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
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Slide1
Population Genetics
Starter - write biological definitions for the following key words:
Gene
Allele
Population
Gene
pool
Slide2Population 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
Slide3Population
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
Slide4Allele 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
Slide5Allele 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%
Slide6Hardy-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
Slide7Hardy-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
Slide8Using 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
Slide9Using 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
Slide10Examples
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)
Slide11correct 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;
Slide12Two
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;