Genetic Variation and Meiosis

Study Notes

Factors that Give Rise to Genetic Variation

The behavior of chromosomes during meiosis and fertilization reshuffles alleles and chromosomes every generation
Three mechanisms contribute to chromosomal genetic variation:
- Independent assortment of chromosomes
- Crossing over
- Random fertilization

Independent Assortment

The "2n rule" states that the number of possible chromosome sorting combinations is 2^n
For humans (n = 23), there are 2^23 = 8,388,608 possible combinations of chromosomes based on independent assortment alone
With n=2, there are 4 possibilities and 8 possible combinations of chromosomes

Random Fertilization

8,388,608 possible gametes x 8,388,608 possible gametes = >70 trillion possible offspring (excluding cross over)

Human Metaphase Plate

Cell cycle is arrested at metaphase
Chromatids are purified, Giemsa stained, and analyzed for number and rearrangements

Cross Over

Early in Meiosis I, a pair of homologs is held together during synapsis
A single crossing over event leads to 4 genetically unique daughter cells

When Things Go Wrong

Aneuploidy: any deviation from the normal number of chromosomes, usually meaning a cell nucleus possessing too many or too few chromosomes
Somatic aneuploidy: non-heritable and has a localized effect

Genetic Disorders

Down Syndrome:
- Low muscle tone
- Single deep crease across center of palm
- Looseness of joints
- Small skin folds at the inner corners of the eyes
- Excessive space between first and second toe
- Mental retardation (mild to severe)
Trisomy 21: results in characteristic facial and body patterns, varying mental abilities
Kleinfelter's Syndrome:
- Male child with an extra X chromosome
- Development of breast tissue
- Little body hair
- Tall stature
- Small testes
- Infertility
- Evidence of mental retardation may or may not be present
Patau Syndrome (Trisomy 13):
- Polydactyly
- Deformed/rocker-bottom feet
- Neurological problems
- Microcephaly
- Severe mental deficiency
- Facial defects
Edwards' Syndrome (Trisomy 18):
- Low birth weight
- Prominent occiput
- Clubbed hands
- Neurological problems
- Severe development delays
- Facial defects
Turner Syndrome:
- Nondisjunction of the X
- Sterile females with webbed necks, shortened torso
- 45 chromosomes - XO not XX

Characteristics of Turner Syndrome

Short stature
Failure of ovaries to develop (90-95%)
Webbed neck (25%) or short neck (40%)
Abnormal fingernails and toenails (70%)
Low hairline at neck (40%)
Heart defect (30%)
Kidney or urinary tract defect (30%)
Hearing disorders (50-90%)
Frequent ear infections in childhood (75%)
Shortening of bones in the hands (35%)
Lower jaw smaller than normal (60%)
Drooping eyelids (ptosis), wandering eyes (strabismus)

Terms to Know

Mitosis: Clonal division - somatic cells
Meiosis: Reductive division-gametes
Somatic cell: all the cells of a multicellular organism except the gamete-forming cells
Chiasmata: the structure that forms at the crossover points after genetic material is exchanged
Crossing over: the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents
Recombinant: describing something composed of genetic material from two sources
Reductive division: a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus
Synapsis: the formation of a close association between homologous chromosomes during prophase I
Tetrad or Bivalent: two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I

Description

This quiz covers the mechanisms that contribute to genetic variation, including independent assortment, crossing over, and random fertilization. It explains how these processes occur during meiosis and fertilization.