7.3 Terminology Used

1. Factor : A particle or unit in the organism which is responsible for the inheritance and expression of a particular character.

2. Gene : Mendel’s factor is now known as gene. A gene is a particular segment of a DNA molecule which determines the inheritance and expression of a particular character.

3. Alleles or Allelomorphs : Two or more alternative forms of a gene are called alleles or allelomorphs. For example in pea, the gene for producing seed shape may occur in two alternative forms: round (R) and wrinkled (r). Round and wrinkled forms of the gene are alleles of each other. Alleles occupy same locus on homologous chromosomes.

4. Dominant : Of the two alternating forms (allomorphs) of a trait, the one which appears in the F₁ hybrid is called the dominant trait (Dominant Allele).

5. Recessive : Of the two alternating allomorphs of a trait, one which is suppressed (does not appear) in the F₁ hybrid is called the recessive trait (recessive allele).

6. Genotype : The genetic make-up or genic constitution of an individual (which he/she inherits from the parents ) is called the genotype, e.g., the genotype of pure round seeded parent will be RR.

7. Phenotype : The external (morphological) appearance of an individual for any trait or traits is called the phenotype, e.g. for seeds, round shape or wrinkled shape is the phenotype.

8. Homozygous : An individual possessing (receiving from parents) identical alleles for a trait is said to be homozygous or pure for that trait, e.g. plant with RR alleles is homozygous for the seed shape. A homozygous always breeds true for that trait.

9. Heterozygous : An individual receiving dissimilar alleles for a trait is said to be heterozygous or impure for that trait, e.g. a plant with Rr alleles is heterozygous for the seed shape. Heterozygous is also called a hybrid.

10. Parent generations : The parents used for the first cross represent the parent (or P1) generation.

11. F₁ generation : The progeny produced from a cross between two parents (P1) is called First Filial or F₁ generation.

12. Inbreeding : When the individuals of a progeny (e.g. F₁ generation) are allowed to cross with each other, it is called inbreeding.

13. F₂ generation : The progeny resulting from self hybridization or inbreeding of F₁ individuals is called Second Filial or F₂ generation.

14. Monohybrid cross : The cross between two parents differing in a single pair of contrasting characters is called monohybrid cross and the F₁offspring as the hybrid(heterozygous for one trait only).

15. Monohybrid ratio : The phenotypic ratio of 3 dominants : 1 recessive obtained in the F₂ generation from the monohybrid cross is called monohybrid ratio.

16. Dihybrid cross : The cross between two parents in which two pairs of contrasting characters are studied simultaneously for the inheritance pattern. The F₁ offspring is described as dihybrid or double heterozygous (i.e. with dissimilar alleles for two characters).

17. Dihybrid ratio : The phenotypic ratio obtained in the F₂ generation from a dihybrid cross is called dihybrid ratio. In Mendelian experiments, this ratio is 9:3:3:1.

18. Homologues or Homologous chromosomes : The morphologically similar looking chromosomes in a diploid cell (one chromosome coming from the male parent and the other from the female parent) are called homologous chromosomes. They have identical gene loci bearing alleles.

Table of Contents

7.0 Introduction
7.1 Gregor Mendel
7.2 Mendel's Experiment on Sweet Pea
7.3 Terminology Used
7.4 Law of Dominance
7.5 Monohybrid Ratio
7.6 Law of Segregation
7.7 Dihybrid Ratio
7.8 Law of Independent Assortment
7.9 Test Cross or Back Cross
7.10 The Concept of "Factor"

Chapter 8

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