Pearson Edexcel GCSE Combined Science · B3

Genetics

50 revision questions with answers for B3 Genetics in Pearson Edexcel GCSE Combined Science (spec 1SC0). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official Pearson material.

All 50 questions and answers

Question 1 · Spec B3.3
What is the role of meiosis?
Answer:
It produces haploid gametes with half the chromosome number, enabling sexual reproduction without doubling chromosome number each generation.
Question 2 · Spec B3.3
How many daughter cells does meiosis produce, and are they identical?
Answer:
Four genetically different haploid cells.
Question 3 · Spec B3.3
A diploid cell has 46 chromosomes. How many are in each gamete?
Answer:
23 chromosomes.
Question 4 · Spec B3.3
What happens to chromosome number when two gametes fuse?
Answer:
The diploid chromosome number is restored in the zygote.
Question 5 · Spec B3.3
How do mitosis and meiosis differ in their products?
Answer:
Mitosis produces two genetically identical diploid cells; meiosis produces four genetically different haploid cells.
Question 6 · Spec B3.4
Describe DNA's overall shape.
Answer:
Two strands coiled into a double helix.
Question 7 · Spec B3.4
What are the three parts of a DNA nucleotide?
Answer:
A sugar, a phosphate group and one nitrogen-containing base.
Question 8 · Spec B3.4
Which bases pair together in DNA?
Answer:
Adenine with thymine, and cytosine with guanine: A-T and C-G.
Question 9 · Spec B3.4
What holds complementary DNA bases together?
Answer:
Weak hydrogen bonds between the bases on opposite strands.
Question 10 · Spec B3.4
What forms the backbone of each DNA strand?
Answer:
Alternating sugar and phosphate groups.
Question 11 · Spec B3.4
Find the complementary DNA sequence to A-T-G-C-C-A.
Answer:
T-A-C-G-G-T.
Question 12 · Spec B3.5
How do gene and genome differ?
Answer:
A gene is a DNA section coding for a specific protein; the genome is an organism's entire DNA.
Question 13 · Spec B3.5
How are chromosomes, DNA and genes related?
Answer:
Chromosomes contain long DNA molecules with many genes along them.
Question 14 · Spec B3.6
Why crush fruit before extracting DNA?
Answer:
Crushing breaks up the tissue, allowing the extraction solution to reach more cells.
Question 15 · Spec B3.6
What does detergent do during DNA extraction?
Answer:
It disrupts cell and nuclear membranes, releasing DNA into the solution.
Question 16 · Spec B3.6
Why filter the fruit mixture before collecting DNA?
Answer:
Filtration removes large pieces of tissue while dissolved material passes through.
Question 17 · Spec B3.6
Why add cold ethanol carefully to the filtered fruit mixture?
Answer:
DNA is poorly soluble in ethanol and precipitates as pale strands at the liquid boundary.
Question 18 · Spec B3.12
Why can individuals inherit different versions of a characteristic?
Answer:
They may inherit different alleles, which are different versions of a gene.
Question 19 · Spec B3.13
Define chromosome.
Answer:
A structure containing DNA and many genes.
Question 20 · Spec B3.13
Define dominant and recessive alleles.
Answer:
A dominant allele is expressed with one copy; a recessive allele is expressed when no dominant allele is present.
Question 21 · Spec B3.13
Define homozygous and heterozygous.
Answer:
Homozygous: two identical alleles for a gene. Heterozygous: two different alleles.
Question 22 · Spec B3.13
Define genotype and phenotype.
Answer:
Genotype is the allele combination; phenotype is the characteristic expressed through genes and environmental influences.
Question 23 · Spec B3.13
What are a gamete and a zygote?
Answer:
A gamete is a haploid reproductive cell; a zygote is the diploid cell formed when gametes fuse.
Question 24 · Spec B3.14
For a dominant allele A, which genotypes show the dominant trait?
Answer:
AA and Aa; aa shows the recessive trait.
Question 25 · Spec B3.14
What gametes can an Aa individual produce for this gene?
Answer:
A-bearing and a-bearing gametes, usually with equal probability.
Question 26 · Spec B3.14
What offspring genotypes result from AA x aa?
Answer:
