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You now know that genes code for proteins, which in turn shape an organism's characteristics. This lesson explains how those characteristics are passed from parents to offspring. It introduces the precise vocabulary that geneticists use — allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype — shows you how to predict the outcome of a cross using a Punnett square, and explains how the sex of a baby is decided by the sex chromosomes. Monohybrid crosses (following one gene at a time) are one of the most reliable sources of marks in Topic B5 of your OCR Gateway Combined Science course, and one where careful, methodical working really pays off, so we will build the technique step by step.
By the end of this lesson you should be able to use all the key genetic terms correctly, complete a Punnett square for a monohybrid cross, express the outcomes as ratios, fractions and percentages, and explain how sex is inherited.
This lesson develops AO1 (understanding the language of genetics) and AO2 (applying the maths skill of constructing Punnett squares and converting outcomes into ratios, fractions and percentages).
Most genes come in slightly different versions, and which versions you inherit decides your characteristics. To describe this precisely you need a small set of terms. Learn them carefully — examiners use them exactly, and most marks in this topic depend on using them correctly.
| Term | Meaning |
|---|---|
| Gene | A section of DNA that codes for a particular protein / characteristic |
| Allele | A different version of a gene (for example the allele for brown eyes or the allele for blue eyes) |
| Dominant | An allele that is expressed (shows in the phenotype) even if only one copy is present; written as a capital letter (e.g. B) |
| Recessive | An allele that is only expressed when two copies are present; written as a lower-case letter (e.g. b) |
| Homozygous | Having two of the same allele for a gene (e.g. BB or bb) |
| Heterozygous | Having two different alleles for a gene (e.g. Bb) |
| Genotype | The alleles an organism has (e.g. Bb) — its genetic make-up |
| Phenotype | The physical characteristic that results (e.g. brown eyes) — what you can observe |
Two ideas are worth dwelling on:
Exam Tip: Use capital letters for dominant alleles and lower-case for recessive, and choose letters where the capital and lower-case look clearly different — B/b works well, but S/s or C/c can be hard to tell apart in handwriting. Always write the dominant and recessive forms of the same letter (never B and r).
The difference between genotype (the alleles) and phenotype (the visible characteristic) is central to the whole topic. Consider a gene for fur colour where B (brown) is dominant and b (white) is recessive:
| Genotype | Homozygous or heterozygous? | Phenotype |
|---|---|---|
| BB | Homozygous dominant | Brown |
| Bb | Heterozygous | Brown |
| bb | Homozygous recessive | White |
Notice that two different genotypes (BB and Bb) give the same phenotype (brown). This is exactly why two brown-furred parents can sometimes have a white offspring: if both parents are Bb, each can pass on a b allele, and an offspring that inherits b from each parent is bb (white). Being able to explain this is a frequent exam question.
Exam Tip: A common misconception is that "heterozygous" (Bb) gives an in-between phenotype — for example a pale-brown fur. It does not: a single dominant allele is enough to show the dominant characteristic fully, so Bb looks exactly like BB.
A Punnett square is a simple grid that shows all the possible combinations of alleles when two parents reproduce. It lets you predict the genotypes and phenotypes of the offspring, and the ratio in which they are expected.
The method is the same every time:
Two mice, both heterozygous for fur colour (Bb), are crossed. B (brown) is dominant to b (white). Predict the genotypes and phenotypes of the offspring.
Step 1 — parents' genotypes: Bb × Bb.
Step 2 — gametes: each parent can pass on B or b.
Step 3 and 4 — the Punnett square:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Step 5 — read off the results. The four boxes are BB, Bb, Bb, bb.
Answer: the expected offspring ratio is 3 brown : 1 white. As a fraction, 43 are brown and 41 are white; as a percentage, 75% brown and 25% white.
Common error: counting BB and bb as the "same" because both are homozygous. They give different phenotypes — BB is brown, bb is white. Always read the phenotype from the alleles, using the rule that one dominant allele is enough to show the dominant characteristic.
A heterozygous brown mouse (Bb) is crossed with a white mouse (bb). Predict the offspring.
Step 1 — parents' genotypes: Bb × bb.
Step 2 — gametes: the Bb parent passes on B or b; the bb parent can only pass on b.
Step 3 and 4 — the Punnett square:
| B | b | |
|---|---|---|
| b | Bb | bb |
| b | Bb | bb |
Step 5 — read off the results. The four boxes are Bb, bb, Bb, bb.
Answer: the expected offspring ratio is 1 brown : 1 white — that is 21 (50%) brown and 21 (50%) white.
Exam Tip: When one parent is homozygous (e.g. bb), both its gametes carry the same allele, so two rows (or columns) of your grid are identical. That is correct — do not "tidy" it into a single row. The repeated boxes are what give you the right ratio.
A homozygous brown mouse (BB) is crossed with a white mouse (bb). Predict the offspring.
Step 1 — parents' genotypes: BB × bb.
Step 2 — gametes: the BB parent can only pass on B; the bb parent can only pass on b.
Step 3 and 4 — the Punnett square:
| B | B | |
|---|---|---|
| b | Bb | Bb |
| b | Bb | Bb |
Step 5 — read off the results. All four boxes are Bb.
Answer: all the offspring are heterozygous Bb and brown (100%). Even though one parent was white, none of the offspring are white, because every offspring inherits the dominant B allele.
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