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Population Genetics Starter - write biological definitions for the following key words: Population Genetics Starter - write biological definitions for the following key words:

Population Genetics Starter - write biological definitions for the following key words: - PowerPoint Presentation

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Population Genetics Starter - write biological definitions for the following key words: - PPT Presentation

Gene Allele Population Gene pool Population Genetics Starter write biological definitions for the following key words Gene a section of DNA that codes for a particular proteinpolypeptide ID: 778717

population allele alleles gene allele population gene alleles probability frequency hardy weinberg 2pq recessive 00063 pool principle 9937 genetics

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Presentation Transcript

Slide1

Population Genetics

Starter - write biological definitions for the following key words:

Gene

Allele

Population

Gene

pool

Slide2

Population Genetics

Starter - write biological definitions for the following key words:

Gene – a section of DNA that codes for a particular

protein/polypeptide

Alleles – versions of a gene

Population – all the individuals of a species in the same habitat at the same time

Gene pool – all the alleles of the all the genes in a population at a given time

Slide3

Population

Genetics

– key words to learn

Population genetics

is the study of genes and alleles in populations

Gene

pool

- all the alleles of the all the genes in a population at a given

time

Allele frequency

the number of times an allele occurs within the gene pool is referred to as the allelic frequency

Slide4

Allele Frequencies - Cystic Fibrosis

F

– allele for normal mucus production

f

– cystic fibrosis allele for thicker mucus production

Individuals have two of these alleles – one on each of the pair of homologous chromosomes

We only count one pair of alleles per gene per individual e.g. a population of 1000 will have 2000 alleles in the gene pool for this gene

Slide5

Allele Frequencies - Cystic fibrosis

People can be

FF

,

ff

,

Ff

or

fF

If all individuals in a population were

FF

:

Probability of being

FF

is 1.0 and

ff

is 0.0

The frequency of

F

allele is 100% and f is 0%

If all individuals were

Ff

:

Probability of being

Ff

is 1.0

Frequency of

F

allele is 50% and

f

is 50%

Slide6

Hardy-Weinberg Principle

Mathematical equation used to calculate the frequencies of alleles of a gene in a population

Assumptions:

The proportion of dominant and recessive alleles stays the same from one generation to the next.

No mutations

Isolated population

No selection

Large population

Random mating

Slide7

Hardy-Weinberg

Example: one gene with two alleles

A

and

a

Probability of allele

A =

p

Probability of allele

a

=

qp + q

= 1.0

There are 4 possible arrangements of two alleles so:

AA

+

Aa

+

aA

+

aa

= 1.0

As a probability this is:

p

2

+

2pq

+

q

2

=

1.0

Slide8

Using the Hardy-Weinberg Principle

Can use to determine the probability of any allele in the population

E.g. a recessive disease affects 1 in 25 000 people

Probability of

aa

is 1/25000

= 0.00004

Probability

of

aa

is

q

2

If

q

2

is 0.00004 then

q

= √0.0004 or 0.00063

If

p

+

q

=

1.0 and

q

= 0.00063

p

= 1.0 -

0.00063 = 0.9937

Probability of allele

A

is 0.9937

Slide9

Using the Hardy-Weinberg Principle

q

= 0.00063

p

=

0.9937

Probability of heterozygous is

2pq

2pq

= 2 x 0.9937 x 0.00063 = 0.0125

Slide10

Examples

In a population of sea otters, the allele frequency for the recessive allele, t, was found to be 0.2.

a)

Use the Hardy-Weinberg equation to calculate the percentage of homozygous recessive sea otters in this population. Show your working.

(2)

 b)

What does the Hardy-Weinberg principle predict about the frequency of the t allele after another 10 generations?

(1

)

2. Some

humans have a genetic resistance to infection. A recessive allele gives increased resistance to infection by the malarial parasite. In a population, the proportion of babies born who are homozygous for this allele is 0.01. Use the Hardy-Weinberg equation to calculate the expected proportion of heterozygotes in this population. Show your working

.

(4)

Slide11

correct answer = 0.18;

And three marks for three of:

p + q = 1 and p

2

+ 2pq + q

2

= 1;

0.01 = q

2

;

q = 0.1;

p = 0.9frequency of heterozygotes = 2pq = 2 × 0.1 × 0.9 / 2 × candidatesp × candidates q;

Slide12

 

Two

marks for correct answer of 4;;

One

mark for calculation involving 0.2 × 0.2 or 0.04;

0.2 / the frequency remains the same;