Maleness determining in man

Chromosmes of man

Man has 46 chromosomes in the form of 23 homologous pairs . When we compare the karyotype of a human male with the female, we observe that 22 pairs are common in both.These 22 pairs of chromosomes are called autosomes. In human male and female, the 23rd pair is different from rest of the 22 pairs of autosomes. This pair of chromosomes is called mail determining chromosome pair. A woman has two similar chromosomes in her 23rd pair. These are called X- chromosomes. 

Thus, she is XX. , The chromosomes of 23rd pair of a man are morphologically different from each other. One is X-chromosome, while the other much shorter is Y chromosome. Thus, a man is XY.

 Male Determining Gene:

 'SRY' is the male determining gene. It is located at the tip of short arm of Y-chromosome. Its name SRY stands for "mail determining regions of Y". 

Human x chromosome

Types of Gametes in Man and Woman:

 Like autosomes, these chromosomes also segregate during gamete formation. A woman produces only X gametes, so she is homogametic i.e. XX, while man produce two types of gametes, one carrying X chromosome and other carrying Y chromosome. He is heterogametic, i.e XY. Inheritance If a X carrying sperm fertilizes the egg (X), the zygote will be "XX" and baby-girl is born. Similarly Y carrying sperm on fertilization of egg (X) produces "XY" zygote and baby-boy will be born, In man, X and Y chromosomes are different in size. In addition to sex- determining genes they also carry genes for many other traits. Many genes are present only on X-chromosome. These have no corresponding alleles on Y-chromosome. 

These are X-linked genes and their traits are X-linked traits, e.g., colour-blindness, and haemophilia. X-linked traits are also commonly called sex-linked traits . A few genes are preset only on Y-chromosome, having no counterpart on X-chromosome. These are called Y-linked genes and their traits are called Y-linked traits. Y-linked traits are found only in males, e.g., maleness. 

Some genes are present on both X and Y- chromosomes. These are X-and-Y linked genes. As their pattern of inheritance is like autosomal genes, these are also called Pseudo autosomal genes. X and Y chromosomes have two distinct regions each due to their linked genes . 

 A pairing region with common loci of X-and-Y linked genes due to which they synapse during meiosis. A non-pairing or differential region with different gene loci of either X-linked or Y-linked genes. Human X-and Y-chromosome. 

X-linked genes: 

 The genes, which are located on the X chromosome are called X-linked , and 'their traits are called X-linked traits. 

 Examples: 

Genes for colour blindness, haemophilia are X-linked genes. 

Pattern of Inheritance of Sex-linked Traits: 

The pattern of inheritance of sex-linked traits is very specific. It is in a zig-zag manner. This trait passes from maternal grandfather to daughter and from that daughter to her son. It never comes direct from father to son. As a woman is XX, she carries two alleles for any X-linked trait. 

She can be homozygous or heterozygous for the trait. But a man being XY can carry only one allele of any X-linked trait on his single X-chromosome. His Y-chromosome does not carry its corresponding allele. He is hemizygous for the trait.' As Y-chromosome does not carry any counterpart, even a single recessive allele on his X- chromosome can fully express itself. A sex-linked recessive trait can appear in a woman only when she is homozygous for its alleles. 

That is why chances for a male to be affected by a sex-linked recessive trait are double than for a female. This is the reason why sex- linked recessive traits are more common in males than jn females.

 A woman is XX, she carries two alleles for any X- linked trait. She can be homozygous or heterozygous for the trait. But a man is XY, can carry one allele of any linked trait on his single chromosome. His Y chromosome does not carry its corresponding (similar) allele. He is called "hemizogous" for the trait. 

Even a single recessive allele on his single X-chromosome can fully express 'ts Jf.- This is the reaso why linked recessive traits are more common in males than female. X-linked Recessive Trait X-linked recessive is a trait which is determined by,a X-linked recessive genes. Examples: (1) Colour Blindness (ii) Haemophilia