A genetic diagram is a prediction machine: write the parents' genotypes, list every type of gamete, combine them in a Punnett square and read off the ratio. The same method handles dominance, codominance, multiple alleles, sex linkage, test crosses, linkage and epistasis.
A gene is a length of DNA at a fixed position, its locus, coding for a polypeptide; alleles are different versions of it. The genotype is the alleles present (homozygous if the two are the same, heterozygous if different) and the phenotype is the observable feature. A dominant allele is expressed in the heterozygote; a recessive one only when homozygous; codominant alleles are both expressed. In a genetic diagram, write parental phenotypes and genotypes, gametes (one allele of each gene per gamete), a Punnett square, then offspring genotypes, phenotypes and ratio. A monohybrid cross of two heterozygotes gives 3 : 1; a dihybrid cross of two double heterozygotes gives 9 : 3 : 3 : 1 if the genes assort independently. A test cross with a homozygous recessive reveals an unknown genotype. Autosomal linkage (genes on the same chromosome) and epistasis (one gene affecting the expression of another) change the expected ratios.
Fig. 1: a sex-linked cross for haemophilia; alleles are written as superscripts on the X chromosome
Section 01
The vocabulary and the monohybrid cross.
Every genetics answer starts with the same words. Learn them precisely; the examiners do.
Term
Meaning
gene / locus
a length of DNA coding for a polypeptide / the fixed position of a gene on a chromosome
allele
one of two or more alternative forms of a gene
dominant / recessive
expressed in the phenotype even when only one copy is present / expressed only when no dominant allele is present
codominant
both alleles in a heterozygote are fully expressed (for example HbAHbS, or blood group AB)
genotype / phenotype
the alleles an organism has / its observable or measurable characteristics
homozygous / heterozygous
two identical alleles of a gene / two different alleles
F1 / F2
offspring of the parental cross / offspring of crossing F1 individuals together
test cross
crossing an organism of unknown genotype with a homozygous recessive
linkage
genes on the same chromosome, which tend to be inherited together
Codominance: use a capital gene letter with superscripts. HbAHbS × HbAHbS gives 1 unaffected : 2 sickle cell trait : 1 sickle cell anaemia, because the heterozygote has its own phenotype.
Multiple alleles: the ABO gene has three alleles, IA and IB (codominant) and Io (recessive to both). IAIo × IBIo can give all four blood groups, A, B, AB and O, in a 1 : 1 : 1 : 1 ratio.
Sex linkage: a gene on the X chromosome (with no matching locus on the Y). Males have one X, so a single recessive allele shows in a male. Write alleles as superscripts: XHXh, XhY (Fig. 1).
Examiner trap
Write a full genetic diagram: parental phenotypes, parental genotypes, gametes (one allele of each gene), Punnett square, offspring genotypes and phenotypes, ratio. For sex-linked genes always attach the allele to the X, and give the Y no allele.
Section 02
Two genes at once: 9 : 3 : 3 : 1 and the test cross.
Choose 9:3:3:1 and read the PHENOTYPE RATIO for RrYy × RrYy peas (round R, wrinkled r, yellow Y, green y). Then choose test cross and see what RrYy × rryy gives.
Gametes: each gamete gets one allele of each gene. RrYy makes RY, Ry, rY and ry in equal numbers when the genes are on different chromosomes (independent assortment).
F2 of a dihybrid cross: RrYy × RrYy fills a 4 × 4 Punnett square: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
Test cross: cross with the homozygous recessive (rryy), whose gametes are all ry, so each offspring phenotype shows exactly which gamete came from the other parent.
Unknown parent (round yellow)
Test cross offspring
RRYY
all round yellow
RrYY
1 round yellow : 1 wrinkled yellow
RRYy
1 round yellow : 1 round green
RrYy
1 : 1 : 1 : 1 (all four phenotypes)
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 simulator, what phenotype ratio does RrYy × RrYy give?
Quick check+10 XP
Why does the test cross in the simulator give 1 : 1 : 1 : 1?
Quick check+10 XP
With the linkage option, which gametes does each RrYy parent mainly make?
Quick check+10 XP
In the epistasis example, why are all cc mice albino?
Section 03
When the ratio breaks: linkage and epistasis.
Back in the simulator, choose linkage, then epistasis, and watch the phenotype ratio change.
Autosomal linkage: two genes close together on the same (non-sex) chromosome. If R and Y are on one homologue and r and y on the other, RrYy makes mainly RY and ry gametes, so RrYy × RrYy gives close to 3 round yellow : 1 wrinkled green instead of 9 : 3 : 3 : 1.
Recombinants: crossing over between the loci makes a few Ry and rY gametes, so small numbers of the other two phenotypes appear. In a linked test cross the parental phenotypes are common and the recombinants rare.
Showing linkage in a diagram: write the alleles on each chromosome together, for example RY/ry, and show only the parental gametes unless crossing over is mentioned.
Epistasis: one gene affects the expression of another gene. In the mouse example, cc mice cannot make any pigment, so they are albino whatever alleles of gene A they carry; AaCc × AaCc gives 9 agouti : 3 black : 4 albino.
Exam approach: you are not expected to memorise epistatic ratios. Build the 16-box Punnett square, then use the information given to assign each genotype a phenotype and count.
Examiner trap
Linkage means two genes on the same chromosome; sex linkage means one gene on the X chromosome. Do not use the words interchangeably.
Stage 2 · Exam
Exam-style questions
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Exam style+20 XP
A woman with blood group A, whose father was group O, has children with a man of blood group AB. What is the probability that their first child has blood group B?
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Red-green colour blindness is sex-linked recessive. A colour-blind man and a woman homozygous for normal vision have children. Which statement is correct?
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A test cross of a double heterozygote gives 44 AB, 6 Ab, 5 aB and 45 ab phenotypes. What is the best explanation?
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Two pea plants heterozygous for flower colour (Pp, purple dominant) are crossed. What fraction of the purple offspring are homozygous?
Skill unlocked
Monohybrid and dihybrid crosses, mastered.
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