Biology Chapter 9: Sexual Reproduction

Chapter 9: Lecture Outline

• Sexual Reproduction: Animals and plants reproduce sexually, passing chromosomes to offspring.
• Offspring Variation: Each child receives unique chromosome combinations from parents, resulting in differences from parents.
• Meiosis: Meiosis is a type of cell division crucial for sexual reproduction and creating variation among offspring.
• Homologous Chromosomes: Chromosomes of each parent exist in pairs called homologues; they are identical in size, shape, construction, and the genes they hold, although versions of those genes (alleles) might differ.
• Human Chromosomes: Humans have 23 pairs of homologous chromosomes, a total of 46 chromosomes.
  • 22 pairs of autosomes
  • 1 pair of sex chromosomes (XY in males, XX in females)
• Diploid/Haploid Numbers:
  • Diploid number is the total chromosome count (46 in humans).
  • Haploid number is the number of chromosome pairs (23 in humans).
• Human Life Cycle: The human life cycle involves mitosis and meiosis.
  • Mitosis is crucial for growth and tissue repair.
  • Meiosis leads to gamete production (sperm and egg).
  • Gametes have only one chromosome from each pair (23 in humans).
• Sperm/Egg Production: In males, meiosis is part of spermatogenesis (sperm production); in females, oogenesis (egg production).
• Fertilization: The egg and sperm unite during fertilization, restoring the diploid number in the zygote.
• Meiosis Overview: Meiosis involves two cellular divisions, resulting in four daughter cells with one chromosome from each original pair.
  • Meiosis I: Homologous chromosomes pair and exchange genetic material (crossing over); then the pairs separate to produce two haploid cells.
  • Meiosis II: Sister chromatids separate into new cells, producing a total of four haploid cells.
• Crossing-Over: During meiosis' prophase I, homologous chromosomes exchange genetic material, creating new combination of genes in the resulting gametes
• Importance of Meiosis: Meiosis maintains chromosome number in each new generation and creates new allele combinations for each new generation. The process also increases genetic variability.
• Abnormal Chromosome Inheritance: Errors in meiosis (nondisjunction) can lead to abnormal chromosome numbers in offspring. This can result in various conditions like Down syndrome (trisomy 21).
• Nondisjunction: Nondisjunction can occur during meiosis I or meiosis II, where chromosomes or chromatids fail to separate properly, resulting in gametes with too many or too few chromosomes.
• Down Syndrome (Trisomy 21): Trisomy 21 is a common example of an abnormal chromosome number caused by nondisjunction, leading to unique physical attributes and mild to moderate intellectual disability.
• Sex Chromosome Abnormalities:
  • Turner syndrome (XO): Female with monosomy. Physical characteristics of short stature and webbed neck, underdeveloped sexual characteristics.
  • Klinefelter syndrome (XXY): Male with trisomy. Physical characteristics such as breast development, underdeveloped testes, long limbs, and reduced facial hair.