Populations (A Level Only)

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The Species Concept

A species is a group of similar organisms that can reproduce to give fertile offspring.
Organisms of the same species have the same number of chromosomes in their cells.
Different species cannot produce fertile offspring because their cells have different numbers of chromosomes, so gametes cannot combine correctly.
For example, a horse (64 chromosomes) and a donkey (62 chromosomes) produce a mule with 63 chromosomes; an odd number means chromosomes cannot pair properly in meiosis, so the offspring is infertile.
Defining a species can be difficult because physical differences (variation) may exist within a species, and some different-looking organisms can still interbreed successfully.
Fertility of offspring is often the key test for species, but it is not always practical to observe.

Populations

A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
Members of a species do not live alone; they live in populations.
Populations are the unit in which allele frequencies and gene pools are studied.

Gene Pools and Allele Frequency

A gene pool is the collection of genes within an interbreeding population at a particular time.
It 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 is how often different alleles occur in the gene pool of a population.
Allele frequencies can change over time due to processes such as natural selection.
When the gene pool or allele frequencies within a species population change sufficiently over time, the characteristics of the population will also change.
Over time, these changes lead to evolution or can become so great that a new species forms.

Phenotype Frequency

Phenotype frequency is the number of individuals in a population showing a particular observable trait.
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 frequency is calculated by counting individuals showing a specific phenotype and dividing by the total number of individuals in the population or sample.
The answer is usually expressed as a percentage.
Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100.
For example, in 9 pea plants with 7 purple and 2 white flowers, the phenotype frequencies are 78% purple and 22% white.

The Hardy-Weinberg Principle

The Hardy-Weinberg principle states that allele frequencies in a population will remain constant from one generation to the next if specific conditions are met.
The equations can be used to calculate allele and genotype frequencies and to predict how these frequencies will change across generations.
The principle assumes: organisms are diploid; reproduction is sexual only; there is no overlap between generations; mating is random; the population is large; there is no migration, mutation, or selection; and allele frequencies are equal in both sexes.
No selection refers to both natural and artificial selection.
No migration means no individuals entering (immigration) or leaving (emigration) the population.
The Hardy-Weinberg principle is useful for building models and making predictions, but its assumptions are very rarely, if ever, all present in nature.

Hardy-Weinberg Equations

For a gene with two alleles, p represents the frequency of the dominant allele and q represents the frequency of the recessive allele.
The allele frequency equation is p + q = 1.
The genotype frequency equation is p² + 2pq + q² = 1.
p² represents the frequency of the homozygous dominant genotype (e.g. BB).
2pq represents the frequency of the heterozygous genotype (e.g. Bb).
q² represents the frequency of the homozygous recessive genotype (e.g. bb).
All frequencies are expressed as proportions between 0 and 1.
For example, if every individual has the homozygous dominant genotype, its frequency is 1; if half do, the frequency is 0.5.

Using the Hardy-Weinberg Equations

Start with q²: the frequency of the recessive phenotype is the only genotype you can determine directly, as it must be homozygous recessive (bb).
Identify known and unknown values: check what the question gives you (e.g. q² or p) and work out what it asks for (e.g. 2pq).
Use the appropriate equation(s) to connect them: if q² is known, find q, then use p + q = 1 to find p, then calculate p² and 2pq.
Check your calculations by substituting the three genotype frequencies into p² + 2pq + q² = 1; they should add up to 1.
Do not confuse the equations with the principle: the equations estimate allele and genotype frequencies, while the principle assumes no change in allele frequencies between generations (genetic equilibrium).

Worked Example: Recessive Phenotype Known

In a population of birds, 10% exhibit the recessive phenotype of white feathers, so q² = 0.10.
Find q: q = √0.10 = 0.32 (to 2 decimal places).
Find p: p = 1 − q = 1 − 0.32 = 0.68.
Find p²: p² = 0.68² = 0.46 (frequency of homozygous dominant genotype).
Find 2pq: 2 × 0.68 × 0.32 = 0.44 (frequency of heterozygous genotype).
Check: 0.46 + 0.44 + 0.10 = 1.
In summary: allele frequencies p = 0.68, q = 0.32; genotype frequencies p² = 0.46, 2pq = 0.44, q² = 0.10.

