Monohybrid inheritance — Cambridge IGCSE Biology
Genetic terms, genetic diagrams and pedigrees, the test cross, codominance and ABO blood groups, and sex linkage.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Genotype is the genetic make-up of an organism in terms of the alleles present, while phenotype is its observable features. Homozygous means two identical alleles, such as TT or tt, and heterozygous means two different alleles, Tt. A dominant allele is expressed if present; a recessive allele is only expressed when no dominant allele is present.
A monohybrid cross shows the inheritance of one characteristic controlled by a single gene, using a genetic diagram or Punnett square of the parents' gametes. Crossing two heterozygous parents gives a 3 : 1 phenotypic ratio, and heterozygous crossed with homozygous recessive gives 1 : 1. Pedigree diagrams trace a characteristic through a family to show which allele is dominant.
The test cross
SupplementA test cross finds the unknown genotype of an organism showing the dominant phenotype by crossing it with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous, giving about 1 : 1. If all offspring show the dominant phenotype, the unknown parent is probably homozygous dominant.
Codominance and ABO blood groups
SupplementCodominance is when both alleles in a heterozygous organism contribute to the phenotype. The ABO blood group is controlled by one gene with three alleles, IA, IB and Io. IA and IB are codominant, so a person with IA IB has blood group AB. Io is recessive to both, so group O needs two Io alleles.
Sex linkage and colour blindness
SupplementA sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome, making it more common in one sex. Red-green colour blindness is caused by a recessive allele on the X chromosome. Males have only one X, so one recessive allele makes them colour blind. Females with one copy are carriers.
Exam questions
7 exam-style questions (27 marks), each with its mark scheme.
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Quick check questions
Define phenotype.
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The observable features of an organism.
What is a heterozygous genotype?
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Two different alleles of a particular gene, for example Tt.
Two parents without a condition have a child with it. Is the allele dominant or recessive?
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Recessive, and both parents are heterozygous carriers.
Supplement Which genotype is used as the tester in a test cross?
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Homozygous recessive.
Supplement Why is red-green colour blindness more common in males?
Show answer
Males have one X chromosome, so one recessive allele is enough. Females need two.
Frequently asked questions
What is a genotype and phenotype?
A genotype is the genetic make-up of an organism in terms of the alleles present, such as Tt. A phenotype is the observable features of an organism, such as being tall. A heterozygous Tt pea plant has the tall phenotype, because the dominant allele T is expressed even though a recessive allele is present.
What is a Punnett square used for in genetics?
A Punnett square is used to show the possible offspring of a genetic cross. The gametes of one parent go along the top and the other along the side, and the boxes show the possible offspring genotypes. From these you work out the phenotypes and the ratio, such as 3 : 1 or 1 : 1.
How do you find out if an organism is homozygous or heterozygous?
Supplement You use a test cross: cross the organism showing the dominant phenotype with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous. If all the offspring show the dominant phenotype, it is probably homozygous dominant. A large number of offspring gives a more reliable result.
What is codominance?
Supplement Codominance is when both alleles in a heterozygous organism contribute to the phenotype, so neither is hidden and they do not blend. The ABO blood group is an example. The alleles IA and IB are codominant, so a person with genotype IA IB has blood group AB. The allele Io is recessive to both.
Why is colour blindness more common in males?
Supplement Red-green colour blindness is more common in males because the recessive allele is on the X chromosome, and males have only one X. So one recessive allele is enough to make a male colour blind. Females have two X chromosomes and need two recessive alleles; with one, they are carriers with normal colour vision.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026