Family Pedigrees

A family pedigree (sometimes called a pedigree diagram or simply a family tree) is a diagram that shows how a single inherited characteristic — usually a genetic disorder — has been passed down through several generations of one family.

Each person in the family is drawn as a symbol. Lines join the symbols together to show who is married/partnered to whom and who their children are. By looking at the pattern of affected and unaffected people, you can work out useful genetic information, such as:

• whether the allele that causes the feature is dominant or recessive, and
• the likely genotype of each individual in the family.

The generations are usually drawn as rows, with the oldest generation at the top. These rows are often labelled with Roman numerals (I, II, III), and the individuals within a row may be numbered (1, 2, 3 …) so each person can be referred to exactly (e.g. "individual II-3").

Exam tip. Read the question carefully to find out what the shaded symbols mean — the key (legend) on the diagram tells you which feature "affected" refers to (for example colour blindness, cystic fibrosis or a particular flower colour).

To read a pedigree you must know what each symbol and line means. There is a small, fixed set to learn:

Symbol / line	Meaning
Circle ⃝	A female
Square ☐	A male
Shaded (filled-in) symbol	An affected individual (has the feature/disorder)
Unshaded (empty) symbol	An unaffected individual (does not show the feature)
Horizontal line between a circle and a square	A couple — the two parents
Vertical line dropping down from a couple	Links the parents to their offspring (children)

Here is a simple worked pedigree. The shaded symbols show people affected by a recessive disorder.

Exam tip. Lock in the two shape rules with a memory trick: a circle is "round like a female symbol ♀", a square is the male. Mixing up circle and square is one of the easiest marks to throw away.

The most important skill in this topic is using a pedigree to decide whether the allele for the feature is dominant or recessive. There is one golden rule that almost always cracks it:

If two UNAFFECTED parents have an AFFECTED child, the allele for the feature must be RECESSIVE.

Why? If the feature were dominant, then any individual carrying even one copy of the allele would show it. So an affected child could only be born to a parent who is also affected. If both parents are unaffected but the child is affected, the allele cannot be dominant — it must be recessive, "hidden" in the parents.

The opposite clue points to a dominant allele:

If the feature appears in every generation and an affected child always has at least one affected parent, the allele is likely dominant.

A neat way to remember the recessive rule is the phrase "two clear parents, an affected child → recessive". Examiners love this exact situation because it is the clearest piece of evidence in the whole diagram.

Exam tip. Always point to the specific people in your answer — for example, "individuals I-1 and I-2 are unaffected but their child II-2 is affected, so the allele must be recessive." Naming the individuals shows the examiner you have used the evidence, not just guessed.

Once you know whether the allele is dominant or recessive, you can work out the genotype of individuals. Choose a letter pair, using a capital for the dominant allele and the same letter in lower case for the recessive allele. For a recessive disorder we often use B (unaffected, dominant) and b (affected, recessive).

Work through the people like this:

1. Affected individuals (recessive disorder). They must be homozygous recessive — they have two recessive alleles, e.g. bb. This is the only way a recessive feature can show.
2. Unaffected parents of an affected child. They do not show the feature, so they have at least one dominant allele, but they must each have passed on a recessive allele to make the affected (bb) child. So they must be heterozygous carriers, e.g. Bb.
3. Other unaffected individuals. They have at least one dominant allele, so they are BB or Bb. Sometimes the pedigree gives enough information to be sure which; sometimes you can only say "BB or Bb".

A carrier is an unaffected person who is heterozygous (Bb) — they carry one recessive allele but do not show the feature. Carriers are crucial because two carriers can have an affected child.

You can confirm a cross using a simple genetic diagram (Punnett square). For two carrier parents (Bb × Bb):

	B	b
B	BB	Bb
b	Bb	bb

This gives BB : Bb : bb = 1 : 2 : 1, so 3 unaffected : 1 affected — a ¼ (25%) chance each child is affected (bb), and ¾ unaffected, of whom 2 in 3 are carriers.

Exam tip. When you cannot be certain, write "BB or Bb" rather than guessing one — examiners accept the honest answer and you keep the mark. For an affected person with a recessive disorder, always write the homozygous recessive genotype (bb).