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KS3 Science · Year 9Chapter 2 · Inheritance, DNA and genes
Year 9 · Chapter 2

Inheritance, DNA and genes

By the end of this chapter you will be able to

  • Describe chromosomes, genes and DNA, and how they are inherited
  • Explain the base-pairing rule and how DNA codes for proteins
  • Describe sexual reproduction, gametes and fertilisation, with chromosome numbers
  • Use the words allele, dominant, recessive, genotype and phenotype correctly
  • Complete a Punnett square and use it to give a probability, including an inherited disorder

You should already know

  • Cells and the nucleus (Year 7 and Year 8 Science: cell structure)
  • Reproduction and fertilisation in humans (Year 8 Science, organs and systems)

Where this leads

  • Variation and natural selection in Chapter 3: why genetic variation matters for evolution

National Curriculum for England, Key Stage 3 science, Year 9. Content: 9B1 chromosomes in the nucleus carry genes; DNA as a double helix of bases that codes for proteins (basic), Sexual reproduction, gametes and fertilisation, Alleles, dominant and recessive; simple genetic crosses with Punnett squares (one gene, two alleles), Inherited disorders (for example cystic fibrosis) as the result of inheriting particular alleles; careful wording on screening and ethics.

Exercise P  Do you remember?

1
FWhich part of a cell controls what the cell does? Write the word.
2
FWhat is the name of the process by which plants make food using light? Write the word.
3
FWhat is 3 x 15? Write the number.
4
FWhich is the male sex cell in animals and humans? Write the words.
5
FWhat is 25 percent of 100? Write the number.
2.1

Chromosomes, genes and heredity

Year 9
9B1: chromosomes in the nucleus carry genes; heredity passes genetic information from one generation to the next
Video lesson: Chromosomes, genes and heredityComing soon
Story. Lina has her grandmother’s nose and her father’s hair. “How does that happen?” she asks. Her teacher explains that the information for these features is passed on in the nucleus of every cell, on chromosomes.

Inside the nucleus of most cells are chromosomes. A chromosome is made of a long molecule called DNA. A gene is a short section of DNA that controls one characteristic, such as hair colour. Heredity is the way genetic information is passed from one generation to the next.

Human body cells have 46 chromosomes. They are arranged in 23 pairs: one chromosome in each pair comes from each parent.

cellnucleuschromosomes carry genesa gene
A cell with a nucleus containing chromosomes, which carry genes

Key ideas

  • Chromosomes are in the nucleus and are made of DNA.
  • A gene is a short section of DNA that controls one characteristic.
  • Human body cells have 46 chromosomes, in 23 pairs.
  • Heredity passes genetic information from one generation to the next.
Worked example 2.1.1
How many chromosomes are in a human body cell? Write the number.

Method. Recall the number for a human body cell.

  1. Recall. Human body cells have 23 pairs of chromosomes.
    23 pairs
  2. Count. Each pair has 2 chromosomes, so 23 x 2 = 46.
    23 x 2 = 46
  3. Write the answer.
    46

Check. 46 divided by 2 is 23 pairs. ✓

Worked example 2.1.2
An imaginary animal has 12 chromosomes in each body cell. How many pairs is this? Write the number.

Method. Divide by 2, because each pair has two chromosomes.

  1. Divide. Share 12 chromosomes into pairs of two.
    12 / 2
  2. Calculate. So there are 6 pairs.
    12 / 2 = 6
  3. Write the answer.
    6

Check. 6 pairs x 2 = 12 chromosomes. ✓

Worked example 2.1.3
Name the part of a cell in which the chromosomes are found. Write the word.

Method. Find where the chromosomes sit.

  1. Look. Chromosomes are inside the control centre of the cell.
    control centre
  2. Name it. The control centre of the cell is the nucleus.
    nucleus
  3. Write the answer.
    nucleus

Check. Genes are on the chromosomes inside the nucleus. ✓

Watch out

Watch out: a gene is not the same as a chromosome. A chromosome is long; a gene is a short section of it.

Exam tip

Exam tip. A human body cell has 46 chromosomes, not 46 pairs. There are 23 pairs.

Try it (with your teacher, or a grown-up at home). With a grown-up, look at a photo of your family. Make a list of three features that look like one parent. Explain in one sentence why the genes for each feature came from that parent.

