There are one or two things which make a biology teacher’s (and indeed an exam marker’s) blood pressure rise. Well in fact in my case there are many dozens of things, as some of you know, but let’s keep it to the things candidates write in genetics answers in exams. This post is an attempt to encourage you to avoid the commonest “howler”.
The dominant allele does not have to be the more common one in a population.
Just because an allele is dominant, it does not mean it will be the most common in a population. I often hear answers in which people think that in a population 3/4 of the population will have the dominant phenotype, 1/4 will be recessive. This is utter nonsense of course. The ratio of 3:1 only applies to the probabilities of offspring produced by mating two heterozygous individuals.
There is a gene in humans in which a mutation can cause polydactyly: this rare condition results in babies born with an extra digit on each hand. Anne Boleyn was a famous sufferer in the past. But the allele of the gene that causes polydactyly is dominant – it is a P allele. I would imagine everyone reading this post, (all 12 of you…..), will probably have the genotype pp. The p allele that causes a normal hand to form is very very common in our population whereas the P allele is very very rare.
Don’t ever believe that just because an allele is common, it must be dominant.
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…..
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……..
The science of genetics looks at how inherited characteristics are passed from one generation to the next. The father of genetics was the Moravian monk, Gregor Mendel, who showed with his breeding experiments in peas that individual, discrete “particles” are passed from one generation to the next. We now know that these “particles” are actually small sections of a DNA molecule called genes.
Mendel worked out that there were always two such “particles” in any cell which acted together to determine the feature described. But he knew that gametes (sex cells such as pollen grains and egg cells) only contained one “particle” for each feature. You should understand why this is.
The discrete particles that are passed from generation to generation are genes: these are sections of a DNA molecule and are located on chromosomes. Chromosomes in most organisms are found in pairs within the nucleus of a cell. The word for a cell that contains pairs of homologous chromosomes is a diploid. The gametes do not have pairs of chromosomes: they are haploid cells that contain one member of each pair. This ensures that at fertilisation when two gametes fuse, a diploid zygote is produced.
iGCSE candidates can find genetics a difficult topic and one reason is that there is lots of jargon. Have a look at my definitions for these jargon words and ensure that you understand what they mean. Genetics is not a topic in which rote learning and memorisation are helpful – the very top candidates at iGCSE will understand what is going on, and can then answer all possible questions with ease.
Gene: ” a section of a DNA molecule that codes for a single protein”
Allele: “an alternative version of a gene found at the same gene locus”
Gene locus: “the place on a chromosome where a particular gene is found”
Phenotype: “the appearance of an organism, e.g tall, short, blue eyes etc.”
Genotype: “the combination of alleles at a single gene locus that an organism possesses – e.g TT, Tt”
Homozygous: “a gene locus where the two alleles are identical is said to be homozygous – e.g. TT, tt”
Heterozygous: “a gene locus where the two alleles are different is heterozygous – e.g. Tt”
Dominant allele: “a dominant allele is the one that determines the phenotype in a heterozygous individual”
Recessive allele: ” a recessive allele can only determine the phenotype in a homozygous individual”
Codominance: “two alleles are codominant if they both contribute to the phenotype in a heterozygous individual”