lecture 13

Terminologies: gene, allele, locus, homozygous, heterozygous, hemizygous, genotype, phenotype, monohybrid, dihybrid, trihybrid, polyhybrid.

Gene: Hypothetical unit of inheritance located at (Johannsen) a fixed position (i.e. Locu) on a chromosome. (Factor - (Bateson) determines a character.

Allele: (Allelon - one another, Morphus - Form) Alternative form of a gene. Mendel recognised the presence of constant differentiating characters. These contrasting characters are attributed to the presence of allelomorphs, situated at the same locus of homologous chromosomes.

Locus: Gene occupying a fixed position in the chromosome.

Gene symbols: Dominant gene is represented by capital letter and its recessive allele by the corresponding small letter.

Homozygote: (Bateson) As organism derived from the union of gametes of similar genetic constitution e.g. RR, rr

Heterozygote: (Bateson) An organism derived from the union of gametes of dissimilar genetic constitution e.g. Rr.

Hemizygous: Having only a single copy of a gene instead of the customary two copies. All of the gene on the single X chromosome in the male are in the hemizygous state.

Phenotype (Pheno- appear) - (Johannsen) It is the external appearance of an organism. It is the result of the interaction between genotype and environment.

Genotype: The entire genetic constitution of an organism e.g. TT – Genotype. Character - Phenotype.

Monohybrid: A cross between parents differing in a single gene. An individual heterozygous for one pair of alleles.

Dihybrid: A cross between parents differing in two genes, an individual heterozygous for two pairs of alleles

Trihybrid:

   A trihybrid cross is between two individuals that are heterozygous for three different traits. Example: Pea shape and pea colour and then new trait: pod shape.

Poly hybrid:

   The F1 offspring produced by crossing two true breeding parents, which differ for more than three characters, is known as poly hybrid.