Exercise 2A  Chromosomes, genes and heredity

1
FWhich part of a cell contains the chromosomes? Write the word.
2
FWhat is a gene? Write the words.
3
FWhat is the name of the long molecule that makes up a chromosome? Write the word.
4
FHow many chromosomes are in a human body cell? Write the number.
5
FHow many pairs of chromosomes do human body cells have? Write the number.
6
FAre genes found on chromosomes? Write yes or no.
7
PSAn imaginary cell has 4 pairs of chromosomes. How many chromosomes are there in total? Write the number.
8
PSAn imaginary animal has 20 chromosomes in each body cell. How many pairs does it have? Write the number.
9
FHeredity is the way information passes from one generation to the next. Which word means the same? Write the word for a parent or child in a family line. Accept: generation.
10
PSA child has one chromosome from each parent in each pair. How many chromosomes come from the mother in a human body cell? Write the number.
11
RExplain why a child can have features from both parents. Write one sentence.
12
RChallenge: a gene controls hair colour. Write one sentence saying where the gene is found in the cell. Accept any sentence saying the gene is a section of DNA on a chromosome in the nucleus.
2.2

DNA, base pairs and proteins

Year 9
9B1: DNA as a double helix of bases that codes for proteins (basic); the part played by Watson, Crick, Wilkins and Franklin
Video lesson: DNA, base pairs and proteinsComing soon
Story. In a school museum display, a poster tells the story of the DNA model. Franklin and Wilkins took X-ray pictures of DNA. Watson and Crick used those pictures, and others, to build a model of the double helix in 1953.

DNA is a double helix: two strands twisted around each other, like a ladder that has been twisted. Each strand is a chain of bases. There are four bases: A, T, C and G. The bases pair up in a fixed way: A always pairs with T, and C always pairs with G.

The order of bases in a gene is a code. The code tells the cell which protein to make. Proteins do most of the work in cells.

A simple model of DNAATCGGCTAsugar-phosphatebackbonebase pairs
A simple model of DNA: base pairs A with T and C with G

Key ideas

  • DNA is a double helix made of two strands.
  • The four bases are A, T, C and G.
  • A always pairs with T; C always pairs with G.
  • The order of bases is a code for making a protein.
Worked example 2.2.1
A strand reads A T G C. Write the base that pairs with each one, in order. Write the letters.

Method. Pair each base using the rule.

  1. A pairs with T. The first base A pairs with T.
    A - T
  2. T pairs with A. The second base T pairs with A.
    T - A
  3. G pairs with C; C pairs with G. The third base G pairs with C, and the fourth base C pairs with G.
    G - C, C - G
  4. Write the answer.
    TACG

Check. Check: each pair has one A or T, and one C or G. ✓

Worked example 2.2.2
Which base pairs with C? Write the letter.

Method. Use the base-pairing rule.

  1. Find the rule. C always pairs with G.
    C - G
  2. Name the base. So the base that pairs with C is G.
    G
  3. Write the answer.
    G

Check. G pairs with C. ✓

Worked example 2.2.3
Which scientists used X-ray pictures of DNA to help build the double helix model? Write one name.

Method. Recall the story of the model.

  1. Recall. Rosalind Franklin and Maurice Wilkins made the X-ray pictures.
    X-ray pictures
  2. Name one. Watson and Crick used these pictures to build the model.
    Watson and Crick
  3. Write the answer.
    Accept Franklin or Wilkins; Watson and Crick used the pictures to build the model.

Check. The pictures were key evidence, but the model was built by Watson and Crick. ✓

Watch out

Watch out: the bases pair in fixed pairs. A and T, and C and G. Do not pair A with C, or G with T.

Exam tip

Exam tip. A DNA strand and the strand it pairs with are the same length, and each A is matched to a T.

Try it (with your teacher, or a grown-up at home). With your teacher, use paper cut-outs or coloured counters to build a short DNA ladder. Use the base-pairing rule to fill in the other strand. Check that every A is paired with T and every C with G.

