Inheritance
Inheritance is how characteristics are passed from parents to offspring through genes. This chapter covers monohybrid and dihybrid inheritance, genes and alleles, and inheritance in humans such as sex determination and blood groups.
Punnett squares and correct genetic terms are essential, the genetics tool pages give extra practice.
A test cross is used to find out whether an individual showing the dominant phenotype is homozygous or heterozygous: it is crossed with a homozygous recessive individual, and the ratio of phenotypes among the offspring reveals the unknown genotype. If all offspring show the dominant phenotype, the tested individual was homozygous dominant; if about half show the recessive phenotype, it was heterozygous.
Not every characteristic follows a simple dominant-recessive pattern. The ABO blood group system shows codominance and multiple alleles: the alleles IA and IB are both fully expressed when present together, giving blood group AB, while the allele i is recessive to both.
Some conditions, such as red-green colour blindness and haemophilia, are sex-linked, carried on the X chromosome, which is why they occur far more often in males than in females.
Incomplete dominance is different again: neither allele is fully dominant, so the heterozygote shows a blended or intermediate phenotype. A cross between red-flowered and white-flowered snapdragons, for example, produces pink-flowered offspring, because neither the red nor the white allele completely masks the other.
A person who carries one copy of a recessive allele for a condition, without showing the condition themselves, is called a carrier. Two carrier parents can each pass on either their dominant or their recessive allele, so predicting the chance that a particular child inherits the condition uses exactly the same Punnett square logic as any other monohybrid cross.
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Content standards in this chapter
- 26.1 Monohybrid Inheritance
- 26.2 Dihybrid Inheritance
- 26.3 Genes and Alleles
- 26.4 Inheritance in Humans
Key concepts
- Genes and alleles
- A gene is a section of DNA that codes for a characteristic; alleles are its different versions, one from each parent.
- Dominant and recessive
- A dominant allele shows in the phenotype whenever present; a recessive allele shows only when both alleles are recessive.
- Genotype and phenotype
- The genotype is the alleles an organism has; the phenotype is the characteristic that shows.
- Monohybrid inheritance
- The inheritance of a single characteristic, worked out with a Punnett square to predict ratios.
- Dihybrid inheritance
- The inheritance of two characteristics at once, giving a 9:3:3:1 ratio in a typical cross.
- Inheritance in humans
- Sex is determined by the X and Y chromosomes; blood groups and some conditions follow specific inheritance patterns.
- Test cross
- Crossing an organism showing a dominant phenotype with a homozygous recessive individual reveals whether the unknown parent is homozygous or heterozygous, based on whether any offspring show the recessive phenotype.
- Codominance and multiple alleles
- In codominance, two different alleles are both fully expressed in the heterozygote, as seen in the ABO blood group system, where a gene has three possible alleles, IA, IB and i, rather than just two.
- Sex-linked inheritance
- A characteristic controlled by a gene on the X chromosome, such as red-green colour blindness or haemophilia, is called sex-linked; because males have only one X chromosome, a single recessive allele is enough to produce the recessive phenotype in a male.
- Pedigree diagram
- A pedigree is a family tree showing which relatives have a particular characteristic or condition, used to trace how a trait, especially a sex-linked or recessive one, has been inherited across generations.
- Incomplete dominance
- Unlike simple dominance, neither allele is fully expressed over the other, so a heterozygote shows an intermediate phenotype, such as pink flowers from a cross between red and white parents.
- Carrier
- A carrier has one dominant and one recessive allele for a condition and does not show the condition, but can pass the recessive allele on to offspring, who may show the condition if they inherit a recessive allele from both parents.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Completing a Punnett square and giving the genotype and phenotype ratios.
- Explaining sex determination or blood group inheritance.
- Defining genetic terms precisely.
- Interpreting a test cross to determine an unknown genotype.
- Completing a Punnett square for the ABO blood group system or another codominant trait.
- Explaining why a sex-linked recessive condition is more common in males than females.
- Distinguishing incomplete dominance from codominance using a described cross.
- Calculating the probability that a child of two carrier parents will show a recessive condition.
Common mistakes
What students write: Confusing genotype and phenotype.
What earns the mark: Genotype is the alleles (e.g. Tt); phenotype is the visible characteristic (e.g. tall).
What students write: Writing a recessive trait shows with one recessive allele.
What earns the mark: A recessive trait shows only when both alleles are recessive (e.g. tt).
What students write: Giving a 3:1 ratio as a genotype ratio.
What earns the mark: A monohybrid cross gives a 3:1 phenotype ratio but a 1:2:1 genotype ratio.
What students write: Using the same letter cases carelessly.
What earns the mark: Use a capital letter for the dominant allele and the same letter in lower case for the recessive one, e.g. T and t.
What students write: Assuming a test cross always uses another individual with the dominant phenotype.
What earns the mark: A test cross always uses a homozygous recessive individual, since only this genotype reliably reveals the unknown parent's alleles from the offspring ratio.
What students write: Writing that blood group AB shows one allele is dominant over the other.
What earns the mark: Blood group AB arises because the IA and IB alleles are codominant, both are expressed together, neither masks the other.
What students write: Assuming a sex-linked condition is equally common in males and females.
What earns the mark: A recessive allele on the X chromosome produces the recessive phenotype more often in males, since a male has only one X chromosome and no second allele to mask it.
What students write: Confusing a pedigree diagram with a Punnett square.
What earns the mark: A pedigree diagram records the actual family history of who has a trait; a Punnett square predicts the probability of offspring genotypes from a given cross.
What students write: Confusing incomplete dominance with codominance.
What earns the mark: In incomplete dominance the heterozygote shows a blended intermediate phenotype, such as pink; in codominance both phenotypes appear together and fully, unblended, such as the AB blood group.
What students write: Assuming a carrier shows a mild form of the condition.
What earns the mark: A carrier does not show the condition at all, since their one dominant allele is enough to produce the normal phenotype; they simply carry the recessive allele and can pass it on.
What students write: Assuming an individual can carry all three ABO alleles, IA, IB and i, at once.
What earns the mark: An individual only ever carries two alleles for a gene, one from each parent, even though the ABO gene has three possible alleles in the population as a whole.
Study this chapter
Frequently asked questions
What is the difference between genotype and phenotype?
How do you predict the offspring of a monohybrid cross?
How is the sex of a baby determined?
How does a test cross work?
Why are sex-linked conditions like colour blindness more common in men?
What is the difference between incomplete dominance and codominance?
How do two carrier parents produce a child with a recessive condition?
Can one person have all three ABO alleles?
Source:SRC-DSKP-EN, SRC-FORMAT
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