POPULATION GENETICS OR QUANTITATIVE GENETICS
The study of genetic changes in groups of individuals particularly over periods of several generations. It is the study of similarities b/w the parent population and the progenies as well as the differences within the progeny populations.
The meaning of word population in genetics is the group of individual related common descent.
Slow changes in population are called evolution (the theoretical process by which all species develop from earlier forms of life. According to this theory, natural variation in the genetic material of a population favors reproduction by some individuals more than others, so that over the generations all members of the population come to possess the favorable traits).
– “Mutations are sudden heritable changes in genes, chromosomes or genome (the set of chromosomes) of an organism”.
– The changes, which are not heritable, are excluded from mutations
– Mutations involve all kinds of changes in the genotype whether they are small or large.
– Large and sudden mutations with large phenotypic effects are called “macromutations” and small mutations with less effect are called “micromutations”.
– Most of the mutations are micromutations and are continuous. Mutations with large heritable changes are called discontinuous mutations
– Mutations occur spontaneously in nature (because of very low and very high ranged rays ie ultraviolet, infrared) or they may be artificially induced in the laboratory e.g. exposure to x-rays (this process is called RADIATION).
– The changes induced by both artificial and natural mutations may be external or internal affecting the structure or physiology of an organism. But mutations will be heritable only when it affects the germ cells.
– The mutation may increase or decrease the resistance of organisms, both plants, and animals.
Importance of Mutation:
– Mutations are important for evolution. Because of mutation, new varieties are produced, bring changes in the old ones and thus cause evolution.
There are two types of mutations:
1. Gene mutation
2. Chromosomal mutations
– Gene mutation is any change in number structure and size of genes.
– All cells have a limited number of chromosome pairs ranging from 3 to 100. But the number of genes is always very high reaching about 20,000 – 40,000 in man.
– During cell division, each gene synthesizes, by autocatalysis from the materials present inside, an exact copy of itself.
– In this way, all genes present on the chromosomes of a nucleus reproduce itself.
– Sometimes, a slight slip occurs in gene reproduction and a newly produced gene is not exactly identical with the original that produced it. It is somewhat different. This change in gene reproduction is called gene mutation.
– The original gene and the mutated gene are always present at the same point locus on the homologous chromosomes.
– A gene mutation produces multiple alleles, for example, hair type in human is common of three types i.e. straight, wavy and curly. These are controlled by multiple alleles of the same gene.
– Similarly, 4 classical blood groups A, B, AB and 0 are also examples of gene mutation.
– Sometimes a gene that has mutated may mutate back to its (original) normal. Such mutations are called reverse mutations and are very rare.
– A chromosome is an assemblage of genes arranged in a linear order.
– The number of genes on any particular chromosome is fixed, and the relative positions and distances from one another are also fixed.
– Occasionally, changes occur which result in the change in the number sequence of genes within a chromosome without causing any harm to the number of chromosome i.e. the only structure of a chromosome is changed and the number remains the same.
– Any structural change in the chromosome is called chromosomal aberration or chromosomal mutations.
The various kinds of chromosomal aberration are as under:
1. Deficiency or Deletion
– Deficiency is the loss of a part of a chromosome.
– Depending upon the size of the segment lost, deficiency may contain a single gene or a block of genes.
– In case of deficiency, the chromosome lacks the genes contained in the deleted or missing segment. The deleted segment is without a centromere and therefore, unable to survive.
Depending upon the position of the deleted segment deletion may be terminal or intercalary,
- Terminal deletion is one in which the terminal part of a chromosome is deleted (deficient), while when the deleted part is from middle or inside of the two ends, the deletion will be intercalary.
- The deletion occurs in nature but can be produced by x-rays as well. Deletion may occur only in one member of the homologous chromosomes of a pair and in rare cases in both homologous chromosomes.
The significance of Deletion:
- Deleted chromosomes result in the production of new types. Hence deficiency has a role in evolution.
- Duplication is a type of chromosomal aberration in which a segment of a chromosome becomes part of another homozygous or heterozygous chromosome.
- The detached segment may be terminal or intercalary.
- Example duplication in two homologous chromosomes.
The significance of Duplication:
- Duplication has a role in evolution, because of duplication of new gene combinations arise which results in arise of new characteristics.
- In this type of chromosomal aberration, one chromosome is deleted, while other is duplicated.
- In this type of chromosomal aberration exchange of segments, b/w two non-homologous chromosomes take place.
- Reciprocal translocation or simply translocation is like crossing over, but the difference is in the type of chromosome, i.e. here the exchange of segments is between the non-homologous chromosomes while in crossing over the exchange of segments is in between homologous chromosomes, naturally but can also be induced by x-rays.
The significance of Translocation:
- Translocation also produces new varieties. Thus have a role in evolution.
- Inversion is a type of chromosomal aberration in which a reversal of a segment of a chromosome takes place.
- Suppose a chromosome has a linear arrangement of genes ABCDEFGH breaks into three segments AB, CDE, and FGH and the three segments join again, but with a difference that the middle segment has rotated through 180 degrees. The linear order of the genes in the reconstituted chromosome will be ABEDCFGH. This is known as inversion in a chromosome.
- Movement of genes by pollens, seeds or transplanting trees.
- The gene frequency in a population may be changed by migration of individual b/w various populations or various groups which belong to a large population.
- The degree of change is little frequency in one population caused by immigration (new arrival) which is dependent upon the degree of gene frequency of immigration.
- Migration is more rapid than mutation.
- Migration leads to change in the gene pool (the total of all genes carried by all individuals in an interbreeding population) of the population this factor enhances/ increases the spread of advantageous alleles throughout the spp. As a result, the individuals possessing these alleles survive greater and leave more offsprings
- It is the reverse of Migration.
- It is the prevention of crossing among populations.
- It occurs due to Geographic Isolation, Ecological Isolation, Phenological Isolation, and Genetic Isolation.
- Theory of Natural Selection
- By Darwin in 1858 published in Origin of Species
Salient Features of TNS:
- Individuals in a population have different capabilities to exploit the resources. No two individuals of a population are exactly alike. Variations are of two types ie heritable and non-heritable. Only heritable variations produced by mutation provide the raw material for evolution.
- The population of all organisms increases rapidly but their number remains constant under natural conditions due to competition for food, space, and other necessities.
- Cells production is always more; there is a struggle for survival. Only the fittest will survive.
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