Page 137 - NCERT Science Class 10 English Medium

inherited. This is explained by the fact that each gene set is present, not
as a single long thread of DNA, but as separate independent pieces,
each called a chromosome. Thus, each cell will have two copies of each
chromosome, one each from the male and female parents. Every germ-
cell will take one chromosome from each pair and these may be of either
maternal or paternal origin. When two germ cells combine, they will
restore the normal number of chromosomes in the progeny, ensuring
the stability of the DNA of the species. Such a mechanism of inheritance
explains the results of the Mendel experiments, and is used by all
sexually reproducing organisms. But asexually reproducing organisms
also follow similar rules of inheritance. Can we work out how their
inheritance might work?

8.2.4 Sex Determination

We have discussed the idea that the two sexes participating in sexual
reproduction must be somewhat different from each other for a number
of reasons. How is the sex of a newborn individual
determined? Different species use very different strategies
for this. Some rely entirely on environmental cues. Thus,
in some animals like a few reptiles, the temperature at
which fertilised eggs are kept determines whether the
animals developing in the eggs will be male or female. In
other animals, such as snails, individuals can change sex,
indicating that sex is not genetically determined. However,
in human beings, the sex of the individual is largely
genetically determined. In other words, the genes inherited
from our parents decide whether we will be boys or girls.
But so far, we have assumed that similar gene sets are
inherited from both parents. If that is the case, how can
genetic inheritance determine sex?
The explanation lies in the fact that all human
chromosomes are not paired. Most human chromosomes
have a maternal and a paternal copy, and we have 22
such pairs. But one pair, called the sex chromosomes, is
odd in not always being a perfect pair. Women have a
perfect pair of sex chromosomes, both called X. But men
have a mismatched pair in which one is a normal-sized X
while the other is a short one called Y. So women are XX,
while men are XY. Now, can we work out what the
inheritance pattern of X and Y will be?
As Fig. 8.6 shows, half the children will be boys and
half will be girls. All children will inherit an X chromosome
from their mother regardless of whether they are boys or
girls. Thus, the sex of the children will be determined by
what they inherit from their father. A child who inherits
Figure 8.6
Figure 8.6
Figure 8.68.6
Figure 8.6
Figure
Sex determination in an X chromosome from her father will be a girl, and one
human beings who inherits a Y chromosome from him will be a boy.
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