Definitions

Gamete

A reproductive cell of an animal or plant.

Fertilization

Mating of 2 cells.

n + n = 2n

Polyploidy

Haploid

1n, one complete sets of chromosomes.

Diploid

2n, two complete sets of chromosomes.

Triploid

3n, three complete sets of chromosomes.

Tetraploid

4n, four complete sets of chromosomes.

Mitosis

Mitosis

Produces a whole set of chromosomes

Mitosis produces 2 identical daughter cells.

Meiosis

Produces a half-set of chromosomes.

• So that a fertilized egg has a whole set of chromosomes.

Meiosis produces 4 non-identical daughter cells.

Homologous pairs stack on top, only sister chromatids separate.

Male vs. Female Meiosis

1 large egg takes all nutrients, 3 un-viable eggs. 4 small sperm cells are all viable.

Meiosis

Goes through PMAT twice. Short interphase between the two cycles, no dna replication in this short interphase.

Chromosomes from mom or dad may be on left or right side at random. So genetic variation occurs (Independent Assortment, 8+ million combinations).

Second major difference (crossing over) occurs.

• Creates even more genetic diversity.
• Happens during prophase I.

Why genetic diversity matters ?

• Adaptation to environmental changes.
• Disease resistance in populations
• Avoids inbreeding problems
• Raw material for evolution

Meiosis I

Metaphase

Homologous pairs line up next to each other rather than stacking on top of each other as in mitosis.

Meiosis II

Same as meiosis I, but no new dna replication.

Metaphase

There are no homologous chromosomal pairs at this point.

Sister chromatids from each chromosome line up next to each other. All chromosomes stack.

Crossing Over

Crossing over occurs on the sides touching each other (only the side of the sister chromatids facing each other).

• Homologous pairs, when touching, exchange equivalent dna segments, leaving each chromosome with mixed genetic material.

Chiasma Chiasmata

The crossover point.

Synaptonemal Complex

Protein "zipper" holding chromosomes together.

X and Y Sex Chromosomes

XX

Female

XY

Male

Has SRY gene that starts testosterone production.

Sex determination is not the same in all species.

Hermaphrodites

One individual produces both sperm and eggs

Earthworms, snails, most flowering plants

ZW System (Birds)

Opposite of humans

Mother determines offspring sex!

Males: ZZ (homogametic)

Females: ZW (heterogametic)

Haplodiploidy Number of chromosomes

Fertilized eggs (diploid) = Females

Unfertilized eggs (haploid) = Males

(Bees, Ants, Wasps):

Temperature-Dependent

Warmer eggs = one sex

Cooler eggs = other sex

(Turtles, Alligators)

Chromosomal Disorders

M-phase checkpoint fails

Non-Disjunction

Meiosis Fails

Happens in Anaphase

• In Meiosis I - homologous pairs don't separate.
• In Meiosis II - sister chromatids don't separate.

Autosome

Any chromosome that is not a sec chromosome.

Sex chromosome aneuploidy are usually survivable, or the 21st chromosome (down's syndrome). They are chromosomes with a relatively small number of genes. Smaller chromosomes aneuploidy are usually survivable.

Trisomy

3 copies of a chromosome.

Monosomy

1 copy of a chromosome.

Chromosomal Aberrations

Structural Changes

Deletion

Loss of chromosome segment (can lose genes).

Duplication

Segment repeated (extra gene copies).

Inversion

Segment flipped around (same genes, different order).

Translocation

Segment moved to different chromosome.

Concepts

Balanced aberrations

No gain or loss of genetic material - often okay.

Some males with XX chromosomes have part of Y chromosome (with SRY gene) translocated to an X chromosome!

Unbalanced aberrations

Gain or loss of genes - usually problematic.

Down's Syndrome