Biology Reference
In-Depth Information
BOX 10.1 SEX DETERMINATION
The sex of an individual can be determined by sex chromosomes, the
environment, or even change during its lifespan (Bull, 1983).
In species with genetic (chromosomal) sex determination, such as birds or
mammals, sex is determined by whether individuals have two of the same kind
of sex chromosome (homogametic) or two distinct sex chromosomes
(heterogametic). In mammals, females are the homogametic sex (XX) and males
the heterogametic sex (XY). Consequently, all ova are X and sex is determined by
whether an ovum is fertilized by an X or a Y sperm. In birds, females are the
heterogametic sex (ZW) and males the homogametic sex (ZZ). Consequently, all
sperm are Z and sex is determined by whether the ovum is Z or W. To control the
sex of her offspring, a female would, therefore, need to bias either acceptance of
X or Y sperm (mammals), or the production of W or Z ova (birds).
(b)
(a)
(c)
(d)
Fig. B10.1.1
Temperature dependent sex determination in reptiles. In many reptiles
sex is determined by the temperature during development. For example, in (a) the box
turtle (
Terrapene ornate
) and (b) the green turtle (
Chelonia mydas
), males are
produced at cool incubation temperatures and females at warm incubation
temperatures. In other species, such as (c) the Australian freshwater crocodile
(
Crocodylus johnstoni
), the opposite pattern occurs, with males produced at relatively
high temperatures. Finally, one sex may be preferentially produced at extreme
temperatures (both hot and cold), such as (d) the frill-necked dragon (
Chlamydosaurus
kingii
), where both sexes are produced at intermediate temperatures, but only females
at extreme temperatures. Photo (a) © Fred Janzen; (b) © Annette Broderick; (c) and
(d) © Ruchira Somaweera.
