Inheritance Cambridge IGCSE Co-ordinated Sciences (9–1) revision
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In plain words
You get one copy of each gene from each parent. Sometimes the two copies disagree, and one wins: it's called dominant. The other, recessive, only shows if you have two of it. That is why two brown-eyed parents can have a blue-eyed child.
Three things to know
- Genotype: the alleles an organism has (such as Bb). Phenotype: what it looks like (brown eyes). Homozygous: two identical alleles (BB or bb). Heterozygous: two different alleles (Bb).
- A dominant allele shows even if only one copy is present. A recessive allele shows only when there are two copies.
- A Punnett square shows the possible offspring. Two heterozygous parents (Bb × Bb) give a 3:1 ratio of dominant to recessive. A heterozygous parent with a homozygous recessive (Bb × bb) gives 1:1.
Worked example
In pea plants, tall (T) is dominant to short (t). Two heterozygous plants are crossed. What fraction of the offspring are expected to be short?
- Both parents are Tt, so each gives gametes T or t.
- The Punnett square gives TT, Tt, Tt, tt.
- Only tt is short: one in four, or 25%.
Tips and tricks
- Set it out the same way every time: parents' genotypes, gametes, Punnett square, offspring genotypes, phenotypes, ratio.
- Use a capital letter for the dominant allele and the same letter in lower case for the recessive one.
It lands in your notebook with its questions as flashcards.
Inheritance: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What does heterozygous mean?
For example Bb.
When does a recessive allele show in the phenotype?
A dominant allele would mask it.
Two heterozygous parents (Bb × Bb) have offspring. What ratio of dominant to recessive phenotypes is expected?
The offspring are BB, Bb, Bb and bb.
What is the phenotype?
It results from the genotype (and the environment).
A plant with genotype Tt is crossed with one that is tt. What proportion of offspring are expected to be tt?
The offspring are Tt, Tt, tt, tt.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 8 marks. Write your answers on paper, then check them.
Inheritance
Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 8 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by genotype and by phenotype.[2]
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Genotype is the genetic make-up of an organism (the alleles it has). Phenotype is its observable features.
In guinea pigs, black fur (B) is dominant to white fur (b). Two heterozygous black guinea pigs breed. Use a genetic diagram to work out the expected ratio of black to white offspring.[4]
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Parents Bb × Bb. Gametes B, b and B, b. Offspring BB, Bb, Bb, bb. Three black to one white (3:1).
Explain why a recessive condition can appear in a child when neither parent has it.[2]
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Both parents are heterozygous (carriers). Each can pass on the recessive allele, and a child who inherits two recessive alleles has the condition.
Answers: Inheritance
- 1. Genotype is the genetic make-up of an organism (the alleles it has). Phenotype is its observable features.
- 2. Parents Bb × Bb. Gametes B, b and B, b. Offspring BB, Bb, Bb, bb. Three black to one white (3:1).
- 3. Both parents are heterozygous (carriers). Each can pass on the recessive allele, and a child who inherits two recessive alleles has the condition.