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Chromosomes
In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes.
The first 22 pairs are called autosomes.
Autosomes are homologous chromosomes i.e. chromosomes which contain the same genes (regions of DNA) in the same order along their chromosomal arms.
The 23rd pair of chromosomes are called allosomes.
These consist of two X chromosomes in most females, and an X chromosome and a Y chromosome in most males.
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Females therefore have 23 homologous chromosome pairs, while males have 22.
The X and Y chromosomes have small regions of homology called pseudoautosomal regions.
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What are sex chromosomes?
A sex chromosome is a type of chromosome involved in sex determination.
Humans and most other mammals have two sex chromosomes, X and Y, that in combination determine the sex of an individual.
Females have two X chromosomes in their cells, while males have one X and one Y.
X chromosome:
Females have two X chromosomes, while males have one X and one Y chromosome.
The chromosome X represents about 5% of a cell's total DNA and likely contains about 900 genes.
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An X chromosome is always present as the 23rd chromosome in the ovum.
Y chromosome:
The Y chromosome is very small.
It likely contains 50 to 60 genes and represents 2% of a cell's total DNA.
Either an X or Y chromosome may be present in an individual sperm.
Because only males have the Y chromosome, the genes on this chromosome tend to be involved in male sex determination and development.
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The Y chromosome, often referred to as the “master of maleness”.
Sex linked diseases are passed down through families through one of the X or Y chromosomes.
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