Genetics follow up: Grade 9 understanding for IGCSE Biology 3.19 3.20 3.21B

In an earlier post, written for my own Y11 students at a previous school, I asked them to check that they knew the answers to 5 questions before attempting to answer any practice questions involving inheritance and genetics. I think that as this blog is now read by a much wider audience, it is probably time to answer the 5 questions myself! Here goes……

1) What is the difference in meaning between a gene and an allele?

People who I have taught know that there is just one thing that every A* student should memorize for IGCSE Biology and that is the definition of a gene.

“A gene is a sequence of DNA that codes for a single protein”

Genes are located on chromosomes and exist in alternate versions called alleles. So for example in pea plants there is a gene that codes for a protein that determines the height of the pea. This gene exists in two possible versions: a T version that makes the plant tall and a t version that makes the plant short. These alternate versions of a single gene are called alleles.

2). Why does the genotype of a person, plant or rabbit always contain two alleles for each gene?

In order to understand this, you need to understand something about chromosomes. Humans, plants and rabbits are all diploid organisms – this means that they have pairs of chromosomes one inherited from mum, one from dad. Because genes are found on chromosomes, this means that a genotype (combination of alleles) will also have two alleles. Alleles are versions of genes, genes are found on chromosomes and chromosomes come in pairs! Simple…

3) What is different about the genotype of a gamete compared with other body cells? Why are gametes different?

Gametes only contain one allele for each gene. This is because gametes are cells that do not contain pairs of chromosomes like every other cell in the body. Gametes are haploid – they only have one member of each pair of chromosomes. One chromosome per pair means only one version of each gene…. Gametes have to be different because they have a different fate or destiny to every other body cell. (Just typing the word destiny means I can hear the Star Wars theme as clear as anything in my head!). Luke’s destiny was to unite the light and dark sides of the force. A gamete’s destiny is less exciting but it is to fuse with another gamete in the act of fertilisation. If both gametes had pairs of chromosomes like every other body cell, then the act of fertilisation would result in a doubling of the chromosome number in every generation and that is clearly unlikely to do anyone any good!

4) How would you explain what is meant by a recessive allele?

A recessive allele is always given the lower case symbol, for example t. The best way to explain what is meant by a recessive allele is to say that recessive alleles only determine the phenotype (the appearance of the organism) if there are two of them. An individual with two identical alleles for a gene is described as being homozygous so a recessive allele will only determine the phenotype in a homozygous individual. In the example I have used so far, because the dwarf allele, t is recessive, the only genotype possible for a short pea plant is tt.

5) What does it mean if two alleles are codominant?

Codominant alleles are alleles that both contribute to the phenotype in a heterozygous individual. Heterozygous is the adjective used if the two alleles present are different to each other.

A good example is the genetics of the ABO blood group system in humans. There are three possible alleles present for this gene in the human population: IA, IB and IO.

The IA allele is dominant to the IO allele.

The IB allele is dominant to the IO allele.

But if you are heterozygous with the genotype IAIB then you have an intermediate blood group called AB. Both alleles are contributing to your blood group and so you are neither blood group A, nor blood group B but a different phenotype called AB. This is because the alleles IA and IB are codominant to each other.

This is a complex topic (not tested at all in the 2018 Paper 1B) so it is worth trying to get your head around all the jargon here over half term…….

If you have any questions at all, please ask me in the “Leave a Comment” box at the foot of this post.


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