Form 5 · Common mistakes

Inheritance, common mistakes

The mistakes SPM students make on Inheritance, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Drawing a Punnett square with both parents' alleles listed along the same axis instead of one across the top and one down the side.	A Punnett square works by pairing one gamete from each parent in every box; putting both parents' alleles on the same axis does not represent the fertilisation of one parent's gamete with the other's, and produces a meaningless grid.	List one parent's gametes across the top of the grid and the other parent's gametes down the side before filling in each box.
Drawing a dihybrid Punnett square without first listing all four gamete types a double heterozygote can produce.	A double heterozygote such as PpIi produces four gamete types, PI, Pi, pI and pi, in equal proportion; skipping this step often leads to a grid with the wrong number of columns or rows and an incorrect ratio.	List all four gamete types for each parent systematically before drawing a 4 x 4 grid.
Assuming a genotype ratio such as 1:2:1 always converts directly into a matching phenotype ratio.	A 1:2:1 genotype ratio only becomes a 3:1 phenotype ratio when the heterozygote shows the same phenotype as one of the homozygotes, which is true for simple dominance but not for incomplete dominance, where the heterozygote instead shows its own blended phenotype.	Check whether the trait shows simple dominance, incomplete dominance or codominance before converting a genotype ratio into a phenotype ratio.
Assuming the individual with the recessive phenotype in a test cross is the one whose genotype is being investigated.	A test cross investigates an individual with the dominant phenotype but unknown genotype; the homozygous recessive partner is the known tool used to reveal that unknown genotype, not the subject of the investigation.	Identify the unknown individual in a test cross as the one showing the dominant phenotype.
Believing that a 9:3:3:1 ratio means there are 16 different genotypes among the offspring.	The number 16 refers to the number of equally likely boxes in a 4 x 4 Punnett square, not the number of distinct genotypes; a standard dihybrid cross of two double heterozygotes actually produces 9 distinct genotype combinations, which group into 4 phenotype classes.	Distinguish the 16 boxes of the Punnett square from the 9 genotypes and 4 phenotypes those boxes represent.
Treating an ABO blood group cross as involving only two alleles, as in a standard monohybrid cross.	The ABO gene has three possible alleles across the population, I^A, I^B and i, although any single individual carries only two of them; a cross can therefore need more than the two alleles used in a simple dominant-recessive problem.	Identify which two of the three possible ABO alleles each parent actually carries before working through the cross.
Assuming a person with blood group AB (genotype I^A I^B) always passes on both I^A and I^B to a child.	During gamete formation, a parent's two alleles for a gene separate from each other exactly as in any other cross, so an I^A I^B parent's gametes carry either I^A or I^B, never both together in the same gamete.	Apply the law of segregation to codominant alleles in the same way as to any other pair of alleles.
Concluding that a condition is sex-linked simply because more males than females in one family happen to show it.	A condition appearing in more males purely by chance in a single small family is not sufficient evidence of sex-linkage; sex-linkage should be confirmed by a consistent inheritance pattern across a full pedigree, such as an affected father never passing an X-linked recessive condition to his sons.	Use the specific pattern of transmission across a complete pedigree, not a simple headcount in one family, to identify sex-linked inheritance.
Assuming a shaded individual in a pedigree diagram is always homozygous for the responsible allele.	A shaded individual showing a recessive trait must indeed be homozygous recessive, but for a dominant trait a shaded individual could be either homozygous dominant or heterozygous, so genotype cannot always be read directly from a shaded symbol.	Only conclude a homozygous recessive genotype directly from a shaded symbol when the trait itself is already known to be recessive.
Assuming incomplete dominance always produces a 1:2:1 phenotype ratio regardless of which parents are crossed.	The 1:2:1 phenotype ratio results specifically from crossing two heterozygotes; crossing a heterozygote with a homozygote, for example, gives a different ratio, such as 1 pink : 1 red or 1 pink : 1 white.	State the actual parental cross being used before predicting the specific ratio it will produce.
Describing a carrier as someone who is mildly affected by the condition they carry.	A carrier of a recessive condition shows no trace of the condition at all, because the single dominant allele in the heterozygote is enough to produce a fully normal phenotype; there is no intermediate 'mild' phenotype in simple dominant-recessive inheritance.	State that a carrier's phenotype is completely normal and distinct from the phenotype of an affected individual.
Assuming the probability of a second child being affected is reduced because an earlier child was already affected.	Each pregnancy is an independent event with a probability determined solely by the parents' genotypes; the outcome of one pregnancy has no effect on the probability for the next.	Apply the same probability, such as one in four for two carrier parents, to each pregnancy independently, regardless of earlier outcomes.

How to avoid these mistakes

Learn the genetic terms and use them correctly.
Practise monohybrid crosses and read off the phenotype and genotype ratios.
Work through a dihybrid cross and explain the 9:3:3:1 ratio.
Work through a test cross example and explain what each possible outcome tells you about the unknown genotype.
Complete a Punnett square for the ABO blood group system involving I^A, I^B and i.
Practise reading a simple pedigree diagram and identifying whether a trait is likely dominant or recessive.
Draw a Punnett square for a cross between red- and white-flowered snapdragons and predict the phenotype ratio.
Calculate the probability that a child of two carrier parents inherits a recessive condition, using a Punnett square.

Frequently asked questions

What is the difference between genotype and phenotype?

The genotype is the pair of alleles an organism has for a characteristic, written with letters such as TT, Tt or tt. The phenotype is the characteristic that is actually shown, such as being tall or short. Two organisms can have different genotypes (TT and Tt) but the same phenotype (both tall), because T is dominant.

How do you predict the offspring of a monohybrid cross?

Write the genotypes of the two parents, work out the gametes each can produce, and combine them in a Punnett square. The square shows all the possible offspring genotypes and their proportions. From these you read off the phenotype ratio (often 3:1) and the genotype ratio (often 1:2:1) for a cross between two heterozygotes.

How is the sex of a baby determined?

Sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y (XY). All eggs carry an X, while sperm carry either an X or a Y. If an X-carrying sperm fertilises the egg the baby is female (XX); if a Y-carrying sperm does, the baby is male (XY). There is an equal chance of each.