Page 31 - CB1.3 Genetic Varation sciPAD

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CB 1.3 GENETIC VARIATION
TRACKING GENES
Genotype and Phenotype
An organism’s ‘phenotype’ is its observable characteristics. Phenotype is a result of the interaction
between an organisms genes and its environment. For example, eye colour is observable so it is
part of your phenotype.
An organism’s ‘genotype’ is the set of genes/alleles that it carries. The phenotype of an organism
is an expression of its genotype. Remember that for each gene you will inherit two alleles (one
from your mother and one from your father), and these alleles may be different.
If one allele is dominant over the other, it will always be expressed in the phenotype. Dominant
alleles are usually shown with a capital letter. A recessive allele will only be expressed in the
phenotype if no dominant alleles are present. Recessive alleles are shown with a lower-case
letter.
If both alleles are the same they are called ‘homozygous’ (homo means ‘the same’). If the alleles
are different they are called ‘heterozygous’ (hetero means ‘different’).
For example, in rats, the colour of their fur coat is controlled by a gene. There are two forms (or
alleles) of this gene: the allele for black fur (B) is dominant over the allele for white fur (b).
1. The length of fur on a rat is also controlled by a gene. The long fur allele is (L), while the short
fur allele is (l).
(a) Which allele is:
(i) dominant?
(ii) recessive?
(b) Describe the phenotype of individuals with the following genotype.
(i) LL
(ii) ll
(iii) Ll
(c) State the genotype and phenotype of the rat that is:
(i) homozygous recessive.
(ii) heterozygous.
(iii) homozygous dominant.
(d) Explain why a rat that has long fur can have two possible genotypes.
(e) Explain why a rat that has short fur can only have one possible genotype.
Genotype: BB
Phenotype: Black
Homozygous dominant
Genotype: bb
Phenotype: White
Homozygous recessive
Genotype: Bb
Phenotype: Black
Heterozygous
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