Genetics is that branch of biology which deals with heredity and variations. It tells about the inborn traits of organisms.
In genetics we study—
(a) Gene structure and function.
(b) Production of transgenic plants and animals.
(c) Production of useful proteins by new methods.
Genes : Genes are present on chromosomes and are carriers of characters.
Heredity : Heredity is the transmission of characters from one generation to the successive generations of living beings. It also tells about the similarities and differences between the offsprings of parents.
Variation : The structure and functional aspects of all living beings are not alike and vary in many respects. This is known as variation.
Causes of Variations
Following are the causes of variations in clones—
(i) Environment : Environment brings the changes.
(ii) Mutations : These are sudden changes and they are discontinuous inheretable as produced due to changes in genetic make up.
(iii) Identical Twins arising from one fertilized egg sime.
Heritable Variations : These variations affect the germ cells. They are transmitted to the next generation. They are also called germinal variations.
Non-heritable Variations : They affect somatic cells. They are lost with the death of organisms. They are produced by three types of factors: environment, use and disuse of organs and conscious efforts.
Phenotypic Variations : These are the differences in the size, structure, colour, texture, etc. of organism.
Genotypic Variations : The differences in the genetic constitution of an organism constitute genotypic variations.
Continuous Variations : These are small, indistinct differences from the normal conditions and are called fluctuations.
Discontinuous Variations : These are large distinct differences from the parents and termed as mutations or sport.
Germinal Variations : These variations are caused due to changes in germ cells.
Somatic Variations : They are caused due to changes in the somatic cells.
Allele (Allelomorph) : Two optimal members or forms of a gene. Number of members may be more.
Dominant and Recessive : In one pair of characters, one is effective and visible on the other and present in the progeny and is called dominant over the one which is hidden and is called recessive.
Homozygote : When two members of gene are same in living organism.
Heterozygote : When two members of the gene are not same in the living organism.
Emasculation : The removal of anthers of a flower before pollination is called emasculation.
Homozygous : Two alleles of one gene are same. Only one type of gametes are produced.
Heterozygous : Two alleles of one gene are different. Two different types of gametes are produced.
Phenotype : Physical appearance in living organism.
Genotype : Genetic constitution in living organism.
Genome : A complete set of genes (coded information in DNA) contained in the haploid set of chromosomes and inherited as a unit from one parent.
Gene Pool : All the genotypes of all the individuals in a population make gene pool.
Factors : The characters which are carried by an organism in the form of hereditary units are called factors.
Punnet Square : (Proposed by R.C. Punnet). It is a chequer board used to show the results of a cross between two organisms and depicts both genotype and phenotype of the progeny.
Hybrid : The cross of two living organism is called hybridization and the organism formed is called hybrid.
F1 Generation : First generation after hybridization.
F2 Generation : Second generation after hybridization.
Back Cross : In this type of hybridization, one of the parent is crossed with F1 generation hybrid.
Test Cross : In this type of hybridization, one recessive parent is crossed with F1 generation hybrid.
Mendel’s Laws of Inheritance
Mendel gave the following laws of inheritance :
Law of Unit Character : According to this law, all the characters of body are represented in the gametes by certain units called factors or determiners which always occur in pair. These genes are present on different chromosomes of homologous pairs at the same locus.
Law of Dominance : In crossing between pure organisms for two contrasting characters, only one character of the pair appears in F1 generation which is dominant, while the other is recessive.
Law of Segregation : If the hybrids of F1 generation are crossed, the F2 generation shows ratio 1 : 2 : 1 which indicates that the characters have their identity and segregate from each other, thus it is also called the law of purity of gametes or law of splitting of hybrids.
Law of Independent Assortment : When F1 hybrids were crossed among themselves; they produced four types of seeds in the ratio 9 : 3 : 3 : 1.
Incomplete Dominance : The expression of heterozygous alleles, different from those of the parents, producing distinguishable hybrids is called incomplete dominance.
Reciprocal Cross : These crosses involve the crosses concerning the same characters but with reversed sexes. It means if a cross A is used as female parent and B as male parent, then in 2nd or reciprocal crosses A will be used as male parent and B as female parent.
