A pedigree chart is a family tree with the genetics left in. Given one, you can usually work out whether a condition is dominant or recessive, whether it is sex-linked, and the genotype of almost everyone on the page — without a single calculation.
📚 What you need to know
Squares are males, circles are females. Shaded (or filled) means affected; unshaded means unaffected.
A horizontal line between two people means they had children; the children hang from a line beneath.
Generations are numbered with Roman numerals (I, II, III) and individuals are numbered left to right.
If two unaffected parents have an affected child, the allele must be recessive.
If affected children appear in every generation and always have an affected parent, the allele is probably dominant.
If nearly all the affected people are male, suspect X-linked recessive. If both sexes are affected about equally, it is probably autosomal.
Reading the symbols
Every pedigree uses the same shorthand, so learn it once and you can read any of them.
Symbol
Meaning
Square
Male
Circle
Female
Shaded shape
Affected — shows the condition
Unshaded shape
Unaffected — does not show the condition
Horizontal line joining two shapes
A couple who have had children
Vertical line down to a horizontal sibling line
Their children, eldest on the left
Individual II-4 must be homozygous recessive, so I-1 and I-2 are both heterozygous carriers — even though neither shows the condition.
How to work out the pattern
🧩 A method that always works
Look for affected children with unaffected parents. If you find even one, the allele is recessive — the parents were hiding it. This is the fastest clue on the page.
If every affected person has an affected parent and the condition never skips a generation, the allele is probably dominant.
Count affected males and females. Roughly equal numbers point to an autosome. Mostly males points to the X chromosome.
Fill in every affected person’s genotype first — for a recessive condition they must be homozygous recessive.
Work outwards to the parents. Every allele in a child came from a parent, so an unaffected parent of an affected child has to be a carrier.
Write “or” where you cannot be certain. An unaffected person in a recessive pedigree is often AA or Aa, and saying so is the correct answer.
Do not guess when the evidence runs out. If a pedigree cannot tell you whether someone is AA or Aa, write both and say why. That is a full-mark answer — picking one and hoping is not.
Why the X-linked clue works. A male has only one X. If the recessive allele is on it, nothing masks it and he is affected. A female needs two copies, which is much less likely. So a pedigree where almost all the shaded shapes are squares is a strong hint at X-linked recessive inheritance.
Worked examples
WORKED EXAMPLE 1
In the pedigree above, deduce whether the allele causing the condition is dominant or recessive, and state the genotypes of I-1, I-2 and II-4.
Step 1: find the giveaway
I-1 and I-2 are both unaffected, but their son II-4 is affected
Step 2: reason it throughA dominant allele always shows — so if it were dominant, at least one parent would be affected. It is not. So the allele is recessive.Step 3: genotype of the affected son
Affected + recessive means aaStep 4: trace his alleles back
He got one a from each parent, but neither parent shows the condition, so each must also carry A
Recessive. II-4 is aa; I-1 and I-2 are both AaII-3 and II-5 are unaffected, so they are AA or Aa — the chart cannot narrow it down.
WORKED EXAMPLE 2
In a different family, six people are affected by a condition: five males and one female. Explain what this suggests, and state what would need to be true for the affected female to exist.
Step 1: what does the sex bias mean?
Far more affected males than females suggests the gene is on the X chromosome and the allele is recessive
Step 2: explain why
A male has one X and no matching allele on the Y, so a single recessive allele is enough to affect him
Step 3: the affected female
She has two X chromosomes, so she needs two copies of the recessive allele — XaXaStep 4: where do those come from?
One from each parent, so her father must be affected and her mother must be a carrier or affected
X-linked recessive; the affected female must be XaXa with an affected fatherSay “suggests”, not “proves”. A sex bias is strong evidence, but a small family can look biased by chance.
Why pedigrees matter beyond the exam
Genetic counsellors use pedigrees to estimate the chance that a couple’s child inherits a condition, using exactly the reasoning above.
They also explain a wider point. Close relatives share more alleles than strangers do, including rare harmful recessive ones. If both partners carry the same rare recessive allele, the chance of a child inheriting two copies rises sharply. That is the biological reason many countries restrict marriage between close relatives.
📚 Inductive and deductive reasoning
Deductive reasoning goes from a general rule to a specific case: two unaffected parents with an affected child means the allele is recessive and both parents are carriers.
Inductive reasoning goes the other way — from specific observations to a general conclusion: seeing affected males in family after family suggests the condition is sex-linked.
Reading a pedigree uses both, and IB may ask you to name which one you used.
💡 Exam tip
Annotate the chart itself. Pencil the genotypes onto the shapes as you go — it is much faster than holding them in your head.
Start with the affected individuals in a recessive pedigree; they are the only certain genotypes.
Give genotypes as “Aa or AA” when both are possible. That is the right answer, not a cop-out.
Use “explain” properly. Say which individuals prove your point, by number.
Check the sexes of the affected people before deciding autosomal or sex-linked.
Use the letters the question gives you. If it says use A and a, do not switch to B and b.
⚠ Common mix-up
Reading circles as males. Circles are female, squares are male — check the key every time.
Deciding it is dominant just because the condition looks common in that family. Look for the skipped generation instead.
Assuming shaded always means affected. Some charts shade the opposite way; the key is there for a reason.
Giving one genotype where two are possible. That loses the mark even if you happen to pick the right one.
Ignoring a person who married into the family. They bring in alleles from outside, and they are often the source of a new one.
Calling a male a carrier for an X-linked condition. He is affected, not a carrier.
Up next: Continuous Variation (Skills) — discrete versus continuous variation, polygenic features, and how to draw and read a box plot.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
