A pedigree chart is a puzzle with the answer hidden inside it. Nobody tells you whether the allele is dominant or recessive, or whether it sits on an autosome or the X — you have to deduce both from the pattern. There is a reliable method, and once you have it these become some of the most satisfying marks in the paper.
📘 What you need to know
A pedigree traces a characteristic, usually a disease, through the generations of a family, and lets you work out the probability that someone will inherit it.
Males are squares and females are circles. Affected individuals are shaded (or shown in a different colour).
A horizontal line between two individuals shows they have had children, who are linked underneath.
Roman numerals label generations; individuals are numbered within each generation, eldest on the left.
An affected child of two unaffected parents proves the allele is recessive.
An affected female with an unaffected father rules out X-linked recessive, so the gene is autosomal.
Reproducing with close relatives raises the chance both parents carry the same harmful recessive allele, which is why such marriages are restricted in many countries.
The conventions
The symbols are fixed, and using them correctly is worth marks in its own right if you are asked to complete a chart.
Feature
What it means
Square
Male
Circle
Female
Shaded or cross-hatched symbol
Individual affected by the condition
Unshaded symbol
Individual not affected (though possibly a carrier)
Horizontal line joining two symbols
A couple who have produced children
Vertical line dropping to a horizontal bar
The children of that couple, eldest on the left
Roman numeral at the side
The generation number
Reading a chart
Here is a three-generation family in which a condition appears twice. Everything you need to identify the type of inheritance is present, and it takes two observations to get there.
Individual II-1 is doing all the work here. She proves the allele is recessive (her parents are unaffected) and she rules out X-linkage (an X-linked recessive female would need an affected father).
Deducing the mode of inheritance
Rather than staring at the whole chart, look for the specific pieces of evidence below. Each one lets you eliminate a possibility, and two of them together are usually enough.
The first check is the strongest, because it is a proof rather than a suggestion. The fourth is only a hint — in a small family, an all-male pattern can happen by chance.
🧩 A method that works every time
Find an affected individual whose parents are both unaffected. If one exists, the allele is recessive and both parents are carriers.
If instead every affected person has an affected parent, and the condition appears in each generation, treat it as dominant.
Now test for sex linkage. For an X-linked recessive condition, an affected female must have an affected father. If she does not, the gene is autosomal.
Write the known genotypes onto the chart: every affected individual under a recessive model is homozygous recessive. That is your anchor.
Work outwards from there. An unaffected parent of an affected child must be heterozygous.
For anyone unaffected whose genotype is not forced, give both possibilities: homozygous dominant or heterozygous.
Step 6 catches people out. If an individual is unaffected and nothing in the chart forces the issue, the honest answer is “AA or Aa”. Writing a single genotype you cannot justify loses the mark; writing both, with a reason, gains it.
Why relatives are a genetic risk
Pedigree charts make plain something that is otherwise abstract. Harmful recessive alleles are usually rare, so two unrelated people are unlikely to carry the same one. Close relatives, however, share a recent common ancestor and therefore share a much higher proportion of their alleles.
That raises the chance that both partners are carriers of the same harmful recessive allele, which in turn raises the chance of a child inheriting two copies and being affected. This is the biological reason marriage between close relatives is prohibited in many countries.
Inductive and deductive reasoning
Pedigree work is a good illustration of two kinds of scientific reasoning, and the distinction sometimes appears in a nature-of-science question.
Deductive reasoning moves from known evidence to a specific conclusion. If two unaffected parents have an affected child, you can deduce that the condition is recessive and that both parents are carriers. That conclusion follows necessarily.
Inductive reasoning moves from a limited sample to a general conclusion. If you notice across several pedigrees that affected individuals are nearly always male, you might generalise that the condition is sex-linked. That is a reasonable inference, but it could be overturned by the next family you look at.
Worked examples
WORKED EXAMPLE
In a pedigree tracing albinism, two unaffected parents have a son with albinism. Deduce the type of allele responsible and explain your reasoning. [3]
Identify the informative individualsthe affected son and his two unaffected parentsReason from themthe allele cannot be dominant, because a dominant allele would show in at least one parentso the son must have inherited one recessive allele from each parent, making both parents carriersState the conclusionAlbinism is caused by a recessive allele; the parents are both heterozygousif females are affected elsewhere in the chart, add that it is autosomal rather than X-linked
WORKED EXAMPLE
In the same pedigree, state the possible genotypes of an unaffected sister of the affected son, and explain why more than one is possible. [2]
Fix what is certainboth parents are Aa, so their children can be AA, Aa or aaUse the information you are givenshe is unaffected, so she is not aa; she must have at least one dominant alleleAA or Aa — there is no way to distinguish them from her phenotype aloneamong unaffected children of two carriers, two out of three are heterozygous
💡 Exam tips
Annotate the chart itself. Write genotypes onto every symbol you can be certain about — the affected ones first, since they are fixed.
Give both possible genotypes when the chart does not determine one, and say why.
Justify the mode of inheritance by naming the individuals that prove it: “II-1 is affected but her parents are not”.
Watch the sex of affected individuals. Roughly equal numbers of affected males and females points to autosomal inheritance.
“Deduce” and “explain” both want the reasoning written out. The conclusion alone rarely gets full marks.
⚠️ Common mix-ups
Assuming a shaded symbol means a carrier. Shading means affected. Carriers look exactly like unaffected individuals on a chart.
Swapping the symbols. Squares are male, circles are female. Getting this backwards can invert the whole deduction.
Concluding “sex-linked” just because more males are affected. In a small family that can easily be chance. Look for a female who rules it out.
Giving one genotype when two are possible. Unaffected individuals are often ambiguous, and the mark is for recognising that.
Reading the horizontal line as a sibling link. A horizontal line between two symbols is a couple; siblings hang from a bar below.
Forgetting that a condition can skip a generation. That is the signature of recessive inheritance, not an error in the chart.
Up next: Continuous Variation — what happens when a characteristic is controlled by many genes at once, and the statistics you need to describe it.
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