Page 102 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

94 Chapter 4 The Chromosome Theory of Inheritance

4.2 Sex Chromosomes Figure 4.6 The X and Y chromosomes determine sex
and Sex Determination in humans. (a) This colorized micrograph shows the human X
chromosome on the left and the human Y on the right. (b) Children
can receive only an X chromosome from their mother, but they can
inherit either an X or a Y from their father.
learning objectives a: © Biophoto Associates/Science Source
(a)
1. Predict the sex of humans with different complements
of X and Y chromosomes.
2. Describe the basis of sex reversal in humans.
3. Compare the means of sex determination in different
organisms.

Walter S. Sutton, a young American graduate student at
Columbia University in the first decade of the twentieth cen-
tury, was one of the earliest cytologists to realize that
particular chromosomes carry the information for determin-
ing sex. In one study, he obtained cells from the testes of the
great lubber grasshopper (Brachystola magna; Fig. 4.5) and
followed them through the meiotic divisions that produce
sperm. He observed that prior to meiosis, precursor cells (b)
within the testes of a great lubber grasshopper contain a total
of 24 chromosomes. Of these, 22 are found in 11 matched
pairs and are thus autosomes. The remaining two chromo-
somes are unmatched. He called the larger of these the X
chromosome and the smaller the Y chromosome.
After meiosis, the sperm produced within these testes
are of two equally prevalent types: one-half have a set of 11
autosomes plus an X chromosome, while the other half have XX XY
a set of 11 autosomes plus a Y. By comparison, all of the
eggs produced by females of the species carry an 11-plus-X
set of chromosomes like the set found in the first class of
sperm. When a sperm with an X chromosome fertilizes an X Y
egg, an XX female grasshopper results; when a Y-containing X XX XY
sperm fuses with an egg, an XY male develops. Sutton con-
cluded that the X and Y chromosomes determine sex.

Figure 4.5 The great lubber grasshopper. In this mating Several researchers studying other organisms soon veri-
pair, the smaller male is astride the female. fied that in many sexually reproducing species, two distinct
© L. West/Science Source chromosomes—known as the sex chromosomes—provide
the basis of sex determination. One sex carries two copies of
the same chromosome (a matching pair), while the other sex
has one of each type of sex chromosome (an unmatched
pair). The cells of normal human females, for example, con-
tain 23 pairs of chromosomes. The two chromosomes of
each pair, including the sex-determining X chromosomes,
appear to be identical in size and shape. In males, however,
one unmatched pair of chromosomes is present: the larger of
these is the X; the smaller, the Y (Fig. 4.4 and Fig. 4.6a).
Apart from this difference in sex chromosomes, the two
sexes are not distinguishable at any other pair of chromo-
somes. Thus, geneticists can designate women as XX and
men as XY and represent sexual reproduction as a simple
cross between XX and XY.