Monohybrid inheritance

Objectives

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• Explain the meaning of the key terms relating to inheritance.
• Explain the principle underlying dominant and recessive alleles
• Draw dominant/ recessive monohybrid crosses to predict offspring genotypes and phenotypes

Definitions

• Gene: A sequence of DNA bases at a specific locus on a chromosome that codes for one polypeptide
• e.g. Pea plants have a gene that controls the shape of the pea
• Allele: An alternative form of a gene, found at the same locus but may produce a different phenotype
• e.g. The gene for pea shape has two alleles: S and w

Definitions

Definitions

• Genome: the complete set of genes in a cell.
• Genotype: the genetic constitution of an organism i.e. the specific genes/alleles an organism possesses.
• e.g. A wrinkled pea has the genotype ww.
• Phenotype: The observable characteristics of an organism. A product of interactions between the genotype and the environment.
• e.g. The genotype ww results in the phenotype of wrinkled peas. Though soaking in water could turn the peas smooth.

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Genotype → Phenotype

• Genes code for polypeptides (proteins).
• The proteins control phenotype in their roles as enzymes, pumps, hormones or structural elements. E.g.

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Definitions

• Homozygous: An organism that has the same alleles for a particular gene on each homologous chromosome.
• e.g. Wrinkled peas have the same allele on both homologous chromosomes (ww)
• Heterozygous: An organism that has two different alleles for the same gene (characteristic).
• e.g. Smooth peas can have different alleles on each homologous chromosome (Sw)

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Definitions

• Dominant: When one allele masks or prevents the expression of another.
• e.g. The allele for smooth peas (S) is dominant over the allele for wrinkled peas (w), so smooth peas can have either Sw or SS alleles
• Recessive: When one allele is masked or prevented from expression by another.
• e.g. The allele for smooth peas (S) is dominant over the allele for wrinkled peas (w), so wrinkled peas must have ww alleles

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Genetic crosses

AKA Punnett squares

• E.g. a cross between two smooth peas with a heterozygous genotype
• What is the likelihood of there being a wrinkled pea in the offspring?

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25% chance of the wrinkled phenotype

Genetics question

• The fruit fly (Drosophila) is a small insect, one variety of which has a light-coloured body and the other a black coloured body.
• A cross was made between a pure breeding Drosophila with a light coloured body and one with black coloured body. The resulting offspring all had light-coloured bodies.
• When these were offspring were interbred their offspring consisted of 380 light coloured flies and 116 black coloured flies.
• Assuming that body colour is controlled by one pair of alleles, use genetic diagrams to give an explanation for these results.

Light = A Black = a

Genetics question

Can you determine all of the genotypes, assuming a straight finger is the recessive phenotype

Definitions

• Monohybrid inheritance: The study of the inheritance of alleles for one gene.
• e.g. Studying the inheritance of the alleles for smooth and wrinkled peas
• Dihybrid inheritance: The study of the inheritance of alleles for two genes at the same time.
• e.g. Studying the inheritance of the alleles for smooth and wrinkled peas, and the inheritance of the alleles for green and yellow peas

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Activity

• Answer the exam questions on monohybrid inheritance

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Activity

• First 15 minutes of next lesson:
• Go to L3
• Read pages 429 - 432 in your textbook
• Answer the summary Qs on pages 431 and 432
• Mr McCall will supervise you until I arrive

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Codominance and multiple alleles

Objectives

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• Define what is meant by co-dominant alleles
• Draw co-dominant monohybrid crosses to predict offspring genotypes and phenotypes
• Describe how to represent alleles in multiple allele crosses
• Draw multiple allele crosses to predict offspring genotypes and phenotypes

Definitions

• Co-dominant: When neither of a pair of different alleles masks the other, both are expressed equally.
• e.g. In snapdragons the C^R allele produces red flowers, the C^w allele produces white flowers, but the C^RC^w heterozygote has pink flowers

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Genetics question

• Another example of codominance is feather colour in chickens. The feather colour gene F has two alleles: F^B (black) and F^W (white). A heterozygote has chequered feathers.
• Use a genetic cross to describe the offspring of a cross between two chickens with chequered feathers.

