Chromosome, sex determination & chromosomal disorders
Chromosomes and sex determination
Why do plants and animals living around us are not alike? What makes them to bear such diversity in their features? Why does the seed of maize germinate into maize plant but not into millet plant? Why does a dog give birth to puppy but not the piglet? How do characters of parents gets transmitted to offspring from generation to generation? Body of all living organism is made of large number of cells. Each cells consists of nucleus. Thread like substances are present in nucleoplasm of nucleus. These thread like substances are called chromosomes. These chromosomes are responsible for the passing of characters from parents to offspring. Each chromosomes contains large number of genes. Because of the genes present in chromosomes characters of the parents get transmitted into offspring. These genes are made up pf DNA (Deoxyribonucleic acid). In this article we are going to study about chromosomes and process of sex determination.
Chromosomes
Chromosomes are thread like hereditary materials which store, replicate and transmit coded informations of biological importance. They are present in nucleoplasm and made up of nucleic acids, proteins and basic genetic materials.
The word chromosome is derived from two Greek words 'Chromo' meaning colour and 'soma' meaning body. Chromosomes are normally visible under the microscope only when a cell is undergoing mitosis. The full chromosome containing both sister chromatids become visible only in metaphase of mitosis. In prokaryotic cells chromosomes are found in cytoplasm but in eukaryotic cell they are found in nucleoplasm.
A chromosome consists of two similar thread called chromatids. Two chromatids of a chromosome are attached to each other at a point called centromere. The centromere may lie at centre, near centre or terminal position. Depending upon the position of centromere chromosomes may have different shapes. The surface of chromosomes bears number of small swellings called chromomeres. Chromosomes were discovered by swiss botanist Karl Wilhelm Von Nageli. The name chromosome was coined by Edourad Van Benden. On the basis of position of centromere chromosomes can be divided into four parts.
Metacentric Chromosome
Such chromosomes have centromere at the centre so that both section of chromatids are of equal length.
Submetacentric
If the centromere is lying near the centre, the chromosome is called submetacentric chromosome.
Arocentric chromosome
If the centromere shifts away from the centre giving a very long and very short section of chromatids, the resulting chromosome is called acrocentric chromosome.
Telocentric
If the centromere is at the end of the chromatids, the resulting chromosome is called telocentric. Telocentric chromosome is not found in human being.
Number of chromosomes:-
A cell of each organism contains a fixed number of chromosomes. Generally somatic cell of living organism contains diploid (2n) chromosomes but the gametic cells contain halploid (n) number of chromosomes. Out of the diploid chromosomes half are maternal chromosomes and half are parental chromosomes. Number of chromosomes in some living being are given below:-
Frog:- 26
Lion:- 38
Human being:- 46
Gorilla:- 48
Goat:- 60
Pigeon:- 80
Sex Determination
In a baby genetic sex is determined at the time of conception. Human body consists of 23 pairs of chromosomes. These are divided into two groups :- autosomes (body chromosome) and allosome ( sex chromosome). Out of 23 pairs of Chromosomes 22 pairs are autosomes and rest one pair are allosome. Certain genetic traits are related to person sex determination. These traits are passed through sex chromosomes. The rest of the hereditary information and characters are passed through autosomes.
The 22 pairs of chromosomes look the same in male and female. The 23rd pair called sex chromosomes is different in male and female. Female contains two copies of X chromosome in 23rd pair but male contains one X and one Y chromosomes in 23rd pair.
During the time of gamete formation female produces only one type of gamete but male produces two types of gamete. It means gamete (eggs) produced by female are alike. They contain 22 autosomes and one x - chromosomes. The gamete (spermatozoan) produced by male contain two types of spermatozoan. Out of these half of spermatozoans carry 22 autosome and one x - chromosome (same as female) and rest half spermatozoans contain 22 autosomes and only y - chromosome. If x-spermcell (gamete) of male combine with female gamete (egg) it gives a zygote of 44 + xx combination and results into female baby. If y-sperm cell of male combine with female gamete (egg) during fertilization it yields zygote of 44 + xy composition and results into male baby.
