or neurofibrillary tangles
  • Alzheimer's disease
  • Parkinson dementia

These are two microscopic pictures of nerve cells from the brain of an Alzheimer's patient. The neurons are encased by amyloid plaques.

Polygenic Effects on Behaviour

People are behaviourally different

Most behaviour differences not explained by single gene or gross chromosomal errors

Range of behaviours in a population

• Continuous variation
• Normal distribution (bell curve)

Polygenic characteristic

• Affected by many genes
• Varies in continuous manner
  • Combination of genetic and environmental effects

Heritability

The degree to which variation among individuals on a particular trait, in a particular population, can be attributed to genetic differences among those individuals.

Heritability coefficient

h2 varies from 0.0 to 1.0

• 0.0 means no variance due to genes
• 1.0 means all variance due to genes

Heritability applies to:

• Differences among individuals
  • It is a population measure

Heritability does not apply to:

• Effect of genes and environment on an individual

What is h2 fpr a group of clones?