Transcript Genetics Stand Alone Instructional Resource

Introduction to Genetics
Developed by Mr. Neil Syrek
This Stand-Alone Instructional Resource is intended to introduce
students to basic concepts in the field of Mendelian genetics, such as
terminology, the segregation of parental alleles, and the use of
punnett squares to predict genotypic and phenotypic ratios of
offspring.
This project was designed to be used with general education students
in high school Biology. Typically these students would be in grades 9
or 10.
The following Michigan High School Content Expectations will be met
by this project:
B4.1c Differentiate between dominant and recessive
B4.1d Explain the genetic basis for Mendel’s laws of segregation and
independent assortment.
B4.1e Determine the genotype and phenotype of monohybrid crosses using a
Punnett Square.
In addition, the learners will understand the meanings of the terms gene,
allele, homozygous and heterozygous. Furthermore they will be able to
predict phenotypic and genotypic ratios of offspring and be able to predict
the appearance of certain traits in individuals.
Consider this…
A mother and father both
have brown eyes
But…
Their child has blue eyes
Is this even possible? How could
this happen?
To begin to solve our problem…
we will need to learn some genetics terminology
• Gene—the molecular information that codes for a certain trait (like eye
color)
• Allele—a form or variation of a given gene (like blue or brown eyes)
– Side note: In most cases, individuals have a pair alleles for each gene—sometimes the
alleles are the same, sometimes they are different
• Homozygous—what its called when the two alleles in the pair are the
same (like two brown eye alleles)
• Heterozygous—what its called when the two alleles in the pair are
different (like one brown eye allele, and one blue eye allele)
• Genotype—the genetic makeup or plan of an individual (can refer to a
single trait or a collection of traits)
• Phenotype—the observable appearance of a trait or collection of traits
that is the result of the genotype
Try to figure out these analogies
1. A blueprint is to a building, as a genotype is
to ____________.
A. A gene
B. An allele
C. A phenotype
Fantastic!!
Genotype
Phenotype
An individual’s phenotype is the result of his genotype—just like a
building is the result of a blueprint or plan
Sorry
Let’s review:
• Gene—the molecular information that codes for a certain trait (like eye color)
• Allele—a form or variation of a given gene (like blue or brown eyes)
– Side note: In most cases, individuals have a pair alleles for each gene—
sometimes the alleles are the same, sometimes they are different
•
•
Genotype—the genetic makeup or plan of an individual (can refer to a single trait
or a collection of traits)
Phenotype—the observable appearance of a trait or collection of traits that is the
result of the genotype
2. A convertible is to an automobile, as an allele
is to a ___________.
A. heterozygous
B. gene
C. phenotype
Way to go!!
An allele is a variation of a gene like a convertible
is a variation of an automobile
Sorry
Hint: a convertible is a variation or type of automobile
• Gene—the molecular information that codes for a certain trait (like
eye color)
• Allele—a form or variation of a gene (like blue or brown eyes)
• Heterozygous—what its called when the two alleles in the pair are
different (like one brown eye allele, and one blue eye allele)
• Phenotype—the observable appearance of a trait or collection of
traits that is the result of the genotype
More on Genotype
For every gene, an individual has two alleles.
In the case of eye color, the possible
combinations are:
Brown/Brown
Brown/Blue
Blue/Blue
*Note: Eye color is a complex trait that in actuality is controlled by multiple genes. For this
example, we must act as if there were only one gene for eye color.
What color eyes do you think someone with
the Brown/Brown allele combination would
have?
or
Right
• …so a Blue/Blue combination will cause what
color eyes?
or
Nope. It was brown
• …so a Blue/Blue combination will cause what
color eyes?
or
Good
I think you’ve got it.
Well now, what color eyes will a Brown/Blue
allele combination cause?
Yes!!
In the case of the Brown/Blue allele
combination, brown eye color shows up at the
expense of blue.
Thus we can say that the brown allele is
dominant over the blue one.
The term used to describe the allele that is
being masked, the blue one in this case, is
recessive.
In most cases, one allele is dominant over the
other.
Sorry
It was brown—but don’t worry you had no way of
knowing
In the case of the Brown/Blue allele combination,
brown eye color shows up at the expense of blue.
Thus we can say that the brown allele is dominant over
the blue one.
The term used to describe the allele that is being
masked is recessive.
In most cases, one allele is dominant over the other.
Scenario
Luis has a genetic disorder called Huntington’s
Disease (HD), which causes a degeneration of
the central nervous system. Luis is married to
Sandra who does not have the disorder.
Before they decide to have a baby, they want
to know the probability that the baby will
inherit Luis’ disease.
For more information on HD, visit the National Institutes of Health homepage
Some more info
• There are two alleles for the gene that causes HD:
affected and normal
– The affected allele is dominant
– The normal allele is recessive
• For simplicity, we can abbreviate these alleles:
– ‘A’ for affected
– ‘a’ for normal
• We use the capital and lowercase forms of the same
letter because they are variations of it, just as alleles
are variations of a single gene.
• Now we must figure out Luis and Sandra’s genotype
so we can work on probability.
Luis’ genotype
• Through a study of family history, we find out
that Luis is heterozygous for Huntington’s
Disease. In other words, he has one affected
allele and one normal allele
• What do you think his genotype would be?
AA
Aa
aa
Yes!!
Nice job. Since Luis is heterozygous for the trait,
he has two different alleles.
Aa
Sorry
• Think about it. If Luis is heterozygous, that
means he has two different alleles (or two
different forms of the same letter).
• Remember: ‘A’ for affected
‘a’ for normal
Sandra’s genotype
• Sandra does not have the disease, so she is
homozygous for the recessive trait.