Chromosomal disorders
Any abnormality of a chromosome due to loss, duplication or rearrangement of it's segment is called chromosomal disorders. In these disorder entire chromosome, or large segment of them are missing, duplicated or altered. Numerical disorder of chromosome is called aneuploidy. It occurs when a chromosome is missing from a pair or has more than two chromosomes in a pair. Some of the chromosomal disorders are:-
Down syndrome (DS)
It is a chromosomal disorder caused due to the presence of an extra copy of chromosome. It was described by joln Langdon Down in 1866. The parent of the affected individual are normal. The extra chromosome occurs by chance
Down syndrome is typically associated with delay physical growth, characteristic facial features and mild to moderate intellectual disability. In this syndrome mental ability of adult is similar to 8 or 9 years child. It is the most common chromosomal abnormalities in human. It occurs about one per thousand babies.
Turner's syndrome (TS)
The condition in which female is partly or completely missing an 'X' chromosome is called Turner syndrome. Turner's syndrome is not inherited. Environmental risk and mother's age does not play role in it. Human beings has 46 chromosomes but a person suffering from this syndrome has only 45 chromosome. This syndrome was described by Henry Turner in 1938. Short webbed neck, low set ears, short stature are some symptoms of this syndrome. Female suffering from this syndrome can develop only menstrual period and breast but not capable to have a child.
Klinefelter's Syndrome (KS)
This symptom is described by Harry Klinefelter's in 1942. It is the genetic disorder that affect the male. It occurs when a body is born with one more extra 'X' chromosome. A normal male have one 'X' and one 'X' chromosomes. Having an extra X chromosome causes to have some physical traits unusual to males. Total number of chromosomes becomes 47 or more.
It brings sterility. The other symptoms are weak muscles, greater height, less body hair, smaller genitals, breast growth and less interest in sex. This Syndrome is not inherited. This problem is diagonalized by genetic test known as Karyotype.
During the time of gamete formation female produces only one type of gamete but male produces two types of gamete. It means gamete (eggs) produced by female are alike. They contain 22 autosomes and one x - chromosomes. The gamete (spermatozoan) produced by male contain two types of spermatozoan. Out of these half of spermatozoans carry 22 autosome and one x - chromosome (same as female) and rest half spermatozoans contain 22 autosomes and only y - chromosome. If x-spermcell (gamete) of male combine with female gamete (egg) it gives a zygote of 44 + xx combination and results into female baby. If y-sperm cell of male combine with female gamete (egg) during fertilization it yields zygote of 44 + xy composition and results into male baby.
Chromosomal disorders
Any abnormality of a chromosome due to loss, duplication or rearrangement of it's segment is called chromosomal disorders. In these disorder entire chromosome, or large segment of them are missing, duplicated or altered. Numerical disorder of chromosome is called aneuploidy. It occurs when a chromosome is missing from a pair or has more than two chromosomes in a pair. Some of the chromosomal disorders are:-
Down syndrome (DS)
It is a chromosomal disorder caused due to the presence of an extra copy of chromosome. It was described by joln Langdon Down in 1866. The parent of the affected individual are normal. The extra chromosome occurs by chance
Down syndrome is typically associated with delay physical growth, characteristic facial features and mild to moderate intellectual disability. In this syndrome mental ability of adult is similar to 8 or 9 years child. It is the most common chromosomal abnormalities in human. It occurs about one per thousand babies.
Turner's syndrome (TS)
The condition in which female is partly or completely missing an 'X' chromosome is called Turner syndrome. Turner's syndrome is not inherited. Environmental risk and mother's age does not play role in it. Human beings has 46 chromosomes but a person suffering from this syndrome has only 45 chromosome. This syndrome was described by Henry Turner in 1938. Short webbed neck, low set ears, short stature are some symptoms of this syndrome. Female suffering from this syndrome can develop only menstrual period and breast but not capable to have a child.
Klinefelter's Syndrome (KS)
This symptom is described by Harry Klinefelter's in 1942. It is the genetic disorder that affect the male. It occurs when a body is born with one more extra 'X' chromosome. A normal male have one 'X' and one 'X' chromosomes. Having an extra X chromosome causes to have some physical traits unusual to males. Total number of chromosomes becomes 47 or more.
It brings sterility. The other symptoms are weak muscles, greater height, less body hair, smaller genitals, breast growth and less interest in sex. This Syndrome is not inherited. This problem is diagonalized by genetic test known as Karyotype.
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