Important Notes Of Principles Of Inheritance and Variation For NEET



        Chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Baveri in 1902.

        This theory states that Mendelian factors or genes have specific loci (position) on chromosomes and it is the chromosomes that undergo segregation and independent assortment, so the chromosomes carry the information from one generation to the next generation.

        The experimental proof of chromosomal theory was given by C Bridges in1916 through his non-disjunction experiment in Drosophila flies.

        The first definite evidence of chromosomal theory came from sex determination. The establishment of sex through differential development in an individual at the time of zygote formation, is called sex determination.

        The various mechanisms of sex determination are

        i.            Sex chromosome mechanism This was worked out by EB Wilson and Stevens (1902-1905). It is the method by which the distinction between male and female is established in a species. It is usually under genetic control of specific chromosomes celled sex chromosomes or allosomes. This mechanism is seen in various organisms.

a)     XX-XY type is seen in insects like Drosophila melanogaster and humans. Males have XY-type while females have XY-type chromosomes.

b)    XX-XO type is seen in insects like grasshopper, etc. It is an example of male heterogamety because in this male produce two different types of gametes either with or without X-chromosomes.

c)     ZZ-ZW type is seen in birds, fowls and fishes. It is an example of female heterogamety because female produces two different types of gametes.

     ii.             Genic balance mechanism it was given by CB Bridges. According to him, Y-chromosome plays no role in the sex determination of Drosophila, it is the ratio of the number of X-chromosomes to the set of autosomes which determines the sex of fly.

   iii.            Haplodiploidy mechanism it is found in honeybees, ants, wasps, etc. In this mechanism, the unfertilised egg develops into male parthenogenetically and the egg if fertilised given rise to the female fly. So, female flies are diploid while male flies are haploid.

   iv.             Hormonal and environmental mechanism of sex determination in higher vertebrates fishes, amphibians, birds and even in some mammals, it was seen that females secrete certain hormones (gonadal hormones), which induces sex in the organism. In marine worm, i.e. Bonnelia viridis environmental control of sex determination is seen.

      v.            Single gene mechanism This mechanism is seen in certain plants (i.e. maize), Asparagus, Neurospora, yeast, etc. In this mechanism single gene pair is found to be responsible for the determination of sex.

        Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation. In other words, it is the tendency of genes staying together during inheritance.

        Morgan and his group found that even when genes were grouped on the same chromosome, some genes are tightly linked. Therefore, if the frequency of recombination is low, linkage is stronger between two genes and if the frequency of recombination is high, linkage is weak.

        Crossing over is the process of interchanging of segment between non-sister chromatids of homologous chromosomes. It produces new combination of genes and usually occurs in pachytene stage of cell division during meiosis.

        Sex chromosomes are primarily concerned with the determination of sex, but these also carry the genes also for other characters. Such body characters whose genes are located on the sex chromosomes are known as sex-linked characters. The genes which govern these sex-linked characters are known as sex-linked genes and the phenomenon of their inheritance is known as sex-linked inheritance.

        Multiple allelism or Multiple alleles can be defines as set of 3, 4 or more allelomorphic genes or alleles which have arisen as a result of mutation of the normal gene and occupy the same locus in the homologous chromosomes.

        Isoalleles are those alleles, which have the ability to express themselves within the same phenotypic range.

        Pseudoalleles are actually genes, which seem to behave like alleles and have minimum chances of crossing over.

        Cytoplasmic inheritance is the transmission of characters taking place through the cytoplasm of the parents to their offsprings. The total self-replicating hereditary material of cytoplasm is called plasmon and cytoplasmic unit of inheritance are described as plasma genes.

Cytoplasmic inheritance have two distinct features

a)     It is a maternal inheritance, i.e. only maternal parent contribute for inheritance.

b)    The reciprocal crosses are not same due to participation of female parents only, e.g. sigma particle inheritance in Drosophila, kappa particle inheritance in Paramecium and breast tumour in mice, etc.

        Mutation is any heritable change in the genetic make up of an individual. It is a sudden change of a gene or chromosome from one form to another. It may be visible as discontinuous variation in genotype and phenotype of an organism. The organism, which undergoes mutation is called mutant. The agent of mutation is called mutagen, e.g. UV radiations and chemicals, etc.

        In most of the cases, mutations are silent mutations, i.e. any gene mutation which does not results in phenotypic expression.

On the basis of occurrence, mutations can be subdivided under the following main headings as

        Chromosomal aberrations These are changes that occur at the level of morphology of chromosomes and results in the change of sequence of genes on chromosomes. In them there can be change in chromosome number or change in chromosome structure.

        Euploidy is the case where variation occurs in the entire set of chromosomes of an individual. The euploidy can be either monoploidy, i.e. haploid set of chromosomes or polyploidy, i.e. more than two sets of chromosomes.

        Polyploidy occurs when there is failure in separation of the duplicated chromosomes into daughter nuclei. In this phenomenon, the cell has loss or gain of 3, 4 or more sets of chromosomes, e.g. triploids (3n), tetraploids (4n), pentaploids (5n), etc

        Aneuploidy occurs when the members of a homologous pair of chromosome fails to segregate during meiosis. It means that there is a loss or gain of one or more chromosomes.

        Gene mutations These are sudden stable change in the structure of a gene. These mutations may arise due to substitution in the nitrogenous base of DNA, i.e. base pair substitution or due to shift in reading frame of DNA, i.e. frameshift mutation.