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GCSE Biology · GCSEChapter 15 · DNA, genes and inheritance
GCSE · Chapter 15

DNA, genes and inheritance

By the end of this chapter you will be able to

  • Describe DNA, genes and alleles, and the structure of a chromosome
  • Complete a Punnett square and state the genotypes and phenotypes of the offspring
  • Calculate the probability of an outcome in a cross, and of a child having a condition
  • Read a pedigree diagram and say how a condition is inherited
  • [H] Explain meiosis, base pairing and the link between a gene and a protein

You should already know

  • Cells, the nucleus and chromosomes (Chapters 2 and 4)
  • Mitosis produces identical cells (Chapter 4)
  • Fractions and percentages (Chapter 1)

Where this leads

  • Variation and natural selection (Chapter 16)
  • Mutation and antibiotic resistance (Chapter 14)
  • Sex and reproduction in plants (Chapter 8)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the nucleus, chromosomes, DNA, genes and alleles, Describe dominant and recessive alleles, and use Punnett squares for a single gene, Calculate the probability of an inherited characteristic and interpret genetic diagrams, Explain sex determination, cystic fibrosis and polydactyly as examples of inheritance, Describe mutations as changes in DNA, [H] Describe the four bases and base pairing, meiosis and gametes, and protein synthesis (idea).

Exercise P  Do you remember?

1
FWhich part of a cell holds the chromosomes? Write the word.
2
FHow many chromosomes are in a human body cell? Write the number.
3
FWhat is 1 divided by 2, written as a fraction? Write the answer.
4
FName the process that makes identical body cells for growth. Write the word.
5
FWhat is 3 x 4 divided by 2? Write the number.
15.1

DNA, genes and alleles

GCSE
Describe the nucleus, chromosomes, DNA and genes, Describe alleles, [H] Describe the four bases and base pairing
Video lesson: DNA, genes and allelesComing soon
Context. Ali has blue eyes, like his grandmother, but his brother has brown eyes. “How do we get our features from our parents?” he asks. The answer is in the DNA in the nucleus of each cell.

DNA is a long molecule that carries the genetic information of an organism. It is found in the nucleus, where it is packed into chromosomes. A gene is a section of DNA that codes for a characteristic, such as eye colour. Different versions of a gene are called alleles.

DNA is shaped like a double helix: two strands twisted together. Each strand is made of a chain of bases. [H] The four bases are A, T, C and G. A always pairs with T, and C always pairs with G.

ATTACGGCATCGa double helix: two strands held together by base pairs (A with T, C with G)
A simplified DNA double helix with base pairs

Key ideas

  • DNA carries the genetic information; it is found in the nucleus, in chromosomes.
  • A gene is a section of DNA that codes for a characteristic.
  • An allele is one version of a gene.
  • [H] The four bases are A, T, C and G; A pairs with T, and C pairs with G.
Worked example 15.1.1
Name the molecule that carries genetic information. Write the word.

Method. Recall the molecule.

  1. Recall. DNA carries the genetic information of an organism.
    carries the genetic information
  2. Name it. So the molecule is DNA.
    DNA
  3. Write the answer.
    DNA

Check. DNA is found in the nucleus. ✓

Worked example 15.1.2
Name a section of DNA that codes for a characteristic. Write the word.

Method. Recall the unit of inheritance.

  1. Recall. A section of DNA that codes for a characteristic is a gene.
    codes for a characteristic
  2. Name it. So the word is gene.
    gene
  3. Write the answer.
    gene

Check. Genes are arranged along chromosomes. ✓

Worked example 15.1.3
[H] Which base pairs with A in DNA? Write the letter.

Method. Recall the base pairing rule.

  1. Recall. A always pairs with T in DNA.
    A pairs with T
  2. Name it. So the base that pairs with A is T.
    T
  3. Write the answer.
    T

Check. C always pairs with G. ✓

Watch out

Watch out: a gene is a section of DNA, and an allele is one version of a gene. Do not use the two words as if they mean the same thing.

Exam tip

Exam tip. Name the structure in order: DNA, then chromosome, then gene, then allele.

Try it (with your teacher). With your teacher, look at a labelled diagram of a chromosome. Write the order from the nucleus to the chromosome, to the gene, to the allele.

