12th Class Biology Principle Of Inheritance And Variation Important Terms Used In Inheritance Studies

Gene : (Mendel called them factor) In modern sense an inherited factor that determines a biological character of an organism is called gene (functional unit of hereditary material).

Allelomorphs or alleles : Alleles indicates alternative forms of the same gene. e.g., Tall TT and dwarf tt are alternation forms of the same gene etc.

Gene locus : The specific place on a chromosome where a gene is located.

Wild and mutant alleles : An original allele, dominant in expression and wide spread in the population is called wild allele. An allele formed by a mutation in the wild allele, recessive in expression and less common in the population is termed as mutant allele.

Homozygous (Bateson and Saunders, 1902) : Both the genes of a character are identical is said to be homozygous or genetically pure for that character. It gives rise to offspring having the same character on self-breeding e.g., TT (Homozygous dominant) or tt (Homozygous recessive).

Heterozygous (Bateson and Saunders, 1902) : Both the genes of a character are unlike is said to be heterozygous or hybrid. Such organisms do not breed true on self fertilization e.g., Tt.

If we know the number of heterozygous pairs we can predict the following :

Number of types of gametes\[={{2}^{n}}\]

Number of \[{{F}_{2}}\] phenotype\[={{2}^{n}}\]

(Where n is the number of heterozygous pairs).

Number of \[{{F}_{2}}\] genotype\[={{3}^{n}}\]

Genotype : The genotype is the genetic constitution of an organism. TT, Tt and tt are the genotypes of the organism with reference to these particular pairs of alleles.

Phenotype : External feature of organisms, colour and behaviour etc.

Pure line : Generations of homozygous individuals which produce offsprings of only one type i.e., they breed true for their phenotype and genotype.

Monohybrid, dihybrid and polyhybrid : When only one allelic pair is considered in cross breeding, it is called monohybrid cross. Similarly when two allelic pairs are used for crossing, it is called dihybrid cross and more than two allelic pairs in a cross are called polyhybrid cross.

Reciprocal cross : The reciprocal crosses involve two crosses concerning the same characteristics, but with reversed sexes.

Genome : Total set of genes (DNA instructions) in the haploid set of chromosomes and inherited as unit from parents to offspring is called genome.

Gene pool : All the genotypes of all organisms in a population form the gene pool.

\[{{\mathbf{F}}_{\mathbf{1}}}\]Generation : \[{{F}_{1}}\] or first filial (filus–son, filia–daughter; Bateson, 1905) generation is the generation of hybrids produced from a cross between the genetically different individuals called parents.

\[{{\mathbf{F}}_{\mathbf{2}}}\]Generation (Bateson, 1905) : \[{{F}_{2}}\] or second filial generation is the generation of individuals which arises as a result of inbreeding or interbreeding amongst individuals of \[{{F}_{1}}\] generation.

Punnet square : It is a checker-board used to show the result of a cross between two organisms, it was devised by geneticist, R.C. Punnet (1927). It depicts both genotypes and phenotypes of the progeny.

Back cross : It is cross which is performed between hybrid and one of its parents. In plant breeding, back cross is performed a few times in order to increase the traits of that parent.

Test cross : It is a cross to know whether an individual is homozygous or heterozygous for dominant character. The individual is crossed with recessive parent. The ratio will be 50% dominant and 50% recessive in case of hybrid or heterozygous individual. In case of double heterozygote (e.g., RrYy) crossed with recessive (rryy) the ratio will be 1:1:1:1. Test cross helps to find out genotype of parents.

Self cross/selfing : It is the process of fertilization with pollen or male gametes of the same individual.

Observed Vs expected results : Experimental results confirm to the ones expected through the theory of probability if the size of the sample is small but they tend to approach the latter if the sample size is large.

Hybrid : The organism produced after crossing of two genetically different individuals is called hybrid.

Heredity and variations in sexual and asexual reproduction

Sexual reproduction : Variations are common in animals and plants which reproduce by sexual means. The reason for this is that the sexual reproduction is biparental, involves meiosis and fertilization, and the offspring receives some traits from father and some from mother.

Asexual reproduction : Those organisms which reproduce by asexual means e.g., bacteria, amoeba, euglena, rose etc. The asexual reproduction is monoparental, involves mitosis and the organism produced by it, inherits all the traits of its single parent. With the result, it is almost a carbon copy of the parent and is known as ramet. A group of ramets is called a clone.