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Genetics: DNA, Genes, Chromosomes and Punnett Squares — Answers

Answer sheet with model answers and success criteria for the Year 10 cells & classification worksheet.

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Success TutoringWorksheetsAnswer sheetYear 10 · Science · Cells & Classification

Genetics: DNA, Genes, Chromosomes and Punnett Squares — Answers

Part A: DNA, genes and chromosomes

DNA (deoxyribonucleic acid) is made of two strands twisted into a double helix. Each strand is a chain of units containing one of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The bases on the two strands pair up: A with T and C with G.

The order of the bases is a code. A gene is a section of DNA that codes for a protein, and proteins help determine characteristics. Each chromosome is one very long DNA molecule carrying many genes. Chromosomes come in pairs: one of each pair comes from each parent.

1.Match each term to its meaning.
  • DNA
  • Gene
  • Chromosome
  • Allele
  • Mutation
  • One of the different versions of the same gene
  • A change in the sequence of bases in DNA
  • A long, tightly coiled strand of DNA found in the nucleus
  • The molecule that carries genetic information, shaped like a twisted ladder (a double helix)
  • A section of DNA that carries the instructions for making a particular protein

Answer: DNA → The molecule that carries genetic information, shaped like a twisted ladder (a double helix); Gene → A section of DNA that carries the instructions for making a particular protein; Chromosome → A long, tightly coiled strand of DNA found in the nucleus; Allele → One of the different versions of the same gene; Mutation → A change in the sequence of bases in DNA

2.In DNA, the base A always pairs with (answer space), and C always pairs with (answer space).

Answer: T; G

A (adenine) pairs with T (thymine) and C (cytosine) pairs with G (guanine).

3.Write the complementary strand for this DNA sequence: ATGCCGTA

Answer: TACGGCAT

4.Write the complementary strand for this DNA sequence: GGATCCTA

Answer: CCTAGGAT

5.Human body cells contain 46 chromosomes. How many pairs is this, and how many chromosomes are in a human sperm or egg cell?

Answer: 23 pairs. A sperm or egg cell contains 23 chromosomes (one from each pair), so when they join at fertilisation the new cell has 46.

6.Put these in order from largest to smallest.
  1. ANucleus, chromosome, gene, base
  2. BChromosome, nucleus, gene, base
  3. CGene, chromosome, nucleus, base
  4. DNucleus, gene, chromosome, base

Answer: Nucleus, chromosome, gene, base

The nucleus contains chromosomes; each chromosome is a long DNA molecule containing many genes; each gene is a sequence of many bases.

Part B: Alleles, genotype and phenotype

A capital letter stands for a dominant allele and the same letter in lower case stands for the recessive allele.

7.Match each inheritance term to its meaning.
  • Genotype
  • Phenotype
  • Homozygous
  • Heterozygous
  • Dominant allele
  • Recessive allele
  • An allele that shows in the phenotype even if only one copy is present
  • An allele that shows in the phenotype only if two copies are present
  • The alleles an organism has for a characteristic, such as Pp
  • The characteristic that shows, such as purple flowers
  • Having two different alleles, such as Pp
  • Having two identical alleles, such as PP or pp

Answer: Genotype → The alleles an organism has for a characteristic, such as Pp; Phenotype → The characteristic that shows, such as purple flowers; Homozygous → Having two identical alleles, such as PP or pp; Heterozygous → Having two different alleles, such as Pp; Dominant allele → An allele that shows in the phenotype even if only one copy is present; Recessive allele → An allele that shows in the phenotype only if two copies are present

8.In pea plants, purple flowers (P) are dominant over white flowers (p). What are the possible genotypes of a purple-flowered plant?

Answer: PP or Pp.

9.What is the genotype of a white-flowered pea plant? Explain how you know.

Answer: pp. White is recessive, so it only shows when the plant has two copies of the recessive allele.

Part C: Punnett squares

Draw a Punnett square for each cross: write one parent's gametes across the top and the other parent's down the side, then fill in each box.

10.Two heterozygous purple-flowered pea plants (Pp × Pp) are crossed. Complete a Punnett square and give the ratio of genotypes and of phenotypes in the offspring.