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1.Define the term population.
Medium
- AA group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
- BAll the different species living in a particular habitat at a particular time
- CA group of organisms of different species that can interbreed to produce fertile offspring
- DThe sum of all the alleles of the genes of a population at a particular time
2.Define the term gene pool.
Medium
- AThe sum of all the alleles of the genes of a population of a single species at a particular time
- BThe number of times that an allele occurs within a population
- CAll the genes in a single individual organism
- DThe total number of organisms in a population at a particular time
3.Which of the following are assumptions made by the Hardy-Weinberg principle? (select all that apply)
Medium
- AOrganisms are diploid
- BMating is random
- CThe population is large
- DThere is no migration, mutation, or selection
- EAllele frequencies differ between the sexes
4.The Hardy-Weinberg principle states that allele frequencies in a population will remain constant from one generation to the next if specific conditions are met.
Easy
True or false?
5.In the Hardy-Weinberg equation p + q = 1, what does p represent?
Easy
- AThe frequency of the dominant allele
- BThe frequency of the recessive allele
- CThe frequency of the heterozygous genotype
- DThe frequency of the homozygous recessive genotype
6.In the Hardy-Weinberg equation p² + 2pq + q² = 1, what does the term 2pq represent?
Medium
- AThe frequency of the heterozygous genotype
- BThe frequency of the homozygous dominant genotype
- CThe frequency of the homozygous recessive genotype
- DThe frequency of the dominant allele
7.A species of goat has coat colour controlled by one gene with alleles for brown (C^B), black (C^A) and white (C^W). The allele for brown is dominant to the other two, and the allele for black is dominant to the allele for white. Which statement defines a recessive allele?
Medium
- AAn allele that is only expressed in the phenotype when two copies are present
- BAn allele that is always expressed in the phenotype when present
- CAn allele that is never expressed in the phenotype
- DAn allele that is only expressed in heterozygous individuals
8.Match each term to its correct definition.
Medium
- Species
- Population
- Gene pool
- Allele frequency
- A group of similar organisms that can reproduce to give fertile offspring
- A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
- The sum of all the alleles of the genes of a population of a single species at a particular time
- How often different alleles occur in the gene pool of a population

