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Genetic Terms
- Dihybrid Cross. It is a cross between two organisms of a species which is made to study the inheritance of two pairs of factors or alleles of two genes.
- Dihybrid Ratio. It is a ratio which is obtained in the F2 generation when a dihybrid cross is made and the offspring of F1 generation are self - bred. Dihybrid ratio is 9 : 3 : 3 : 1 (phenotypic ratio) where 9/16 individuals carry both the dominant traits, 3/16 first dominant and second recessive, 3 /16 first recessive and second dominant and 1/16 carry both the recessive traits.
- Trihybrid Cross. It is a cross between the two individuals of a species for studying inheritance of three pairs of factors or alleles belonging to three different genes.
- Trihybrid Ratio. It is the ratio obtained in F2 generation when three independently assorting genes are studied . There are eight phenotypes in the ratio of 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.
- Gene Interaction. It is the influence of an allele over another allele of the same or different gene that results in change of its expression, phenotype or normal mendelian ratio. It is of two types : -
- (I) Intragenic (Interallelic) Interaction. The alleles of the same gene have an interaction other than dominance - recessive, e.g. , incomplete dominance, codominance, multiple alleles, pleiotropy . ( ii ) Intergenic ( Nonallelic ) Interaction.
- Alleles of different genes interact to produce a new expression, e.g. , supplementary genes, complementary genes, epistasis, inhibitory genes, lethal genes, duplicate genes.
- Pure Line. The term was coined by Johannsen in 1900, the same year Mendel's paper was discovered. Pure line is the progeny of a single, homozygous self pollinated plant.
- Members of pure line are homozygous for one or more characters.
- In homozygous form both the alleles express the same effect. Such characters do not show any change on continued selfing. Hence, these organisms are said to breed true.
- Natural pure lines do not produce vigorous offspring because several of the defective genes also occur in homozygous state or become so due to continued selfbreeding.
- Pure lines of all the available varieties of important plants are maintained by plant breeding centres for one or more useful homozygous characters like grain colour, nutrient content, disease resistance, size, number of flowers, number of seeds, tillers, root system, etc. They are used for cross breeding in order to get the desired improvement in crops.
- Mendel's Mode of Calculations. Mendel used algebraic methods for calculating the ex pected combinations. For example, inbreeding of hybrid (say Aa) involves separation of its factors in the gametes and then their random fusion : (A + a) (A + a) or (A + a) ? or AA +2 Aa + aa. Similarly, in a test cross the hybrid (say Aa) is crossed with recessive parent (aa). The cross will result in (A + a) × a or Aa + aa.
- Theory of Probability. (I)Out of the two alternate events, the probability of occurrence of each one of them is 50 %.( ii ) Two events are independent if occurrence of one does not affect the probability of occurrence of the other.( iii ) The probability of joint occurrence of two independent events is the product of their individual probabilities. (iv) For an event which can happen through two independent pathways, the probability of its occurrence is the sum of separate probabilities.
- Offspring or siblings are the product of sexual reproduction and commonly biparental in origin. They resemble their parents, family, tribe, race, variety and species sufficiently that one can recognise them to belong to their particular grouping. However, this resemblance is never 100 %.
- Only monozygotic twins are genetically alike. Otherwise children of the same parents, including bizygotic twins, possess genetic variations. This is not true for asexually reproducing organisms.
- Here the progeny is monoparental or derived from a single parent through the process of mitosis As a result, the progeny forms a uniform population.
- It possesses exact copies of genetic characteristics of the parent.
- These individuals which are carbon copies of one another and/or the parent are called ramets while the group of such individuals is known as clone.
- Identical twins are also clones of each other. They are, of course, offspring of their parents.
- Species. A group of organisms capable of interbreeding to produce fertile offspring
- Antigen. A substance usually protein or polysaccharide molecule usually induces spe cific antibody production
- Antibody. A protein in a tissue or fluid of the body , produced in response to the presence of some foreign substance, protects the organism against antigen.
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