• What would Sandra’s genotype be?
AA
Aa
aa
Yes!!
• Since Sandra is homozygous, her alleles are
the same. Also, because she has the recessive
trait, we use the lowercase letters.
aa
Sorry
• Remember, Sandra is homozygous, so her
alleles are the same. In addition, she has the
recessive trait, which is represented by
lowercase letters.
• Now that we know their genotypes, we can
figure out the probability that their offspring
will have HD. To do this, we can use a tool
called a Punnett square.
Punnett squares
• To begin, insert the father’s alleles at the top of the
table. Remember, Luis’ genotype was A a
A
a
• Now insert the mothers alleles along the side.
Sandra’s genotype is a a.
A
a
a
a
Filling in the table
• The four remaining squares represent the possible genotypes
of the offspring
• To fill them in, simply drop the father’s allele down from the
top, and drag the mothers allele over from the side—sort of
like a multiplication table.
a
A
a
Aa
aa
a
• What letters do you think would go in this space?
aa
AA
Aa
Right on!!
• Since Luis would donate the ‘A’ allele, and
Sandra would send the ‘a’ allele, that square
would be filled with A a.
Good try…but no
• The allele that Luis would donate to that
square is all the way at the top of the table
(A), while the allele that Sandra would donate
is all the way to the left (a).
• Try again
Filling in the table
• Almost finished…
A
a
a
Aa
aa
a
Aa
• What letters do you think would fill in the last space?
aa
AA
Aa
Way to go!!
• Both Luis and Sandra donated the ‘a’ allele to
that space
Try again
• Hint: look at the top of the table above that
square for one allele, and all the way to the
left for the other.
The results
• Okay, we’ve got our Punnett square filled in Now
what does it all mean?
a
a
A
a
Aa aa
Aa aa
• Some simple math will allow us to interpret the
results.
a
a
A
a
Aa aa
Aa aa
• Two of the four squares representing the
offspring show the genotype ‘A a,’ which
results in the affected phenotype.
• Since two out of four is the same as 50%, we
can say there is a 50% chance that Luis and
Sandra will have a child with Huntington’s
Disease.
Try this
• If instead of the square we just completed, we had
one that looked like this,
A
a
A AA Aa
a Aa aa
what would be the probability of an offspring being
affected with Huntington’s Disease?
A) 25%
B) 50%
C) 75 %
Nice job!!
• Since HD is a dominant trait, either the ‘A A’ or
the ‘A a’ genotype will result in an affected
individual.
• 3 out of the 4 squares, or 75%, coded for the
affected phenotype.
Try again
• Remember, since HD is a dominant trait,
either the ‘A A’ or the ‘A a’ genotype will result
in an affected individual.
• Also, 3 out of the 4 squares coded for the
affected phenotype.
Now its time for the big quiz
• I’m ready. Lets go!
• I want to review the vocabulary first.
• I want to review Punnett squares again.
Let’s revisit our opening scenario
• Recall that two parents, each with brown eyes, had a blue
eyed baby.
• The brown eyes allele is dominant.
• The blue eyes allele is recessive.
• We’ll use ‘B’ for the brown allele, and ‘b’ for the blue allele
• Which of these genotypes would result in the blue eyes
phenotype?
BB
Bb
bb
Yes!!
• Blue eyes is the recessive trait, so it is
represented by two lowercase letters.
Sorry
• Remember, blue eyes is the recessive trait.
• Even one brown eyes allele will result in the
brown eyes phenotype.
Next question
• Recall that two parents, each with brown eyes, had a blue
eyed baby.
• The brown eyes allele is dominant.
• The blue eyes allele is recessive.
• We’ll use ‘B’ for the brown allele, and ‘b’ for the blue allele
• Which of these Punnett squares could represent this
scenario?
B
b
b
Bb
bb
b
Bb
bb
(click here)
B
b
B
BB
Bb
b
Bb
bb
(click here)
B
b
B
BB
Bb
B
BB
Bb
(click here)
Great!!
That was the only choice showing both parents with
brown eyes, and a possibility of blue eyed offspring.
B
b
B
BB
Bb
b
Bb
bb
Next question
• In this scenario, both parents had a ___________ genotype.
A) homozygous
B) heterozygous
C) phenotype
Think again
• Remember, both parents have brown eyes
Think again
• You’re partially right. Both parents have
brown eyes, but…
• Remember, the baby has blue eyes
Well done!!
• Since the parents each had different alleles,
they would be heterozygous
Try again
• The genotype of both parents is ‘B b’
• The alleles are different
Last question
• What was the probability that these parents
would have an offspring with blue eyes?
A) 100%
B
b
B
BB
Bb
b
Bb
bb
B) 50%
C) 25%
Spot on!!
• Only 1 of the 4 squares showed the blue eyed
phenotype. That’s 25%.
• You are a genetics genious!
Back to
beginning
Try again
• The only genotype that will result in blue eyes
is ‘b b’
• Gene—the molecular information that codes for a certain trait (like eye
color)
• Allele—a form or variation of a given gene (like blue or brown eyes)
– Side note: In most cases, individuals have a pair alleles for each gene—sometimes the
alleles are the same, sometimes they are different
• Homozygous—what its called when the two alleles in the pair are the
same (like two brown eye alleles)
• Heterozygous—what its called when the two alleles in the pair are
different (like one brown eye allele, and one blue eye allele)
• Genotype—the genetic makeup or plan of an individual (can refer to a
single trait or a collection of traits)
• Phenotype—the observable appearance of a trait or collection of traits
that is the result of the genotype
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