Plus de questions

9.A horse has 64 chromosomes and a donkey has 62 chromosomes. Their offspring has 63 chromosomes. Why is the offspring infertile?
Easy
- AIt has an odd number of chromosomes, so they cannot pair up properly in meiosis and it cannot make gametes
- BIt has too many chromosomes, so its cells cannot divide
- CIt has different numbers of chromosomes from both parents, so its cells cannot fuse
- DIt has an even number of chromosomes, so meiosis cannot occur
10.Two organisms from different species can produce fertile offspring.
Easy
True or false?
11.In a population of birds, 10% of individuals show the recessive phenotype of white feathers. What is the frequency of the recessive allele (q)?
Medium
- A0.32
- B0.10
- C0.90
- D0.68
12.Place the steps for solving a Hardy-Weinberg problem in the correct order.
Medium
- Identify the frequency of the recessive phenotype (q²)
- Calculate q by taking the square root of q²
- Calculate p using p = 1 - q
- Calculate the required genotype frequency using p², 2pq or q²
13.Why is it easier to calculate a phenotype frequency than an allele frequency?
Medium
- APhenotypes are observable, whereas alleles can only be inferred from genotype data
- BPhenotypes are always controlled by a single gene
- CAllele frequencies are always the same as phenotype frequencies
- DPhenotypes can be counted directly, but alleles cannot be observed
14.In a population of species X, the frequency of the recessive allele is 0.21. Use the equation p + q = 1 to calculate the frequency of the dominant allele.
Medium
- A0.79
- B0.21
- C0.41
- D0.58
15.Which of the following conditions must be met for the Hardy-Weinberg principle to apply? (select all that apply)
Hard
- ANo migration
- BNo mutation
- CNo selection
- DRandom mating
- ESmall population size
16.The Hardy-Weinberg principle can be applied to any population in nature because its assumptions are always met.
Easy
True or false?
17.Which of the following best describes the relationship between allele frequency and evolution?
Medium
- AWhen allele frequencies within a population change sufficiently over time, the characteristics of the population change, leading to evolution or even a new species
- BAllele frequencies always remain constant, so evolution cannot occur
- CEvolution occurs only when allele frequencies remain unchanged
- DAllele frequencies change only due to mutation, not natural selection
18.In a population of snails, 84% have pink shells (dominant phenotype). What percentage of the population would you expect to be heterozygous? Use the Hardy-Weinberg equation.
Medium
- A48%
- B36%
- C16%
- D84%
19.What does the term 'gene pool' refer to?
Medium
- AThe sum of all the alleles of the genes of a population of a single species at a particular time
- BThe total number of genes in an individual organism
- CThe number of different species in a habitat
- DThe frequency of a single allele in a population
20.Allele frequency is the number of times that an allele occurs within a gene pool.
Easy
True or false?
21.A chi-squared test gave a probability of more than 0.05. What can be concluded?
Medium
- AThere is no significant difference between the observed and expected results
- BThere is a significant difference between the observed and expected results
- CThe null hypothesis should be rejected
- DThe observed results are exactly as expected
22.Which of the following is NOT an assumption of the Hardy-Weinberg principle?
Medium
- ASmall population size
- BRandom mating
- CNo mutation
- DNo migration
23.In a population of 100 individuals, 16 have the homozygous recessive genotype (bb). What is the frequency of the heterozygous genotype?
Medium
- A0.48
- B0.36
- C0.16
- D0.84
24.Which of the following are conditions required for the Hardy-Weinberg principle to be correctly applied to a population? (select all that apply)
Medium
- AThe population is large
- BMating is random
- CThere is no migration, mutation or selection
- DOrganisms reproduce asexually
- EThere is overlap between generations
25.Which of the following statements about the Hardy-Weinberg principle are correct? (select all that apply)
Medium
- AAllele frequencies remain constant from one generation to the next if conditions are met
- BIt can be used to calculate allele and genotype frequencies
- CIt can predict how allele frequencies will change across generations
- DIt assumes that natural selection occurs
- EIt requires that organisms are haploid
26.Match each term to its correct description.
Easy
- Species
- Population
- Gene pool
- Allele frequency
- A group of similar organisms that can reproduce to give fertile offspring
- A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
- The sum of all the alleles of the genes of a population at a particular time
- How often different alleles occur in the gene pool of a population
27.Place the following steps in the correct order to calculate genotype frequencies using the Hardy-Weinberg equation when given the percentage of individuals showing the recessive phenotype.
Medium
- Identify the frequency of the recessive phenotype (q²)
- Calculate q by taking the square root of q²
- Calculate p using p = 1 - q
- Calculate the heterozygous frequency using 2pq
28.A population of 200 individuals has 72 individuals with the homozygous recessive genotype (aa). Which of the following are correct? (select all that apply)
Medium
- Aq² = 0.36
- Bq = 0.6
- Cp = 0.4
- DThe frequency of heterozygous individuals is 0.36
- EThe frequency of homozygous dominant individuals is 0.16
29.In a population of 500 individuals, 45 have the recessive phenotype. Which of the following are correct? (select all that apply)
Hard
- AThe frequency of the recessive allele is 0.3
- BThe frequency of the dominant allele is 0.7
- CThe number of heterozygous individuals is 210
- DThe number of homozygous dominant individuals is 245
- EThe frequency of the recessive allele is 0.09
30.Which of the following are assumptions of the Hardy-Weinberg principle? (select all that apply)
Medium
- ANo mutation occurs
- BMating is non-random
- CThe population is large
- DNo natural selection occurs
- EThere is migration into the population
31.Match each genotype frequency term in the Hardy-Weinberg equation to what it represents.
