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Sex-linked Characters

JESSICA MAWU-KOENYA BANSAH
JESSICA MAWU-KOENYA BANSAH

Sex-linked character, an observable feature of an organism controlled by the genes on the chromosomes that determine the organism's sex. Simply, it is a trait expressed when a gene is located on a sex chromosome.

Did you know that you can be at risk to obtain certain conditions due to your sex? Yes, it is not a myth. Certain genes are found on your sex chromosomes that will influence your behaviour. To understand this, let us go back to the basics.

A human being is formed from the fusion of the male and female gamete. In humans, the gamete contains chromosomes that determines what you will look like. There are 46 chromosomes: 23 is obtained from the mother through the egg and the other 23 is derived from the sperm. The 23 chromosomes in one cell join together as matched pairs with the 23 in the other cell, so that genes for the same trait (such as height) are situated together on the same chromosome.

Out of this 46, 44 are called autosomal chromosomes/ autosomes whilst the remaining 2 are called sex chromosomes. It is these last 2 chromosomes that will determine if you will be a male or a female.

The sex chromosomes are represented by X and Y. An individual can either be an XX or XY. In humans and some species, females have two X chromosomes(XX) whilst males have one X chromosome and one Y chromosome(XY).

Hence, during fertilization (during the fusion of the egg and sperm), your mother can give only one of two XX chromosomes whilst the sperm given by the male (XY) can give either an X or Y. Thus, it is the sperm that ultimately depicts the gender of the baby. If a male gives a sperm that has an X chromosome, it will combine with the egg to give a female child but if it gives a Y, then a male child will be formed.

Once fertilization has occurred, the chromosomes find their corresponding pairs and attach. For instance, the chromosome that is responsible for complexion from the egg will attach to the other chromosome that is also accountable for complexion from the sperm. The combined effect of the 2 chromosomes for complexion is what will determine a person’s colour. This happens to only the 22 chromosomes from each gamete leaving the sex chromosomes.

This is because the X chromosome is much larger than the Y chromosome. Since it is larger, it naturally has room for more genes, which carry specific traits. Therefore, when the sex chromosomes pair off and an X chromosome matches up with a Y chromosome (to make a male), many of the genes on the X chromosome do not have matching partners on the Y chromosome. Since the X chromosome is larger and has space for genes that cannot fit on the smaller Y chromosome, males carry slightly less genetic information than females.

Thus, a male's sex chromosomes (XY) are different from a woman's (XX) and can carry fewer genes. This is very significant and will lead to different inheritance patterns between the sexes.

Sex-linked trait

This implies that, if a female has a recessive genetic mutation(change) in one of her X chromosomes, the other X chromosome can override that change and make her live normally. However, if a male has a recessive genetic mutation(change) in the X chromosome, that mutation will affect him if the Y chromosome does not have a matching gene on it which is usually the case. Thus, if there is a gene on the X chromosomes of males which has no pairing on the Y chromosome, that gene will always be expressed.

Examples of such genes are baldness, colour blindness, haemophilia and so on. Ever wondered why guys find it difficult to identify colours? This is the reason.

Colour blindness

It is the reduced ability to distinguish between different colours. The typical form of this condition is an inability to tell the colour green apart from the colour red. The condition is caused by a recessive gene carried on the X chromosome. This trait(characteristic) only shows up when there is no dominant gene to override it.

This is the reason why colour blindness shows up most often in males. If a female has an X chromosome with the defective gene carrying colour blindness, chances are her other X chromosome has a gene with normal vision, and she will not be colour blind. Males have no "other" X chromosome, and so whatever recessive gene they inherit on their one X chromosome is expressed.  Thus, females can carry that gene but won’t express it unless both of their X chromosomes contain the defective gene.

Baldness

A man is bald if he has only one gene for baldness, but a woman is bald if two genes are present. If ‘B’ presents a gene for baldness and ‘b’ for non-baldness, and the sex- influences are such that ‘B’ is dominant in men and recessive in women, the genotypes of various individuals will be :

Haemophilia

It is a medical condition in which the blood does not clot normally during injuries making the sufferer bleed severely even from a slight injury. A condition such as this could lead to death if not attended to immediately. Unfortunately, it is caused by an abnormal gene on the X sex chromosome. Just like the other conditions, it can be overridden if there is another X chromosome to dominate it. However, if this condition is transferred to a male, there won’t be any X chromosome to offset it.

Muscular dystrophy

It is a group of inherited diseases that damage and weaken your muscles over time In muscular dystrophy, abnormal genes (mutations) interfere with the production of proteins needed to form healthy muscle. These genes are found on the X sex chromosomes. It uses the same mechanism as the others. There's no cure for muscular dystrophy. But medications and therapy can help manage symptoms and slow the course of the disease.

References

  1. Sex-linked traits. Retrieved from https://www.encyclopedia.com/social-sciences/applied-and-social-sciences-magazines/sex-linked-traits

2. Biology discussion- Baldness. Retrieved from https://www.biologydiscussion.com/human-genetics/is-baldness-an-example-of-sex-linked-genes-genetics/67113

JESSICA MAWU-KOENYA BANSAH

A young lady who is excited to influence the society and world with the knowledge she has acquired.