Factor : A particle or unit in the organism
which is responsible for the inheritance and expression of a
Gene : Mendelís 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.
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.
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).
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).
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.
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.
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.
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.
Parent generations : The parents used
for the first cross represent the parent (or P1) generation.
: The progeny produced from a cross between two parents
(P1) is called First Filial or F1
Inbreeding : When the individuals of
a progeny (e.g. F1 generation)
are allowed to cross with each other, it is called inbreeding.
: The progeny resulting from self hybridization or inbreeding
of F1 individuals
is called Second Filial or F2
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).
Monohybrid ratio : The phenotypic ratio
of 3 dominants : 1 recessive obtained in the F2
generation from the monohybrid cross is called monohybrid
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).
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.
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.