Exercise 15A  DNA, genes and alleles

1
FName the molecule that carries genetic information. Write the letters.
2
FIn which part of the cell is DNA found? Write the word.
3
FName the section of DNA that codes for a characteristic. Write the word.
4
FWhat is an allele? Write the words.
5
FHow many strands make up the DNA double helix? Write the number.
6
FName the shape of the DNA molecule. Write the words.
7
FWhat are the names of the chromosomes that make up a pair in a human body cell? Write the number of pairs. Write the number.
8
F[H] Name the four bases in DNA. Write the letters.
9
F[H] Which base pairs with G? Write the letter.
10
PSWhy is a gene described as a section of DNA? Write one sentence. Accept any sentence that says a gene is part of a DNA molecule that carries the code for one characteristic.
11
PSAli and his brother have different eye colours. Explain why, using the word allele. Write one sentence. Accept any sentence that says they have different alleles of the eye colour gene.
12
RChallenge: explain why a child has features from both parents, in two sentences. Accept any answer that says each parent passes on one allele for each gene, and the child has a combination from both.
15.2

Dominant, recessive and Punnett squares

GCSE
Describe dominant and recessive alleles, Complete a Punnett square for one gene, Describe genotype, phenotype, homozygous and heterozygous
Video lesson: Dominant, recessive and Punnett squaresComing soon
Context. Ruth is testing a cross between two pea plants. She writes the alleles as letters and fills in a square to find the possible offspring. “A capital letter means dominant,” her teacher says, “and a small letter means recessive.”

A dominant allele shows its characteristic whenever it is present. A recessive allele only shows its characteristic when there are no dominant alleles. We write dominant alleles as capital letters, such as A, and recessive alleles as small letters, such as a.

The genotype is the combination of alleles, such as AA, Aa or aa. The phenotype is the characteristic it produces. A genotype with two of the same allele is homozygous; with two different alleles, it is heterozygous.

A Punnett square shows all the possible combinations of alleles from two parents. For a cross between two heterozygous parents (Aa and Aa), the square gives 1 AA : 2 Aa : 1 aa.

Aa
AAAAa
aAaaa
Punnett square for a cross between two heterozygous parents, Aa and Aa

Key ideas

  • A dominant allele shows its characteristic if it is present.
  • A recessive allele shows only if there is no dominant allele.
  • Genotype is the allele combination; phenotype is the characteristic.
  • Aa x Aa gives 1 AA : 2 Aa : 1 aa, and 3 dominant : 1 recessive phenotype.
Worked example 15.2.1
Complete the Punnett square for Aa x Aa. Write the number of offspring with the genotype aa, out of four. Write the number.

Method. Count the aa squares.

  1. Look. The four squares are AA, Aa, Aa and aa.
    AA, Aa, Aa, aa
  2. Count. One square is aa, so 1 out of 4.
    1 out of 4
  3. Write the answer.
    1

Check. 1 out of 4 is 25%. ✓

Worked example 15.2.2
Find the phenotype ratio of Aa x Aa, as dominant to recessive. Write the ratio.

Method. Count the phenotypes in the square.

  1. Count dominant. Three squares show the dominant phenotype (AA, Aa and Aa).
    3 dominant
  2. Count recessive. One square shows the recessive phenotype (aa).
    1 recessive
  3. Ratio. The ratio is 3 : 1.
    3 : 1
  4. Write the answer.
    3 : 1

Check. 3 + 1 = 4 squares. ✓

Worked example 15.2.3
State the genotype of a plant with two different alleles, Aa, and say whether it is homozygous or heterozygous. Write the word.

Method. Recall the terms.

  1. Recall. A genotype with two different alleles is heterozygous.
    two different alleles
  2. Name it. So Aa is heterozygous.
    heterozygous
  3. Write the answer.
    heterozygous

Check. AA and aa are homozygous. ✓

Watch out

Watch out: a recessive allele can be present without showing. A heterozygous plant (Aa) looks like the dominant phenotype.

Exam tip

Exam tip. Write both parents on the top and side of the square, and fill in each square by combining one allele from each parent.

Try it (with your teacher). With your teacher, use two coins to model a monohybrid cross: one coin is one parent’s allele, the other is the other parent’s. Toss both coins 20 times and record the outcomes.

