Inheritance and ecology · GCSE Biology

Inheritance, variation and evolution

DNA, genes and alleles, Punnett squares, sex determination, meiosis, mutation and natural selection.

UNDERSTAND→RETRIEVE→REMEMBER
THE MEMORY HOOK
A gene is a length of DNA that codes for a protein. Alleles are different versions of that gene. A Punnett square is just a grid of possible fertilisations — not a prediction for one baby.

The important bits

What you need to know

  1. 1

    DNA is a polymer of nucleotides made of two strands in a double helix. A gene is a section of DNA that codes for a sequence of amino acids in a protein. Chromosomes are structures of DNA in the nucleus; humans have 23 pairs.

  2. 2

    Alleles are different forms of the same gene. Genotype is the alleles present (for example Bb). Phenotype is the characteristic expressed. Dominant alleles are expressed even if only one copy is present; recessive alleles need two copies.

  3. 3

    Homozygous means two identical alleles; heterozygous means two different alleles. Genetic diagrams and Punnett squares show predicted ratios, not a guaranteed result for one offspring.

  4. 4

    Sexual reproduction involves meiosis, producing gametes with half the chromosome number. Fertilisation restores the full number and mixes alleles, so offspring vary. Asexual reproduction uses mitosis and produces clones.

  5. 5

    Sex determination in humans: XX female, XY male. A Punnett square of XX × XY gives a 50% chance of a boy or a girl.

  6. 6

    Mutations change DNA. Most have no effect; some change a protein and therefore a phenotype. They increase variation. Evolution by natural selection: variation, competition, survival of better-adapted individuals, reproduction, alleles become more common.

  7. 7

    The fossil record, antibiotic-resistant bacteria and classification into the three-domain system (archaea, bacteria, eukaryotes) are evidence used in this topic. Embryo screening raises ethical issues about genetic conditions.

Go deeper

Punnett squares are probabilities

Write the parents’ genotypes, circle the gametes, then fill every box. A heterozygous cross for a dominant condition, Bb × Bb, gives 1 BB : 2 Bb : 1 bb, so three quarters show the dominant phenotype. That is a prediction across many offspring. One child is still a single random fertilisation. Use the language the mark scheme wants: allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype. “The gene for brown” is sloppy; brown is a phenotype caused by an allele of a gene. If a pedigree chart is given, look for a trait that skips generations — that often flags a recessive allele hidden in heterozygotes.

Go deeper

Natural selection is a sequence, not a slogan

Start with variation, often from mutation or sexual reproduction. Individuals compete for limited resources. Those with advantageous alleles are more likely to survive and reproduce. Their offspring inherit those alleles, so the proportion in the population rises. That is evolution. Do not write that organisms try to adapt, or that they develop a feature because they need it. The variation has to exist first. Antibiotic resistance is the cleanest modern example: a mutation, antibiotics as the selection pressure, resistant bacteria survive, the allele becomes more common. Speciation, on Higher or Triple routes, adds isolation and so much change that two groups can no longer interbreed.

WORKED EXAMPLE

See the idea in action

Cystic fibrosis is caused by a recessive allele, f. Two heterozygous parents are Ff × Ff. Gametes are F or f from each parent. Offspring: FF, Ff, Ff, ff. Probability of cystic fibrosis (ff) is 1 in 4, or 25%. Probability of being a carrier (Ff) is 50%. The parents do not have the condition because the dominant allele F produces a working phenotype. A single family may not match 25% because each child is an independent event.

Exam technique

Turn knowledge into marks

Define gene, allele, genotype and phenotype in the first sentence if the question allows. Show working on genetic diagrams. In natural selection answers, use variation, survival, reproduction and alleles becoming more common — avoid “need” and “try”.

Common mistakes

Do not give these marks away

  1. 01

    Treating a 3:1 Punnett ratio as a promise about one child, or mixing up genotype with phenotype.

  2. 02

    Writing that organisms adapt because they need to, instead of selection acting on existing variation.

  3. 03

    Saying asexual reproduction produces variation, or that meiosis makes identical body cells.

QUICK RETRIEVAL

Two heterozygous parents (Bb × Bb) are crossed. What fraction of the offspring are expected to show the recessive phenotype?

AAll of them

BThree quarters

COne quarter

DNone of them

Show the answer

One quarter. The Punnett square is BB, Bb, Bb, bb. Only bb shows the recessive phenotype, which is one in four offspring on average.

Quick questions

If this is the bit you searched

What is the difference between a gene and an allele?

A gene is a section of DNA that codes for a protein. Alleles are different versions of that gene, such as B and b.

How is sex determined in humans?

Females are XX and males are XY. Sperm carry X or Y, so there is a 50% chance of a boy or a girl at fertilisation.