Meiosis; Chapter 6.2 I. Purpose of meiosis is to create a gamete that is haploid (half the normal number of chromosomes), from a diploid cell (complete set of chromosomes). Upon fertilization, a male gamete (sperm) will combine with a female gamete (egg), to produce a zygote (single fertilized cell).

II. Phases; Divided into Meiosis I and Meiosis II A. Prophase I; Almost like prophase of mitosis. Homologous chromosomes line up with each other, gene by gene to form a tetrad.

During this phase, crossing over occurs. During crossing over homologous chromosomes can actually break apart and exchange genetic material.

Crossing over results in new allele combinations, and produces genetic variation.

B. Metaphase I; Spindle fibers attach to centromeres. Homologous chromosomes line up as tetrads (side by side) across the middle of the cell.

C. Anaphase I; Homologous chromosomes separate and C. Anaphase I; Homologous chromosomes separate and move to opposite ends of the cell. Centromeres, holding sister chromatids together, do not split.

D. Telophase I; Spindle fibers break down, and chromosomes uncoil. Cytoplasm divides to form two new cells. Each new cell has half the genetic info. of the original cell.

E. Prophase II; Spindle fibers form and attach to the E. Prophase II; Spindle fibers form and attach to the chromosomes in each new cell.

F. Metaphase II; Chromosomes (still made of sister F. Metaphase II; Chromosomes (still made of sister chromatids) line up along the center of the cell.

G. Anaphase II; Centromeres of each chromosome split G. Anaphase II; Centromeres of each chromosome split. Sister chromatides separate, and move to opposite poles.

H. Telophase II; Nuclei reform, spindle fibers break H. Telophase II; Nuclei reform, spindle fibers break down, and cytoplasm divides. New cells are haploid (n).

III. Nondisjunction; The failure of homologous chromosomes III. Nondisjunction; The failure of homologous chromosomes to separate properly during meiosis.

A. Human disorders due to chromosome alterations in autosomes (Chromosomes 1-22). There only 3 trisomies that result in a baby that can survive for a time after birth; the others are too devastating and the baby usually dies in utero. 1. Down syndrome (trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are 47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely: characteristic facial features, short stature; heart defects susceptibility to respiratory disease, shorter lifespan prone to developing early Alzheimer's and leukemia often sexually underdeveloped and sterile, usually some degree of mental retardation. Down Syndrome is correlated with age of mother but can also be the result of a translocation of the father's chromosome 21. By age 45, a woman’s risk is about 1 in 30.

2. Patau syndrome (trisomy 13): serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months. This particular boy survived, but is blind, deaf and physically disabled. Other physical defects from Patau Syndrome include; Cleft palate, polydactyly and/ or cyclopia.

3. Edward's syndrome (trisomy 18): almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months. B. Nondisjunction of the sex chromosomes (X or Y chromosome): Can be fatal, but many people have these karyotypes and are just fine! 1. Klinefelter syndrome: 47, XXY males. Males, usually sterile. Breast enlargement and other feminine body characteristics. Normal intelligence 2. 47, XYY males: Individuals are somewhat taller than average and often have below normal intelligence. Often severe acne. At one time (~1970s), it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time.

C. Trisomy X: 47, XXX females C. Trisomy X: 47, XXX females. 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female except by karyotype Monosomy X (Turner's syndrome): 1:5000 live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)

A. Autosomal Trisomy- Somatic cell ends up with an extra chromosome. 1. Trisomy 21- Down Syndrome 1 in 700 live births Increased risk with older mother. By age 45, risk increases to 1 in 30. 2. Trisomy 13 - Patau Syndrome 1 in 5000 live births 3. Trisomy 18- Edwards Syndrome 1 in 10,000 live births B. Trisomy in gametes- 1. Trisomy X- metafemales 3 X chromosomes 2. Klinefelter’s Syndrome- Male with XXY combination. 3. Super males- XYY combination. C. Monosomy- zygote is missing a chromosome 1. Turner Syndrome- XO affects females 2. OY lethal to males