Download Welcome to Genetics This is the science of genes, heredity and

Welcome to Genetics
This is the science of genes, heredity
and variation of organisms.
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Students will analyze (figure out) how biological
traits (things that make us look different) are
passed on to successive generations (our children
and their children and theirs...).
You will collect class data for the traits listed and
then compare them to data collected at other
schools.
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DNA This macromolecule holds an organism's hereditary information.
gene A segment of DNA on the chromosome that is coded for a particular trait.
dominant An allele that can mask the recessive allele.
recessive An allele that can be masked by a dominant one.
trait A specific, observable, characteristic that varies from one individual to another. allele Alternative forms of a gene, like tall and short in Mendel's pea plants.
homozygous Having two identical alleles for the same genetic trait, eg. TT or tt.
heterozygous Having two different alleles for the same genetic trait, eg. Tt.
genotype The specific combination of alleles an individual has, eg. TT or Tt
phenotype The physical expression of genes, eg. tall or short.
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Allele
Dominant
An allele that can mask
the recessive allele.
B
Alternative forms of a
gene, like tall and short in
Mendel's pea plants.
Recessive
An allele that can be
masked by a dominant
one.
b
Trait
Trait
Genotype
Phenotype
BB brown
Gene
bb blue DNA
Bb brown
Heterozygous Homozygous
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Mom
B
b
B
b
B
B
b
b
Dad
b B
Brown
blue
B
b
b
b
B
B
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Punnett Square The grid diagram that shows the gene combinations that might result from a genetic cross.
b
B
What fraction of kids will have brown eyes?
B
b
What fraction will have blue eyes?
BB brown
Bb brown
bb blue
Genotype Phenotype
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Punnett Square
B
B
b
B
What fraction of kids will have brown eyes?
What fraction will have blue eyes?
Bb brown
BB brown
Genotype Phenotype
bb blue
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Punnett Square
b
B
b
b
What fraction of kids will have brown eyes?
What fraction will have blue eyes?
Bb brown
BB brown
Genotype Phenotype
bb blue
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Punnett Square
b
B
B
b
What fraction of kids will have brown eyes?
What fraction will have blue eyes?
Bb brown
BB brown
Genotype Phenotype
bb blue
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Genotype
ss
S__
Phenotype
s
S
s
ss
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Free (dominant) or
attached(recessive)
earlobes
White forelock
(dominant)
or
no white forelock
(recessive)
Dimples (can be one
side or both) dominant
or
no dimples
(recessive)
Straight
(dominant)
or curved thumbs
(recessive)
Straight (recessive)
or bent pinky
(dominant)
Mid-digit hair
(dominant) or
no mid-digit hair
(recessive)
red - green
colour
blindness
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Questions to answer.
1. For each of the 7 traits surveyed, calculate the total number of individuals surveyed. (KE and online database)
2. For each trait, calculate the total number of individuals exhibiting the dominant trait.
3. For each trait, calculate the total number of individuals exhibiting the recessive trait.
4. Calculate the percentage of students who have each of the phenotypes surveyed. 5. Do most people have the dominant trait? 6. For one of the traits surveyed, what are the possible genotypes for individuals showing the dominant trait?
7. For one of the traits surveyed, what are the possible genotypes for individuals showing the recessive trait?
8. What might cause a recessive trait to be more common in a population?
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What percent of the population is heterozygous
dominant for the traits we measured?
e.g. albinism.
A
a
A
a
We need the Hardy-Weinberg equation to help us solve
such problems.
p2 + 2pq + q2 = 1
p = frequency of homozygous dominant individuals (e.g. AA)
q- frequency of homozygous recessive individuals e.g (aa)
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ALBINISM: A SAMPLE HARDY-WEINBERG PROBLEM
The average human frequency of albinism in North America is only
about 1 in 20,000.
Referring to the Hardy-Weinberg equation (p² + 2pq + q² = 1), the
frequency of homozygous recessive individuals (aa) in a population is q².
Therefore, in North America the following must be true for albinism:
q² = 1/20,000 = .00005
By taking the square root of both sides of this equation, we get:
(Note: the numbers in this example are rounded off for simplification.)
q = .007
In other words, the frequency of the recessive albinism allele (a) is .
00707 or about 1 in 140. Knowing one of the two variables (q) in the
Hardy-Weinberg equation, it is easy to solve for the other (p).
p = 1 - q
p = 1 - .007
p = .993
The frequency of the dominant, normal allele (A) is, therefore,
.99293 or about 99 in 100.
The next step is to plug the frequencies of p and q into the HardyWeinberg equation:
p² + 2pq + q² = 1
(.993)² + 2 (.993)(.007) + (.007)² = 1
.986 + .014 + .00005 = 1
This gives us the frequencies for each of the three genotypes for
this trait in
the population:
p² =predicted frequency of homozygous dominant individuals = .986 =
98.6%
2pq =predicted frequency of heterozygous individuals = .014 = 1.4%
q² =predicted frequency of homozygous recessive individuals
(the albinos) = .00005 = .005%
With a frequency of .005% (about 1 in 20,000), albinos are extremely
rare. However, heterozygous carriers for this trait, with a predicted
frequency of 1.4% (about 1 in 72), are far more common than most
people imagine. There are roughly 278 times more carriers than albinos.
Clearly, though, the vast majority of humans (98.6%) probably are
homozygous dominant and do not have the albinism allele.
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Now use the Hardy- Weinberg equation and our
collected data to find the percentages of homozygous
dominant, heterozygous dominant and homozygous
recessive individuals for each trait.
Create a graph to display the final results.
Can you determine the genotype of yourself for one or
two the traits we examined? You will need to find the
phenotypes of parents and both sets of grandparents.
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Widow's peak: Free ear lobes: Dimples: Cleft chin: longer second toe: Rolling tongue: Bent Pinkies: mid digit hair
Taster
http://chroma.gs.washington.edu/outrea
ch/genetics/traits/index.html
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