Edexcel IGCSE Biology 3.25 Monohybrid inheritance

Practise predicting probabilities from monohybrid crosses, including sex combined with inherited phenotypes in diagrams and tables.

Exam points

use a genetic diagram to find offspring genotype ratios and convert them into probabilities
calculate combined probabilities,
compare observed offspring counts with expected ratios from a monohybrid cross

[Maximum number: 2]

Fur colour in rats is controlled by a gene with two alleles.

One allele codes for black fur colour. The other allele codes for agouti fur colour.
Several female rats with agouti coloured fur are mated with several male rats with black coloured fur.

All of the offspring have agouti coloured fur.

Calculate the probability of any one offspring from this second cross being male with agouti coloured fur.

probability of agouti / probability of male

- multiplied

$\frac{3}{8}$ or 0.375 or 37.5%

allow full marks for correct answer no working

1 mark for 0.5 or $\frac{1}{2}$ or 50\%

or 0.75 or $\frac{3}{4}$ or 75%

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3.25 Monohybrid cross probabilities question 1