Question 4
(a) Explain the following terms:
(i) test cross;
(ii) monohybrid cross. [4 marks]
(b) In a mango plant, the allele for bean-shaped seed is r and is recessive to round-shaped seed R. With the aid of a genetic diagram, determine the genotypes of the offspring if a homozygous bean shaped parent is crossed with a homozygous round shaped parent. [16 marks]
Observation
This was the most unpopular question among the candidates. Many candidates could not explain Test cross satisfactorily. Majority of the candidates that attempted 4b scored good marks with the exception of those that could not cross properly.
The expected answers are:
(a) (i) Test cross
This is the crossing/mating of an organism of unknown genotype; with a homozygous recessive/double recessive organism; to determine the genotype of an organism.
(ii) Monohybrid cross
Is a genetic cross between parents; that differ in the alleles they possess; for one particular gene
(b) Crosses
(i) R- Round seed
r- bean-shaped seed
Parental genotype (homozygous) RR X rr
Gametes R R X r r
Correct crossing (CC)
F1 generation (Genotype) Rr Rr Rr Rr