CHAPTER 12 Summary
In-Class Activity
Click here to access the Test Banks specifically designed for chapter 12.
Click here to access Active Learning Activities: Complex Inheritance specifically designed for chapter 12.
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.
279