Principles of Inheritance and Variation Test - 11

In human beings 23 pairs of chromosomes are present out of which 22 pairs are called autosomes and 1 pair is called sex chromosome.

Genes are segment of DNA present on chromosome. Large numbers of genes are located on single chromosome. A gene or genes determines the traits of the individual.

Somatic cells in human male contain 22 pairs of autosomes and 1 pair (XY) of sex chromosome. Y chromosome is smaller in size and do not have allelomorphic pair of X chromosome. So, only one sex-linked gene is available for each character.

During cell division, chromosomes get condensed in shape. In metaphase stage of mitosis, the chromosomes are located at center of cell in linear fashion. It can be easily studied at this stage.

Most of the organisms are diploid, contain two set of chromosome (2n) but a number of organisms contains multiple of chromosome set like 3n,4n, 6n etc. This condition of having multiple sets chromosome is called Euploidy.

The exchange of segments of chromosome between homologous chromosomes during reduction division is called crossing over. It creates variation in the individuals of same parents.

Somatic cells of human beings contain 23 pairs of chromosomes, 23 each from male and female gametes. These chromosomes always occurs as homologous chromosome as contain same kinds of trait.

Monosomic is an aneuploidy in which a chromosome is deficient of its homologous causing genetic imbalance resulting in weaker than normal form. In normal form(2n) 24 chromosome should be present but 1 homologous pair is absent so, only 23 chromosome will be present in monosomic individual.

Recessive genes are expressed only in case of homozygous condition but sex chromosomes of males do not have opposite alleles on Y-chromosome. So, the recessive gene located on X-chromosome get expressed in males.