This great article which i found was written by Dr Ibrahim.B.Syed
Professor of medicine @ Louisville University KY.
I am gaining the pleasure of sharing this article with readers of my blog.(Kitty)
The Noble Qur’an which was revealed to Prophet Muhammad(pbuh) about 1400 years ago mentions how the sex of a human being is determined. The following two verses or Ayat of the Qur’an are very clear on this matter.(A. Yusuf Ali translation)
“And of him He made
Two sexes, male and female.” Surah Qiyamat (the Resurrection), 75: 39
In the above Ayah, ” of him He made two sexes, male and female” means that of the human male (he), He (Allah)
Created both the sexes male and female. Hence human male is responsible for the birth of a male child or a female child. The human female is not responsible for the sex determination of the child. However, in many Muslim countries the woman is blamed for the birth of a female child as in these countries they welcome the arrival of a male child.
“That He did create in pairs,-male and female,
From a seed when lodged (in its place); Surah Najm(the Star) 53:45-46
To understand Sex determination we need to understand the Chromosomes, particularly the X and Y chromosomes.
The accepted chromosome number for humans in 1935 was forty-eight. The year 1956 is considered to mark the beginning of modern human Cytogenetics. Until this time the number of chromosomes in the normal human cell was considered to be 48. Due to improvements in techniques it was discovered that the correct number is 46 (Tjio and Levan, 1956)
Reference: Tjio JH, Levan A (1956) The chromosome number of man. Am J Obstet Gynecol 130:723-724
What is a Chromosome?
A chromosome is composed of a very long molecule of DNA and associated proteins that carry hereditary information. The centromere, which is at the center of the chromosome, is a specialized structure that appears during cell division and ensures the correct distribution of duplicated chromosomes to daughter cells. Telomeres are the structures that seal the end of a chromosome. Telomeres play a critical role in chromosome replication and maintenance by counteracting the tendency of the chromosome to otherwise shorten with each round of replication.
Chromosomes are composed of protein and DNA, and they are distinct dense bodies found in the nucleus of cells. Genetic information is contained in the DNA of chromosomes in the form of linear sequences of bases (A, T, C, G). The DNA in an individual chromosome is one, long molecule, which is highly coiled and condensed. The total number of bases in all the chromosomes of a human cell is approximately six billion and individual chromosomes range from 50 to 250 million bases. The DNA sequence for a single trait is called a gene. Each chromosome contains a few thousand genes, which range in size from a few thousand bases up to 2 million bases.
During most of the cell cycle, interphase, the chromosomes are somewhat less condensed and are not visible as individual objects under the light microscope. However during cell division or mitosis, the chromosomes become highly condensed and are then visible as dark distinct bodies within the nuclei of cells. The chromosomes are most easily seen and identified at the metaphase stage of cell division.
Sex Chromosomes X and Y
Somatic cell is any cell other than a sperm or egg cell. Somatic cells in humans and most other animals are diploid. Diploid cells are cells containing two sets of chromosomes. Each somatic cell in humans contains 46 chromosomes which can be distinguished from one another by their appearance under a microscope. Chromosomes differ in size, position of the centromere, and staining pattern. A pair of chromosomes that have the same size, centromere position and staining pattern are called Homologous chromosomes (homologues). The chromosomes of a homologous pair carry genes controlling the same inherited traits. Each locus (the position of a gene along the length of a chromosome) is in the same position on homologues.
One exception to homologous chromosomes for human somatic cells is the two distinct sex chromosomes which are important in sex determination. All other chromosomes are referred to as autosomes.
Sex chromosomes in humans are the X and Y chromosomes.
Females have a homologous pair of X chromosomes.
Males have one X and one Y chromosomes.
Thus, humans have 22 pairs of autosomes and 1 pair of sex chromosomes.
