AQA A-level Biology — Genetics, Populations and Ecosystems flashcards
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AQA A-level Biology topic 7: genetic crosses including sex linkage, linkage and epistasis, chi-squared, Hardy–Weinberg, selection, drift and speciation, and populations in ecosystems with sampling and succession (required practical 12).
Learn deck · part of AQA A-level Biology · written against: AQA AS and A-level Biology (7401/7402) specification, version 1.6, July 2026 (eight topics). Updated 2026-10-10.
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What is the difference between genotype and phenotype?The genotype is an organism's alleles; the phenotype is the characteristics that result when it is expressed and interacts with the environment.Genetics Populations and Ecosystems › Inheritance
Two organisms with the same genotype can differ in phenotype if their environments differ. -
A gene has three alleles in a population. How many can one diploid individual carry?At most two — one at the locus on each homologous chromosome.Genetics Populations and Ecosystems › Inheritance
Example: the ABO blood group gene has three alleles (Iᴬ, Iᴮ, Iᴼ), but each person has only two. -
What does it mean for alleles to be codominant?Both alleles are expressed in the phenotype of a heterozygote.Genetics Populations and Ecosystems › Inheritance
Example: a person with genotype IᴬIᴮ has blood group AB. -
What is the difference between homozygous and heterozygous?Homozygous: the two alleles at a locus are the same; heterozygous: they are different.Genetics Populations and Ecosystems › Inheritance
A recessive allele shows in the phenotype only when no dominant allele is present: when homozygous, or in a male for a gene on the X chromosome. -
Two heterozygous pea plants (Tt) are crossed. What phenotypic and genotypic ratios are expected in the offspring?Phenotypes 3 tall : 1 short; genotypes 1 TT : 2 Tt : 1 tt.Genetics Populations and Ecosystems › Inheritance
Each parent makes T and t gametes in equal numbers, and fertilisation is random. -
How can you find out whether an organism showing a dominant phenotype is homozygous or heterozygous?Carry out a test cross with a homozygous recessive organism.Genetics Populations and Ecosystems › Inheritance
If any offspring show the recessive phenotype, the parent must be heterozygous. -
What phenotypic ratio is expected from a dihybrid cross between two double heterozygotes (AaBb × AaBb) with unlinked genes?9 : 3 : 3 : 1Genetics Populations and Ecosystems › Inheritance
Each parent makes four gamete types (AB, Ab, aB, ab) in equal numbers. -
A double heterozygote (AaBb) is crossed with a double homozygous recessive (aabb), and the genes are unlinked. What ratio is expected?1 : 1 : 1 : 1Genetics Populations and Ecosystems › Inheritance
The recessive parent only gives ab gametes, so the offspring show each of the four gamete types of the other parent. -
A man with blood group A (IᴬIᴼ) and a woman with blood group B (IᴮIᴼ) have children. Which blood groups are possible, and in what ratio?AB, A, B and O in a 1 : 1 : 1 : 1 ratio.Genetics Populations and Ecosystems › Inheritance
Gametes Iᴬ or Iᴼ combine with Iᴮ or Iᴼ to give IᴬIᴮ, IᴬIᴼ, IᴮIᴼ and IᴼIᴼ. -
Why are recessive sex-linked conditions such as haemophilia far more common in males?The gene is on the X chromosome; males have only one X, so a single recessive allele is always expressed.Genetics Populations and Ecosystems › Inheritance
Females usually need two copies, one on each X, to be as severely affected as males; a carrier with one copy can have milder symptoms.
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About these cards
Review status: Second-reader reviewed, with corrections applied.
Checked against:
- Topic outline follows the AQA AS and A-level Biology (7401/7402) specification, version 1.6 (July 2026) (outline only; no specification text reproduced).
- OpenStax Biology 2e, consulted as a reference; no text copied.
- Also checked against the AQA AS and A-level Biology required practical handbook, version 2.3 (January 2026), the NIST/SEMATECH e-Handbook of Statistical Methods and the NHS website (reference only; no text copied).
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