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Haryana 22. Rajasthan

What are genes? …

Genes are the basic units of heredity in an organism which are passed down by an organism to its offspring. They act as strorehouses of the heritable characteristics of an organism.

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1. Genes are made up of a complex organic molecule called

deoxyribonucleic acid

(DNA) which comprises a linear molecule made up of four types of building blocks, or nucleotide bases. A strand of DNA has two strings of nucleotides twisted around each other, with loose attachments between nucleotide pairs. 2. Genetic diversity arises due to the variation in the sequence of nucleotide pairs of DNA strands. 3. In higher organisms, the bulk of DNA is packaged in a linear form within chromosomes located in the cell nuclei of these organisms. 4. Chromosomes occur in pairs in each cell of an organism. One of each pair is inherited from the father while the other is received from the mother. Humans have 23 pairs of chromosomes. Genetic diversity is the “fundamental currency of diversity” that is responsible for variation. This is the diversity of basic units of hereditary information which are passed down generations found within a species (e.g. different varieties of the same species). Genetic diversity underlies the variability (differences) among individuals of a given species. For example no two individuals even in the same family are identical, unless of course they are identical twins with the same genome (i.e. complete genetic makeup). This is true of all species, because any individual resulting from sexual reproduction, except identical twins, has a slightly different combination of genes from another individual. The differences in the genetic makeup of organisms also form the basis for variation within a population, or between populations, of that species. Thus, genetic diversity within a species will increase with the number of individuals of a given species. It is estimated that there are about 10 9 different genes distributed throughout the world’s biota (i.e. living component).

It is genetic diversity that allows a species to adapt to changing environmental conditions such as a lower rainfall, a higher temperature year round, etc. The effective conservation of genetic diversity within a species cannot, however, be achieved by merely conserving the species. This is because the conservation of one or few population(s) of a species cannot lead to conservation of genetic diversity. For example conserving one or more populations of a species may conserve the species as a whole, but may not serve to capture its entire genetic diversity. For example losing even a single population or a group of individuals in a population, may sometimes result in the loss of genetic variation. Sometimes the lost variation of a gene may even be crucial for the future survival of the species in the face of changing environmental conditions.

Box 5.2: The problem of survival among cheetahs

Once quite widespread throughout Asia and Africa, there has been a large reduction of the cheetah’s habitat and numbers though it is still seen in widely separated areas of Africa. Recent surveys of cheetah populations have shown that there is almost no genetic variation among individuals, even among those taken from the furthest points of its range. This is due to an evolutionary “bottleneck” caused by the fact that at one point cheetah populations crashed to very low numbers and then expanded from the same stock. As a result cheetahs in the wild, as well as in captivity, show several problems due to inbreeding which is threatening its future survival. The problems encountered are low sperm counts leading to reproductive difficulties, low survival of juveniles, morphological aberrations and high susceptibility to diseases because of low genetic diversity in the immune system. Thus, even when numbers seem relatively adequate for a given species, a low genetic diversity can increase its vulnerability to extinction.

Genetic diversity is, thus, important for the preservation of species diversity, and hence biological diversity. A knowledge of the variability of genes also allows us to understand how closely individuals or species are related to each other. Genetic

diversity between species becomes greater with increasing distance in the

relationship of species. Consequently more distantly related species will represent a higher range of genetic diversity than closely related species. Genetic variability responsible for these different traits, interacts with local environmental conditions to determine the extent to which populations can adopt to environmental changes. Isolated population such as those on ocean: islands or in small patches of habitats cut off from the surrounding environment tend to have less genetic variation, hence more susceptible to extinction.

5.3.2 Species Diversity

Species diversity means the differences between species (both domesticated and wild). It is the most visible component of biodiversity as implied by the word

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