Medium
- p²
- 2pq
- q²
- Frequency of homozygous dominant genotype
- Frequency of heterozygous genotype
- Frequency of homozygous recessive genotype
32.Order the steps to calculate the percentage of heterozygous individuals in a population where 16% show the recessive phenotype.
Medium
- q² = 0.16
- q = 0.4
- p = 0.6
- 2pq = 2 × 0.6 × 0.4 = 0.48
- Convert to percentage: 48%
33.A population of 1000 individuals has 90 individuals with the recessive phenotype. Which of the following are correct? (select all that apply)
Hard
- AThe frequency of the recessive allele is 0.3
- BThe frequency of the dominant allele is 0.7
- CThe number of heterozygous individuals is 420
- DThe number of homozygous dominant individuals is 490
- EThe frequency of the recessive allele is 0.09
34.Which of the following statements about the Hardy-Weinberg principle are true? (select all that apply)
Medium
- AIt assumes that allele frequencies remain constant from generation to generation
- BIt can be used to calculate the frequency of a recessive allele from the frequency of the recessive phenotype
- CIt assumes that natural selection is acting on the population
- DIt requires that the population is small
- EIt assumes random mating
35.In a population of 250 individuals, 40 individuals show a recessive phenotype. What is the frequency of the recessive phenotype in this population?
Medium
- A0.16
- B0.40
- C0.84
- D0.64
36.A population has a recessive allele frequency q = 0.4. What is the expected frequency of heterozygous individuals?
Medium
- A0.24
- B0.48
- C0.40
- D0.60
37.In a population of 200 individuals, 72 are homozygous recessive. What is the frequency of the homozygous dominant genotype?
Medium
- A0.16
- B0.36
- C0.48
- D0.60
38.A recessive condition affects 1 in 10,000 individuals. What is the frequency of carriers (heterozygotes) in the population?
Hard
- A0.0001
- B0.0099
- C0.0198
- D0.9801
39.In a population of 200 individuals, 72 have the homozygous recessive genotype (aa). What is the frequency of the recessive allele (q)?
Medium
- A0.36
- B0.60
- C0.48
- D0.40
40.In a population of chickens, 64% are resistant to brodifacoum (dominant allele B^R). Calculate the percentage of chickens that are heterozygous.
Hard
- A16%
- B32%
- C48%
- D64%
41.In a population of cattle, 12 individuals are white, 15 are red, and 28 are roan. Calculate the frequency of the red phenotype. Give your answer as a decimal to two decimal places.
Medium
42.State the use of the Hardy-Weinberg equation.
Easy
43.Complete the sentence about the Hardy-Weinberg principle.
Medium
The Hardy-Weinberg principle states that allele frequencies in a population will remain ____ from one generation to the next if specific conditions are met.
44.In a population of 100 individuals, there are 200 alleles for a particular gene. If there are 120 dominant alleles, calculate the frequency of the dominant allele (p).
Medium
45.In a population of 160 chickens, 36 have genotype B^R B^R, 118 have genotype B^R B^V, and 6 have genotype B^V B^V. Calculate the frequency of the allele B^R. Give your answer as a decimal to two decimal places.
Hard
46.Complete the Hardy-Weinberg equation for genotype frequencies.
Medium
p² + ____ + q² = 1
47.In a population, 1 in 3500 individuals are born with Tay Sachs disease (a recessive condition). Calculate the frequency of the recessive allele (q). Give your answer to three decimal places.
Hard
48.In a population of 500 individuals, 45 have the homozygous recessive genotype. Calculate the frequency of the recessive allele (q) to two decimal places.
Medium
49.In a population of 1000 individuals, 90 have the recessive phenotype. Calculate the number of heterozygous individuals expected.
Medium
50.In the general population, 1 in 320,000 individuals are born with Tay Sachs disease. In the Ashkenazi Jewish community, 1 in 3,500 are born with it. Calculate how many times higher the frequency of the Tay Sachs allele is in the Ashkenazi Jewish community compared to the general population. Give your answer to two significant figures.
Hard
51.Define the term species.
Medium
52.Explain one reason why the frequency of an allele might change over time.
Medium
53.State three assumptions made by the Hardy-Weinberg principle.
Hard
54.Explain why the frequency of an allele might change over time.
Hard
55.In a population of chickens, 64% are resistant to brodifacoum. Resistance is conferred by the dominant allele B^R. Calculate the percentage of chickens that are heterozygous. Show your working.
Hard
56.In a population of cattle, 12 individuals are white, 15 are red, and 28 are roan. Calculate the frequency of the red phenotype. Give your answer as a decimal to two decimal places.
Medium
- A0.27
- B-0.73
- C1.27
- D2.27
57.In a population of 100 individuals, there are 200 alleles for a particular gene. If there are 120 dominant alleles, calculate the frequency of the dominant allele (p).
Medium
- A2.6
- B1.6
- C0.6
- D-0.4
58.In a population of 160 chickens, 36 have genotype B^R B^R, 118 have genotype B^R B^V, and 6 have genotype B^V B^V. Calculate the frequency of the allele B^R. Give your answer as a decimal to two decimal places.
Hard
- A-0.41
- B2.59
- C1.59
- D0.59
59.In a population, 1 in 3500 individuals are born with Tay Sachs disease (a recessive condition). Calculate the frequency of the recessive allele (q). Give your answer to three decimal places.
Hard
- A0.017
- B-0.983
- C1.017
- D2.017
60.In a population of 500 individuals, 45 have the homozygous recessive genotype. Calculate the frequency of the recessive allele (q) to two decimal places.
Medium
- A0.3
- B2.3
- C1.3
- D-0.7
61.In a population of 1000 individuals, 90 have the recessive phenotype. Calculate the number of heterozygous individuals expected.
Medium
- A378
- B504
- C420
- D462
62.In the general population, 1 in 320,000 individuals are born with Tay Sachs disease. In the Ashkenazi Jewish community, 1 in 3,500 are born with it. Calculate how many times higher the frequency of the Tay Sachs allele is in the Ashkenazi Jewish community compared to the general population. Give your answer to two significant figures.
Hard
- A8.6 times
- B11.6 times
- C10.6 times
- D9.6 times

