A pedigree diagram lets you track a feature through a family and work out who must carry a hidden allele. Extended: some alleles are codominant, as in the ABO blood groups, and some genes sit on the X chromosome, which is why red-green colour blindness is far more common in males.
In a pedigree diagram, two unaffected parents with an affected child show that the allele is recessive and both parents are carriers. Extended: in codominance both alleles in a heterozygote contribute to the phenotype (IAIB is blood group AB). A sex-linked gene is on a sex chromosome, so the feature is more common in one sex, as with red-green colour blindness.
Fig. 1: a genetic diagram for red-green colour blindness: a carrier mother and a father with normal vision
Section 01
Reading a pedigree.
This family tree follows albinism, caused by a recessive allele (a). Squares are males, circles are females and dark symbols are affected people. Click each person to see their genotype, or press Reveal all.
Start with the affected people: for a recessive feature they must be homozygous recessive (aa).
Work outwards: each parent of an aa child must have given an a; each child of an aa parent must have received an a.
The key clue: two unaffected parents (I-1 and I-2) with an affected child (II-1) show the allele is recessive and both parents are heterozygous carriers (Aa).
Sometimes you cannot tell: II-2 is unaffected with Aa parents, so he could be AA or Aa.
Examiner trap
An unaffected person is not automatically AA. Look at their parents and children: an aa child or an aa parent proves they carry a.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the pedigree sim, what genotype must the affected people have?
Quick check+10 XP
I-1 and I-2 are unaffected but their daughter II-1 is affected. What does this show?
Quick check+10 XP
Extended: in the ABO sim, which genotype gives cells carrying both antigen A and antigen B?
Quick check+10 XP
Extended: in the sex linkage sim, choose mother XBXb and father XBY. What do the sons show?
Section 02
Codominance and blood groups.
Extended. Choose the two parents' genotypes. Yellow pyramids on the red blood cells are antigen A and blue cubes are antigen B.
Genotype
Blood group (phenotype)
IAIA or IAIo
A
IBIB or IBIo
B
IAIB
AB
IoIo
O
Codominance: both alleles in a heterozygous organism contribute to the phenotype. IA and IB are codominant, so IAIB cells carry both antigens: group AB.
Three alleles: IA, IB and Io. Io is recessive to both of the others, so only IoIo is group O.
All four groups at once: IAIo × IBIo gives 1 A : 1 AB : 1 B : 1 O.
Examiner trap
A person can only have TWO of the three alleles. Group A is not always IAIA: it can be IAIo, which is how two group A parents can have a group O child.
Section 03
Sex linkage: colour blindness.
Extended. Choose the mother's and father's genotypes and read the daughters and sons readouts. Tick the box to see the test plate as a colour-blind person sees it.
Sex-linked characteristic: a feature in which the gene responsible is located on a sex chromosome, which makes it more common in one sex than in the other.
Colour blindness: the gene for red-green colour vision is on the X chromosome; the Y carries no copy. XB (normal vision) is dominant to Xb (colour blind).
Males (XBY or XbY) have only one X, from their mother, so one recessive allele makes them colour blind.
Females need Xb from both parents (XbXb) to be colour blind; XBXb females are carriers with normal vision.
Carrier mother × normal father (XBXb × XBY): half the sons are colour blind; no daughters are, but half are carriers.
Examiner trap
A son can never inherit colour blindness from his father: a father passes his Y, not his X, to every son.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
In a pedigree, an unaffected man and woman have an affected son. The feature is caused by:
Exam style+20 XP
Extended: which pair of parents could have children of all four blood groups?
Exam style+20 XP
Extended: why is red-green colour blindness more common in males than in females?
Exam style+20 XP
Extended: a colour-blind woman (XbXb) has children with a man with normal vision (XBY). Which is correct?
Skill unlocked
Pedigrees, codominance and sex linkage, mastered.
This skill is now recorded on your profile. Keep the chain going.