Monohybrid Inheritance

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).

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The Language of Genetics

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:

• Because chromosomes come in pairs, you inherit two alleles of each gene — one from each parent. They may be the same (homozygous) or different (heterozygous).
• A dominant allele needs only one copy to show in the phenotype, so a heterozygous organism (e.g. Bb) shows the dominant characteristic. A recessive characteristic appears only when the organism is homozygous recessive (e.g. bb).

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).

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Genotype and Phenotype

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.

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Punnett Squares: Predicting a Cross

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:

1. Write the genotypes of the two parents.
2. Identify the gametes each parent can make — remember a gamete carries only one allele of the gene (because of meiosis).
3. Draw a 2 × 2 grid. Put one parent's gametes along the top and the other parent's down the side.
4. Fill each box by combining the allele from its row with the allele from its column.
5. Read off the genotypes and phenotypes, and write the ratio.

Worked Example 1 — A heterozygous cross (Bb × Bb)

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.

• Genotype ratio = 1 BB : 2 Bb : 1 bb.
• Phenotypes: BB, Bb and Bb are all brown (3 boxes); bb is white (1 box).
• Phenotype ratio = 3 brown : 1 white.

Answer: the expected offspring ratio is 3 brown : 1 white. As a fraction, $\frac{3}{4}$43​ are brown and $\frac{1}{4}$41​ are white; as a percentage, $75\%$75% brown and $25\%$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.

Worked Example 2 — A test cross (Bb × bb)

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.

• Genotype ratio = 2 Bb : 2 bb, which simplifies to 1 Bb : 1 bb.
• Phenotypes: Bb is brown (2 boxes); bb is white (2 boxes).
• Phenotype ratio = 1 brown : 1 white.

Answer: the expected offspring ratio is 1 brown : 1 white — that is $\frac{1}{2}$21​ ($50\%$50%) brown and $\frac{1}{2}$21​ ($50\%$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.

Worked Example 3 — A homozygous-dominant cross (BB × bb)

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.

• Genotype: all offspring are Bb (heterozygous).
• Phenotype: all are brown.

Answer: all the offspring are heterozygous Bb and brown ($100\%$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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Ratios, Fractions, Percentages and Probability