A gene is a length of inherited material that helps determine a characteristic. Alleles are different forms of the same gene.
The idea
In these crosses a letter stands for a gene. A capital letter is the dominant allele. A small letter is the recessive allele. An organism with two different alleles is heterozygous. Two identical alleles make it homozygous. The genotype is the pair of alleles. The phenotype is what you see. A recessive phenotype appears only when both alleles are recessive, because a single dominant allele masks it.
At gamete formation the two alleles of a pair separate, so each gamete carries one of them. That is segregation. In a cross of two heterozygous parents, Tt and Tt, the offspring genotypes are TT, Tt, Tt and tt, in the ratio 1 to 2 to 1. If T gives the dominant phenotype, three of the four show it and one shows the recessive phenotype, a ratio of 3 to 1. The ratio describes a large number of offspring. A family of four need not match it exactly.
What earns the marks
Show the gametes, then the combinations. A ratio with no diagram or grid can still score if the gametes are written. Use one letter. Do not use T for tall and S for short in the same cross. They are alleles of one gene, so T and t. Distinguish genotype from phenotype when the question uses the words. 3 to 1 is phenotypes. 1 to 2 to 1 is genotypes. Swapping those ratios is a common loss of the last mark.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
4 marks
In a plant, T is the allele for tall and t is the allele for short. Tall is dominant. A heterozygous tall plant is crossed with another heterozygous tall plant.
(a) State the genotype of each parent. (1 mark)
(b) List the gametes from one parent. (1 mark)
(c) Give the phenotypic ratio in the offspring. (2 marks)
Model answer
(a) Both parents are Tt.
(b) T and t.
(c) 3 tall to 1 short.
Mark scheme
Tt and Tt1 mark
Gametes T and t1 mark
3 tall1 mark
1 short. A genotype ratio of 1 to 2 to 1 does not answer a phenotype question1 mark
2Question 2
2 marks
Define the terms genotype and phenotype. (2 marks)
Model answer
The genotype is the alleles an organism carries. The phenotype is the characteristic that is expressed.
Mark scheme
Genotype: the genetic makeup, or the alleles present1 mark
Phenotype: the observable characteristic1 mark
3Question 3
2 marks
Explain why a short plant in this cross must have the genotype tt. (2 marks)
Model answer
Short is recessive. A plant shows the short phenotype only when no dominant allele is present, so both alleles are t.
Mark scheme
Short is recessive, or T masks t1 mark
Both alleles must be t. One T would make the plant tall1 mark
Practise Biology with Anna
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
What is the difference between genotype and phenotype?+
The genotype is the pair of alleles, such as Tt. The phenotype is what you see, such as tall. Two different genotypes, TT and Tt, can share the dominant phenotype.
Why is a 3 to 1 ratio not guaranteed in four offspring?+
The ratio is what you expect in a large number of offspring, because each fertilisation is independent. In four offspring you might see any mix. The ratio is a probability, not a promise for one small family. All WASSCE Biology past questions