Key Terms in Genetics

Before the six key terms make sense, you need two ideas already familiar from earlier in the genetics topic.

• A gene is a section of DNA that controls a characteristic (such as eye colour).
• Most genes come in slightly different versions called alleles. For eye colour there might be a "brown" allele and a "blue" allele of the same gene.

You inherit two copies of each gene — one from each parent — so you have two alleles for each characteristic. These two alleles may be the same or different.

How we write alleles. Geneticists use a clever shorthand so that you can see at a glance which allele is dominant:

• the dominant allele is written as a capital (upper-case) letter, e.g. B;
• the recessive allele is written as the same letter in lower case, e.g. b.

We always use the same letter for both alleles of one gene, because they are versions of the same gene. So for eye colour we might choose:

Symbol	Allele	Type
B	brown	dominant
b	blue	recessive

Because you have two alleles, the possible combinations a person could have are BB, Bb or bb.

Exam tip. Always pick a letter where the capital and lower-case look clearly different (B and b are good; S and s are risky because they look almost the same). Write capitals clearly larger than lower-case so the examiner is never in doubt.

These two terms describe how an allele behaves when it is paired with a different allele.

A dominant allele is one that shows its effect even when only one copy is present. So if a person has the alleles Bb, the dominant brown allele (B) is expressed and the person has brown eyes — the single recessive b is "masked" or hidden.

A recessive allele is one that only shows its effect when two copies are present (i.e. when no dominant allele is there to hide it). So only a person with bb has blue eyes.

Putting the three genotypes together for our eye-colour example:

Genotype	Alleles present	Eye colour shown
BB	two dominant	brown
Bb	one of each	brown (B is dominant)
bb	two recessive	blue

Notice that both BB and Bb give brown eyes — one dominant allele is enough. Blue eyes need bb, two recessive alleles.

This is why recessive characteristics can seem to "skip" a generation: two brown-eyed parents who are each Bb can have a bb blue-eyed child, because each parent can pass on their hidden b allele.

Exam tip. Define dominant as "an allele that is expressed / shows even if only one copy is present" and recessive as "an allele that is only expressed when two copies are present". The words "only when two copies are present" are what secure the recessive mark.

These two terms describe whether the two alleles are the same or different. The word roots help:

• homo- means "same" → homozygous = the two alleles are the same;
• hetero- means "different" → heterozygous = the two alleles are different.

So for eye colour:

Genotype	Same or different alleles?	Term
BB	two dominant — the same	homozygous (dominant)
bb	two recessive — the same	homozygous (recessive)
Bb	one of each — different	heterozygous

A few useful consequences to understand:

• An organism that is homozygous recessive (bb) is the only genotype that shows the recessive characteristic (blue eyes).
• A heterozygous (Bb) organism shows the dominant characteristic but carries a hidden recessive allele — it is sometimes called a carrier.
• You can be homozygous dominant (BB) or heterozygous (Bb) and look exactly the same (both brown). You cannot tell them apart just by looking — only by their offspring or genotype.

A quick diagram to fix the words in your memory:

Exam tip. Tie the definition to the word root: homozygous = two identical alleles, heterozygous = two different alleles. Don't say "homozygous = dominant" — you can be homozygous recessive (bb) too.

These last two terms are the ones examiners test most often — and the ones students mix up most often.

• The genotype is the alleles an organism has for a characteristic — written in letters, e.g. BB, Bb or bb.
• The phenotype is the physical characteristic that you can actually observe — the result of the genotype, e.g. brown eyes or blue eyes.

A simple way to remember it: genotype = the letters; phenotype = the photo. The genotype is the genetic "code" inside; the phenotype is what you'd see in a photograph.

The key relationship to understand is that different genotypes can produce the same phenotype:

Genotype (alleles present)	Phenotype (characteristic seen)
BB	brown eyes
Bb	brown eyes
bb	blue eyes

Here BB and Bb are two different genotypes but the same phenotype (brown), because the dominant B allele is expressed whether there is one copy or two. Only bb gives the blue phenotype.

This is exactly why a worded question might say "a brown-eyed person could have the genotype BB or Bb" — from the phenotype alone you cannot be sure which. But a blue-eyed person must be bb, because that is the only genotype that gives the blue phenotype.

Exam tip. Whenever you are asked to give a genotype, answer in letters (BB, Bb, bb). Whenever you are asked for a phenotype, answer in words describing what is seen (brown eyes, blue eyes). Mixing the two up — giving "Bb" when asked for a phenotype — throws marks away.

Let's describe one person with brown eyes whose genotype is Bb, using all six terms at once. This is exactly the kind of "describe using genetic terms" question that appears on Paper 1.

For the genotype Bb:

1. The person has two alleles: B (brown) and b (blue).
2. B is the dominant allele — it shows its effect even though there is only one copy.
3. b is the recessive allele — it is hidden here because there is only one copy.
4. Because the two alleles are different, the person is heterozygous.
5. The genotype is Bb (the alleles present).
6. The phenotype is brown eyes (the characteristic you can see, because the dominant B is expressed).

Now compare with a blue-eyed person, genotype bb:

• Both alleles are recessive (b) and the same, so the person is homozygous recessive.
• The genotype is bb; the phenotype is blue eyes.
• This is the only genotype that gives the blue phenotype, because the recessive allele only shows when there are two copies.

A handy summary table linking every term:

Term	Means	Eye-colour example
Dominant	allele shown even with one copy	B (brown)
Recessive	allele shown only with two copies	b (blue)
Homozygous	two alleles the same	BB or bb
Heterozygous	two alleles different	Bb
Genotype	the alleles present	BB, Bb or bb
Phenotype	the characteristic seen	brown or blue eyes

Exam tip. Practise writing all six definitions from memory with the B/b example next to each. In the exam, a single eye-colour example sentence (e.g. "B = dominant brown, b = recessive blue") lets you answer almost any 3.20 question quickly and correctly.