Sex is the most distinct kinds of hereditary differences observed between individuals of the same species which is determined at the time of fertilization when the female gamete and male gamete fuse together. The mechanism which establishes the sex of an individual when the life begins is called sex determination. Sex is genetically determined in most of the animals as well as in humans. It is based on the formation of two gametes in one sex. The organisms that produce only one kind of gamete i.e. the organisms having two similar sex chromosomes (xx) are known as homogametic organisms. For example: Female of humans. Similarly, the organisms which produce two kinds of gametes (x and y) i.e. the organisms having two dissimilar chromosomes (XY) are known as heterogametic organisms. For example: Male of humans. The sperms containing x- gamete is called gynosperms and the sperm containing x- gamete is called androsperms. Both gametes are produced in equal proportion.
Sex Determination in Humans
Altogether there are 23 pairs of chromosomes in a human body. In the body of a male individual, there are 22 pairs of autosomes and one pair of autosomes having one X and one Y gamete (44 + XY). Similarly, in the body of a female individual, there are 22 pairs of autosomes and one pair of X chromosome (44 + XX). So, females are homogametic as they produce only one type of chromosomes (X) and males are heterogametic as they produce two types of chromosomes (X and Y).
Sex determination is done at the moment of fertilization by the kind of sperm that fertilizes the egg. When the egg with X- chromosome (22 + x) fertilizes with sperm with x- chromosome (22 + x), it produces a zygote with xx chromosome (44 + xx) which develops into a female child. Similarly, when the egg with X- chromosome (22 + x) fertilizes with the sperm with Y- chromosome (22 + y), it produces the zygote with xy chromosome (44 + xy) which develops into a male child.
The small segments of DNA of the chromosome within the nucleus is known as genes which work as building blocks of heredity. They are responsible to control biological characteristics and regulate cellular activities as they are passed from parent to a child. Sometimes, mutation can cause changes in the structure and number of genes which results in dysfunctioning of the cells. Such medical condition is called genetic disorder. A genetic disorder is an illness caused by defects or changes in the DNA of a person. It may be heritable or non- heritable in humans. They are classified into: Mendelian Disorders and Chromosomal Disorders.
- Mendelian Disorders
The disorder caused by the mutation in a single gene of autosomes and sex determination is said to be Mendelian disorder. The principle of Mendelian genetics is followed by their mode of inheritance and it affects both male and female equally. Some of the Mendelian disorders are muscular dystrophy, sickle cell anemia, albinism, etc. It can be predicted.
The Mendelian disorders caused by changes in the genes lying in sex chromosomes are called sex-linked disorders. Sex-linked inheritance is the inheritance of sex-linked characters or disorders from parents to the offsprings. Some of the sex-linked disorders are color blindness or daltonism, night blindness and haemophilia or bleeder's disease.
Haemophilia or Bleeder's Disease
The genetic disorder caused by one of the clotting factor defective genes on the X- chromosome is known as haemophilia or bleeder's disease. It is a rare condition in which the blood does not clot properly which is mostly seen in males since the gene can be passed from mother to son. As male lack an X- chromosome so they are unable to make up for the defective gene. Although female are carriers of haemophilia, they are unlikely to have the disorder. For a female to have haemophilia, she must have the abnormal gene of her X-chromosome which is very rare.
Those who suffer from haemophilia lack protein called clotting factor, which works with platelets to stop bleeding at the site of an injury. This is why the person suffering from haemophilia tend to bleed for a longer time even after a small injury. As haemophilia suffering people tend to have internal bleeding, it can be fatal as internal bleeding occurs within internal organs such as the brain.
- excessive bleeding both internal and external in any small cut, wound, bite or other injuries
- bleeding from the nose is common
- even after the previous bleeding is ceased, there may be prolonged or continue bleeding
When the haemophilia is suspected in a person, the physician asks about the person's family and personal medical history as it helps to identify the cause of bleeding problems.
- Chromosome Disorders
Chromosomal disorders are the disorders which are caused by the changes in the number or structure of the chromosomes. It may occur due to autosomal or sex chromosomal abnormalities. The abnormal condition in which more than one or more chromosomes of a normal set of chromosomes are missing or present in more than their usual number of chromosomes is said to be aneuploidy. Aneuploids, organisms where aneuploidy occurs, arises due to the failure of separation of the homologous chromosome during meiosis which results in either missing a particular chromosome or excess of a particular chromosome. These are abnormal gametes which on fertilization produce chromosomal disorders. In simple language, we can define aneuploidy as the loss or gain of chromosomes due to the failure of the segregation of chromatids during cell division. Some of the autosomal disorders are: Down's syndrome, Edward's syndrome, etc. Some of the sex chromosomal disorders are: Turner's syndrome, Klinefelter's syndrome, etc.
a. Down's Syndrome
The genetic disorder caused due to autosomal abnormality is known as Down's syndrome. As it is an aneuploid trisomic disease, there is a presence of an additional copy of chromosome number 21 (Trisomy of 21). Both the chromosomes of 21 pass into a single egg, which instead of having 23 chromosomes has 24 chromosomes and the offsprings have 47 chromosomes (45 + XY in male and + XX in female) instead of 46. It is also known as 21- trisomy or Mongolian Idiocy or Mongolism. It is usually seen in the ovaries of the women above the age of 40 years, so these women tend to give birth to a child with Down's syndrome. The first Down's syndrome was reported by Langdon Down in 1886.
- round face, broad forehead, partially opened mouth, furrowed tongue, short stature, small round head, flat hands, short neck, small stubby fingers and Mongolian- type eyelids
- underdeveloped gonads
- congenital heart disease
- mental retardation
It can be confirmed by a blood test to examine the chromosomes, chest x- rays, echocardiography to check for the heart's problem. It is suspected on physical appearance at birth.
b. Klinefelter Syndrome
The sex chromosomal disorder which is occurred by the additional copy of the X chromosome is Klinefelter syndrome. Generally, when an abnormal egg (22, XX) and a normal sperm (22, Y) or a normal egg (22, X) and an abnormal sperm (22, XY) fuses the embryo thus produced will have one extra set of X- sex chromosome. Hence, the baby developed from this type of embryo will have three sex chromosomes (44 + XXY) instead of two sex chromosome in their karyotype. So, the offsprings will have 47 chromosomes instead of 46 chromosomes. It was first described in the USA in 1942 by Dr. Harry Klinefelter.
- sterile male with mental retardation, less muscular body, underdeveloped testes, sparse body hair and long limb with some female characteristics like enlarged breast
- children and adults tend to be taller than the peers
It is diagnosed when a man is being investigated for sterility or infertility. It can be confirmed by chromosome analysis test taking blood sample through which extra sex chromosome can be found. It can also be diagnosed at the time of puberty by finding small tastes, breast tissue development, mental functioning, etc.
c. Turner's Syndrome
The sex chromosomal disorder caused by the fertilization of an abnormal egg (22, 0) and a normal sperm (22, Y) or a normal egg (22, X) and an abnormal sperm (22, 0) is said to be tuner's syndrome. It is caused due to the absence of one X- chromosome i.e. 44, X0 (monosomic for sex chromosome) instead of 46. It was first discovered by Turner. It occurs in about 1 out of every 5000 live female births.
- sterile female with poorly developed ovaries and underdeveloped breasts, small uterus and abnormal intelligence
- webbed neck, broad chest with male characters like heavy neck muscle and narrow hips
- female suffering from this have a short stature without puberty and the menstrual cycle
A special blood test should be done to diagnose Turner's syndrome. It is often done during parental testing because of an abnormal amniocentesis.