Nondisjunction & Sources of Genetic Variation

Define nondisjunction

Failure of chromosomes to separate during meiosis

What does nondisjunction cause?

Aneuploidy (extra or missing chromosome)

Why does nondisjunction affect all body cells?

Occurs in gametes, so every offspring cell inherits abnormal number

List conditions caused by nondisjunction

Down syndrome (trisomy 21), Patau (trisomy 13), Edwards (trisomy 18), Klinefelter (XXY), Turner/Fragile X (monosomy X)

Describe Down syndrome

Three copies of chromosome 21

How does maternal age affect nondisjunction?

Risk increases with maternal age due to oocytes arrested in prophase I

Compare asexual and sexual reproduction

Asexual produces clones

Why did meiosis evolve?

To enhance genetic variation and survival

List three sources of variation from sexual reproduction

Crossing over, random assortment, random fusion of gametes

Describe crossing over

Exchange of DNA between non-sister chromatids during prophase I

Define bivalent (tetrad)

Pair of homologous chromosomes

Define chiasma

Point where crossing over occurs

What is the effect of multiple chiasmata?

Increases variation

Describe random assortment

Homologous chromosomes line up randomly at metaphase I

Describe random fusion of gametes

Any sperm can fertilize any egg, producing unique zygotes

List three key events of prophase I

Condensation, homologous pairing, crossing over