Q1. A recently married couple are expecting their first child. 


(a) Given the information in the partly completed diagram, complete the diagram to show the possible combinations of sex chromosomes that the baby could have.


(b) What will be the probability of the first child being a boy?


From the combinations of X and Y, we can see that the probability of the child being male is 0.5, or 50%, or 1 in 2 if you want to put it that way, any one of these 3 representations will be accepted.


(c) From the information given, draw a punnet square to show the possible combinations of sex chromosomes that the mother and father's baby could have.



Q2. A male child has parents who are heterozygous for the polydactyly allele. Draw a punnet square and identify any offspring with this condition, also write to the ratio of affected to unaffected offspring (4 marks)


First of all let's allocate some letters. As the parents are both heterozygous, they both carry the dominant and recessive characteristic. Let us say that D represents the allele for polydactyly and the d represents an unaffected person. In this case both parents would be Dd.


Drawing the punnet square would reveal the following information:




This square tells us that in three out of four cases where the characteristic is dominant, the offspring will inherit the condition, however in the fourth case where the characteristics are both recessive, the condition will not be inherited.


Q3. Jane and Mark are a couple. Jane has dimples in her cheeks, but Mark does not. The presence of dimples is thought to be caused by the presence of a dominant allele D. The recessive form of the allele is represented as d and we know that Jane is heterozygous for this dimples gene.  (5 marks total).


(a) Should they decide to have children, what is the probability that their first baby will have dimples?


(b) Draw a genetic diagram and/or a punnet square to show the possible inheritance of the dimples gene. 


(c) Identify the phenotype of each possible outcome.


(a) First of all we need to identify the genotype for Jane and Mark, given the information in the question. We are told that Jane is heterozygous for the gene so her genotype will be "Dd" but we are told that Mark does not have the dimples therefore he must be homozygous for the recessive gene "dd". We can identify the probability of the first child having dimples (or not) if we quickly produce a punnet square with the information we now have:



You can see from the diagram that in two out of four cases the child is probably going to inherit dimples, but in the remaining 2 cases, probably not. Therefore the probability is 0.5 or 50%.


(b) We have already drawn a punnet square but just for the sake of practice we will now produce the genetic diagram using the same information.



(c) The phenotypes would be "Dd" - has dimples, "dd" - does not have dimples.


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