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AQA A Level Biology復習筆記7.2.2 Gene Pools & Allele Frequency

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月29日下午4:57

Gene Pools & Allele Frequency

Gene pools

The?phenotype?of an organism is dependent on its?genotype?and the environmental influence on the organism
Members of the?same species?will have the?same genes, of which there may exist different?alleles?(alternate versions of genes)
A?gene pool?is the collection of?genes?within an?interbreeding population?at a?particular time
- As these genes can have different alleles, a?gene pool?can also be thought of as the?sum?of all the?alleles?of the?genes?of a population (of a single species) at a?particular time

Allele frequency

How?often?different?alleles?occur in the gene pool of a population is known as the?allele frequency
The gene pool (or allele frequencies) in a species population can?change?over time due to processes such as?natural selection
When the gene pool (or allele frequencies) within a species population?changes sufficiently?over time, the?characteristics?of the species population will also change
Over time, these changes can become so great that a?new species?forms

Apparatus & Techniques: Collecting Data about the Frequency of a Phenotype

The?frequency?of a?phenotype?is simply the number of individuals in a population that have a specific, observable?trait?(a particular phenotypic?characteristic)
- Many organisms have traits that show?more than one phenotype?(e.g. shell colour in banded snails can be pink or yellow and flower colour in pea plants can be purple or white)
Phenotype frequencies can be calculated by counting the number of times a particular phenotype appears in a population (or sample of a population) and dividing this by the?total number?of individuals in the population (or the sample)
- Phenotype frequencies are normally given as a?percentage?of the total population

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

Worked Example

In a population of 9 pea plants, 7 of the plants have purple flowers, whilst 2 have white flowers. Calculate the phenotype frequencies of purple and white flowers. Give your answers as percentages.

Two phenotypes of pea plant flowers

Pea-plants-phenotype-frequency

Step 1: Calculate the phenotype frequency of purple flowers

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

= (7 ÷ 9) × 100

= 0.78 × 100

= 78%

Step 2: Calculate the phenotype frequency of white flowers

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

= (2 ÷ 9) × 100

= 0.22 × 100

= 22%

Worked Example

In banded snails (Cepaea nemoralis), shell colour is controlled by two alleles of a single gene. The pink allele (C?) is dominant over the yellow allele (C?). Out of a sample of individuals collected, 275 snails have the genotype C?C?, 150 have the genotype C?C?, and 75 have the genotype C?C?. Calculate the relative phenotype frequencies of the banded snails collected.

Two phenotypes of banded snails

Banded-snails-phenotype-frequency