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B1.9a Explaining inheritance
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Class
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A gene may exist in different types – different alleles. There are two alleles for flower colour in pea plants,
purple and white. Dominant alleles always have an effect. Recessive alleles do not have an effect when they
are with dominant alleles.
1
The genetic cross diagram shows the purple allele (R) and the white allele (r).
a Complete the diagram to show the possible genotypes of the offspring.
b Out of every four offspring, how many are likely to have purple flowers?
c Out of every four offspring, how many are likely to have white flowers?
d What colour flowers did both parent plants have?
2
Some pea plants have yellow peas and others have green peas. The allele for yellow peas (Y) is
dominant. The allele for green peas (y) is recessive.
a Complete the Punnett square.
b What is the genotype of parent plant 2?
c Out of every four offspring, how many are likely to have the
yellow phenotype?
d Which parent plant is heterozygous for this characteristic?
3
A new pea plant grows from fertilised a egg cell.
What are the names of the two gametes that fuse to create this cell?
© Pearson Education 2010. Edexcel GCSE Science Activity Pack
This document may have been altered from the original.
B1.9b Inheritance
You do not need to remember the details on this sheet for your exam but you could be asked to apply your
knowledge to unfamiliar situations.
Gregor Mendel found that he could not breed pea plants with a medium height – his plants were either tall or
short. This evidence helped him to develop his theory of inheritance.
He said that ‘factors’ carry a certain characteristic and these are passed down from parents to offspring. He
also said that the factors could not be changed.
He proposed that plants were either tall or short depending on the ‘factors’ that they inherited.
1
a What do we call Mendel’s ‘factors’ today?
b Scientists before Mendel thought that offspring inherited features blended from their parents. How does
Mendel’s work show that this was wrong?
c Tall pea plants have the genotype TT or Tt. Short plants have the genotype tt. Copy and complete the
genetic cross diagram to show what possible genotypes the offspring of the parent plants shown may
have.
d For each genotype in your completed diagram, state its phenotype.
H
e Look at your completed diagram again. What is the probability that an offspring pea plant will be tall?
Give your answer as a ratio.
H
f What is the probability that an offspring pea plant will be homozygous for being tall? Give your answer
as a decimal.
2
Pea plants have yellow seed pods or green seed pods. The green
characteristic is dominant.
a Copy and complete the Punnett square for breeding together a homozygous
dominant pea plant and a homozygous recessive pea plant, for this
characteristic. Choose your own genotype letters.
H
b What is the percentage probability of one of the offspring pea plants having
the green pod phenotype.
Extra challenge
H3 Pea seeds (inside the pods) can be round or smooth, green or yellow. Round (R) is dominant and so is
yellow (Y). Work out the probability of a pea plant offspring having green, wrinkled pea seeds if both
parents have the genotypes RrYy. You will need a Punnett square with four squares for the gametes for
each parent (and so 16 squares for the offspring).
© Pearson Education 2010. Edexcel GCSE Science Activity Pack
This document may have been altered from the original.