Assignment 3

Assignment 3

1.1 Role of Genes and Chromosomes in the Transmission of Characteristics

Genes

Gene is the fundamental physical and functional unit of heredity. A gene is composed not only of a stretch of DNA within a chromosome, but it also includes a particular sequence of nucleotides at a particular location on a chromosome that code for a specific protein. Similar to chromosomes, genes are also present in the form of pairs, called alleles. The alleles reside on each part of a chromosome and their role is to establish the biological features of an individual. There are approximately around 65,000-75,000 genes in the human body (Demerec, 1933).

Chromosomes

The structural unit of inheritance, and the functional unit of characteristics is called chromosome. Genes reside on chromosomes at the locus. Every species has a particular number of chromosomes. The human body contains approximately 20,000 to 25,000 genes with 23 pairs of chromosomes, with 22 pairs of homologous autosomes, and 1 sex chromosome. The sex chromosome is contributed by each parent, and the transmission of characteristics is done to the offspring by the exchange of genetic material between both parent chromosomes called, crossing over (Higgins, 2001).

Structure of chromosomes; Source of Image: BBC (2013)

1.2 Mendelian Laws of Heredity

Based on skill and speculations, various theories came into existence to describe the possible mechanism of inheritance, nevertheless none of these theories were successful. Furthermore, during the year 1866, Gregor J. Mendel formulated the theory of genetics based on the various experiments conducted on a garden pea. Mendel is also called the 'Father of Genetics.' There were three laws of inheritance proposed by Mendel, which were much later recognized in 1900 by the Dutch scientist, German scientist Correns and, Austrian scientist Tschermark.

Mendel's rules of inheritance

According to the first law of inheritance proposed by Mendel, the law of segregation, “the allelic genes present in a zygote do not combine with each other; rather they remain segregated during meiotic cell division and are transmitted into separate gametes”. Whereas, Mendel's second law of inheritance called the law of independent assortment proposed, “When the process of meiosis is initiated, one chromosome among each pair is supplied to the gamete without the influence of the pair's other chromosomes or other cytoplasmic factors”. In short, this law states that, since both chromosomes and the genes occur in the form of pairs, therefore, during division and gamete formation, when they go undergo reduction, one of the genes is contributed to one gamete, whereas the other the other gamete receives the second gene of the pair. As a result, even though the resultant gamete created contains the exact amount of chromosomes, but there is a variety among the genes. To simplify it further, it can be considered that if specie has two pairs of chromosomes containing a solitary allelic gene each, therefore, four types of gametes with a diverse genetic makeup would be present in the two pairs of allelic genes. Hence, when n numbers of allelic pairs are present, then 2n number of gametes would ...