How to score full marks on a genetics question in iGCSE Biology? 3.16 3.17 3.18 3.20

Few things in life are certain, famously just death and taxes.  Northampton Town flirting with relegation can perhaps be added to this list.  But you can be pretty certain that tucked away somewhere in your iGCSE Biology exam there will be a genetics question that asks you to draw a genetic diagram.  There are usually four or even five marks available and so learning how to ensure you get all these marks is vital in your quest for an A* grade.

GCSE candidates are terrible at doing genetic diagrams: they fill the space with messy scribbles, doodles, strange tables and lines and then confidently write 3:1 at the bottom… Not a recipe for success.  So learn how to do it, be neat, take your time and you can guarantee full marks.

If the question doesn’t do it for you, you should start by defining what the letters you will use for the alleles.  If one allele is dominant over the other, it is conventional to use the upper case letter for the dominant allele, the lower case letter for the recessive one.  It will tell you in the question which allele is dominant.

Start your genetic diagram by writing the phenotype of the parents in the cross.

e.g.          Parental Phenotype:             Tall                         Tall

Underneath the phenotype, write the genotype of the parents.

Parental Genotype:                Tt                            Tt

Then you need to think about which alleles are present in the gametes.  Gametes are haploid and so will contain one of each pair of homologous chromosomes – in this example there can only be one allele in each gamete (as we are only looking at one gene)

Gametes:           T               t                       T                   t

Next show random fertilisation.  I think it is much better to draw a Punnett square that has the male gametes down one side, the female gametes down the other and then carefully pair them up.  This is a stage where mistakes can be made if you rush so however simple you think this process is, take your time…..

Random Fertilisation


Finally you need to copy out the offspring genotypes from your Punnet square, like so

Offspring Genotypes:        TT            Tt              Tt             tt

And underneath each one, write the offspring phenotype

Offspring Phenotypes:       Tall           Tall            Tall           Dwarf

Finally, answer the question.  If it asks for a probability, express your answer as either a percentage or a decimal or a fraction.  So if I were asked what is the probability of a homozygous pea being produced, the answer is 50% or 0.5 or 1/2

Follow these rules and you will always score full marks – happy days……..


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s