Page 34 - CB1.3 Genetic Varation sciPAD

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34 CB 1.3 GENETIC VARIATION
TRACKING GENES
Genotypic and Phenotypic Ratios
The possible results of a genetic cross are usually expressed as a ratio. This may be a genotypic ratio
(ratio of the possible allele combinations), or a phenotypic ratio (ratio of the possible phenotypes).
B
BB
Bb
Bb
bb
B
b
b
The genotypic ratio takes into account all the possible
genotypes and is written as:
1BB : 2Bb : 1bb
The phenotypic ratio is an expression of the possible
phenotypes and is written as:
3 Black : 1 White
A cross between two heterozygous black rats (Bb x Bb).
1. Using the same alleles from the previous exercise, show the possible phenotypic and
genotypic ratios of the following crosses. You should use a Punnett square in your answer.
(b) A homozygous dominant black rat crossed with a heterozygous black rat.
Genotypic ratio of the offspring:
Phenotypic ratio of the offspring:
Genotypic ratio of the offspring:
Phenotypic ratio of the offspring:
Genotypic ratio of the offspring:
Phenotypic ratio of the offspring:
Genotypic ratio of the offspring:
Phenotypic ratio of the offspring:
(a) A homozygous dominant black rat crossed with a homozygous recessive white rat.
(c) A homozygous recessive white rat crossed with a heterozygous black rat.
(d) A heterozygous black rat crossed with a heterozygous black rat.
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