CHAPTER 12 Summary

• Humans have 23 pairs of chromosomes. One of these pairs is the sex chromosomes: XX in females and XY in males. It is the presence of the Y chromosome that determines maleness, and therefore fathers determine the sex of a baby.
• Sex determination has genetic, hormonal, anatomical, and behavioral aspects that all interact to produce one’s sense of sexual identity; variations can lead to intersex.
• Because the Y chromosome in a male does not have a homologous partner, it does not recombine during meiosis. The Y chromosome a son inherits from his father is essentially identical to the Y chromosome his father inherited from his father (the grandfather), a fact that can be used to establish paternity.
• Disorders and other traits inherited on X chromosomes are called X-linked traits, and are more common in males than in females.
• Hair texture is an example of incomplete dominance, a form of inheritance in which heterozygotes have a phenotype intermediate between homozygous dominant and homozygous recessive.
• ABO blood type is an example of a codominant trait—both maternal and paternal alleles contribute equally and separately to the phenotype.
• Many traits are polygenic—that is, they are influenced by the additive effects of multiple genes. Polygenic traits often show a continuous, bell-shaped distribution in the population. Human height is an example.
• In many cases, a person’s phenotype is determined by both genes and environmental influences; this type of inheritance is described as multifactorial. Depression and cardiovascular disease are examples of multifactorial illnesses.
• Some genetic disorders result from having a chromosome number that differs from the usual 46. Down syndrome, or trisomy 21, is caused by having an extra copy of chromosome 21.

• MORE TO EXPLORE

  • Colapinto, J. (2000) As Nature Made Him: The Boy Who Was Raised as a Girl. New York: HarperCollins.
  • Foster, E. A., et al. (1998) Jefferson fathered slave’s last child. Nature 396:27–28.
  • Griffiths, A. J. F., et al. (2012) Introduction to Genetic Analysis. New York: W. H. Freeman.
  • Caspi, A., et al. (2003) Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science 301(5631):386–9.
  • Allen, E. G., et al. (2009) Maternal age and risk for trisomy 21 assessed by the origin of chromosome nondisjunction: a report from the Atlanta and National Down Syndrome Projects. Human Genetics 125(1):41–52.