Exercise 15B  Dominant, recessive and Punnett squares

1
FWhat is a dominant allele? Write the words.
2
FWhat is a recessive allele? Write the words.
3
FWrite the genotype of a plant with two dominant alleles, using the letter A. Write the letters.
4
FIs Aa heterozygous or homozygous? Write the word.
5
FIn Aa x Aa, how many of the four squares are aa? Write the number.
6
FIn Aa x Aa, how many of the four squares are Aa? Write the number.
7
FComplete the Punnett square for AA x aa. What is the genotype of every offspring? Write the letters.
8
FWhat is the phenotype ratio of Aa x Aa, dominant to recessive? Write the ratio.
9
PSWhat is the genotype ratio of Aa x Aa? Write the ratio, in the form A : B : C.
10
PSComplete the Punnett square for Aa x aa. How many of the four offspring are aa? Write the number.
11
PSA plant that is Aa is crossed with an aa plant. What is the phenotype ratio of the offspring, dominant to recessive? Write the ratio.
12
RChallenge: explain why a child with two recessive alleles shows the recessive characteristic, but a child with one dominant allele does not, in two sentences. Accept any answer that says the recessive allele only shows without a dominant allele, and the dominant allele masks it when present.
15.3

Probability and sex determination

GCSE
Calculate the probability of an outcome in a genetic cross, Explain how sex is determined by XX and XY
Video lesson: Probability and sex determinationComing soon
Context. Hana’s parents are both carriers of a recessive condition. “What is the chance that a child will have it?” she asks. A Punnett square gives the answer as a probability. The same method explains how sex is inherited.

The probability of an outcome is the number of ways it can happen, divided by the total number of outcomes. A probability of 1 in 4 is 25%, or one quarter.

In humans, females have two X chromosomes (XX) and males have an X and a Y (XY). The mother always gives an X, and the father gives either an X or a Y. So each child has a 1 in 2 chance of being a boy or a girl. The father determines the sex of the child.

GenotypePhenotypeNotes
FFdoes not have cystic fibrosishomozygous dominant
Ffdoes not have cystic fibrosisheterozygous: a carrier
ffhas cystic fibrosishomozygous recessive
Genotypes and phenotypes for a gene with a dominant and a recessive allele
father / motherXX
XXX (female)XX (female)
YXY (male)XY (male)
Sex determination: the father gives an X or a Y; the mother always gives an X

Key ideas

  • Probability = number of favourable outcomes / total outcomes.
  • Females are XX; males are XY.
  • The father determines the sex of the child: a 1 in 2 chance of each.
  • Two carriers (Aa) have a 1 in 4 chance of a child with the recessive condition.
Worked example 15.3.1
Two carriers (Aa) have a child. What is the probability that the child has the recessive condition? Write the number as a fraction.

Method. Use the Punnett square for Aa x Aa.

  1. Square. One square out of four is aa.
    aa is 1 of 4
  2. Probability. So the probability is 1 in 4.
    1/4
  3. Write the answer.
    1/4

Check. 1/4 is 25%. ✓

Worked example 15.3.2
What is the probability, as a percentage, that a child from two carriers is a carrier (Aa)? Write the number, with the percentage sign.

Method. Count the Aa squares.

  1. Count. Two out of four squares are Aa.
    2 of 4
  2. Percentage. 2 / 4 = 0.5, which is 50%.
    50%
  3. Write the answer.
    50%

Check. 50% of 4 is 2. ✓

Worked example 15.3.3
Name the sex chromosomes in a human male. Write the letters.

Method. Recall the male pair.

  1. Recall. Males have one X and one Y chromosome.
    one X and one Y
  2. Name them. So the sex chromosomes are XY.
    XY
  3. Write the answer.
    XY

Check. Females are XX. ✓

Watch out

Watch out: the probability is for each child separately. Having two girls does not change the chance of the next child being a boy: it is still 1 in 2.

Exam tip

Exam tip. Give a probability as a fraction, a decimal or a percentage, as asked. A 1 in 4 chance is 1/4, 0.25 or 25%.

Try it (with your teacher). With your teacher, toss a coin 40 times to model the sex of a child, recording heads for a girl and tails for a boy. Compare your totals with the expected 1 in 2.

