Transcript Human Genetics - Chapter 14

Human Genetics
Concepts and Applications
Tenth Edition
RICKI LEWIS
14
Constant
Allele
Frequencies
PowerPoint® Lecture Outlines
Prepared by Johnny El-Rady, University of South Florida
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Introduction
Population = An interbreeding group of the
same species in a given geographical area
Gene pool = The collection of all alleles in
the members of the population
Population genetics = The study of the
genetics of a population and how the
alleles vary with time
Gene Flow = Movement of alleles between
populations when people migrate and mate
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Allele Frequencies
# of particular allele
Allele frequency =
Total # of alleles in
the population
Count both chromosomes of each individual
Allele frequencies affect the genotype
frequencies
- The frequency of the two homozygotes
and the heterozygote in the population
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Phenotype Frequencies
Frequency of a trait varies in different
populations
Table 14.1
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Microevolution
The small genetic changes due to changing
allelic frequencies in populations
Five factors can change genotypic
frequencies:
1) Nonrandom mating
2) Migration
3) Genetic drift
4) Mutation
5) Natural selection
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Macroevolution
Refers to the formation of new species
Occurs when enough microevolutionary
changes have occurred to prevent
individuals from one population to
successfully produce fertile offspring
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Hardy-Weinberg Equation
Developed independently by an English
mathematician and a German physician
Used algebra to explain how allele
frequencies predict genotypic and
phenotypic frequencies in a population of
diploid, sexually-reproducing species
Disproved the assumption that dominant
traits would become more common, while
recessive traits would become rarer
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Hardy-Weinberg Equation
p = allele frequency of one allele
q = allele frequency of a second allele
All of the allele frequencies
together equals 1
p+q=1
p2 + 2pq + q2 = 1
p2 and q2
2pq
All of the genotype frequencies
together equals 1
Frequencies for each homozygote
Frequency for heterozygotes
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Figure 14.2
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Source of the Hardy-Weinberg
Equation
Figure 14.3
Figure 14.3
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Solving a Problem
Figure 14.4
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Solving a Problem
Figure 14.4
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The allele and genotypic frequencies do not
change from one generation to the next
Thus, this gene is in Hardy-Weinberg equilibrium
Table 14.2
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Applying the Hardy-Weinberg
Equation
Used to determine carrier probability
For autosomal recessive diseases, the
homozygous recessive class is used to
determine the frequency of alleles in a
population
- Its phenotype indicates its genotype
Figure 14.3
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Calculating the Carrier Frequency
of an Autosomal Recessive
Figure 14.5
Figure 14.3
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Calculating the Carrier Frequency
of an Autosomal Recessive
Table 14.3
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Calculating the Carrier Frequency
of an Autosomal Recessive
What is the probability that two unrelated
Caucasians will have an affected child?
Probability that both are carriers =
1/23 x 1/23 = 1/529
Probability that their child has CF = 1/4
Therefore, probability = 1/529 x 1/4 =
1/2,116
Figure 14.3
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Calculating the Risk with
X-linked Traits
For females, the standard Hardy-Weinberg
equation applies
p2 + 2pq + q2 = 1
However, in males the allele frequency is
the phenotypic frequency
p + q= 1
Figure 14.3
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Calculating the Risk with
X-linked Traits
Figure 14.6
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Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium is rare for
protein-encoding genes that affect the
phenotype
However, it does apply to portions of the
genome that do not affect phenotype
These include repeated DNA segments
- Not subject to natural selection
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DNA Repeats
Short repeated segments are distributed all
over the genome
The repeat numbers can be considered
alleles and used to classify individuals
Two types of repeats are important
- Variable number of tandem repeats
(VNTRs)
- Short tandem repeats (STRs)
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DNA Repeats
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DNA Profiling
A technique that detects differences in
repeat copy number
Calculates the probability that certain
combinations can occur in two sources
of DNA by chance
DNA evidence is more often valuable in
excluding a suspect
- Should be considered along with other
types of evidence
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Comparing DNA Repeats
Figure 14.7
Figure 14.7
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DNA Profiling
Developed in the 1980s by British
geneticist Sir Alec Jeffreys
Also called DNA fingerprinting
Identifies individuals
Used in forensics, agriculture, paternity
testing, and historical investigations
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Differing number of copies of the same repeat
migrate at different speeds on a gel
Figure 14.8
Figure 14.8
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Jeffreys used his
technique to
demonstrate that
Dolly was truly a
clone of the 6year old ewe
that donated her
nucleus
Figure 14.9
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DNA Profiling Technique
1) A blood sample is collected from
suspect
2) White blood cells release DNA
3) Restriction enzymes cut DNA
4) Electrophoresis aligns fragments by size
5) Pattern of DNA fragments transferred to
a nylon sheet
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DNA Profiling Technique
6) Exposed to radioactive probes
7) Probes bind to DNA
8) Sheet placed against X ray film
9) Pattern of bands constitutes DNA profile
10) Identify individuals
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Box Figure 14.1
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DNA Sources
DNA can be obtained from any cell with a
nucleus
STRs are used when DNA is scarce
If DNA is extremely damaged, mitochondrial
DNA (mtDNA) is often used
For forensics, the FBI developed the
Combined DNA Index System (CODIS)
- Uses 13 STRs
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CODIS
Figure 14.10
The probability that any two individuals have same
thirteen markers is 1 in 250 trillion
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Population Statistics Is Used to
Interpret DNA Profiles
The power of DNA profiling is greatly
expanded by tracking repeats in different
chromosomes
The number of copies of a repeat are
assigned probabilities based on their
observed frequency in a population
The product rule is then used to calculate
probability of a certain repeat combination
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To Catch A Thief With A Sneeze
Table 14.6
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To Solve A Crime
Table 14.6
Figure 14.11
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Table 14.6
Figure 14.11
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Using DNA Profiling to Identify
Victims
Recent examples of large-scale disasters
- World Trade Center attack (2001)
- Indian Ocean Tsunami (2004)
- Hurricane Katrina (2005)
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Challenges to DNA Profiling
Figure 14.12
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Figure 14.12
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Genetic Privacy
Today’s population genetics presents a
powerful way to identify individuals
Our genomes can vary in more ways than
there are people in the world
DNA profiling introduces privacy issues
- Example: DNA dragnets
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