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Alleles, dominant and recessive traits, Punnett squares, sex determination and inherited disorders such as cystic fibrosis.
Specification references this topic covers: AQA 4.6.1; Edexcel B3.12, B3.13, B3.14, B3.15, B3.16, B3.19.
Why do you look a bit like your parents, but not exactly like either of them? The answer is in your genes. Genes are instructions that are passed from parents to children.
This matters because genes explain family features such as eye colour. They also explain some illnesses that run in families, such as cystic fibrosis.
By the end you will be able to use the key words, such as allele and genotype. You will complete a genetic cross diagram and work out a ratio or probability. You will also describe how sex is decided and discuss embryo screening.
A gene is a section of DNA that holds the instructions for one protein. Think of a gene as a recipe. Genes come in different versions called alleles, like two versions of the same recipe.
A body cell has two copies of each gene, one from each parent. So it has two alleles for each characteristic. Gametes, which are sex cells, have only one allele of each gene.
Your genotype is the combination of alleles you have. Your phenotype is the characteristic you actually show, such as eye colour.
A dominant allele is always shown, even when there is only one copy. A recessive allele is shown only when there are two copies and no dominant allele.
A capital letter stands for a dominant allele, such as B. A small letter stands for a recessive allele, such as b.
Two identical alleles make a person homozygous, such as BB or bb. Two different alleles make a person heterozygous, such as Bb. A Bb person shows the dominant feature, but still carries the recessive allele.
A Punnett square shows the possible offspring from a cross. Write one parent's gametes across the top and the other parent's down the side. Fill each box with the alleles from its row and column. Each box is equally likely.
For Bb × Bb the boxes are BB, Bb, Bb and bb. Three show the dominant feature and one shows the recessive. The ratio is 3 : 1. The chance of bb is 1 in 4, or 25%.
For Bb × bb the ratio is 1 : 1. These are chances for each child. A small family can differ from the ratio.
A family tree, or pedigree, shows how a condition is passed on. Squares are males and circles are females, and shaded symbols show people with the condition. If two parents without the condition have a child with it, the condition is recessive and both parents are carriers (heterozygous).
Use the cross to find the chance for each child. Do not use the pedigree to say what will happen in a small family.
Humans have 23 pairs of chromosomes. One pair decides sex. Females have XX. Males have XY.
All eggs carry an X chromosome. Half of the sperm carry X and half carry Y. The sperm that fertilises the egg decides the sex of the baby. The chance of a boy or a girl is 50% each.
Think of a coin toss. Each time, heads and tails are equally likely. The last toss does not change the next one.
Some disorders are inherited. Polydactyly means having extra fingers or toes. It is caused by a dominant allele.
Cystic fibrosis is a disorder that makes thick, sticky mucus in the lungs and gut. It is caused by a recessive allele. A person needs two copies, ff, to have it. A person with Ff is a carrier. They do not have the disorder, but can pass on the allele.
If both parents are carriers, Ff × Ff, there is a 25% chance that each child has cystic fibrosis.
Embryos made by IVF can be screened for genes that cause disorders. IVF means eggs are fertilised in a laboratory. Only healthy embryos are put in the womb. This can reduce suffering and the cost of care.
There are concerns too. Some people say that embryos are destroyed. Some say it is a step towards designer babies, chosen for traits. Screening also cannot guarantee a healthy baby.
A good answer gives both sides and uses the word because to give reasons.
Alleles are versions of a gene. A dominant allele shows if one copy is present.
Bb × Bb gives 3 : 1. Bb × bb gives 1 : 1. These are chances, so a small family can differ.
XX is female and XY is male, with a 50% chance of each.
Cystic fibrosis is recessive. Two carriers have a 25% chance of an affected child.
A plant has the genotype BB. State whether it is homozygous or heterozygous.
Answer: homozygous
In pea plants tall (T) is dominant to short (t). Two heterozygous tall plants are crossed. Give the ratio of tall to short offspring.
Answer: 3 : 1
Cystic fibrosis is caused by a recessive allele, f. Two parents who do not have the disorder have a child with cystic fibrosis. Explain how this is possible and give the probability that their next child has the disorder.
Answer: Both parents are carriers, Ff, and the probability is 25%
Inheritance: genetic crosses, sex and inherited disorders practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
What word describes two identical alleles, such as bb?
Answer: homozygous
What word describes two different alleles, such as Bb?
Answer: heterozygous
What are the sex chromosomes of a human male?
Answer: XY
Is cystic fibrosis caused by a dominant or a recessive allele?
Answer: recessive
A plant has the genotype Tt, where T is dominant. Does it show the dominant or the recessive feature?
Answer: dominant
What name is given to sex cells such as an egg or a sperm?
Answer: gametes
Two Bb parents have a child. What is the chance the child has the genotype bb? Give a percentage.
Answer: 25%
Give the ratio of dominant to recessive offspring from the cross Bb × Bb.
Answer: 3:1
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
In pea plants the allele for tall stems (T) is dominant to the allele for short stems (t). Two heterozygous tall plants are crossed. (a) Complete a genetic cross diagram to show the genotypes of the offspring. [2 marks] (b) State the probability that an offspring is short. [1 mark] (c) A gardener grows 8 offspring and finds 5 tall and 3 short. Explain why this is not exactly the ratio predicted. [1 mark]
Answer: (a) gametes T and t from each parent, giving TT, Tt, Tt and tt (b) 0.25 or 25% (c) fertilisation is random so a small sample may not match the predicted ratio
Cystic fibrosis is caused by a recessive allele. A man and woman do not have cystic fibrosis, but their first child does. Explain how this can happen and calculate the probability that their second child will also have cystic fibrosis. [4 marks]
Answer: Both parents must be carriers with the genotype Ff. Each passed on a recessive allele f to the child, who is ff. In a cross Ff × Ff the offspring are FF, Ff, Ff and ff, so the probability of a child with cystic fibrosis is 25%.
Saying a recessive allele cannot be passed on
A carrier passes it on, even if they do not show the feature.
Mixing up genotype and phenotype
Genotype is the alleles, such as Bb. Phenotype is the feature that shows.
Saying the ratio 3 : 1 means every family has exactly 4 children
It is a probability for each child, not a rule for a family.
Saying the egg decides the sex of the baby
All eggs carry X. The sperm carries X or Y and decides.
Inheritance: genetic crosses, sex and inherited disorders is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 6.
Saying a recessive allele cannot be passed on. A carrier passes it on, even if they do not show the feature.
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