Sources of human variation: WACE Year 12 Human Biology Unit 4
“Explain the genetic and environmental sources of variation within human populations, including mutation, meiosis and random fertilisation”
Human variation comes from genetic sources and the environment. New alleles arise only from mutation, while meiosis (crossing over and independent assortment) and random fertilisation reshuffle existing alleles into new combinations. The environment then modifies how genes are expressed, producing continuous and discontinuous variation.
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.
Mutation is the only process that creates new alleles. Meiosis and random fertilisation produce new combinations of existing alleles but do not create new ones. A full answer names mutation as the source of new variation and the other two as the source of new combinations.
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.
Two children of the same parents
Each parent produces gametes through meiosis. Crossing over swaps alleles between homologous chromosomes, and independent assortment means each gamete receives a random selection of the 23 maternal and paternal chromosomes, so no two gametes are identical. At fertilisation, one random sperm fuses with one random egg. Because both the gamete formation and the fertilisation step are random, each child receives a unique combination of alleles, which is why siblings (other than identical twins) differ even though they share the same two parents. No new alleles are created here; the differences come from new combinations of alleles that already existed in the parents.
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.
The most common error is claiming that crossing over, independent assortment or fertilisation create new alleles. They create new combinations of existing alleles; only mutation creates new alleles. Another error is treating all variation as genetic; continuous traits such as height are shaped by both genes and the environment.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksExplain how meiosis and random fertilisation produce genetic variation among the offspring of human parents.Show worked answer →
A 6 mark response needs the variation-generating events of meiosis plus fertilisation.
- Crossing over
- During prophase I, homologous chromosomes exchange segments of DNA, producing new combinations of alleles on the chromosomes (recombination).
- Independent assortment
- During metaphase I, homologous chromosome pairs line up and separate independently of one another, so each gamete receives a different mix of maternal and paternal chromosomes.
- Result for gametes
- Together these mean every gamete is genetically unique, carrying a different combination of alleles.
- Random fertilisation
- Any one of a huge number of genetically different sperm can fertilise any one of many genetically different eggs, so the combination of alleles in the zygote is itself a matter of chance, multiplying the variation.
Markers reward crossing over, independent assortment producing unique gametes, and random fertilisation combining them.
WACE 2023-style4 marksExplain why mutation is considered the ultimate source of new genetic variation, and distinguish between genetic and environmental causes of variation using one example of each.Show worked answer →
A 4 mark answer needs the role of mutation plus the genetic-versus-environmental distinction.
Mutation as the source. A mutation is a random change to the DNA base sequence that creates a new allele. Meiosis and fertilisation only reshuffle existing alleles, so mutation is the only process that produces genuinely new genetic variation.
Genetic versus environmental. Genetic variation comes from differences in alleles, for example ABO blood group. Environmental variation comes from external conditions acting on the phenotype, for example differences in body mass due to diet, which are not inherited.
Markers reward mutation as the only source of new alleles and a correct genetic and environmental example.