Tuesday 2 December 2014

Genetics-notes

             Allele : Various or slightly different forms of a gene, having same position on chromosomes.
Phenotype : The observable or external characteristics of an organism
Genotype : The genetic constitution of an organism.
Monohybrid cross : A cross between two individuals of species, considering the inheritance of single pair of contrasting character e.g., a cross between pure tall (TT) and Dwarf (tt).
Dihybrid cross : A cross between two individuals of a species, considering the inheritance of two pairs of contrasting traits/characters e.g., a cross between Round and Yellow (RRYY) and wrinkled and green (rryy) pea seeds
Co-dominance : When two alleles of a gene are equally dominant and express themselves even when they are together.
Multiple allelism : When a gene exists in more than two allelic forms e.g., gene for blood group exist in three allelic forms, IA, IB and i.
Aneuploidy : The phenomenon of gain or loss of one or more chromosome(s), that results due to failure of separation of homologous pair of chromosomes during meiosis.
Trisomy : The condition in which a particular chromosome is present in three copies in a diploid cell/ nucleus.
Male heterogamety : When male produces two different types of gametes/ sperms e.g., In human beings X and Y.
Mutation : The sudden heritable change in the base sequence of DNA, or structure of chromosome or a change in the number of chromosomes.
Pedigree Analysis : The analysis of the distribution and movement of trait in a series of generations of a family.
Female heterogamety : When female produces two different types of gametes/ova e.g., female bird produces Z and W gametes.
Law of Dominance : When two individuals of a species differing in a pair of contrasting characters/traits are crossed, the trait that appears in the F1 hybrid is dominant and the alternate from that remain hidden, is called recessive.
Law of Segregation : The members of allelic pair that remained together in the parent, segregate/separate during gamete formation and only one of the factors enters a gamete.
Law of Independent Assortment : In the inheritance of two pairs of contrasting characters, the factors of each pair of characters segregate independently of the factors of the other pair of characters.
Test Cross : When offspring or individual with dominant phenotype, whose genotype is not known, is crossed with an individual who is homozygous recessive for the trait.

The progeny of monohybrid test cross ratio is 1 : 1 while the dihybrid test cross ratio is 1 : 1 : 1 : 1.