Exercise 2B  DNA, base pairs and proteins

1
FWhat shape is DNA? Write the words.
2
FHow many different bases are there in DNA? Write the number.
3
FWhich base pairs with T? Write the letter.
4
FWhich base pairs with G? Write the letter.
5
FWhat does the order of bases in a gene code for? Write the word.
6
FName one scientist who helped to develop the DNA model. Accept Watson, Crick, Wilkins or Franklin.
7
PSA strand reads GGA. Write the complementary strand, using the base-pairing rule. Write the letters.
8
PSComplete the other strand for the strand AGCT. Write the letters.
9
PSA DNA strand has 12 bases, and 3 of them are A. How many T bases are on the matching strand? Write the number.
10
PSA short gene has 6 bases on one strand. How many bases are there on the matching strand? Write the number.
11
RExplain why A always pairs with T, and C with G. Write one sentence.
12
RChallenge: explain in one sentence how the base order in a gene affects the body. Accept any sentence saying the base order codes for a protein, and proteins control the body’s structures and processes.
2.3

Sexual reproduction, gametes and fertilisation

Year 9
9B1: sexual reproduction, gametes and fertilisation; a gamete has half the chromosomes of a body cell
Video lesson: Sexual reproduction, gametes and fertilisationComing soon
Story. Lina asks: “Why do I have only one set of features from each parent, not two?” Her teacher draws a diagram. Each parent gives one sex cell, and when the two sex cells join the new cell has a full set from both.

In sexual reproduction, two parents each make a gamete, which is a sex cell. The female gamete is the egg cell. The male gamete is the sperm cell. Each gamete has half the number of chromosomes of a body cell: 23 in humans.

Fertilisation is when a sperm cell and an egg cell join. The new cell, called a zygote, has 46 chromosomes: 23 from the mother and 23 from the father.

CellChromosomes in a human
Body cell46 (23 pairs)
Gamete (egg or sperm)23
Fertilised cell46
Chromosome numbers in human cells

Key ideas

  • A gamete is a sex cell; the female gamete is the egg, the male is the sperm.
  • Gametes have half the number of chromosomes of a body cell.
  • Fertilisation joins a sperm and an egg to make a zygote.
  • The zygote has a full set: half from each parent.
Worked example 2.3.1
How many chromosomes does a human gamete have? Write the number.

Method. Gametes have half the chromosomes of a body cell.

  1. Start. A body cell has 46 chromosomes.
    46
  2. Halve it. Half of 46 is 23.
    46 / 2 = 23
  3. Write the answer.
    23

Check. 23 + 23 = 46. ✓

Worked example 2.3.2
A sperm cell with 23 chromosomes fertilises an egg cell with 23 chromosomes. How many chromosomes does the zygote have? Write the number.

Method. Add the chromosomes from both gametes.

  1. Add. The zygote gets 23 from each parent.
    23 + 23
  2. Calculate. So the zygote has 46 chromosomes.
    23 + 23 = 46
  3. Write the answer.
    46

Check. 46 is the number in a human body cell. ✓

Worked example 2.3.3
Name the process in which a sperm cell and an egg cell join. Write the word.

Method. Recall the name.

  1. Recall. When two gametes join, the process is called fertilisation.
    process is called fertilisation
  2. Name it. So the word is fertilisation.
    fertilisation
  3. Write the answer.
    fertilisation

Check. Fertilisation makes a zygote. ✓

Watch out

Watch out: a gamete has 23 chromosomes, not 23 pairs. The zygote has 46 chromosomes, which is 23 pairs.

Exam tip

Exam tip. Keep gamete and zygote separate. The gamete has half; the zygote has a full set.

Try it (with your teacher, or a grown-up at home). With your teacher, use 23 pairs of beads, or coloured cards, to show how a gamete from each parent makes a full set. Count the chromosomes in the new set. This is a model, so it does not show the real genes.

