O-Level Biology · Secondary 3 and 4
Inheritance
Inheritance explains how characteristics pass from parents to offspring. You learn the language of genetics, such as gene, allele, genotype and phenotype, and use genetic diagrams to predict the results of monohybrid crosses. The topic also covers test crosses, how sex is determined, and the difference between continuous and discontinuous variation. Genetics questions reward careful, set-out working, so this guide shows the layout examiners expect, together with the ratios and probabilities you need.
What the syllabus expects
- Define gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype
- Use genetic diagrams to predict the results of monohybrid crosses
- Calculate expected ratios and probabilities of offspring
- Explain how a test cross shows whether an organism with a dominant phenotype is homozygous or heterozygous
- Describe the determination of sex in humans
- Distinguish between continuous and discontinuous variation and describe mutation as a source of variation
The key words
A gene is a length of DNA that codes for one protein and so controls a characteristic. Genes sit on chromosomes in the nucleus. An allele is one of the different forms of the same gene, for example the allele for tall stems and the allele for short stems in pea plants. Body cells contain chromosomes in pairs, so each cell has two alleles for every gene. A dominant allele shows its effect even when only one copy is present and is written with a capital letter, such as T. A recessive allele only shows its effect when two copies are present and is written with the lower case of the same letter, t. The genotype is the combination of alleles, such as TT, Tt or tt. The phenotype is the observable characteristic, such as tall. An organism with two identical alleles is homozygous, and one with two different alleles is heterozygous.
Monohybrid crosses and genetic diagrams
A monohybrid cross follows the inheritance of one characteristic. Set it out in the same order every time: parental phenotypes, parental genotypes, gametes, a Punnett square, offspring genotypes, offspring phenotypes and the ratio. Gametes carry only one allele of each gene, so a Tt parent makes gametes T and t, and a TT parent makes only T. Crossing two heterozygous tall plants (Tt × Tt) gives offspring genotypes TT, Tt, Tt and tt in the ratio 1 : 2 : 1, and phenotypes in the ratio 3 tall : 1 short. Crossing Tt × tt gives 1 Tt : 1 tt, so 1 tall : 1 short. Each ratio is a probability for every offspring, not an exact count. A 3 : 1 ratio means each offspring has a 3/4 chance of being tall, and with 200 offspring you expect about 150 tall and 50 short.
Test crosses and sex determination
A tall plant could be TT or Tt, and you cannot tell from its phenotype. In a test cross, it is crossed with a homozygous recessive plant, tt. If all the offspring are tall, the unknown parent is probably TT, because every offspring receives a T from it. If about half are short, the unknown parent must be Tt, since short offspring need a t from each parent. Sex in humans is determined by one pair of sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y (XY). All egg cells carry an X. Half the sperm carry an X and half carry a Y. A genetic diagram of XX × XY gives XX, XX, XY and XY, so each child has a 1/2 chance of being a girl and a 1/2 chance of being a boy, whatever the sexes of earlier children.
Variation and mutation
Variation is the differences between individuals of the same species. Discontinuous variation has a few distinct categories with no intermediates, such as the ABO blood groups. It is controlled by one or a few genes and is not affected much by the environment, and it is shown on a bar chart. Continuous variation has a complete range of values between two extremes, such as height and body mass. It is controlled by many genes together with the environment, such as diet, and it is shown on a histogram, often with a bell-shaped curve. A mutation is a sudden, random change in a gene or in a chromosome. Gene mutations can produce new alleles, such as the allele that causes sickle cell anaemia. A chromosome mutation can change the number of chromosomes, as in Down syndrome, where a person has an extra chromosome 21. Ionising radiation and some chemicals increase the rate of mutation.
Worked example
In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t). Two heterozygous tall plants are crossed and 240 seeds grow into plants. Use a genetic diagram to predict the number of tall plants and short plants.
- Parental phenotypes: tall × tall. Parental genotypes: Tt × Tt.
- Gametes: each parent produces T and t gametes in equal numbers, because the two alleles separate when gametes form.
- Punnett square: T with T gives TT, T with t gives Tt, t with T gives Tt, t with t gives tt.
- Offspring genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: TT and Tt are tall, tt is short, so 3 tall : 1 short.
- Expected tall plants: 3/4 × 240 = 180. Expected short plants: 1/4 × 240 = 60.
- Check: 180 + 60 = 240, and 180 : 60 simplifies to 3 : 1.
Answer: About 180 tall plants and 60 short plants, a ratio of 3 tall : 1 short. Real results will be close to these numbers but not exact, because fertilisation is random.
Common mistakes
- Using different letters for the two alleles, such as T for tall and S for short. Use one letter, capital for dominant and lower case for recessive.
- Writing gametes with two alleles, such as Tt. Each gamete carries only one allele of each gene, so write T and t, each in a circle.
- Treating a ratio as an exact result. A 3 : 1 ratio gives expected numbers, and real counts such as 172 tall and 68 short are normal.
- Saying a family with three boys is 'more likely' to have a girl next. Each child has a 1/2 chance of either sex, independent of earlier children.
- Confusing genotype and phenotype. Tt is a genotype. 'Tall' is a phenotype. Questions asking for one do not accept the other.
Exam tips
- Always label each line of a genetic diagram: parental phenotypes, parental genotypes, gametes, offspring genotypes, offspring phenotypes, ratio. The labels themselves usually carry marks.
- State the key at the start, for example 'T = allele for tall, t = allele for short', especially if the question did not give one.
- When a question gives numbers of offspring, convert the ratio to expected numbers and comment on whether the observed results are close to them.
Practice questions
1. Easy: Define the term allele and give an example.
Show answer
An allele is one of the different forms of the same gene. For example, the gene for stem height in pea plants has an allele for tall stems (T) and an allele for short stems (t).
2. Medium: A black guinea pig (B is dominant for black fur) is crossed with a white guinea pig (bb). They have 10 offspring: 5 black and 5 white. What is the genotype of the black parent? Explain using a genetic diagram.
Show answer
Bb. Cross Bb × bb. Gametes: B and b from the black parent, b only from the white parent. Offspring: Bb, Bb, bb, bb, giving 1 black : 1 white, which matches 5 : 5. If the black parent were BB, all offspring would be Bb and black.
3. Harder: Albinism is caused by a recessive allele, a. Two parents with normal skin colour have an albino child. State the genotypes of the parents and the probability that their next child is albino.
Show answer
Both parents are Aa. The albino child is aa, so it received an a allele from each parent, and as both parents have normal pigmentation each must also carry A. Aa × Aa gives AA, Aa, Aa, aa, so the probability of an albino child is 1/4, or 25%, for each child.
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