Meiosis and Genetic Basis of Sexual Reproduction Flashcards

Changes in Chromosome Number

• Large-scale chromosomal alterations can lead to spontaneous abortion or miscarriage.
• Nondisjunction: Error in mitosis or meiosis where homologous chromosomes or sister chromatids fail to separate.
  • Meiosis I: Both members of a pair go into the same daughter cell; another gets no copy.
  • Meiosis II: Sister chromatids fail to separate.

Aneuploidy

• Aneuploidy: Abnormal number of a particular chromosome (2n-1).
• Monosomic: Aneuploidy where a chromosome is missing (2n-1).
• Trisomic: Chromosome present in triplicate (2n+1).
• Down syndrome (trisomy 21)
• Monosomy and trisomy account for 10-25% of human conceptions, often leading to pregnancy loss.

Nondisjunction in Mitosis

• If it occurs early in embryonic development, the anomaly transmits to all embryonic cells.
• If in many cells, is likely to have substantial effect on the organism.

Polyploidy

• Some organisms have more than two complete chromosomal sets in somatic cells.
  • Triploidy (3n), tetraploid (4n).

Down Syndrome (Trisomy 21)

• Characteristics: Facial features, eyelid fold, short stature, stubby fingers, correctable heart defects, developmental delays, increased risk of leukemia and Alzheimer's; shorter lifespan.
• Lower rate of high blood pressure and certain tumors.
• Risk increases with maternal age (especially after 40).

Aneuploidy of Sex Chromosomes

• Upset genetic balance less than in autosomes.
• Y chromosome carries fewer genes, extra X chromosomes are inactivated (Barr bodies).
• Abnormal sex chromosome number: too few or too many X or Y chromosomes.
• Y determines maleness (SRY gene).
• Newborns with abnormal sex chromosome numbers are more likely to survive than those with abnormal autosome numbers.

Klinefelter Syndrome (XXY)

• Extra X chromosome in males (1 in 500-1,000 live male births).
• Male organs with abnormally small testes; sterile.
• Possible breast enlargement and other female traits; subnormal intelligence.

XYY Condition

• 1 in 1,000 males.
• Normal sexual development.
• Increased risk for learning disabilities and behavioral problems; taller than average.
• Some develop severe cystic acne.

Trisomy X (XXX)

• Occurs in 1 in 1,000 live female births.
• Healthy, slightly taller than average.
• Risk for learning disabilities; fertile.

Turner Syndrome (45, XO)

• Females with monosomy X (1 in 2,500 female births).
• Phenotypically female; sterile (sex organs do not mature).
• Develop secondary sex characteristics with estrogen replacement therapy.
• Normal intelligence.