Definitions

• Multiple alleles: When there are more than two different alleles for a particular gene.
• e.g. In humans blood group is controlled by three alleles I^A, I^B or i (sometimes called I^O)

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i

Genetics question

• In humans the ABO blood groups are determined by three alleles I^A, I^B and i. The I^A and I^B alleles are co-dominant to each other. The i allele is recessive to I^A and I^B .
• A women of blood group A marries a man of blood group B. What are the potential phenotypes of her children?
• A women of blood group A has a child of blood group O. Could the father have been a man of blood group AB?
• Could the father have been a man of blood group B?

Genetics question

• In dogs, coat colour is determined by a series of multiple alleles. The alleles As produces a uniformly dark coat, the allele ay produces a tan coat colour and the allele at produces a spotted coat.
• The dominance hierarchy is As • ay • at which means that As is dominant to both ay and at, but ay is dominant only to at. A family tree for dogs showing these coat colours is given below.
• Determine the genotypes for numbers 1 - 6

= dark male

= tan male

= spotted male

= dark female

= tan female

= spotted female

Dihybrid crosses and epistasis

Objectives

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• Draw dihybrid crosses to predict offspring genotypes and phenotypes
• Explain what is meant by epistasis
• Apply knowledge to calculate the predicted ratios of genotypes and phenotype of offspring, using fully labelled diagrams, when supplied with appropriate information

Definitions

• Monohybrid inheritance: The study of the inheritance of alleles for one gene.
• e.g. Studying the inheritance of the alleles for smooth and wrinkled peas
• Dihybrid inheritance: The study of the inheritance of alleles for two genes at the same time.
• e.g. Studying the inheritance of the alleles for smooth and wrinkled peas, and the inheritance of the alleles for green and yellow peas

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Genetics question

• In a plant the allele for tall (A) is dominant to short (a) and red flower colour (B) is dominant to white (b). A cross was made between a pure breeding tall red plant and a pure breeding short white plant. What would the genotypes and phenotypes be:
• In the F1 generation ?
• In the F2 generation ?
• If you back crossed the F1 with its white parent ?

Parents: AABB x aabb

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All offspring are heterozygotes, so display both dominant traits

Gametes: AB x ab

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Parents: AaBb x AaBb

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Phenotype: Tall, red Tall, white Short, red Short, white

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Ratio: 9 : 3 : 3 : 1

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If there is no linkage, a dihybrid cross of two organisms heterozygous for both traits should always result in a 9:3:3:1 ratio of phenotypes

Gametes: AB, Ab, aB, ab x AB, Ab, aB, ab

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Parents: AaBb x aabb

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Gametes: AB, Ab, aB, ab x ab

Phenotype: Tall, red Tall, white Short, red Short, white

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Ratio: 1 : 1 : 1 : 1

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If there is no linkage, a dihybrid cross of one organism heterozygous for the two traits and one organism homozygous recessive for the two traits will always result in a 1:1:1:1 ratio of phenotypes

Definitions

• Epistasis: when two genes (that are not alleles) interact so that one suppresses the effect of another.
• e.g. If a person has the alleles for albinism, their hair, skin, and/or eyes will little or no have pigment no matter what their alleles for each of these traits code for.

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Epistasis

• Epistasis normally results when two genes are involved in the same pathway.
• Here, the C gene shows epistasis over the A gene for hair colour in mice.

Genetics question

• The production of pigment in Himalayan rabbit fur is controlled by two genes.
• One gene controls whether any pigment is made. This gene has three alleles. Allele A codes an active enzyme which converts a white pigment into a black pigment. Allele A^h codes for a form of the enzyme which becomes inactive at temperatures close to a rabbit’s core body temperature, so only the face, ears, legs and tail are pigmented. A third allele, a, does not code for a functional enzyme.
• The other gene controls the density of pigment in the fur. This gene has two alleles. Allele B is dominant and moves large amounts of pigment, so the fur appears black. Allele b results in less pigment being moved, so the fur appears brown.
• Give all the possible genotypes for a Himalayan rabbit with white fur.

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• Give all the possible genotypes for a Himalayan rabbit with black face, ears, legs and tail.

aaBB, aaBb, aabb

A^hA^h BB, A^haBB, A^hA^h Bb, A^haBb;

The Chi squared test

Objectives

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• Use the chi-squared test to compare the goodness of fit of observed phenotypic ratios with expected ratios.