Notes de révision

## Populations - **Species**: a group of similar organisms that can reproduce to give fertile offspring. - Members of the same species have the same number of chromosomes; different species can't produce fertile offspring (e.g. horse 64, donkey 62 → offspring 63, odd number can't pair in meiosis, infertile). - **Population**: a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed. - Defining a species can be difficult: physical variation exists, different-looking organisms (e.g. hawthorn trees) can interbreed, and fertility of offspring is often not practical to observe. ## Gene Pools & Allele Frequency - **Gene pool**: the collection of genes within an interbreeding population at a particular time; also 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. - Allele frequencies change over time due to processes such as natural selection. Sufficient change alters the population's characteristics, leading to evolution or even a new species. - **Phenotype frequency** = (total individuals with phenotype ÷ total individuals in population) × 100. - It is easier to calculate phenotype frequency than allele frequency because phenotypes are observable, whereas alleles (especially recessive ones in heterozygotes) are not directly visible. ## Hardy-Weinberg - **Principle**: allele frequencies in a population will remain constant from one generation to the next if specific conditions are met. - Used to calculate allele and genotype frequencies and predict how they change across generations. - **Conditions/assumptions**: organisms are diploid; sexual reproduction only; no overlap between generations; random mating; large population; no migration, mutation or selection; allele frequencies equal in both sexes. - These assumptions are very rarely all present in nature. - **Allele frequencies**: p = frequency of dominant allele (B); q = frequency of recessive allele (b). - **p + q = 1** - **Genotype frequencies**: p² = homozygous dominant (BB); 2pq = heterozygous (Bb); q² = homozygous recessive (bb). - **p² + 2pq + q² = 1** - All frequencies are proportions between 0 and 1. ### Calculation tips 1. Start with q² — the recessive phenotype is the only genotype you can determine directly (must be homozygous recessive). 2. Identify known and unknown values; use the appropriate equation(s). 3. Don't confuse the equations (which estimate allele and genotype frequencies) with the principle (which assumes no change between generations, i.e. genetic equilibrium). ### Worked example (birds, 10% recessive white) - q² = 0.10 → q = 0.32 - p = 1 − 0.32 = 0.68 - p² = 0.68² = 0.46 - 2pq = 2 × 0.68 × 0.32 = 0.44 - Check: 0.46 + 0.44 + 0.10 = 1

Species: A group of similar organisms that can reproduce to give fertile offspring.
Population: A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
Gene pool: The collection of genes within an interbreeding population at a particular time; 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.
Phenotype frequency: The number of individuals in a population showing a particular observable trait; calculated as (total individuals with phenotype ÷ total individuals in population) × 100.
Hardy-Weinberg principle: Allele frequencies in a population will remain constant from one generation to the next if specific conditions are met.
p + q = 1: Hardy-Weinberg allele frequency equation: p = frequency of the dominant allele, q = frequency of the recessive allele.
p² + 2pq + q² = 1: Hardy-Weinberg genotype frequency equation: p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive.

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