AP Bio Chapter 12 Vocab

Concept 12.1: Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: scientific inquiry

• Morgan’s work with an eye-color gene in Drosophila led to the chromosome theory of inheritance, which states that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for Mendel’s laws.

• Morgan’s discovery that transmission of the X chromosome in Drosophila correlates with inheritance of an eye-color trait was the first solid evidence indicating that a specific gene is associated with a specific chromosome.

  Concept 12.2: Sex-linked genes exhibit unique patterns of inheritance

  • Sex is often chromosomally based. Humans and other mammals have an X-Y system in which sex is determined by whether a Y chromosome is present.

  • The sex chromosomes carry sex-linked genes, virtually all of which are on the X chromosome (X-linked). Any male who inherits a recessive X-linked allele (from his mother) will express the trait, such as color blindness.

  • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development, becoming highly condensed into a Barr body.

    Concept 12.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome

    • An F₁ testcross yields parental types with the same combination of traits as those in the P generation parents and recombinant types with new combinations of traits. Unlinked genes exhibit a 50% frequency of recombination in the gametes. For genetically linked genes, crossing over accounts for the observed recombinants, always less than 50%.

    • Recombination frequencies observed in genetic crosses allow construction of a linkage map (a type of genetic map).

Concept 12.4: Alterations of chromosome number or structure cause some genetic disorders

• Aneuploidy, an abnormal chromosome number, results from nondisjunction during meiosis. When a normal gamete unites with one containing two copies or no copies of a particular chromosome, the resulting zygote and its descendant cells either have one extra copy of that chromosome (trisomy, 2n + 1) or are missing a copy (monosomy, 2n – 1). Polyploidy (extra sets of chromosomes) can result from complete nondisjunction.

• Chromosome breakage can result in alterations of chromosome structure: deletions, duplications, inversions, and translocations.