Spearman’s rank correlation coefficient

Student’s

t-test

X² test

Start

Looking at data that are measurements

Looking at data that are in categories

Looking for differences in the same variable from different samples

Looking for a correlation between different variables from the same sample

Chi squared test

• When can it be used?
• The chi-squared test is concerned with frequencies i.e. the investigation involves collecting data about the number of individuals in particular categories
• What is it normally used for?
• Genetic cross experiments
• Choice chamber behavioural studies
• Counts of the number of individuals of different species

Chi squared test

Inheritance in corn

• Work through the monohybrid crosses
• Assess whether the actual numbers for each phenotype are as expected, by:
• Calculating the expected numbers for each phenotype
• Using the Chi squared test and p ≤ 0.05 critical tables values
• Work through the dihybrid cross
• Carry out another Chi squared test

Sex-linkage and autosomal linkage

Objectives

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• Explain what is meant by sex-linked alleles, and describe how to represent these
• Draw sex-linked crosses to predict offspring genotypes and phenotypes
• Explain what is meant by autosomal linkage
• Apply knowledge to calculate the predicted ratios of genotypes and phenotype of offspring, using fully labelled diagrams, when supplied with appropriate information

Definitions

• Sex linkage: When a gene is located on a sex chromosome (almost always the x chromosome).
• e.g. genes for red and green colour vision are found on the X chromosome. Mutations in these leads to colour blindness, causing 8% of males but only 0.7% of females to be colour blind. X^rX^r or X^rY
• Carrier: an individual that has one copy of a recessive allele that causes a genetic disorder in individuals that are homozygous for this allele.
• e.g. only females can be carriers of colour blindness. X^RX^r

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Genetics question 5

• In Drosophila fruit flies red eyes (R) are dominant to white eyes (r). The gene controlling the trait is located on the X chromosome. Use genetic crosses to show why:
• When a red-eyed female is crossed with a white-eyed male, the offspring all have red eyes
• When a white-eyed female is crossed with a red-eyed male, the male offspring all had white eyes

Definitions

• Autosomal linkage: When two genes are located on the same chromosome causing the alleles to be inherited together.
• e.g. if the genes for pea shape and pea colour are on the same chromosome then they are autosomally linked

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The Hardy-Weinberg principle

Objectives

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• Explain what is meant when we refer to allele frequencies and a gene pool
• Explain what the Hardy–Weinberg principle predicts
• Explain the conditions under which Hardy–Weinberg principle applies
• Describe and use the mathematical equations used to express allele and genotype frequencies

Definitions

• Gene pool: the complete range of alleles for all genes in the organisms in a population
• Allele frequency: the proportion of a particular type of allele in a population (e.g. A or a)
• Evolution: a change in the allele frequencies in a population.
• Genotype frequency: the proportion of a particular allele combination in a population (e.g. AA, Aa or aa)
• Phenotype frequency: proportion of a particular characteristic in a population (e.g. wrinkled peas or smooth peas)

Hardy-Weinberg principle

• The allele frequency in a population remain constant over time if:
• There are no mutations
• There is no immigration or emigration
• There is no selection
• Mating is random
• The population is large

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Hardy-Weinberg principle

• If the gene pool is stable then we can use an equation to calculate the allele frequencies in a population:

p + q = 1

• Where p is the frequency of the dominant allele and q is the frequency of the recessive allele
• E.g. if 30% of the alleles for a gene are dominant (B) then the remaining 70% must be recessive (b):

0.3 + 0.7 = 1

Hardy-Weinberg principle

• The gametes produced by organisms in this population will only have one allele each – B or b
• We don’t know the allele in any particular gamete but we know that overall the frequencies of the B and b alleles will be the same as in the gene pool:

Hardy-Weinberg principle

• Using this information we can therefore calculate the genotype frequencies:

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p² + 2pq + q² = 1

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• And so also determine the phenotype frequencies

• Let’s take a population of 1000 cats with 840 black cats and 160 white cats.
• The phenotype frequency for black is 0.84 (840/1000) and for white is 0.16 (160/1000)
• We know that white is the recessive allele, so the white cats must be homozygous recessive, so the frequency of the genotype bb is 0.16