Exercise 15C  Probability and sex determination

1
FName the sex chromosomes of a human female. Write the letters.
2
FWhich parent gives the sex chromosome that determines the sex of the child? Write the word.
3
FWhat is the probability of having a son, as a fraction? Write the fraction.
4
FWhat is the probability of an outcome of 1 in 4, as a percentage? Write the number, with the percentage sign.
5
FIn a cross of two carriers, how many of the four possible children have the recessive condition? Write the number.
6
PSTwo parents each have one allele A and one allele a (both are carriers). The child is born and is a boy. What is the probability that the next child is a boy? Write the fraction.
7
PSTwo carriers have 4 children. What is the number of children expected to have the recessive condition, on average? Write the number.
8
PSA child is born to a father who is XY and a mother who is XX. Write the child’s sex chromosomes, if the father gives a Y. Write the letters.
9
PSAli says a family that has four boys in a row must have a girl next. Write yes or no, and give one reason. Accept any answer that says the chance is still 1 in 2 each time, so no.
10
RChallenge: explain why two carriers have a 1 in 4 chance of a child with the condition, but a 1 in 2 chance of a child who is a carrier. Write two sentences. Accept any answer that gives the Punnett square outcomes: one of four is aa, and two of four are Aa.
15.4

Cystic fibrosis, polydactyly and pedigrees

GCSE
Describe the inheritance of cystic fibrosis and polydactyly, Read a pedigree diagram
Video lesson: Cystic fibrosis, polydactyly and pedigreesComing soon
Context. A family tree shows which members of a family have a condition. Ali is reading it to find out how the condition is passed on. “Some of the children have it, and some do not,” he says. “How do I know whether it is dominant or recessive?”

Cystic fibrosis is caused by a recessive allele. A person with cystic fibrosis has the genotype ff. A carrier (Ff) has the allele but does not have the condition. Polydactyly (extra fingers or toes) is caused by a dominant allele, so a person with one dominant allele has the condition.

A pedigree diagram shows the family of several generations. Squares are males, circles are females, and shaded symbols have the condition. A condition that skips a generation is usually recessive.

IIaffectedunaffectedunaffectedaffectedsquares are males; circles are females; shaded symbols have the conditionan invented family: cystic fibrosis (a recessive condition)
A pedigree diagram with two parents and four children, two of whom are affected by cystic fibrosis

Key ideas

  • Cystic fibrosis is recessive: the genotype ff has the condition.
  • Polydactyly is dominant: one dominant allele is enough.
  • Pedigrees: squares are males, circles are females, shaded symbols are affected.
  • A recessive condition can appear in children of two unaffected carriers.
Worked example 15.4.1
In the pedigree, how many of the four children are affected? Write the number.

Method. Count the shaded symbols.

  1. Count. Two of the four children have the shaded symbol.
    two shaded
  2. Answer. So two children are affected.
    2
  3. Write the answer.
    2

Check. 2 out of 4 is one half. ✓

Worked example 15.4.2
Which genotype do the affected children have, if cystic fibrosis is recessive? Write the genotype.

Method. Recall the recessive genotype.

  1. Recall. The recessive condition needs two recessive alleles.
    two recessive alleles
  2. Name it. So the genotype is ff.
    ff
  3. Write the answer.
    ff

Check. A carrier is Ff and does not have the condition. ✓

Worked example 15.4.3
Name the condition in which extra fingers or toes are caused by a dominant allele. Write the word.

Method. Recall the condition.

  1. Recall. Extra fingers or toes are caused by a dominant allele.
    dominant allele
  2. Name it. The condition is polydactyly.
    polydactyly
  3. Write the answer.
    polydactyly

Check. One dominant allele is enough to show it. ✓

Watch out

Watch out: a carrier of a recessive condition does not have the condition. A pedigree shows only the people with the condition, and carriers can be hidden.

Exam tip

Exam tip. When you read a pedigree, look for the condition skipping a generation, and whether the condition is in every generation.

Try it (with your teacher). With your teacher, draw a simple pedigree for three generations in your own family, using only the features your family wants to share. Do not record any medical information about people who have not agreed.

Exercise 15D  Cystic fibrosis, polydactyly and pedigrees

1
FIs cystic fibrosis caused by a dominant or a recessive allele? Write the word.
2
FIs polydactyly caused by a dominant or a recessive allele? Write the word.
3
FWhat is the genotype of a person with cystic fibrosis? Write the letters.
4
FWhat is the genotype of a carrier of cystic fibrosis? Write the letters.
5
FIn a pedigree, what is a square a symbol for? Write the word.
6
FIn a pedigree, what does a shaded symbol show? Write the words.
7
FIn the pedigree, how many of the four children are unaffected? Write the number.
8
PSIn the pedigree, what fraction of the four children are affected? Write the fraction.
9
PSTwo unaffected parents have an affected child. Explain why this shows that the condition is recessive, in one sentence. Accept any sentence that says both parents must have been carriers, passing on the recessive allele.
10
RA parent with polydactyly (Pp) and an unaffected partner (pp) have one child with polydactyly and one without. Explain in one sentence why both are possible. Accept any sentence that says the parent can pass on either the dominant or the recessive allele, and the partner can only pass on the recessive one.
15.5

Mutations, meiosis and protein synthesis [H]

GCSE
Describe mutations as changes in DNA, [H] Describe meiosis and gametes, [H] Describe the link from a gene to a protein
Video lesson: Mutations, meiosis and protein synthesis [H]Coming soon
Context. A baby is born with a change that affects a single protein. “Where does the change come from?” asks the midwife. The answer is a mutation in the DNA, which is then passed on through meiosis.

