Human Sex Chromosomes & Genetic

Human Sex Chromosomes & Genetic

Human Sex Chromosomes and Sex Determination

Sex Chromosomes in Humans

• In human males, the sex chromosome is XY, whereas in females, it is XX.

• During gamete formation in males, half of the sperm contain an ‘X’ chromosome, while the other half contain a ‘Y’ chromosome.

• In females, all gametes contain only one type of chromosome, which is ‘X’.

Determination of Zygote Sex

• When a sperm carrying an ‘X’ chromosome fertilises an ovum, the zygote develops into a female.

• When a sperm carrying a ‘Y’ chromosome fertilises an ovum, the zygote develops into a male.

Barr Body & Fertilization

• The Barr body is found in female somatic cells.

• Fertilisation can be done in a test tube in the laboratory, but further development takes place inside the mother’s womb, resulting in a test tube baby.

Environmental Influence on Sex Determination

• Sometimes, sex determination is regulated by environmental factors.

• In some reptiles, temperature determines the sex at which the fertilised egg is incubated.

Chromosome Number in Humans

• In humans, each cell contains 46 chromosomes.

• Any addition or removal of sex chromosomes or autosomes can cause genetic disorders.

Genes on Sex Chromosomes

• Genes present on the Y chromosome are called holandric genes.

Different Kinds of Syndromes & Disorders

1. Klinefelter Syndrome

• Occurs when a male has an extra X or Y chromosome (XXY or XYY instead of XY).

• The male individual shows masculine development, but feminine development is not completely suppressed.

• The individual becomes sterile.

• In females, when an extra X chromosome is present instead of XX, they show normal development but limited fertility.

• Mental retardation may also be seen.

• The number of chromosomes becomes 47 instead of 46.

2. Turner’s Syndrome

• Occurs when a female has a single sex chromosome (X0).

• Ovaries are rudimentary, and there is a lack of secondary sexual characteristics.

3. Down’s Syndrome

• Occurs when an extra chromosome is added to the 21st autosomal chromosome.

• Leads to Down’s syndrome (mongolism).

• Symptoms include mental retardation, protruding eyes, and irregular physical structure.

4. Patau’s Syndrome

• Caused by an addition of an autosomal chromosome in the 13th chromosome.

• Symptoms include cleft lip and mental retardation.

5. Sickle Cell Anaemia

• In this disorder, erythrocytes are destroyed more rapidly than normal, leading to anaemia.

• Occurs due to a change in the 11th autosomal chromosome.

6. Phenylketonuria

• It is an inborn error of metabolism which results in mental retardation.

• Caused by a change in the 12th autosomal chromosome.

7. Haemophilia

• The gene responsible for this disorder is linked with sex chromosomes.

• This disease leads to failure of blood clotting.

8. Colour Blindness

• This disorder leads to failure to distinguish red and green colour.

• The gene responsible is situated on sex chromosomes.

Final Thoughts

In humans, sex is determined by chromosomes: males have XY, and females have XX. During gamete formation, half of male sperm carry X and half carry Y, while all female eggs carry X. Fertilisation of an egg by X sperm results in a female zygote, while Y sperm produce a male zygote.

Barr bodies appear in female somatic cells, and sometimes environmental factors, like temperature in reptiles, influence sex determination.

Humans normally have 46 chromosomes, and any change can lead to disorders such as Klinefelter, Turner’s, Down’s, or Patau’s syndrome. Holandric genes are on Y chromosomes, while some sex-linked conditions include haemophilia and colour blindness.