⇒ The genotype bb has a frequency q², so q² = 0.16

⇒ q = √q² = √0.16 = 0.4

⇒ (p + q = 1, so) p = 1 – q = 1 – 0.4 = 0.6

• Now we can calculate the genotype frequencies:

⇒ frequency of BB = p² = 0.6² = 0.36

⇒ frequency of Bb = 2pq = 2 x 0.6 x 0.4 = 0.48

⇒ frequency of bb = q² = 0.16 (already found)

⇒ check that the these add up to one = 1.00

• We can convert these frequencies to actual numbers in the population:

⇒ Number of homozygous dominant cats = 0.36 x 1000 = 360

⇒ Number of heterozygous cats = 0.48 x 1000 = 480

Question 1

• If 98 out of 200 individuals in a population express the recessive phenotype, what percent of the population would you predict would be heterozygotes?

Answer 1

• 98/200 = (q²)
• 0.49 = q²
• 0.7 = q

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• p + q = 1
• p = 1 – 0.7
• p = 0.3

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• 2pq = 2(0.3)(0.7) = 0.42 = 42% heterozygotes

Question 2

• Your original population of 200 was hit by a tidal wave and 100 organisms were wiped out, leaving 36 homozygous recessive out of the 100 survivors. If we assume that all individuals were equally likely to be wiped out, how did the tidal wave affect the predicted frequencies of the alleles in the population?

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Answer 2

• 36/100 = q²
• 0.6 = q
• So now 60%, was 70%

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• p + q = 1
• p = 0.4
• So now 40%, was 30%

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Question 3

• Brown fur coloring is dominant to grey fur coloring in mice. If you have 168 brown mice in a population of 200 mice... What is the predicted frequency of
• Homozygous dominants
• Heterozygotes
• Homozygous recessives

Answer 3

• 200 mice in total
• 168 = brown = p² + 2pq
• 32/200 = grey fur = q²
• 0.16 = q²
• 0.4 = q
• p = 0.6 (p + q = 1)

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• p² = 0.36 = 36% homozygous dominant
• 2pq = 0.48 = 48% heterozygotes
• q² = 0.16 = 16% homozygous recessive

Question 4

• If 81% of a population is homozygous recessive for a given trait. Calculate
• Frequency of homozygous dominant
• Frequency of heterozygotes
• Frequency of dominant and recessive alleles

Answer 4

• q² = 0.81
• q = 0.9
• p = 0.1

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• p² = 0.01 = 1% homozygous dominant
• 2pq = 0.18 = 18% heterozygotes

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Question 5

• If 51% of the population carries at least one copy of the recessive allele what is the predicted frequency of the population expressing the dominant phenotype

Answer 5

• 51% = 2pq + q²
• 49% = 0.49 = p²
• 0.7 = p
• 0.3 = q

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• p² + 2pq =
• 0.49 + 0.42 = 0.91 have dominant phenotype

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Question 6

• Albinism is a rare genetically inherited trait that is only expressed in the phenotype of homozygous recessive individuals (aa).  The most characteristic symptom is a marked deficiency in the skin and hair pigment melanin.  This condition can occur among any human group as well as among other animal species.  The average human frequency of albinism in North America is only about 1 in 20,000. Calculate the frequency of the dominant allele in North America.

Answer 6

• q² = 1/20,000
• q = 0.0071
• p = 0.9929

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• dominant phenotype =
• p² + 2pq = 0.9859 + 0.1409

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Question 7

• 1 in 1700 US Caucasian new borns have cystic fibrosis.
• calculate the frequency of the recessive cystic fibrosis allele and the dominant allele in the population
• calculate the frequency of non cystic fibrosis sufferers in the population

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Answer 7

• q² = 1/1700
• q = 0.0243
• p = 0.9757

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• p² + 2pq
• (0.09757)(0.09757) + 2(0.09757)(0.0243)
• 0.9567

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Question 8

• If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?

