17.1.117-18 · Selection, Evolution, Classification and Conservation
Variation- Phenotype — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
17.1.1 Variation: Phenotype — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain that phenotypic variation within a species is caused by genetic factors, environmental factors or both.Phenotype — the observable characteristics of an organism.Phenotypic variation …
- Phenotype = genotype + environment
- Genetic factors only
- Environmental factors only
- Both together
- Genetic variation
- A new allele
Section 1 of 8Phenotype and phenotypic variation
Phenotype and phenotypic variation
Phenotype
- Phenotype the observable characteristics of an organism.
Phenotypic variation
- Phenotypic variation the differences in phenotype between organisms of the same species.
Phenotype
Phenotype genotype + environment
- the complete phenotype results from expression of the genotype and the effect of the environment on it.
Genetic factors only
Three possible causes of phenotypic variation:
- Genetic factors only
- e.g. the four ABO blood groups come from different pairs of the three alleles of the single ABO gene.
- Environmental factors only
- e.g. genetically identical plant clones grow to different heights in different conditions.
- Both together
- e.g. the sickle cell allele is common where malaria is prevalent because heterozygotes resist malaria.
Variation is the raw material for natural selection, which can act only when individuals in a population differ.
Section 2 of 8Genetic variation
Genetic variation
- Genetic variation small differences in DNA base sequence between individuals of a species population.
- Organisms of one species have very similar genotypes, but even twins differ slightly in base sequence.
- The differences are small compared with the size of the genome.
Genetic variation is passed from one generation to the next and generates phenotypic variation within the species.
Processes that produce new combinations of alleles in a gamete or individual:
- Independent assortment of homologous chromosomes in metaphase I.
- Crossing over between non-sister chromatids in prophase I.
- Random fusion of gametes at fertilisation.
Mutation produces completely new alleles.
Section 3 of 8Sources of genetic variation compared
| Process | Mechanism | Consequence |
|---|---|---|
| Independent assortment (metaphase I) | Random alignment of homologous pairs gives different combinations of chromosomes and alleles in each gamete | Variation between gametes |
| Crossing over (prophase I) | Exchange of material between non-sister chromatids gives new allele combinations and can break linkage between genes | Variation between gametes |
| Random fertilisation | Any male gamete can fuse with any female gamete (random mating in the population) | Variation between zygotes and the individuals that develop from them |
| Mutation | Random change in DNA base sequence generates a new allele; it must occur in gametes to be inherited | Variation between individuals in the population |
Section 4 of 8Effects of new alleles and genes
A new allele
- A new allele may be advantageous, disadvantageous or have no apparent effect on phenotype.
- No apparent effect is possible because the genetic code is degenerate.
Hidden alleles
- Hidden alleles a new allele is not always seen in the individual in which it first arises.
- It can stay unexpressed in a population for several generations before it contributes to phenotypic variation.
Genes vary in their effects on phenotype.
- A phenotype may be affected by a single gene or by several genes.
- The effect of a gene may be large, small and/or additive.
Section 5 of 8Environmental causes of variation
1
The environment of an organism affects how it grows and develops.
2
Environments around the world differ in these factors:
- Length of sunlight hours, which may be seasonal.
- Supply of nutrients (food).
- Availability of water.
- Temperature range.
- Oxygen levels.
3
A genotype may not reach its full potential in a poor environment.
- A plant with a tall genotype in soil poor in minerals, light and water cannot grow to the full height its genes allow.
4Variation caused only by the environment
- Variation caused only by the environment cannot be inherited by offspring.
- Only changes to the genetic material of gametes are inherited.
- Genetic variation is inherited because it alters the DNA of the gametes; environmental variation does not.
Section 6 of 8More from the specification
More from the specification
Continuous vs Discontinuous Variation
- Continuous variation shows a quantitative range of phenotypes influenced by polygenes and environment,
- (such as human height)
- whereas discontinuous variation exhibits distinct qualitative categories controlled by single genes.
Student's t-Test for Comparing Means
- The t-test is a statistical tool used to compare the means of two normally distributed samples of continuous data, accounting for sample size and standard deviation.
Statistical Significance and Degrees of Freedom
- By calculating degrees of freedom and determining whether the calculated t-value exceeds the critical value at p = 0.05, researchers assess whether differences between means are statistically significant or arose by random chance, rejecting or accepting the null hypothesis.
Qualitative
- A result describing presence or absence rather than amount.
Quantitative
- Measured with numbers
Continuous variation
- A range of phenotypes between two extremes.
Standard deviation
- Measure of spread around the mean.
Discontinuous variation
- A limited number of distinct phenotypes with no intermediates.
Statistically significant
- A difference with a probability of less than 5% of being due to chance.
t-test
- Test comparing the means of two data sets.
Significant
- Less than 5% probability that the difference is due to chance.
Probability
- The chance that an offspring has a given genotype or phenotype.
Hypothesis
- A testable prediction with a scientific explanation.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Phenotype | The observable characteristics of an organism. |
| Phenotypic variation | Differences in phenotype between organisms of the same species. |
| Genetic variation | Differences in DNA base sequence between individuals of a species population. |
| Independent assortment | Random alignment of homologous chromosomes at metaphase I giving different chromosome combinations in gametes. |
| Crossing over | Exchange of genetic material between non-sister chromatids in prophase I. |
| Mutation | Random change in DNA base sequence that creates a new allele. |
| Random fertilisation | Random fusion of any male gamete with any female gamete. |
Section 8 of 8Exam technique
1
Saying all variation is inherited — only variation due to alleles in the gametes is passed on; environmentally caused differences are not.
2
Saying mutation is the main cause of differences between siblings — most of it comes from meiosis (independent assortment, crossing over) and random fertilisation.
3
Saying the environment changes the genotype — it changes how the genotype is expressed, giving a different phenotype.
4
Forgetting that mutation is the only source of completely new alleles — the other processes only reshuffle existing alleles.
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