International A Level Biology / Development of organisms
Multiple alleles, polygenic inheritance and continuous variation
Edexcel International A Level Biology (YBI11), chapter 9: Development of organisms, specification point 3.21. Free lesson.
Read the revision notes · International A Level Biology past papers
Multiple alleles are different versions of one gene, while polygenic inheritance involves several genes affecting one characteristic. Many genes and environmental effects can produce continuous variation, where values form a range rather than separate categories.
How ABO blood groups show multiple alleles
ABO blood groups are controlled by one gene with three alleles: Iᴬ, Iᴮ and Iᴼ. Each person has two alleles. Each codes for different mRNA and an enzyme: Iᴬ adds antigen A to red blood cells, Iᴮ adds antigen B, and the Iᴼ enzyme does not work. Iᴬ and Iᴮ are codominant, so both are expressed together in group AB. Iᴼ is recessive, so it is expressed as group O only when both alleles are Iᴼ.
How to cross two people with different blood groups
A group A father with genotype IᴬIᴼ has children with a group B mother with genotype IᴮIᴼ. Each parent can pass on either allele.
How polygenic inheritance gives several classes
Polygenic inheritance means several genes at different loci affect one characteristic. A locus is a position on a chromosome. In wheat, three genes affect grain colour. Each allele that adds red pigment darkens the grain a little. When both parents are heterozygous at every gene, more genes create more possible pigment totals. With one gene, there are 0, 1 or 2 pigment alleles, in proportions of 1/4, 1/2 and 1/4. With two genes, there are five classes, from 0 to 4 pigment alleles. The middle classes are most common. With three genes, there are seven classes, from 0 to 6. The middle class,…
How continuous variation arises
Continuous variation means a characteristic can take any value within a range. The birth mass of 1,000 babies forms a smooth, bell-shaped graph called a normal distribution. Many genes each have a small effect, while the environment, such as the mother's diet, blurs the gaps between genetic classes. Most babies are near the mean, or average, of about 3.4 kg. Fewer are towards either end. Label the vertical axis number of babies or frequency. Population is not a variable to put on this axis.
How to distinguish the two types of variation
Discontinuous variation has distinct categories with nothing in between. Continuous variation can take any value within a range. Blood group is discontinuous; height, skin colour and birth mass are continuous. Discontinuous variation is usually controlled by one gene, sometimes with multiple alleles, and has little or no environmental effect. Continuous variation is polygenic and has a big environmental effect, such as diet affecting height. Show discontinuous data in a bar chart with gaps. Show continuous data in a histogram, often with a normal distribution.
Why multiple alleles are not polygenic
Multiple alleles means one gene has more than two versions. Polygenic means several genes at different loci affect one characteristic. Blood group has multiple alleles, but is not polygenic. Human skin colour is polygenic: several genes at different loci each add a little pigment. Sunlight also darkens skin, giving many phenotypes across a continuous range with no separate categories.