Sex Determination Misconceptions and X-linked inheritance

King’s throughout history have prayed for the safety of their reign over the empire. The only way this could be accomplished was by the birth of an heir- the son. If the first wife couldn’t bear sons, she was replaced by a new one. In many countries female foeticide is a prevalent practice. She never gets to see the light of the day. Families are considered incomplete without a son. People keep reproducing till by some chance they are ‘lucky’ enough to give birth to a boy.

Women are often blamed for not birthing sons. Is it really their fault? Is it anyone’s fault at all or is it all purely chance? Purely the divine blessings of god?

It is the man who decides the gender of the child in human beings. If someone had told that to a polygamous king he would have surely been beheaded! Sex determination is a complex process but the easiest way to understand it is by studying the chromosomes of the human body. Lets find out.

Determining Sex in Humans

Humans have 23 pairs of chromosomes. 22 pairs have various functions which are related to development of everything other than sexual characters. The 23rd pair contains information regarding that and decides whether the child will be a boy or a girl. The first 22 pairs of chromosomes are exactly same in both males and females and are called autosomes. The last pair is called sex chromosomes.

Henking in 1891 studied spermatogenesis and fertilization in insects. He could trace a specific body that occurred in 50% of the insects. He named it the X-body. It is now known as the X-chromosome. Human females have a pair of these X-chromosomes as the 23rd pair while males have one X and one Y chromosome for the same. This is called male heterogamety. Y chromosome is another sex chromosome.

The X and Y chromosomes are structurally very different. X-chromosome is larger and carries more genetic information than Y-chromosome. Y has only 231  gene codes on it and is the smallest chromosomes amongst all 46 chromosomes of the cell. X is dominant over Y. this is the reason why men are more prone to diseases than females (will be discussed further in this article).
The following table shows the probability of having a girl or boy child calculated using the Punnett Square Method.

FEMALE/MALE

X

Y

X

XX

XY

X

XX

XY

As can be seen from the table, the probability of a girl or boy child being conceived is 50% and depends entirely on which of the father’s chromosomes fuses with the egg.

During sexual reproduction, one copy of each chromosome from the father and mother is given to the child. This is done by means of a process called meiosis which halves the germ cell’s chromosome count from 46 to 23. The mother produces ovum or egg. This ovum has 22 autosomes and one X-chromosome. The father produces sperm. This sperm also has 22 autosomes and either X-chromosome or Y-chromosome.

When a male and female copulate, the sperms released move through the woman’s uterus to the fallopian tubes where the egg is released. Only one sperm can fertilize one egg. More than one sperm fertilizing is next to impossible and causes various disorders (discussed further in this article). When the egg is fertilized, it forms a protective coat over it which prevents any other sperms from fertilizing it. This zygote is then implanted in the uterine wall and grows to form the baby. Thus, the conception of male or female is purely dependent on which sperm fertilizes the egg, one containing X or Y-chromosome.

It is important for every individual to have knowledge about the sex determination pattern in humans.

Abnormal conceptions (chromosomal disorders)

Though the egg when fertilized is covered by a protective coating to prevent re-fertilization, it is possible that another sperm may x linked disordersfertilize it in some extremely rare cases. This will lead to trisomy of the sex chromosomes.
Klinifelter’s syndrome is a rare genetic disorder caused due to the presence of an additional copy of X-chromosome resulting in a karyotype of 47 chromosomes- XXY. Such an individual has an overall masculine development along with development of breasts (Gynaecomastia). These individuals are sterile.

Turner’s syndrome is caused due to the absence of one X-chromosomes giving a count of 45 chromosomes and X0 sex chromosome configuration. Such females are sterile with rudimentary ovaries and lack secondary sexual characteristics.

X-linked Disorders

Y-chromosome is the weaker chromosome and is recessive compared to X-chromosome. Thus, as mentioned above, males are more prone to diseases like colour blindness and haemophilia. This is not the case in females as even if they have one defective X-chromosome, they have another normal X-chromosome to counter that defect.

Haemophilia is a disease where a single protein that is part of a cascade involved in formation of blood clots at wound sites is affected. Thus, when an affected individual is hurt he loses a large amount of blood due to non-stop bleeding which can be caused even by a simple cut. The female is always a carrier for the disease. Only males can be haemophilic. A haemophilic female dies at embryonic stage but such a conception is rare. The following square shows a cross between a healthy male and carrier female.

FEMALE/MALE

X

Y

X’

X’X

X’Y

X

XX

XY

X’ is the defective chromosome. From the table, we observe that there is a 25% probability each for one normal female and one normal male. There is also a 25% probability for a carrier female and another 25% probability for a haemophilic male.
The following is a cross between a haemophilic male and a carrier female.

FEMALE/MALE

X’

Y

X’

X’X’

X’Y

X

X’X

XY

The above cross shows that a carrier female and an affected male will produce with a 0.25 probability each, a male and female child. Nature does not support an affected female child and she will die in the embryonic stage itself. A carrier female and normal male are also produced. A male child can never be a carrier due to the presence of only one X-chromosome.

By Pedigree Analysis, one can determine traits in sever generations of a family. Queen Victoria’s (House of Windsor)  family’s pedigree chart shows that she was a carrier of haemophilia. Such X-linked diseases are more prevalent in families with a high inbreeding rate. Royals have been prone to such practices in an attempt to keep their bloodline pure. This tradition is also followed in many cultures and religions. This is unhealthy and should be avoided as it destructs the genetic pool and leaves the family’s individuals more prone to diseases. It weakens the person.

Thus, understanding sex determination is very important for protection against X-linked disorders and also for shunning ill societal practices where women are blamed for not giving birth to sons. Also, it should be kept in mind that the rapid increase of the world’s population is placing an extreme stress on the limited resources.