A monohybrid cross is one in which only one character is studied e.g. height, colour etc. The following questions will be good for practice. Be sure to go through them before walking into the next stage (dihybrid crosses). 

1. When a pure breeding bean plant with green pods was crossed with another pure breeding bean plant with red pods, all the offspring produced had green pods. The offspring were allowed to self-pollinate and the following results were obtained, 428 bean plants had green pods while 152 had red pods.

Explain the genetics of the crosses using appropriate diagram and symbols.                           (O/L 2012)

1. When Mendel crossed pure breeding red-flowered pea plant with pure breeding white-flowered pea plant, all the members of the F1 generation had pink flowers.

Using appropriate symbols and diagrams show the possible genotype and phenotype if;

1. 1. The F1 individuals are allowed to self-pollinate
   2. An F1 individual is crossed with a white-flowered parent                                                        (O/L 2015)

1. Outline
   1. Mendel’s first law of inheritance
   2. Mendel’s second law of inheritance

Explain, using appropriate genetic symbols the possible blood groups of children whose father is heterozygous blood group A and mother heterozygous blood group B.

If these parents have two children, what is the probability that both will have blood group A.

                                                                                                                                                (A/L 2021)

1. In cats, black fur colour is determined by a gene B and the allele responsible for yellow fur is L. These genes show co-dominance and the hybrid BL produces an albino. These genes are also sex-linked. Show the gametes and possible phenotypes of the parents of the F1 and F2 generations of a cross between a black furred female and a yellow furred male cat.

To what extend are the male and female offspring produced distinguishable by their fur colour.

                                                                                                                                                (A/L 2019)

1. Mendel proposed much of his genetic theories based on results he obtained from crossing various pea plants. He crossed a tall pea plant with a short pea plant and observed that all the offspring were tall. From this observation he proposed his law of dominance. How would Mendel’s laws have been altered if,
   1. The results of his cross were all short pea plants?
   2. The results of his cross were all plants of medium height?
   3. The results of his cross were plants of man different heights?

Mrs. Doe and Mrs. Roe had babies at the same time. Mrs. Doe took home a girl and named her Nancy. Mrs. Roe received a boy and named him Richard. However, Mrs. Roe was sure she had a girl, and sued the hospital. Blood tests showed that Mr. Roe was type O and Mrs. Roe was type AB.
Mr. and Mrs. Doe were both type B. Nancy was type A and Richard was type O. Had an exchange occurred? Explain your answer

Codominance:

This is the phenomenon whereby 2 or more expressions of a given trait, express themselves phenotypically on a given organism at the same time. In this case, both expressions are dominant. For example, pure black cats and pure white cats are well known but we also know and recognise cats with both black and white fur colourations. This is due to codominance. Let’s actually consider the case of a black cat (BB) and a white cat (WW). Knowing that they are both dominant, the organisms that present only the black or white colour are therefore homozygous for the colour. Also, we aren’t any new colours in this cross.