Skip to main content
SingaporeBiologySyllabus dot point

How are single-gene traits inherited, and how do we predict the outcomes of a monohybrid cross?

Explain monohybrid inheritance using genetic diagrams, including dominant, recessive, codominant and sex-linked alleles

A focused answer to the H2 Biology Inheritance and Evolution outcome on monohybrid inheritance. Alleles, genotype and phenotype, dominant and recessive inheritance, codominance, sex linkage, and how to construct genetic diagrams and predict ratios.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to explain how single-gene (monohybrid) traits are inherited and to construct genetic diagrams that predict offspring genotypes, phenotypes and ratios. You need the key terms (allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive), and the variations of codominance and sex linkage. This is the foundation for dihybrid crosses and population genetics.

The answer

The key terms

  • A gene is a length of DNA coding for a characteristic; an allele is a version of that gene.
  • Genotype is the alleles an organism carries; phenotype is the observable characteristic.
  • Homozygous means two identical alleles; heterozygous means two different alleles.
  • A dominant allele is expressed in the phenotype even when heterozygous; a recessive allele is expressed only when homozygous.

A monohybrid cross

To predict offspring: write the parental genotypes, identify the gametes each produces, combine the gametes (often with a Punnett square), then read off the genotypes and phenotypes and their ratio. A cross between two heterozygotes (Aa x Aa) gives the classic 3:1 phenotypic ratio.

Codominance

In codominance, both alleles are expressed in the heterozygote, so neither masks the other. Crossing two heterozygotes for a codominant pair gives a 1:2:1 phenotypic ratio, because the heterozygote has its own distinct phenotype.

Sex linkage

A sex-linked gene lies on a sex chromosome, usually the X. A male (XY) has only one X, so a single recessive sex-linked allele is expressed; a female (XX) needs two copies. This is why recessive sex-linked conditions are commoner in males.

Examples in context

Example 1. The ABO blood groups. The A and B alleles are codominant with each other and both dominant to O. This single gene produces four blood group phenotypes, illustrating codominance and multiple alleles in one familiar human example.

Example 2. The test cross. To find whether an organism with a dominant phenotype is homozygous or heterozygous, it is crossed with a homozygous recessive. All dominant offspring suggest homozygous; a 1:1 ratio reveals a heterozygote. This shows how a cross deduces an unknown genotype.

Try this

Q1. Define the term heterozygous. [1 mark]

  • Cue. Having two different alleles of a gene at the same locus on homologous chromosomes.

Q2. A cross between two heterozygous tall pea plants (Tt x Tt) is carried out, where T (tall) is dominant to t (short). State the expected ratio of tall to short offspring. [1 mark]

  • Cue. 3 tall to 1 short.

Q3. Explain why a female can be an unaffected carrier of a recessive sex-linked condition but a male usually cannot. [2 marks]

  • Cue. A female has two X chromosomes, so a dominant allele on one X can mask a recessive allele on the other; a male has only one X, so a single recessive allele is expressed with nothing to mask it.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksIn a species, allele R for red flowers is dominant to allele r for white flowers. A heterozygous red-flowered plant is crossed with a white-flowered plant. Construct a genetic diagram to show the expected genotypes and phenotypes of the offspring and their ratio.
Show worked answer →

Examiners want correct symbols, gametes, a cross, and the ratio.

Parental genotypes: the heterozygous red plant is Rr; the white plant is rr.

Gametes: the Rr parent produces gametes R and r; the rr parent produces gametes r and r.

Combining the gametes gives offspring Rr and rr (in equal proportion). Rr plants are red (R is dominant) and rr plants are white.

The expected ratio is 1 red (Rr) to 1 white (rr), so half the offspring are red and half white.

Markers reward correct parental genotypes, correctly identified gametes, a clear combination (a Punnett square or gamete lines), the offspring genotypes, and the 1:1 phenotypic ratio.

Original4 marksHaemophilia is caused by a recessive allele carried on the X chromosome. Explain why haemophilia is much more common in males than in females.
Show worked answer →

The answer should use the inheritance of sex chromosomes.

The allele for haemophilia is recessive and sex-linked, carried on the X chromosome, with no corresponding allele on the smaller Y chromosome.

A male has only one X chromosome (XY), so if his single X carries the recessive allele he has no second X with a dominant allele to mask it, and he shows the condition. He needs only one copy.

A female has two X chromosomes (XX), so she must inherit two copies of the recessive allele (one on each X) to be affected. With only one copy she is an unaffected carrier. Because inheriting two copies is much less likely than inheriting one, females are affected far less often.

Markers reward the recessive sex-linked allele on the X with none on the Y, the single X in males meaning one copy suffices, and the need for two copies in females making them rarely affected.

Related dot points