Exercise 2C  Sexual reproduction, gametes and fertilisation

1
FWhat is the name of the female gamete? Write the words.
2
FWhat is the name of the male gamete? Write the words.
3
FWhat is the name for the joining of a sperm cell and an egg cell? Write the word.
4
FHow many chromosomes does a human gamete have? Write the number.
5
FHow many chromosomes does a fertilised cell (zygote) have? Write the number.
6
FIs sexual reproduction the joining of two gametes? Write yes or no.
7
PSAn imaginary gamete has 7 chromosomes. How many chromosomes does the zygote have after fertilisation with another gamete of 7? Write the number.
8
PSA child has features from both parents. Write the word for the way features are passed on. Write the word.
9
PSA sperm cell has 23 chromosomes. How many chromosomes come from the mother in the zygote? Write the number.
10
PSDo the two gametes in a human zygote have 23 pairs of chromosomes in the zygote? Write yes or no.
11
RExplain why offspring from sexual reproduction vary from their parents. Write one sentence.
12
RChallenge: explain why a gamete has half the chromosomes of a body cell. Accept any sentence saying that the two gametes join at fertilisation, so the full number is restored.
2.4

Alleles, dominant and recessive

Year 9
9B1: alleles, dominant and recessive; the difference between genotype and phenotype
Video lesson: Alleles, dominant and recessiveComing soon
Story. A school gardener grows two kinds of pea plant. One kind is tall and one is short. Tall is dominant, and short only shows when a plant has no tall allele. The gardener writes letters to show the alleles: a capital letter for the dominant one, a small letter for the recessive one.

Each gene can have different versions called alleles. In the pea example, T is the allele for tall and t is the allele for short. The genotype is the letters an organism has, such as Tt. The phenotype is the characteristic it shows, such as tall.

A dominant allele shows its effect when there is at least one copy. A recessive allele shows its effect only when there are two copies.

GenotypePhenotype
TTtall
Tttall
ttshort
Pea plant genotypes and their phenotypes: T is dominant, t is recessive

Key ideas

  • An allele is a version of a gene.
  • Dominant: shows with one copy (capital letter). Recessive: needs two copies (small letter).
  • Genotype is the letters; phenotype is the characteristic seen.
  • Tt is tall because the dominant allele T shows its effect.
Worked example 2.4.1
A pea plant has genotype Tt. Write its phenotype.

Method. The dominant allele shows when there is one copy.

  1. Look. The genotype Tt has one dominant allele T.
    one dominant allele
  2. Decide. T is dominant, so the plant is tall.
    tall
  3. Write the answer.
    tall

Check. The recessive allele t is hidden. ✓

Worked example 2.4.2
A pea plant has genotype tt. Write its phenotype.

Method. A recessive trait needs two copies of the recessive allele.

  1. Look. The genotype tt has two recessive alleles.
    two recessive alleles
  2. Decide. So the plant is short.
    short
  3. Write the answer.
    short

Check. Two copies of t are needed to show short. ✓

Worked example 2.4.3
Write the genotype of a plant that is tall but carries a short allele. Write the letters.

Method. The plant must have one dominant and one recessive allele.

  1. Think. It has the dominant allele for tall, and the recessive allele for short.
    T and t
  2. Write. The genotype is Tt.
    Tt
  3. Write the answer.
    Tt

Check. Tt shows the tall phenotype. ✓

Watch out

Watch out: a capital letter does not mean “bigger”; it means dominant. Tt and TT both look tall.

Exam tip

Exam tip. Write the genotype with the capital letter first, for example Tt, not tT.

Try it (with your teacher, or a grown-up at home). With your teacher, sort a set of coloured cards into groups using a rule like “two cards of one colour is a recessive phenotype”. This is a model, not a real gene: say what the card colours stand for.

Exercise 2D  Alleles, dominant and recessive

1
FWhat is an allele? Write the words.
2
FAn allele that shows its effect with only one copy is called what? Write the word.
3
FHow many copies of a recessive allele must be present for the trait to show? Write the number.
4
FWhat is the phenotype of genotype TT? Write the word.
5
FWhat is the phenotype of genotype tt? Write the word.
6
FWhat is the genotype of a short pea plant? Write the letters.
7
PSA plant is short. Could its genotype be Tt? Write yes or no.
8
PSHow many alleles for this gene does a plant with genotype Tt carry? Write the number.
9
PSWhich phenotype does a plant with genotype TT show? Write the word.
10
PSA plant is tall. Could its genotype be tt? Write yes or no.
11
RExplain why a plant with genotype Tt is tall. Write one sentence.
12
RChallenge: explain why a tall plant does not always tell you its genotype. Accept any sentence that says a tall plant could be TT or Tt, so the phenotype does not show both alleles.
2.5

Punnett squares and inherited disorders

Year 9
9B1: simple genetic crosses with Punnett squares (one gene, two alleles); inherited disorders as the result of inheriting particular alleles
Video lesson: Punnett squares and inherited disordersComing soon
Story. Maya’s parents are both carriers of an allele for cystic fibrosis. A carrier has one copy of the recessive allele and does not have the disorder. Maya’s teacher draws a Punnett square to show the possible genotypes of their children.

