Biology 1001 Spring 2008 (B. Fall), Class notes, topic #10—Genes within individuals III (sex
determination; sex linkage; polygenic traits; pleiotropy)
complete the reading assignments in your text (Freeman, Biological Science, 2
ed.): pp. 281-
Appreciate that there are different mechanisms of sex determination in different organisms.
Understand the concept of chromosomal sex determination and sex linkage.
Appreciate the difference between autosomal and sex-linked genetics crosses.
Appreciate that a gene can influence more than one trait (pleiotropy), and that a trait can be
influenced by more than one gene (polygenic).
Understand that genes interact with their physical and genetic environments, and this interaction
may influence the phenotype.
In some animals (e.g., many reptiles), sex is
determined. For example, whether
individual crocodiles and many turtles become male or female depends on the temperature at
which the eggs are incubated. Many coral reef fish change from one sex to the other as they get
In most animals
(e.g., birds, mammals), sex is determined
. Those in which sex is
determined genetically have
(e.g., X-Y), plus
Mammals and some others (including fruit flies) have an X-Y sex chromosome system. Diploid
human cells, with 46 chromosomes, contain 22 pairs of autosomes, plus one pair of sex
chromosomes—either XX (females) or XY (males).
—produces the same kind of gametes (with respect to sex chromosomes);
e.g., mammal and fruit fly females, XX, produce gametes all of which contain one X
—produces two kinds of gametes (with respect to the sex chromosomes);
e.g., mammal males, XY, produce gametes with
an X or a Y chromosome.
butterflies, moths, and some other animals have an opposite system (ZW): females are
heterogametic, males homogametic.