Principles of Inheritance and Variation 4

Question 1

explain the basics of sex determination

Answer 1

. Henking (1891) could trace a specific nuclear
structure all through spermatogenesis in a few insects, and it was also
observed by him that 50 per cent of the sperm received this structure
after spermatogenesis, whereas the other 50 per cent sperm did not receive
it. Henking gave a name to this structure as the X body but he could not
explain its significance. Further investigations by other scientists led to
the conclusion that the ‘X body’ of Henking was in fact a chromosome

Question 2

explain sex determination mech in insects

Answer 2

It was also observed that in
a large number of insects the mechanism of
sex determination is of the XO type, i.e., all
eggs bear an additional X-chromosome
besides the other chromosomes
(autosomes). On the other hand, some of the
sperms bear the X-chromosome whereas
some do not. Eggs fertilised by sperm having
an X-chromosome become females and,
those fertilised by sperms that do not have
an X-chromosome become males.

Grasshopper is an example of XO type of sex
determination in which the males have only one X-chromosome besides
the autosomes, whereas females have a pair of X-chromosomes.

Question 3

explain sex determination mech in humans

Answer 3

In a number of other insects
and mammals including man, XY type of sex
determination is seen where both male and
female have same number of chromosomes.
Among the males an X-chromosome is
present but its counter part is distinctly
smaller and called the Y-chromosome.
Females, however, have a pair of Xchromosomes. Both males and females bear
same number of autosomes. Hence, the males have autosomes plus XY,
while female have autosomes plus XX. In human beings and in
Drosophila the males have one X and one Y chromosome, whereas females
have a pair of X-chromosomes besides autosomes

Question 4

what is male heterogamety

Answer 4

two types of sex
determining mechanisms, i.e., XO type and XY type. But in both cases
males produce two different types of gametes, (a) either with or without
X-chromosome or (b) some gametes with X-chromosome and some with
Y-chromosome. Such types of sex determination mechanism is designated
to be the example of male heterogamety.

Male heterogamety- two types of gamates are produced by the males. Hence male determines the sex of the offpsring

the parent who possess heterogamety will determine the sex of the offspring.

Question 5

define male heterogamety and female heterogamety

Answer 5

When a male organism produces two different types of sex gametes, the phenomenon is known as male heterogamety. When a female organism produces two different types of sex gametes, the phenomenon is known as female heterogamety.

Question 6

explain sex determination in females

Answer 6

In order to have a distinction with the mechanism of
sex determination described earlier, the two different sex chromosomes
of a female bird has been designated to be the Z and W chromosomes.
In these organisms the females have one Z and one W chromosome,
whereas males have a pair of Z-chromosomes besides the autosomes

Question 7

explain sex determination in humans

Answer 7

Out of 23 pairs of chromosomes present, 22
pairs are exactly same in both males and females; these are the autosomes.
A pair of X-chromosomes are present in the female, whereas the presence
of an X and Y chromosome are determinant of the male characteristic.
During spermatogenesis among males, two types of gametes are produced.
50 per cent of the total sperm produced carry the X-chromosome and
the rest 50 per cent has Y-chromosome besides the autosomes. Females,
however, produce only one type of ovum with an X-chromosome. There
is an equal probability of fertilisation of the ovum with the sperm carrying
either X or Y chromosome. In case the ovum fertilises with a sperm
carrying X-chromosome the zygote develops into a female (XX) and the
fertilisation of ovum with Y-chromosome carrying sperm results into a
male offspring. Thus, it is evident that it is the genetic makeup of the
sperm that determines the sex of the child. It is also evident that in each
pregnancy there is always 50 per cent probability of either a male or a
female child.