A Punnett square shows every combination of alleles that a child can get. The parents’ gametes go along the top and down the side. Each square shows one possible genotype. Each square has the same chance, so the probability can be found by counting squares.

Cystic fibrosis is an inherited disorder. A person who has two recessive alleles (ff) has the disorder. A person with one normal allele (F) does not have the disorder. A person with Ff is a carrier.

This is a simple model for one gene. Doctors give advice and support to families. Decisions about testing are personal.

 Ff
FFFFf
fFfff
Punnett square for a cross of two carriers, Ff and Ff

Key ideas

  • A Punnett square shows each possible combination of alleles.
  • Count the squares to find the proportion or probability.
  • Cystic fibrosis needs two recessive alleles (ff).
  • A carrier (Ff) has one normal allele and does not have the disorder.
Worked example 2.5.1
Cross Bb with bb. What is the chance, as a percentage, that a child has genotype Bb? Write the number.

Method. Fill in the Punnett square, then count.

  1. Fill in. The four squares give Bb, Bb, bb, bb.
    Bb, Bb, bb, bb
  2. Count. Two of the four squares are Bb, which is 2 out of 4.
    2 / 4 = 0.5
  3. Write the answer.
    50%

Check. Two squares out of four is one half, or 50%. ✓

Worked example 2.5.2
Two carriers, Ff and Ff, have a child. What is the chance the child has cystic fibrosis? Write the answer as a fraction.

Method. Fill in the square and count the ff squares.

  1. Fill in. The four squares give FF, Ff, Ff and ff.
    FF, Ff, Ff, ff
  2. Count. One square is ff, so 1 out of 4 children is affected.
    1 out of 4
  3. Write the answer.
    1 in 4

Check. 1 in 4 is the same as 25%. ✓

Worked example 2.5.3
Two carriers have a child. What is the chance the child is a carrier, Ff? Write the answer as a fraction.

Method. Count the Ff squares in the square.

  1. Count. Two of the four squares are Ff.
    2 of 4
  2. Simplify. Two out of four is one half.
    2 / 4 = 1 / 2
  3. Write the answer.
    1 in 2

Check. The two Ff squares are the carriers. ✓

Watch out

Watch out: a chance of 1 in 4 does not mean one child in four is always affected. It is a chance for each child.

Exam tip

Exam tip. Write the alleles of each parent along the top and down the side, then copy each letter into the squares.

Try it (with your teacher, or a grown-up at home). With your teacher, use two coins: one for each parent, each with a capital and small letter. Toss them and record the outcome for 20 pairs. Compare your results with the 1 in 4 expected from a Punnett square.

Exercise 2E  Punnett squares and inherited disorders

1
FIn a cross of Aa and aa, how many of the four squares show aa? Write the number.
2
FIn a cross of Aa and Aa, how many of the four squares show AA? Write the number.
3
FIn a cross of Bb and bb, how many of the four squares show Bb? Write the number.
4
FIn a cross of Ff and Ff, how many of the four squares show ff? Write the number.
5
PSTwo carriers, Ff and Ff, have a child. What is the chance the child has cystic fibrosis? Write the answer as a fraction.
6
PSTwo carriers, Ff and Ff, have a child. What is the chance the child is a carrier? Write the answer as a fraction.
7
PSTwo carriers, Ff and Ff, have a child. What is the chance the child has the normal genotype FF? Write the answer as a fraction.
8
PSIn a cross of Ff and FF, what is the chance the child has cystic fibrosis? Write the number, as a percentage.
9
PSIn a cross of Ff and FF, how many of the four squares are carriers, Ff? Write the number.
10
PSA child has genotype ff. Explain why both parents must carry an f allele. Write one sentence.
11
RExplain why two parents who do not have cystic fibrosis can have a child who does. Write one sentence.
12
RChallenge: give one reason why a family might want information from a doctor about inherited disorders. Accept any sentence that says a doctor can explain the chances and the support available, and the decision belongs to the family.

