Inheritance and genetic crossesIB MYP Biology: Revision notes
Section 1
Key terms
A gene can have different versions called alleles. We write them as letters: a capital letter for the dominant allele (T) and a lower-case letter for the recessive allele (t). A dominant allele shows in the organism even if there is only one copy. A recessive allele shows only if there are two copies.
The genotype is the pair of alleles an organism has, such as TT, Tt or tt. The phenotype is the feature that results, such as tall or short. Two identical alleles (TT or tt) are homozygous, and two different alleles (Tt) are heterozygous.
Dominant does not mean stronger or more common. It only means the allele shows when present.
Section 2
Monohybrid crosses and Punnett squares
A monohybrid cross follows one gene. Each parent makes gametes with one allele each. A Punnett square shows every possible combination at fertilisation.
Worked example: Tt × Tt. Gametes: T or t from each parent. Offspring: TT, Tt, Tt, tt. That gives 3 tall : 1 short, a 3 : 1 ratio.
Tt × tt: gametes T or t, and t only. Offspring: Tt, Tt, tt, tt. That gives 1 tall : 1 short, a 1 : 1 ratio.
Section 3
Probability
Ratios from a cross are probabilities, not guarantees. In Tt × Tt, each offspring has a 1 in 4 (25%) chance of being tt and a 3 in 4 (75%) chance of being tall. Each fertilisation is independent, so even if the first child is tt, the next child still has a 1 in 4 chance.
Small families rarely match the ratio exactly. Larger numbers of offspring come closer to it.
Write the probability as a fraction first (1/4), then convert to a percentage if asked (25%).
Section 4
Family pedigrees
A pedigree is a family tree showing who has a feature. Squares are males, circles are females, and shaded shapes show affected people. Use these clues:
- If two parents without the feature have a child with it, the feature is recessive and both parents are carriers (heterozygous).
- If an affected child has an affected parent, the feature may be dominant.
Section 5
Sex determination
Humans have 23 pairs of chromosomes. One pair is the sex chromosomes: females are XX and males are XY. Every egg carries an X. Half the sperm carry X and half carry Y. At fertilisation, X + X gives a girl and X + Y gives a boy, so there is a 50% chance of each.
Section 6
Inherited disorders
- Cystic fibrosis: caused by a recessive allele. Thick sticky mucus affects the lungs and digestion. A person needs two copies, so parents can be healthy carriers.
- Sickle cell disease: caused by a recessive allele for a faulty form of haemoglobin, which makes red blood cells curved and unable to carry oxygen well.
- Huntington's disease: caused by a dominant allele. One copy is enough, and it damages the nervous system, usually starting after about age 35. A child of an affected parent has a 50% chance of inheriting it.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Inheritance and genetic crosses
- A student in Nairobi studies inheritance in pea plants. The gene for height has two alleles: the allele for tall stems (T) is dominant and the allele for short stems (t) is recessive.Define the terms allele and dominant.2 marks
- A breeder in Lisbon keeps guinea pigs. Black fur is controlled by a dominant allele (B) and white fur by a recessive allele (b). She has two heterozygous black guinea pigs and one white guinea pig.The breeder crosses a heterozygous black guinea pig with the white one. State the expected ratio of black to white offspring and the genotype of the black offspring.2 marks
- Cystic fibrosis is an inherited disorder caused by a recessive allele (f). A healthy couple, Amara and Kwame, have a first child who has cystic fibrosis. Both parents carry one copy of the allele.Explain how two healthy parents can have a child with cystic fibrosis.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).