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Answer 8

• q² = 9%
• q² = 0.09
• q = 0.3
• p = 0.7

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• 2pq = 2(0.3)(0.7) = 0.42 = 42%

Question 9

• The allele y occurs with a frequency of 0.8 in a population of clams. Give the frequency of genotypes YY, Yy, and yy.

Answer 9

• The allele y (recessive) has a frequency q = 0.8.
• p + q = 1, then p = 1 – 0.8 = 0.2

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• genotype:
• YY genotype frequency = p² = 0.04
• Yy genotype frequency = 2pq = 0.32
• yy genotype frequency = q² = 0.64.

Question 10

• In the year 2374, humans finally developed the technology necessary for time travels. You are a scientist interested in the population genetics of extinct animals. Taking advantage of this technological advance, you decide to go to the past 8 million years to conduct a field work in Venezuela to study a population of Phoberomys pattersoni*, the world’s largest extinct rodent weighing approximately 700 kg (1500 lb) and looking vaguely like a giant guinea pig.
• The coat color of this rodent varies between tan (dominant) and brown (recessive). Assume the population is in Hardy-Weinberg equilibrium. You observed 336 tan Phoberomys and 64 brown Phoberomys during your study.
• What is the frequency of the homozygous recessive genotype
• What is the allelic frequency of the dominant (tan) allele in the population?
• Of the animals you observed, how many were heterozygous?

Answer 10

• There are 336 + 64 = 400 animals in the population.
• 64 are homozygous recessive (brown)
• Frequency of homozygous recessive = q² = 64/400 = 0.16

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• Since q² = 0.16, take the square root to get q = 0.4
• p + q = 1 (formula for allele frequencies)
• Frequency of the dominant allele p = 0.6

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• 2pq = 0.192

Question 11

• You make another trip to Venezuela and this time you observe 650 animals.
• How many of the 650 animals would you expect to be tan, assuming the population is still in Hardy-Weinberg equilibrium?
• How many of these 650 animals would you expect to be brown, assuming the population is still in Hardy-Weinberg equilibrium?

Answer 11

• If the population is still in H-W equilibrium, then the allele frequencies would be the same: p = 0.6, q = 0.4
• The tan phenotype = p² + 2pq
• (0.6)² + (2)*(0.6)*(0.4) = 0.84
• 0.84 * 650 = 546 tan

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• p² = (0.6)² = 0.36,
• (0.36)*(650) = 234

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• Brown animals are homozygous recessive
• Frequency of brown is q² = (0.4)² = 0.16
• (0.16)*(650) = 104

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Question 12

• As you observe the animals, you count 200 brown Phoberomys and 450 tan.
• Conduct a chi-square test to determine if your observations are significantly different from what you expect.

Answer 12

• The calculated X² is 105.5
• There are 2 phenotypes (brown and tan), so there is 1 degree of freedom (2 – 1 = 1)
• The theoretical X² for 1 degree of freedom is 3.841, which is much smaller than our calculated one. Therefore, we reject the null hypothesis that the population of 650 is in
• H-W equilibrium. Our observations are significantly different from our expectation, assuming H-W equilibrium.

Variation

Objectives

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• Explain why individuals within a population of a species may show a wide range of variation in phenotype
• Describe variation and continuous or discontinuous based on trends in graphs, and link this to the causes of variation.

Sources of variation

• Causes of variation in the genome of an organism:

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• Individuals are also influenced by the environment

Genotype + environment = phenotype

Discontinuous variation

• There are distinct categories with no overlap between categories
• Are controlled by a small number of genes
• Are largely unaffected by the environment

Continuous variation

• No categories to place individuals
• Are controlled by a large number of genes
• Are significantly affected by the environment

Standard deviation

• Shows how much variation exists around the mean average.
• A small standard deviation indicates that the data points tend to be very close to the mean
• A large standard deviation indicates that the data points are spread out over a large range of values

Activities

• Collect the data specified on the experimental sheet
• Complete the statistical analysis
• Homework: answer the questions

Natural selection

Objectives

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• Explain what is meant by a selection pressure, and which factors can act as selection pressures
• Explain how natural selection is linked to adaptation and inheritance of alleles by the next generation
• Explain the concept of reproductive success

Reproductive Success

• Fact 1: all organisms produce more offspring than can be supported
• Fact 2: the gene pool of a population contains a wide variety of alleles
• Fact 3: all organisms face selection pressures that determine their ability to survive