Chapter summary

SkillMethod
Chromosomes and genesChromosomes are in the nucleus and made of DNA; a gene is a section of DNA that controls a characteristic.
Base pairingDNA is a double helix; A pairs with T and C pairs with G; the base order is a code for proteins.
Gametes and fertilisationGametes have 23 chromosomes in humans; fertilisation gives a zygote with 46.
AllelesDominant alleles (capital letters) show with one copy; recessive alleles need two copies.
Punnett squaresPut the parents’ alleles along the top and side; count the squares to find the chance.
Inherited disordersCystic fibrosis needs two recessive alleles (ff); carriers (Ff) do not have the disorder.

Can you…?

  • I can describe chromosomes, genes and DNA, and where they are found
  • I can pair the bases of DNA correctly, and say what the code does
  • I can describe gametes, fertilisation and chromosome numbers
  • I can use the words allele, dominant, recessive, genotype and phenotype
  • I can complete a Punnett square and give a probability for one gene

Key terms

chromosome
a thread of DNA in the nucleus that carries genes
gene
a short section of DNA that controls one characteristic
DNA
the long molecule that makes up chromosomes and carries the genetic code
base
one of the four chemical units A, T, C and G that make up DNA
double helix
the twisted ladder shape of DNA, made of two strands
protein
a substance made by cells from the instructions in DNA
gamete
a sex cell: an egg cell or a sperm cell
fertilisation
the joining of a sperm cell and an egg cell
allele
a version of a gene
dominant
an allele that shows its effect when there is one copy
recessive
an allele that shows its effect only when there are two copies
genotype
the letters that show the alleles an organism has
phenotype
the characteristic an organism shows
carrier
a person with one recessive allele for a disorder, who does not have the disorder

Exercise 2Z  Check-up

1
Which part of a cell contains genes on chromosomes? Write the word.
2
Which base pairs with A? Write the letter.
3
How many chromosomes does a human gamete have? Write the number.
4
What is the name for the joining of a sperm cell and an egg cell? Write the word.
5
What is the genotype of a short pea plant? Write the letters.
6
A pea plant has genotype Tt. Write its phenotype.
7
In a cross of Bb and bb, what fraction of the four squares show Bb? Write the answer as a fraction.
8
Two carriers, Ff and Ff, have a child. What is the chance the child has cystic fibrosis? Write the answer as a fraction.
9
Name the disorder caused by two copies of a recessive allele that is taught in this chapter. Write the words.
10
Explain why a carrier does not have cystic fibrosis. Write one sentence.

Total 19 marks. Allow about 19 minutes.

Exercise 2X  Practice test questions

1
FWhat is a gene? Write the words.[1 mark]
2
FWrite the base that pairs with T in DNA. Write the letter.[1 mark]
3
FA gamete has 23 chromosomes. A zygote is formed from two gametes. How many chromosomes does the zygote have? Write the number.[1 mark]
4
FName the male gamete in humans. Write the words.[1 mark]
5
FGive the phenotype of a plant with genotype TT. Write the word.[1 mark]
6
PSComplete the Punnett square for a cross of Aa with Aa. Write the number of squares that show aa. Write the number.[2 marks]
7
PSIn a cross of Ff and Ff, what is the chance of a child having cystic fibrosis? Write the answer as a fraction, and give the percentage.[2 marks]
8
FDescribe the structure of DNA in one sentence. Accept any sentence that says DNA is a double helix of two strands, made of four bases that pair as A with T and C with G.[2 marks]
9
RExplain why a child inherits features from both parents. Write one sentence. Accept any sentence that says each parent gives one chromosome from each pair, so the child has genes from both.[2 marks]
10
PSA pea plant with genotype Tt is crossed with a plant with genotype tt. Complete the Punnett square and give the phenotypes of the offspring. Write the number of tall offspring out of four. Write the number.[3 marks]
11
RA family has two parents who are carriers. Give the chance, as a percentage, that a child is a carrier, and explain why carriers do not show the disorder. Accept any answer giving 50%, with an explanation that a carrier has one normal dominant allele.[3 marks]
← Chapter 1: Working scientifically (Year 9)Chapter 3: Variation, natural selection and evolution →

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