Where does the variation between individual humans actually come from?
Explain the genetic and environmental sources of variation within human populations, including mutation, meiosis and random fertilisation
A focused answer to the WACE Year 12 Human Biology Unit 4 dot point on variation. Mutation as the source of new alleles, meiosis and random fertilisation, plus continuous, discontinuous and environmental variation.
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What this dot point is asking
WACE wants you to separate the sources of variation and to know which ones create new genetic material and which only rearrange it. Variation is the differences between individuals in a population, and it is the raw material on which natural selection acts in later topics.
Mutation: the only source of new alleles
A mutation is a permanent change in the DNA base sequence. Mutations are the ultimate source of all new alleles and therefore all new genetic variation. They occur randomly and can be caused or increased by mutagens such as radiation and certain chemicals.
- Gene (point) mutations change one or a few bases by substitution, insertion or deletion.
- Chromosomal mutations alter whole segments or the number of chromosomes, for example the extra chromosome 21 in Down syndrome.
Only mutations in the gametes (germ-line mutations) are passed to offspring. Mutations in body cells (somatic mutations) affect only that individual. Mutations may be harmful, neutral or, occasionally, beneficial, and only the inherited ones contribute to variation in the next generation.
Meiosis: reshuffling existing alleles
Meiosis is the cell division that makes gametes and it introduces variation in two ways.
Crossing over happens in prophase I, when homologous chromosomes exchange segments, producing chromosomes with new combinations of alleles.
Independent assortment happens in metaphase I, when the homologous pairs line up randomly, so each gamete gets a random mix of maternal and paternal chromosomes. With 23 pairs, this alone produces a huge number of possible combinations.
Random fertilisation
Fertilisation combines one gamete from each parent at random. Because each parent can produce an enormous number of genetically different gametes, and any sperm can fertilise any egg, the number of possible offspring genotypes is vast. This multiplies the variation generated by meiosis.
Continuous and discontinuous variation
Variation in a population falls into two patterns.
Continuous variation shows a range of values with no clear categories, such as height, body mass or skin colour. It is usually polygenic (controlled by many genes) and strongly influenced by the environment, producing a normal (bell-shaped) distribution.
Discontinuous variation shows distinct categories with no in-between values, such as ABO blood group or the ability to roll the tongue. It is usually controlled by one or a few genes and is little affected by the environment.
The environment as a source of variation
Not all variation is genetic. The environment affects how genes are expressed. Diet affects height and body mass, exposure to ultraviolet light affects skin colour, and exercise affects muscle development. Identical twins have the same DNA but can differ because of environmental influences. Continuous traits are the most affected by the environment, whereas discontinuous traits such as blood group are almost entirely genetic.
How this maps to the exam
Expect questions that ask you to list and explain the sources of genetic variation, to distinguish which ones create new alleles, or to classify a trait as continuous or discontinuous and explain why. You may also be asked to separate genetic from environmental causes of a trait such as height.