All are Aa and show the dominant trait.
Question 27 · Spec B3.14
What genotype ratio results from Aa x Aa?
Answer:
1 AA : 2 Aa : 1 aa.
Question 28 · Spec B3.16
What phenotype ratio results from Aa x Aa with complete dominance?
Answer:
3 dominant : 1 recessive, corresponding to 75% and 25%.
Question 29 · Spec B3.14
What offspring result from Aa x aa?
Answer:
50% Aa and 50% aa, giving a 1:1 phenotype ratio with complete dominance.
Question 30 · Spec B3.16
What is the probability of an aa offspring from two Aa parents?
Answer:
1/4 or 25% for each conception.
Question 31 · Spec B3.16
Does a 25% probability guarantee exactly one affected offspring in four?
Answer:
No. Each conception is independent, so actual family outcomes can differ from the expected proportions.
Question 32 · Spec B3.14
Two unaffected parents have a child with a recessive trait. What genotypes explain this?
Answer:
Both parents can be heterozygous carriers, Aa, while the child is aa.
Question 33 · Spec B3.14
How are males, females and affected individuals commonly shown in pedigrees?
Answer:
Squares represent males, circles females and shading the characteristic being studied; always check the diagram's key.
Question 34 · Spec B3.15
What sex chromosomes do human females and males usually have?
Answer:
Females XX and males XY.
Question 35 · Spec B3.15
Which sex chromosome is carried by an egg and by a sperm?
Answer:
An egg carries X; a sperm carries either X or Y.
Question 36 · Spec B3.15
Show the expected sex-chromosome outcomes from XX x XY.
Answer:
Half the expected offspring are XX and half XY, so each outcome has approximately 50% probability.
Question 37 · Spec B3.19
Are most phenotypic features controlled by one gene?
Answer:
No. Most result from interactions between multiple genes, often also influenced by environment.
Question 38 · Spec B3.20
Give an example of mainly genetic variation.
Answer:
A person's inherited blood group.
Question 39 · Spec B3.20
Give an example of environmental variation.
Answer:
A scar acquired after an injury.
Question 40 · Spec B3.20
Why can genetically identical plants reach different heights?
Answer:
They may experience different light, water, temperature or nutrient conditions.
Question 41 · Spec B3.20
How does sexual reproduction contribute to genetic variation?
Answer:
Different combinations of parental alleles are brought together in offspring.
Question 42 · Spec B3.22
What is a mutation?
Answer:
A change in DNA that can create a new allele.
Question 43 · Spec B3.22
Why is there usually extensive genetic variation within a species?
Answer:
Mutations create different alleles that accumulate and are mixed through reproduction.
Question 44 · Spec B3.23
Do most mutations strongly change phenotype?
Answer:
No. Most have no phenotypic effect, some have small effects and only rarely does one cause a major change.
Question 45 · Spec B3.21
What was the main outcome of the Human Genome Project?
Answer:
The sequence and mapping of the human genome, providing a reference for studying genes and their functions.
Question 46 · Spec B3.21
How can genome knowledge help diagnose inherited disease?
Answer:
It helps identify DNA variants associated with a condition and assess whether a person carries them.
Question 47 · Spec B3.21
How could genome information improve treatment?
Answer:
It can reveal disease mechanisms and guide development or selection of medicines suited to particular genetic variants.
Question 48 · Spec B3.21
Give an ethical issue raised by access to genome information.
Answer:
Privacy, consent and possible discrimination if genetic risk information is disclosed or misused.
Question 49 · Spec B3.16
A genetic cross predicts 25% aa. How many aa offspring are expected among 200?
Answer:
0.25 × 200 = 50; this is an expectation rather than a guaranteed count.
Question 50 · Spec B3.20
How do inherited and acquired characteristics differ?
Answer:
Inherited characteristics depend on genes passed from parents; acquired characteristics develop through environmental influences during life.

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