Theory of Pangenesis : This theory was given by Charles Darwin for inheritance of characters. He thought that every somatic cell of the body produces a tiny particle called gemmule or pangene and later on migrated to gamete cell. They get mixed up with zygote and guide the formation of respective parts. The young one thus formed has a blend (mixture) of pangenes, hence possesses characters of both the parents.
Three Basic Features of Inheritance
1. The traits could be hidden or masked for one or more generations but ultimately reappear unchanged.
2. Traits may remain together in one generation and separate in subsequent generations.
3. One alternate of a particular trait may be exhibited more frequently than the other alternate form.
Reasons for Selecting Pea Plant by Mendel for His Experiments
(i) The pea plants were true breeding varieties.
(ii) Pea plants being annual, it was possible to cross and study many generations within a short period.
(iii) They had a number of distinguishable, contrasting characters such as tall and dwarf habits, round and wrinkled seeds.
Monohybrid Cross : It is a cross in which only one character is considered at a time, e.g. in a cross between tall and dwarf plant, the size of stem is considered. Mendel made a cross between pure tall (TT) and a pure dwarf (tt) pea plants.
He obtained all tall (hybrid) plants in F1 generation. On selfing produced tall and dwarf in the ratio of 3 : 1. The genotypic ratio of 1 : 2 : 1 and phenotypic ratio of 3 : 1 is termed monohybrid ratio.
Dihybrid Cross : A cross in which two characters are taken into consideration during experimenta-tion, is called dihybrid cross. A cross between a pea plant with yellow smooth seeds and a pea plant with green, wrinkled seeds are considered.
From the dihybrid cross, it can be derived that each gene is assorted independently of the other during its passage from one generation to the other or Law of independent assortment. The ratio obtained in a dihybrid cross is called dihybrid ratio. It is 9 : 3 : 3 : 1.
Complementary Genes (Bateson and Punnet 1906) : These are non-allelic genes which independently show a similar effect but produce a new trait when present together in dominant form. Ratio obtained is 9 : 7, e.g. Flower colour in sweet pea, grain colour in rice, grain colour in Sorgham multiple step reactions.
Supplementary Genes (9 : 3 : 4) : They are a pair of non-allelic genes; one of which produces its effect independently when in dominant state while the dominant allele of the other is without any independent effect but is able to produce a new trait along with the dominant allele of the former, e.g. Glume colour in Sorgham caudatum, Seed colour in Lab lab, coat colour in mice and guinea pig.
Lethal Genes : These genes kill the organisms when they are able to express their effect. Dominant homozygous recessive, e.g. Sickle cell anaemia, Fur colour in mice (first studied by Cuenot in 1905).
Pleiotropic Genes : These genes have multiple effect because they influence number of traits simultaneously.
Polygene : It is defined as a gene which accounts only for partial expression of a trait.
Epistasis : It is the phenomenon of masking or suppressing the expression of a non-allelic gene. The gene which suppresses is called epistatic gene while the gene or locus which is suppressed by non-allelic gene is called hypostatic gene.
Dominant Epistasis (12 : 3 : 1) : The dominant allele at one locus suppresses the expression of another gene regardless of its allelic condition (dominant or recessive), e.g. Fruit colour in Cucurbita (yellow, black and brown).
Recessive Epistasis (9 : 3 : 4) : In this case recessive homozygous genotypic at one locus (aa) suppresses the expression of non-alleles at another locus (B-). The latter can produce their effect only when a dominant allele (A-) occurs at first locus, e.g. Pigmentation in onion bulb, coat colour in mice.
Dominant recessive Epistasis (13 : 3) : Here the dominant alleles at one locus (A-) and recessive allele at another locus (bb) give rise to same effect. (A-B-A-bb, aabb) other gene combination produce different phenotype.
Multiple Alleles : Genes having more than two alternative forms. They are located at the same locus in the chromosome. They regulate a particular character.
Additive Factors : Polygenes affecting the same trait, with each enhancing the phenotype.
Concordance : Identity of matched pairs or groups for a given trait, e.g. identical twins both expressing the same genetic syndrome.
Pseudoalleles : Pseudoalleles are the genes which are located very close on chromosome and have minimum chances of crossing over.
Isoalleles : A dominant gene occurring in two or more than two forms are termed isoalleles.
