Exercise practice: genetics problems

4.1 Monohybrid and test cross problems

A cross between two heterozygous tall plants, Tt × Tt, gives the gametes T and t on each side, so the four squares are TT, Tt, Tt and tt. The genotypic ratio is 1 : 2 : 1 and the phenotypic ratio is 3 tall : 1 dwarf. If the question asks for the probability of dwarf offspring, the answer is one quarter.

A test cross is used when the genotype of a tall plant is unknown. Cross it with a dwarf plant. If the tall plant is TT, all offspring are tall. If it is Tt, half the offspring are dwarfs. A result of 50 percent dwarf offspring proves that the tall parent was heterozygous, and the working should show both possibilities before choosing the one that matches the data.

4.2 Dihybrid cross problems

In a cross between two heterozygotes for two traits, RrYy × RrYy, each parent makes four kinds of gametes: RY, Ry, rY and ry. The 4 × 4 Punnett square has sixteen boxes. The phenotypic ratio is 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. The genotypic ratio is more complicated, but questions usually ask only for the ratio of a specific phenotype or the number of organisms in a class.

A common board problem gives true breeding round yellow crossed with wrinkled green and asks for the F1 and F2 ratios. The F1 is all RrYy, round yellow. The F2 ratio is the 9 : 3 : 3 : 1 above, and the law demonstrated is the law of independent assortment, because new combinations of seed shape and seed colour appear.

4.3 ABO blood group problems

The ABO gene has three alleles, A, B and O. A and B are co dominant and both are dominant over O. The possible genotypes are AA or AO for group A, BB or BO for group B, AB for group AB and OO for group O. A cross between a person of group AB and a person of group O gives children of group A or group B, never group O or group AB. Draw the two alleles of each parent and fill the four boxes; the blood group question then reduces to a simple monohybrid cross with three alleles.

4.4 Sex linked and pedigree problems

For an X linked recessive trait, write the mother's two X chromosomes and the father's X and Y separately. A carrier mother Xx crossed with a normal father gives daughters who are normal or carriers and sons who are normal or affected. This is why an affected son can be born to unaffected parents when the mother is a carrier, a pattern that appears in haemophilia and colour blindness questions.

In a pedigree, a trait that appears in a child when both parents are unaffected must be recessive. If the trait appears mostly in males and passes from a carrier mother to sons, it is X linked recessive. If an affected father passes the trait to all daughters and none of his sons, the trait is X linked dominant; if it appears in every generation in both sexes, it is likely autosomal dominant. State the reason for each conclusion, because the marks sit in the reasoning.

4.5 Common mistakes in genetics answers

Do not write phenotypes when the question asks for genotypes, and do not forget to define the symbols you use. Write the gamete letters in capitals and lower case consistently, put the male gametes across the top and the female gametes down the side, and give the ratio as a ratio rather than a list. Finally, when a question asks for a probability, give it as a fraction or percentage and show the square that produces it.

4.6 A sex linked probability problem

A carrier mother for haemophilia, XH Xh, marries a normal man, XH Y. The four boxes are XH XH, XH Xh, XH Y and Xh Y. The daughters are normal, with one of them a carrier; the sons are one normal and one affected. So the probability that a son is affected is one half, and the probability that a child is an affected son is one quarter. Write the two X chromosomes of the mother and the X and Y of the father as separate gametes, and the square answers the whole question.

4.7 A worked pedigree question

In a pedigree, two unaffected parents have one affected son and two unaffected daughters, and the grandfather on the mother's side was also affected. The unaffected parents point to a recessive trait. The fact that only males are affected, and that the trait travels from an affected grandfather through a carrier mother to her son, points to X linked recessive inheritance. The father must be XH Y, the mother XH Xh, the affected son Xh Y, and the daughters either XH XH or XH Xh. Present the answer in that order, because a pedigree answer is marked for the mode and the reasoning, and also for the genotypes.