Page 105 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.2 Sex Chromosomes and Sex Determination 97

TABLE 4.1 Sex Determination in Fruit Flies and Humans

Complement of Sex Chromosomes
XXX XX XXY XO XY XYY OY

Drosophila Dies Normal female Normal female Sterile male Normal male Normal male Dies
Humans Nearly Normal female Klinefelter Turner female Normal male Normal or Dies
normal male (sterile); (sterile); short, nearly normal
female tall, thin webbed neck male
Humans can tolerate extra X chromosomes (e.g., XXX) better than can Drosophila because in humans all but one X chromosome becomes a Barr body, as discussed
later in this chapter. Complete absence of an X chromosome is lethal to both fruit flies and humans. Additional Y chromosomes have little effect in either species.
Although the Y chromosome in Drosophila does not determine whether a fly looks like a male, it is necessary for male fertility; XO flies are thus sterile males.

Species Vary Enormously in Sex TABLE 4.2 Mechanisms of Sex
Determining Mechanisms Determination
Other species show variations on this XX versus XY chro- ♀ ♂
mosomal strategy of sex determination. In fruit flies, for
example, although normal females are XX and normal Humans and XX XY
Drosophila
males XY (see Fig. 4.2), it is ultimately the number of X
XX
chromosomes (and not the presence or absence of the Y) Moths and (hermaphrodites X0
that determines sex. The different responses of humans and C. elegans in C. elegans)
Drosophila to the same unusual complements of sex chro-
mosomes (Table 4.1) reveal that the mechanisms for sex Birds and ZW ZZ
Butterflies
determination differ in flies and humans. XXY flies are
female because they have two X chromosomes, but XXY Bees and Diploid Haploid
Wasps
humans are male because they have a Y. Conversely, be-
cause they have one X chromosome, XO flies are male, Lizards and Cool Warm
while XO humans are female because they lack a Y. Alligators temperature temperature
The XX = female / XY = male strategy of sex determi- Tortoises and Warm Cool
nation is by no means universal. In some species of moths, Turtles temperature temperature
for example, the females are XX, but the males are XO. In C. Anemone Fish Older adults Young adults
elegans (one species of nematode), males are similarly XO, In the species in the top three rows, sex is determined by sex chromosomes. The
but XX individuals are not females; they are instead self- species in the bottom four rows have identical chromosomes in the two sexes,
fertilizing hermaphrodites that produce both eggs and sperm. and sex is determined instead by environmental or other factors. Anemone fish
(bottom row) undergo a sex change from male to female as they age.
In birds and butterflies, males have the matching sex chro-
mosomes, while females have an unmatched set; in such spe-
cies, geneticists represent the sex chromosomes as ZZ in the and probably many other traits as well. Sutton and other
male and ZW in the female. The sex having two different sex early adherents of the chromosome theory realized that the
chromosomes is termed the heterogametic sex because it perpetuation of life itself therefore depends on the proper
gives rise to two different types of gametes; conversely, the distribution of chromosomes during cell division. In the
sex with two similar sex chromosomes is the homogametic next sections, you will see that the behavior of chromo-
sex. The gametes of the heterogametic sex would contain somes during mitosis and meiosis is exactly that expected
either X or Y in the case of male humans, and either Z or W of cellular structures carrying genes.
in the case of female birds; the gametes of the homogametic
sex would contain only an X (humans) or only a Z (birds). essential concepts
Yet other variations include the complicated sex-determination
mechanisms of bees and wasps, in which females are diploid • Many sexually reproducing organisms have sex
and males haploid, and the systems of certain fish, in which chromosomes that are sex-specific and that determine sex.
sex is determined by changes in the environment, such as • In humans, male sex determination is triggered by a
fluctuations in temperature. Table 4.2 summarizes some of Y-linked gene called SRY; female sex determination
the astonishing variety in the ways that different species have occurs in XX embryos by default.
solved the problem of assigning sex to individuals. • Mechanisms of sex determination vary remarkably; in
In spite of these many differences between species, some species sex is determined by environmental factors
early researchers concluded that chromosomes can carry rather than by specific chromosomes.
the genetic information specifying sexual characteristics—