How to score full marks on IGCSE Genetics questions 3.18 3.20

This will be my final blog entry from Dubai.  I will be flying home tomorrow with spirits refreshed by this amazing country and the positive and dynamic people I have met.

There will be a Mendelian genetics question in one of the two EdExcel IGCSE Biology papers.  Examiners are people who like to stick to tried and tested formulae with setting questions and it’s always worked in the past, so why change now…?

You should welcome the genetics question when it appears for two reasons:

  • If you understand what is going on and
  • if you are prepared to set the answer out correctly (see below)

you can almost guarantee that you will score all the marks!  And that’s what we want as full marks = top grade

The understanding you need for these questions is actually quite detailed and beyond what I can explain in this post.  Check your understanding by answering the following questions:

  1. What is the difference in meaning between a gene and an allele?
  2. Why does the genotype of a person, plant, fruit fly or rabbit contain two alleles for each gene?
  3. What is different about the genotype of a gamete compared with every other cell in the body?  Why are gametes different?
  4. How would you explain what is meant by a recessive allele?
  5. If two alleles are codominant, what does this mean?  Give me a specific example in which this pattern of inheritance is found.

Good, I am assuming you have answered these questions fully using important terms like diploid, homologous chromosomes, phenotype, heterozygous correctly……

In which case, all that remains is to remind you how to set out a genetic diagram.  I am not usually a proponent of slavishly following protocols but in producing a genetic diagram in an exam, you certainly should.  There are usually five marks available for a question like this and only one of the marks is for getting the right answer.  20% = E grade and that is not what we want.

  • Start with the phenotype of the parents – write mother and father’s phenotype down in full
  • Then underneath the phenotype, write the genotype of the parents.  (The letters to use for the two alleles will be given in the question and always use the letters suggested, don’t make up your own.  Slavish following of protocol remember)
  • The next bit is the first tricky bit.  Write the alleles present in the gametes.  Remember gametes are formed by meiosis and so only contain one member of each homologous pair of chromosomes – they will only have one allele from each pair in each cell.  Draw circles around each gamete to show the examiner you understand they are individual cells.
  • Draw a fertilisation table (called a Punnett square after Reginald Punnett – who says you don’t learn anything useful at GCSE?)
  • Write out the offspring genotypes from the table
  • Write out the offspring phenotypes underneath your list of offspring genotypes showing how they match up.

Answer the question.  If asked for a probability, express it as a fraction or percentage.  Those of you who follow the horses are sometimes tempted to write the probability as odds, but “3-1 the dwarf rabbit, 3-1 on the field” is not a good answer in your Biology exam…

If you do this you will always get all the marks.

Please remember:

The ratio of 3:1 is only found in the offspring of two heterozygous parents.  Sometimes students seem to think that all genetic crosses produce offspring in this ratio. This doesn’t make any sense if you think about it for a moment but in an exam, thinking for a moment is not always easy.

If you look at phenotypes in a population, the dominant phenotype is not always more common that the recessive phenotype.  This is something people find really difficult to get their head around.  Think of the disease polydactyly in which suffers have an extra digit (e.g. Anne Boleyn)  Polydactyly js caused by a dominant allele but I bet in your class at school, people with 5 digits on each hand are more common than those with 6.  (A joke about schools in the Fens north of Cambridge has been removed in the interests of good taste)

As fertilisation is random, offspring will never exactly fit the expected Mendelian ratio.  If you are given a cross in which peas produce offspring and 495 are smooth and 505 are wrinkled, you do not have to work out some complicated theory to explain this ratio.  It will be a 1:1 ratio with the small differences due to random fertilisation

Good luck and keep working hard!  Comments welcome as always – it does show me that someone is reading this stuff…….







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