Inheritance and ecology · GCSE Biology
Genetic diagrams
Draw Punnett squares for GCSE Biology: parental genotypes, gametes, offspring ratios, 3:1 and 1:1 crosses, XX × XY sex determination, and why a 25% chance is not a promise about one baby.
Write parents’ genotypes, circle the gametes, fill every box. A heterozygous cross Bb × Bb is 1 BB : 2 Bb : 1 bb, so 75% dominant phenotype, 25% recessive — a prediction across many offspring, not one child.
The important bits
What you need to know
- 1
A genetic diagram or Punnett square shows possible fertilisations. It predicts ratios, not a guaranteed result for a single baby.
- 2
Monohybrid cross method: choose a letter (dominant capital, recessive lower case), write parental genotypes, show gametes (one allele each after meiosis), fill the square, state genotype and phenotype ratios.
- 3
Heterozygous × heterozygous (Bb × Bb) → 1 BB : 2 Bb : 1 bb genotypes, 3:1 phenotypes if B is dominant. Heterozygous × homozygous recessive (Bb × bb) → 1:1 genotypes and phenotypes.
- 4
Homozygous dominant × anything with a recessive phenotype cannot produce a recessive child in a simple dominant/recessive model, because every offspring gets a B.
- 5
Sex determination: female XX, male XY. Punnett square of XX × XY gives two XX and two XY, so 50% chance of a girl or a boy at each fertilisation. The Y-bearing sperm determines maleness.
- 6
Pedigree charts: a trait that skips generations is often recessive (hidden in heterozygote carriers). A trait appearing every generation is more likely dominant, but you must still check the diagram.
- 7
Use the words allele, homozygous, heterozygous, genotype, phenotype, dominant, recessive. “The gene for brown” is sloppy.
- 8
Probability: each child is an independent event. Four children of Bb × Bb might not be exactly 3:1. Over many offspring the ratio approaches the prediction.
Quotations worth analysing
Short evidence. Real method.
“A Punnett square shows the probability of each genotype, not a certainty for one offspring.”
Always say “expected” or “predicted” ratio. Families of four do not have to match 3:1.
“Bb × Bb gives 25% homozygous recessive if b is recessive.”
Show the working: gametes B or b from each parent; one of four boxes is bb.
“XX female, XY male; 50% chance of each sex.”
Sperm are X or Y. Eggs are all X. Do not put YY as a parent.
Go deeper
What is the safest step-by-step for a four-mark Punnett square?
Choose letters from the question (if it says R for red, use R and r). Write parent genotypes beside the square. Meiosis: each gamete gets one allele — write them on the top and side. Fill four boxes as fertilisations. Circle or highlight the phenotype the question asks for and convert to a fraction, ratio or percentage. State “this is a prediction”. If a parent is unknown, work backwards: a recessive child (bb) means both parents gave b, so a brown parent who had a blue child must have been heterozygous. That reverse logic is how pedigree questions work. Do not leave the square blank and guess 50%. Do not use two different letters for two alleles of the same gene (B and r).
Go deeper
How do I read a pedigree without panicking?
Shaded usually means the condition is expressed. If two unshaded parents have a shaded child, the trait is recessive and both parents are carriers (heterozygous). If a shaded parent has an unshaded child, the shaded parent cannot be homozygous recessive for a dominant trait — think it through with alleles. Sex-linked pedigrees show more shaded males; only include that if the paper indicates X-linkage. Always assign alleles to one named individual first, then test whether the rest of the tree still works. If it does not, your dominance assumption is wrong. Write genotypes on the diagram in the exam booklet; the working is part of the marks even if the printout of the tree is given.
Go deeper
Why is “three to one so my next child is safe” a mark-scheme trap?
Each fertilisation is independent, like tossing two coins. After three brown-eyed children, the next child of Bb × Bb still has a 1/4 chance of bb. The 3:1 ratio is the expected proportion in a large sample. Exam questions love “the couple already have three healthy children; what is the chance the fourth has cystic fibrosis?” — still 1/4 if both are carriers. Also, a 50% sex chance does not mean a family of two must be one boy and one girl. State independence clearly. If you cannot explain that, you do not yet own the square; you only copied it.
See the idea in action
Cystic fibrosis is recessive, allele f. Parents Ff × Ff. Gametes F or f from each. Offspring: FF, Ff, Ff, ff. Probability of cystic fibrosis (ff) is 1 in 4 (25%). Probability of a carrier (Ff) is 50%. The parents do not have the condition because F is dominant. If this couple’s first child is ff, the chance the second is ff is still 25%. A student who writes “they already had the 25%, so the next is 0%” has misunderstood independent events.
Exam technique
Turn knowledge into marks
Show the square, label gametes, state the ratio as expected. Use capital/lower case consistently. For sex, XX × XY, 50%. Never treat 3:1 as a promise about one child.
Common mistakes
Do not give these marks away
- 01
Treating a 3:1 ratio as a guarantee for one family, or mixing genotype with phenotype.
- 02
Using two different letters for one gene, or putting two alleles in one gamete.
- 03
Drawing XY × XY, or saying the mother determines the baby’s sex.
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
How do you draw a Punnett square for Bb × Bb?
Gametes are B or b from each parent. The four boxes are BB, Bb, Bb and bb. Phenotype ratio is 3 dominant : 1 recessive if B is dominant.
Why might a real family not match a 3:1 ratio?
Each child is an independent fertilisation. Small families can deviate from the predicted ratio. The ratio appears over many offspring.
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.
What does heterozygous mean?
Two different alleles of a gene, for example Bb. Homozygous means two the same, BB or bb.