Answer: Genotypes 1 PP : 2 Pp : 1 pp; phenotypes 3 purple : 1 white

  1. Gametes from each parent: P or p
  2. Square: PP, Pp, Pp, pp
  3. Genotype ratio: 1 PP : 2 Pp : 1 pp
  4. Phenotype ratio: PP and Pp are purple, pp is white, so 3 purple : 1 white
11.In the cross above, what is the probability that an offspring has white flowers? Give your answer as a fraction and a percentage.

Answer: , or 25%.

12.The same cross (Pp × Pp) produces 400 seeds that grow into plants. About how many would you expect to have white flowers?

Answer: About 100 (one quarter of 400). The actual number will vary a little, because each offspring is a separate chance event.

13.A heterozygous purple plant (Pp) is crossed with a white plant (pp). Give the expected phenotype ratio of the offspring.

Answer: 1 purple : 1 white (50% purple, 50% white)

  1. Gametes: P or p from the first parent; p only from the second
  2. Square: Pp, Pp, pp, pp
  3. 2 Pp (purple) : 2 pp (white) = 1 purple : 1 white
14.A homozygous purple plant (PP) is crossed with a white plant (pp). What percentage of the offspring will have purple flowers?

Answer: 100% (all Pp)

  1. Gametes: P only from the first parent; p only from the second
  2. Every offspring is Pp, so 100% have purple flowers
15.In pea plants, tall (T) is dominant over short (t). A heterozygous tall plant (Tt) is crossed with a homozygous tall plant (TT). What are the expected genotypes and phenotypes of the offspring?

Answer: 1 TT : 1 Tt; all tall

  1. Gametes: T or t from the first parent; T only from the second
  2. Square: TT, TT, Tt, Tt
  3. Genotypes 1 TT : 1 Tt; every offspring has at least one T, so all are tall
16.A gardener has a purple-flowered pea plant but does not know whether it is PP or Pp. She crosses it with a white plant (pp) and some of the offspring have white flowers. What is the genotype of the purple parent? Explain.

Answer: Pp. A white offspring must be pp, so it received a p allele from each parent. The purple parent must therefore carry a p allele. (If it were PP, all the offspring would be Pp and purple.)

Part D: Human inheritance

17.In humans, females usually have the sex chromosomes XX and males usually have XY. Use a Punnett square to show the chance that a baby is female.

Answer: 50% (1 XX : 1 XY)

  1. Gametes: X only from the mother; X or Y from the father
  2. Square: XX, XX, XY, XY
  3. 1 XX : 1 XY, so the chance of a female (XX) is 50% for each pregnancy
18.A couple have three daughters. Because of this, their next child is more likely to be a son than their first child was.

Answer: False

Each pregnancy is a separate chance event. The father's sperm still carry X or Y in roughly equal numbers, so the chance of a son is about the same (about 50%) every time, whatever happened before.

19.Cystic fibrosis is caused by a recessive allele (f); the dominant allele is F. Two parents are both carriers (Ff) but do not have the condition. What is the probability that a child has cystic fibrosis, and what is the probability that a child is a carrier?

Answer: Has cystic fibrosis: 25%; carrier: 50%

  1. Gametes: F or f from each parent
  2. Square: FF, Ff, Ff, ff
  3. ff has cystic fibrosis: 1 in 4 = 25%
  4. Ff is a carrier (does not have the condition): 2 in 4 = 50%

Part E: Explain it

20.Explain how two purple-flowered pea plants can produce a white-flowered offspring. Use the words allele, dominant, recessive, genotype and phenotype, and include a Punnett square.[5 marks]

Model answer: Flower colour is controlled by a gene with two alleles: P (purple) is dominant and p (white) is recessive. Both parents have the purple phenotype but the heterozygous genotype Pp, so each carries a hidden recessive allele. Each parent passes on one allele in each gamete, either P or p. Punnett square: PP, Pp, Pp, pp. If an offspring receives p from both parents, its genotype is pp, and because there is no dominant allele to mask it, its phenotype is white. The chance of this is 1 in 4 (25%) for each offspring.

Success criteria:

  • Identifies both parents as heterozygous (Pp)
  • Correct Punnett square
  • Explains that the white offspring is pp
  • Uses the five key terms correctly
  • States the 1 in 4 probability
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