Natural selection

• Predation, disease and competition means that there is competition for survival - each of these provide a different selection pressure
• Individuals with a certain combinations of alleles have adaptations that make them more likely to survive and reproduce
• Their advantageous alleles are passed onto the next generation
• Over many generations, the frequency of the advantageous alleles increases and the frequency of non-advantageous alleles decrease
• So the frequency of the successful phenotypes also increase in subsequent generations

Examples of natural selection

• Peppered moths
• Bacterial resistance to antibiotics
• Beak size on Daphne Major

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La Nina = cold temperatures and drought, and more larger, harder seeds

El Nino = warmer temperatures and rainfall, and more smaller softer seeds

Activities

• Answer the exam questions on variation and natural selection

Selection

Objectives

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• Recall what is meant by allele frequency
• Explain what is meant by stabilising, directional and disruptive selection in the context of the effect that each has on allele frequencies

Definitions

• Gene pool: all the alleles of all the genes of all the individuals in a population at any one time.
• Allele frequency: the number of times an allele occurs within a gene pool
• Evolution: a change in allele frequency over time (due to selection)

Types of Selection

Activity

• Measure the length of 30 algal cells
• Add your results to the spreadsheet (https://goo.gl/ypaM2i)
• Produce a histogram of algal cell length
• Suggest:
• What causes the stabilising selection for this population?
• What could cause disruptive selection?

Genetic drift and speciation

Objectives

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• Describe genetic drift
• Explain the importance of genetic drift in small populations
• Explain how natural selection and isolation may lead to the formation of a new species
• Explain what is meant by allopatric and sympatric speciation
• Explain possible mechanisms for reproductive isolation resulting in sympatric speciation

Genetic drift

• Genetic drift - The change in frequency of an allele in a population over time.
• Genetic drift occurs most often in smaller populations
• Genetic drift and the population bottlenecks are linked:
• The ‘founding’ population is only made up of a small number of individuals with a small number of alleles
• The new population has non-representative allele frequencies compared to the parent population
• Genetic drift causes allele frequency to become different to the parent population

The Mainland

Island

The Mainland

Island

Population

The Mainland

A few individuals colonise a new isolated area

The Mainland

Genetic drift causes a change in allele frequency

Example of genetic drift

• Work in pairs
• Count out 15 m&ms or midget gems - this is your founder population - record the number and colour of these alleles
• Randomly select (with eyes closed) 5 sweets - record the colours and numbers of these 5 alleles. These sweets bred successfully.
• When each individual in this generation dies it can leave a maximum of 3 offspring (or 3 sweets of the same colour). So replace each sweet with 3 sweets of the same colour - record the new allele frequencies. (If there are not enough sweets of that colour then sadly the offspring died at a young age).
• Repeat steps 2 and 3 five times and keep a record of all allele frequencies.
• Have you experienced genetic drift? Is the drift the same for each population/group?

Speciation

• Speciation is the evolution of a new species from an existing species
• Can you recall the sequence of events you learnt at GCSE?

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Speciation

• Gene pool isolation i.e. no gene flow
• Evolution i.e. changes in allele frequency occur in each population
• Reproductive isolation i.e. organisms can no-longer breed to produce fertile offspring

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• Speciation can occur as a result of:
• Allopatric speciation
• Sympatric speciation

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Allopatric speciation

• Organisms are geographically separated e.g. by mountain ranges or rivers
• Organisms in each population experience different selection pressures
• Different alleles will be advantageous in each population
• Mutations occur differently in each population

Sympatric speciation

• Organisms live in the same habitat at the same time
• Random mutations occur that cause an organism (or organisms) to be reproductively isolated from other individuals
• Normally requires the organism to be able to reproduce asexually (initially, at least)

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Reproductive isolation

• In both allopatric and sympatric speciation, reproductive isolation could be as a result of:
• Seasonal changes - e.g. a change in flowering/mating season
• Mechanical changes - e.g. genitalia are no longer complementary
• Behavioural changes - e.g. a change in courtship behaviour
• Chromosomal changes - e.g. a chromosome divides in two, or an organism becomes polyploid

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