A mutation is a random change in the DNA. Most mutations have no effect, but some change the protein a gene codes for, and this can cause disease. Mutations are the source of new alleles.

[H] Meiosis is a type of cell division that makes gametes (sex cells), each with half the number of chromosomes. In humans, a gamete has 23 chromosomes. When the gametes fuse at fertilisation, the full number of 46 is restored.

[H] The genetic code is carried in the DNA. A gene is used to make a protein: the gene is copied into a message, which is used to build the protein from amino acids. A change in the gene can change the protein.

Key ideas

  • A mutation is a random change in DNA; it can change a protein.
  • [H] Meiosis makes gametes with half the chromosomes (23 in humans).
  • [H] Fertilisation restores the full number of 46 chromosomes.
  • [H] A gene codes for a protein, which is built from amino acids.
Worked example 15.5.1
Name the change in DNA that can change a protein and cause disease. Write the word.

Method. Recall the random change.

  1. Recall. A random change in DNA is a mutation.
    random change
  2. Name it. So the change is a mutation.
    mutation
  3. Write the answer.
    mutation

Check. Most mutations have no effect. ✓

Worked example 15.5.2
[H] How many chromosomes does a human gamete have? Write the number.

Method. Recall the gamete number.

  1. Recall. Meiosis halves the number, so a gamete has half of 46.
    half of 46
  2. Calculate. 46 / 2 = 23.
    23
  3. Write the answer.
    23

Check. 23 + 23 = 46 after fertilisation. ✓

Worked example 15.5.3
[H] Name the process that makes gametes with half the chromosomes. Write the word.

Method. Recall the process.

  1. Recall. Meiosis makes gametes with half the number of chromosomes.
    half the number
  2. Name it. So the process is meiosis.
    meiosis
  3. Write the answer.
    meiosis

Check. Mitosis keeps the number the same. ✓

Watch out

Watch out: meiosis makes gametes; mitosis makes body cells. The gametes have half the number of chromosomes, but the body cells keep the full number.

Exam tip

Exam tip. [H] When you describe meiosis, say that it halves the chromosome number, and that fertilisation restores the full number.

Try it (with your teacher). With your teacher, look at a labelled diagram of meiosis. Count the chromosomes at the start, and at the end, and explain why the number changes.

Exercise 15E  Mutations, meiosis and protein synthesis [H]

1
FName the random change in DNA. Write the word.
2
FDo most mutations have a noticeable effect? Write yes or no.
3
FName the type of change in DNA that can change a protein and cause disease. Write the word.
4
F[H] Name the type of cell division that makes gametes. Write the word.
5
F[H] Name the sex cells that have half the number of chromosomes. Write the word.
6
F[H] What is the chromosome number of a gamete in a human? Write the number.
7
F[H] What is the full chromosome number restored at fertilisation in a human? Write the number.
8
F[H] A gene codes for a protein. What are proteins built from? Write the words.
9
PS[H] Which base pairs with T in DNA? Write the letter.
10
PS[H] Explain why a mutation in a gene might change the protein it codes for. Write one sentence. Accept any sentence that says a change in the DNA code can change the amino acids in the protein.
11
PS[H] Explain why gametes need to have half the chromosome number, in one sentence. Accept any sentence that says fertilisation then restores the full number, so the offspring has the right number.
12
R[H] Challenge: explain why a mutation in a body cell is not passed on to the next generation, but a mutation in a gamete can be. Write two sentences. Accept any answer that says gametes pass their DNA to the offspring, while body cells do not, so only gamete mutations are inherited.

