Genetic inheritance - part one - EdexcelUsing probability, direct proportion and ratios

Mutations can cause a permanent change in the DNA of an organism. In the 19th century, Gregor Mendel determined rules to explain genetic inheritance using pea plants. Genetic crosses using Punnett squares show how likely offspring are to inherit characteristics from their parents.

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Calculations - Using probability, direct proportion and simple ratios

You can express the outcome of a genetic cross using (percentages), direct proportion or ratios. It is important to remember during the process of fertilisation, the allele combinations created are a random process, and that is why probability is used, as nothing is guaranteed. Each of the four possible offspring combinations is as likely to happen during every fertilisation event.

In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.

aa
AAaAa
aaaaa
A
aAa
aAa
a
aaa
aaa

Direct proportion

Half the offspring have the combination Aa and half have aa.

or

Out of 4 offspring 2 have the combination Aa and 2 have aa.

Probability

The probability of the offspring being Aa is 50%.

The probability of the offspring being aa is 50%.

Ratio

The ratio of Aa to aa is 1:1.

Question

Complete the Punnett Square to show the four possible outcomes. Give your answer as a proportion, probability and ratio.

In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.

Bb
B
B
B
B
b
B
B
b

Question

Complete the Punnett Square to show the four possible outcomes. Give your answer as a proportion, probability and ratio.

In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.

cc
C
C
C
c
c
C
c
c

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