Genotype and Phenotype
Homozygous and Heterozygous
Chromosomes come in pairs. Each chromosome in a pair will have a gene at
the same point on the chromosome. There can be more than one alternative
form of the gene at that point. These alternative forms are called alleles.
Both chromosomes in a pair have one allele for the gene. If the two alleles
are the same we say that the individual is homozygous for that gene. If
they are different the individual is heterozygous.
Dominant and Recessive
HH = homozygous dominant
hh = homozygous recessive
Hh = heterozygous
Genotype and Phenotype
The appearance of someone or something , what they look like, is their
phenotype.
The actual combination of alleles, whether they are homozygous or
heterozygous, is called their genotype.
Generations
Each generation is given a name. the first organisms bred are called the
Parental generation, or P1 generation.
Their offspring (or children) are called the First Filial generation, or F1
generation.
Their offspring are called the Second Filial generation, or F2 generation
and so on.
True breeding
These organisms are homozygous for a trait and therefore can be used in
breeding experiments where the genotype is not known.
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Monohybrid crosses
The initial parents are true breeding but of different phenotypes. The
numbers or ratio of the F2 offspring usually fall within expected ratios.
They sometimes differ from the expected because we do not study enough
of the offspring.
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Co-dominance
Polygenic Inheritance
More than one gene is involved and usually a gradation of the characteristic
is seen e.g. seed mass in plants and skin colour in humans. This is due to the
interaction of the alleles of several genes..
The final appearance of an organism (phenotype) is the result of its
genotype and the effects of the environment. Organisms of identical
genotype subject to different environmental conditions show considerable
variation.
Since these differences are not passed from one generation to the next
they do not affect evolution.
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