Chapter summary

SkillMethod
DNA and genesDNA is a double helix in chromosomes; a gene codes for a characteristic; alleles are versions of a gene.
Dominant and recessiveDominant shows if present (capital letter); recessive needs two copies (small letter).
Punnett squaresAa x Aa: 1 AA : 2 Aa : 1 aa; 3 : 1 dominant to recessive; probability 1/4 for aa.
Sex determinationFemales XX, males XY; the father determines the sex; 1 in 2 for each child.
Cystic fibrosis and polydactylyCystic fibrosis is recessive (ff); polydactyly is dominant.
PedigreesSquares are males; circles are females; shaded symbols have the condition.
MutationsA random change in DNA; can change a protein; the source of new alleles.
[H] Meiosis and gametesGametes have half the chromosome number (23 in humans); fertilisation restores 46.

Can you…?

  • I can describe DNA, genes, alleles and chromosomes
  • I can complete a Punnett square and state genotypes and phenotypes
  • I can calculate the probability of an outcome from a genetic cross
  • I can read a pedigree diagram and say how a condition is inherited
  • I can describe mutations as changes in DNA
  • [H] I can describe meiosis, base pairing and protein synthesis

Key terms

DNA
the molecule that carries genetic information
chromosome
a thread of DNA, found in the nucleus
gene
a section of DNA that codes for a characteristic
allele
one version of a gene
dominant
an allele that shows its characteristic if it is present
recessive
an allele that shows only when there is no dominant allele
genotype
the combination of alleles an organism has
phenotype
the characteristic an organism shows
homozygous
having two identical alleles of a gene
heterozygous
having two different alleles of a gene
carrier
a person with one recessive allele who does not show the condition
mutation
a random change in DNA
[H] gamete
a sex cell, with half the number of chromosomes

Exercise 15Z  Check-up

1
Name the section of DNA that codes for a characteristic. Write the word.
2
Name the two alleles of a gene in a heterozygous plant Aa, and which one is dominant. Write the letter.
3
Complete the Punnett square for Aa x Aa. How many of the four offspring are AA? Write the number.
4
What is the phenotype ratio of Aa x Aa, dominant to recessive? Write the ratio.
5
Name the sex chromosomes of a human male. Write the letters.
6
Two carriers have a child. What is the probability that the child has the recessive condition? Write the fraction.
7
Name the condition caused by a dominant allele that gives extra fingers or toes. Write the word.
8
Name the random change in DNA that can change a protein. Write the word.
9
In a pedigree, what does a square represent? Write the word.
10
Explain in one sentence why a carrier of cystic fibrosis does not have the condition. Accept any sentence that says one dominant allele is enough to prevent the recessive condition from showing.

Total 27 marks. Allow about 27 minutes.

Exercise 15X  Exam-style questions

1
FName the section of DNA that codes for a characteristic. Write the word.[1 mark]
2
FName the version of a gene that shows only when two copies are present. Write the word.[1 mark]
3
FName the ratio of the genotypes AA : Aa : aa from an Aa x Aa cross. Write the ratio.[1 mark]
4
PSComplete a Punnett square for Aa x Aa and state the probability of a child with the recessive condition. Write the probability as a fraction, and give one reason it is not 1 in 2. Accept 1/4, and an answer that says only one of the four squares is aa.[2 marks]
5
FDescribe how cystic fibrosis is inherited, using the words recessive, carrier and genotype. Write three sentences. Accept any answer that says the condition needs two recessive alleles (ff), a carrier (Ff) does not have it, and the genotype determines the condition.[3 marks]
6
RRead the pedigree: two unaffected parents have four children, two of whom are affected. Explain what this shows about the inheritance, and say what each unaffected parent must be. Accept any answer that says the condition is recessive, and each unaffected parent is a carrier.[3 marks]
7
RExplain how a mutation can cause a change in an organism, using the words DNA, gene, protein and allele. Write four sentences. Accept any answer that says a mutation changes the DNA, which changes a gene, which may change the protein it codes for, creating a new allele.[4 marks]
8
R[H] Describe the difference between mitosis and meiosis, in terms of the number of cells and chromosomes they produce. Write three sentences. Accept any answer that says mitosis makes two identical cells with the full number, while meiosis makes gametes with half the number.[3 marks]
9
R[H] Explain how a gene becomes a protein, using the words DNA, amino acids and code. Write four sentences. Accept any answer that says the DNA code is copied and used to build a protein from amino acids in a set order.[4 marks]
10
R[H] Challenge: a family has a child with a mutation in a body cell only, and it is not in the gametes. Explain whether the child’s own children could inherit it, and why. Write four sentences. Accept any answer that says the mutation is not in the gametes, so it cannot be passed to the children, though the child may be affected.[5 marks]
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