OK so this is my favourite subject in all of Science, hence why it is the first one I wrote, The story I was told when I was back at school was that the guy who came up with this concept, Gregor Mendel, first wanted to do the experiment on mice, he was told that because he was a monk he he couldn't do this as their sex life was too busy, therefore he would have to do it on pea plants, he was very lucky, pea plants have set, discontinuous characteristics, they either have pink or white flowers, not pinky white. Mice on the other hand have a huge range of the same characteristic, therefore his experiment would not have worked.

Key Terms

Chromosome, gene, allele, heterozygous, homozygous, dominant, recessive, haploid, diploid, phenotype, genotype, gametes, carrier.

Explanation

So you have 46 chromosomes in every nucleus, in every cell (except red blood cells and gametes), that is 23 of pairs, 23 chromosomes from you mother and 23 chromosomes from your father.

The chromosomes from your mother and your father are structured the same, the same genes in the same places, the only difference is that these genes have different forms, these are called alleles.

So the gene for eye colour is always found in the same place on a chromosome, but that gene can be in the form of a blue eyed allele or a brown eyed allele for example.

Got it?

Now when your parents produced their gametes, or the egg and sperm that made you (gross to think of I know) these 46 chromosomes split in to two cell each with 23 chromosomes, therefore a diploid cell turned in to 2 haploid cells. These two haploid cells recombined to create a diploid cell with 46 chromosomes again, also known as you.

So to summarise as this gets confusing, a diploid cell contains 46 chromosomes (in humans), also known as a full set meaning there are two of each chromosome, a haploid contains only one of each of the chromosome (23 in humans).

Moving on...

Let's take my personal example, my dad has blue eyes (his phenotype), therefore he is homozygous recessive and therefore has two the genotype of bb. My mother had brown eyes (her phenotype), as does everyone on that side of my family, so I assume (I can't be sure though) that her genotype was homozygous dominant or BB.

I've given you lots of big words there so let me explain this; homozygous means a gene has two identical alleles, dominant means both these alleles are dominant and recessive means that both are recessive. If someone has a dominant allele and a recessive allele they are known as heterozygous (easy way is to think of homosexual and heterosexual, homosexual meaning two of the same gender, hetero meaning two of different genders!).

A dominant gene is always represented by a capital and recessive by the lower case letter of the dominant gene, for example brown hair is dominant or blonde and ginger, therefore brown will be B, blonde will be b (makes sense, they both begin with a b right?) but ginger will also be represented with a b. when someone has a ginger allele combined with a blonde allele the ginger will be G and the blonde will be g, this is because ginger is dominant!

A dominant gene will always overpower a recessive one, therefore a dominant gene will be displayed, so going back to eye colour I can guarantee my dad has the genotype bb as his phenotype is having blue eyes. My mother having brown eyes could have the genotype Bb or BB as both would give brown eyes, as brown is dominant over blue.

But let's assume for the sake of this my mother was BB. this means each of my dads sperm cells will have a single blue eyed allele(b), and each of my mothers eggs will have a brown eyed allele (B). So I put them into a Punnet square:

As you can see from the diagram every child will have brown eyes as all their genotypes will be heterozygous (Bb), hence both me and my sister have brown eyes.

Now just by a strange coincidence my wife has the same family background and therefore I can guarentee she also has a Bb genotype. When we have children it will produce a Punnet square that looks like this:

meaning that we have a 25% chance of having a blue eyed child. a 50% chance that our child will carry the blue eyed allele but not show it and a 75% chance they will have brown eyes, this is not however a guarantee we will have four children and one will have blue eyes, all could have blue eyes or all could have brown eyes, it is purely down to chance.

Make sense?

Exams

This will come up in a variety of different forms in exams so to paraphrase Ford Prefect...Don't Panic when you see an example you are not used to. The common ones they use are cystic fibrosis, hair colour, eye colour and sickle cell anaemia (this one annoys me as its not the true but they always insist on using it).

This does not mean they won't use a random example, maybe discuss ear lobes or ability to roll ones tongue, just identify the dominant allele and follow the same procedure and make sure you know what the terms mean!

Mistakes.

I cannot stress this enough...LEARN THE KEY WORDS. I so often see students answering utter nonsense and throwing these words left right and center, either because they don't know what they mean or they've panicked.

The most common mistake here however that is easy to solve is with letters that look similar. C and c look the same, just one is bigger. The exam boards go on the assumption that the lower case must be AT MOST half the size of the capital otherwise they refuse to see the difference so be careful with your handwriting. a trick from A level for this is to put an apostrophe next to the lower case so cc becomes c'c'.

Lastly be careful with percentages and fractions. Questions vary greatly on this, if the questions asks for a fraction it is a fraction they want, 75% is not the answer, 3/4 is and vice versa if the question asks for a percentage.

Tricks

LEARN THE KEY WORDS, each word has a route, that helps me, I've already explained homozygous and heterozygous but there are others, e.g. genotype is the combination of genes, therefore starts with gen, phenotype must be the other one or haploid is half a diploid and diploid is double a haploid.

Extension

Often this is linked to malaria or pedigree analysis (which will be another chapter). Sickle cell disease causes a number of problems (see the chapter on genetic diseases) and is caused by two recessive alleles. however if you are heterozygous you don't show these symptoms, however malaria cannot attach to these red blood cells making you more immune to the disease. Therefore in areas where malaria is high there are many more people with sickle cell anaemia as those that have the gene are more likely to survive and pass it on to the next generation.

Personally I question this as there are a number of flaws I could bore you with but the exam board love it as it links genetics to evolution and diseases!