The special type of cell division that produces haploid cells is called meiosis and occurs only in the gonads (ovaries of the female and testes of the male). Meiosis in humans produces sperm and egg cells which contain 23 chromosomes. When fertilization occurs, the zygote contains 46 chromosomes and is diploid. The process of meiosis and fertilization are unique to sexual reproduction and all sexually reproducing organisms follow a basic pattern of alternation between diploid and haploid conditions. Haploid cell is a cell that contains a single chromosome set.
The number of chromosomes in human cells is 46 with 22 autosomal pairs (a total of 44) (one of each type contributed by the mother and one of each type from the father) and 2 sex chromosomes – 2 X chromosomes for females (one from father and one from mother) or an X and a Y chromosome for males (the X from the mother and the Y from the father). Sex chromosomes determine the sex of an organism and some other sex-linked characteristics.
In the process of reproduction, special cells are produced in males and females. Males produce sperm cells and females produce egg cells, or ova. The chromosomal makeup of these cells is interesting. Each sex cell from each parent contains only one member from each pair of chromosomes. Therefore, in ova or egg cells there are 22 autosomes plus an X chromosome. There are two types of sperm cells, each type carrying 22 autosomes plus a gender-determining chromosome, i.e., X or Y.
The banding of chromosomes by using dyes was discovered in the late 1960’s and before that cytogeneticists depended on chromosome length and position of a constriction to identify the individual chromosomes. The band width and the order of bands is characteristic of a particular chromosome – a trained cytogeneticist can identify each chromosome (1,2,3…22, X and Y) by observing its banding pattern under the microscope.
Sperm and egg cells are called gametes and are distinct from somatic cells with respect to their chromosome number. Each human gamete contains a single set of 22 autosomes and a sex chromosome (either an X or a Y). Gametes in humans and other organisms are haploid.
Haploid number: The number of chromosomes in a haploid cell (designated N). The haploid number of humans is 23. Sexual intercourse allows a haploid sperm cell from the father to reach and fuse with an egg cell from the mother in a process called fertilization. Fertilization is the fusion of a sperm cell and an egg cell to form a zygote.
Zygote is a fertilized egg cell and it contains the two haploid sets of chromosomes from the gametes and is diploid.
Half of the genes carried on the chromosomes are contributed by the mother (set of chromosomes from the egg cell) and half by the father (set of chromosomes from the sperm cell).
During development from a zygote to a sexually mature adult, the zygote’s genetic information is passed with precision to all somatic cells by the process of mitosis. Sex organs, which form during development, then produce new gametes which can initiate a new cycle.
A human’s sex is predetermined in the sperm gamete (spermatozoa). The egg gamete mother cell is said to be homogametic, because all its cell possess the XX sex chromosomes. Sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome. In light of this, there are two possibilities that can occur during fertilization between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilizes the egg) they are responsible for determining sex.
Chromosomes X and Y
Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilization, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the ‘extra’ X-chromosomes as indicated below.
Sex Linked Characteristics
These sex linked genes on the X chromosome display a number of characteristics. The following are some examples of phenotypes as a result of these genes:
Red-Green colour blindness and Hemophilia – (a condition which prevents the clotting of the blood)
Humans usually have 46 chromosomes per diploid cell consisting of 22 sets of autosomes and a set of sex chromosomes – either XX or XY. In the usual course of events, individuals with the karyotype 46, XX are female and individuals with 46, XY, are male.
The question arises as to how the sex of an individual is determined. Is sex determined by the number of X-chromosomes – with one X you are male or with 2 X’s you are female? Or is sex determined by the presence or absence of the Y chromosome – the presence of a Y makes for a male or the absence of a Y produces a female?
The answer was provided by individuals resulting from non-disjunction of the sex chromosomes. Some individuals have 45 chromosomes and have only one X chromosome; other individuals have 47 chromosomes and have two X-chromosomes and a Y. The table below indicates the sex of these individuals.
* Female (46, XX) and Male (46, XY) frequencies taken from US Census projection of July 1, 1966 with no correction for chromosome constitution.
It is clear that the presence of a Y chromosome is necessary for male sexual characteristics to develop.