Genetic Mechanisms
of Sex Determination
Hake, L. & O'Connor, C. (2008) Genetic mechanisms of sex
determination. Nature Education 1(1):25
,• Whether an animal will become a male, a female, or a hermaphrodite is
determined very early in development.
• Scientists have worked for hundreds of years to understand the sex-
determination system.
• For instance, in 335 B.C.E., Aristotle proposed that the heat of the male
partner during intercourse determined sex. If the male's heat could
overwhelm the female's coldness, then a male child would form. In contra
the female's coldness was too strong (or the male's heat too weak), a fema
child would form.
• Environmental theories of sex determination, such as Aristotle's, were
popular until about 1900, when sex chromosomes were discovered. As it
turns out, Aristotle was on to something, at least in the case of some repti
in which the temperature of the nest determines the sex of the embryo.
• For most animals, however, sex is determined chromosomally.
,Sex Chromosomes
• Sex determination results in the development of individuals with
characteristics that allow them to be identified as males, females, or in som
cases, hermaphrodites (huh·ma·fruh·dait).
• In certain species, like the solid nematode Caenorhabditis elegans, differen
in sexual characteristics can be very small; in fact, in C. elegans, the only
distinguishing sexual characteristic is the presence of a testis versus an
ovotestis.
• In other species, the phenotypic differences between the sexes can be quite
significant. Consider, for example, the remarkable plumage and display of a
tom turkey (male) versus the rather plain features of a female turkey.
• Female and male mammals are also readily distinguished by many differen
in their internal and external phenotypes, behavior, and metabolism.
, Sex Chromosomes
• The first major breakthrough in understanding sex determination w
the discovery of sex chromosomes in the early 1900s.
• From thorough analyses of male and female insect chromosomes,
scientists discovered that, although most chromosomes were prese
in equal numbers in both males and females, there were one or tw
additional chromosomes that were unequally represented in the tw
sexes.
• Analyses of additional species over the years has revealed that
chromosomal differences are primarily responsible for sex
determination in most animals.
of Sex Determination
Hake, L. & O'Connor, C. (2008) Genetic mechanisms of sex
determination. Nature Education 1(1):25
,• Whether an animal will become a male, a female, or a hermaphrodite is
determined very early in development.
• Scientists have worked for hundreds of years to understand the sex-
determination system.
• For instance, in 335 B.C.E., Aristotle proposed that the heat of the male
partner during intercourse determined sex. If the male's heat could
overwhelm the female's coldness, then a male child would form. In contra
the female's coldness was too strong (or the male's heat too weak), a fema
child would form.
• Environmental theories of sex determination, such as Aristotle's, were
popular until about 1900, when sex chromosomes were discovered. As it
turns out, Aristotle was on to something, at least in the case of some repti
in which the temperature of the nest determines the sex of the embryo.
• For most animals, however, sex is determined chromosomally.
,Sex Chromosomes
• Sex determination results in the development of individuals with
characteristics that allow them to be identified as males, females, or in som
cases, hermaphrodites (huh·ma·fruh·dait).
• In certain species, like the solid nematode Caenorhabditis elegans, differen
in sexual characteristics can be very small; in fact, in C. elegans, the only
distinguishing sexual characteristic is the presence of a testis versus an
ovotestis.
• In other species, the phenotypic differences between the sexes can be quite
significant. Consider, for example, the remarkable plumage and display of a
tom turkey (male) versus the rather plain features of a female turkey.
• Female and male mammals are also readily distinguished by many differen
in their internal and external phenotypes, behavior, and metabolism.
, Sex Chromosomes
• The first major breakthrough in understanding sex determination w
the discovery of sex chromosomes in the early 1900s.
• From thorough analyses of male and female insect chromosomes,
scientists discovered that, although most chromosomes were prese
in equal numbers in both males and females, there were one or tw
additional chromosomes that were unequally represented in the tw
sexes.
• Analyses of additional species over the years has revealed that
chromosomal differences are primarily responsible for sex
determination in most animals.
