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Genetics: Inheritance I
(Mendelian)
I. Some background and definitions
A. Chromosome
B. Locus- a specific location on a chromosome (pl.
“loci”)
C. Gene- each gene is found at a specific locus on a
specific chromosome
D. Allele
• The gene that codes for the beta-subunit polypeptide
of a G-protein is located on this chromosome (15) at
15q21.2
• The gene that codes for a tyrosine kinase receptor
(type 3) used by neurons is located at 15q25.
I. Some background and definitions
A.
B.
C.
D.
E.
Chromosome
Locus
Gene
Allele
Genotype- the alleles an organism has at a gene
I. Some background and definitions
A.
B.
C.
D.
E.
Chromosome
Gene
Allele
Genotype- the alleles an organism has at a gene
Phenotype- the anatomy and physiology of an
organism; the expression of genes in the organism;
the way the organism “looks” and “works.”
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype:
a. Each of us inherits 23 chromosomes from mom and 23 from
dad; they are homologous.
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype:
a.
Each of us inherits 23 chromosomes from mom and 23 from dad; they are
homologous.
*remember that we are using humans as the example. Humans are diploid,
and we have 23 pairs of homologous chromosomes. Each species has a
characteristic number of chromosomes. For example, dogs have 39 pairs,
mosquitoes have 3 pairs, and cabbage plants have 9 pairs. Haploid
organisms do not have homologous pairs; they have only one copy of each
chromosome (therefore one copy of each gene).
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype:
a.
b.
Each of us inherits 23 homologous chromosomes
Genotype: Each pair of homologues has the same genes on the same
location, but may have different ALLELES for those genes. For example,
at the (brown-blue)eye color gene, mom could have given you a blue (b)
allele and dad could have given you a Brown (B) allele. If this were the
case, your GENOTYPE would be Bb. (What would your genotype be if
you got b from mom and b from dad?)
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype:
a. Each of us inherits 23 chromosomes from mom and 23 from
dad; they are homologous.
b. Genotype
c. Phenotype: If you get two of the same allele (ex, bb), you
must express (show) that trait- you will have blue eyes.
However, often when you get two different alleles, one
expresses and the other does not. For example, if your
genotype is Bb, only the Brown allele expresses, and you will
have brown eyes. This is your PHENOTYPE.
Brown eyes
Blue eyes
These are phenotypes; what are their genotypes?
Brown eyes
BB or Bb
Blue eyes
bb
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
a.
Heterozygous and homozygous- when someone has 2 of the same allele at
a gene, we say they are HOMOZYGOUS for that gene. When someone
has 2 different alleles at a gene, we say they are HETEROZYGOUS for
that gene. An individual will be homozygous for some genes, and
heterozygous for other genes. For example, you could have brown eyes
(Bb), and not be able to roll your tongue (tt).
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
a.
b.
Heterozygous and homozygous- when someone has 2 of the same allele at
a gene, we say they are HOMOZYGOUS for that gene. When someone
has 2 different alleles at a gene, we say they are HETEROZYGOUS for
that gene.
Dominant and recessive alleles- When someone is heterozygous for a
gene, and only one allele expresses, we say it is the DOMINANT allele.
The one that does not express is RECESSIVE.
This example considers the genetics of pea color.
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1.
2.
Genotype and Phenotype
Some more definitions, hooray!
a.
b.
c.
Heterozygous and homozygous
Dominant and recessive alleles
Incomplete dominance and codominance: Sometimes there is no
dominance by one allele in heterozygotes. Instead, both get to express.
i.
Incomplete dominance- alleles blend their influence
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1.
2.
Genotype and Phenotype
Some more definitions, hooray!
a.
b.
c.
Heterozygous and homozygous
Dominant and recessive alleles
Incomplete dominance and codominance: Sometimes there is no
dominance by one allele in heterozygotes. Instead, both get to express.
i. Incomplete dominance- alleles blend their influence
ii. Codominance- both alleles express fully
Blood types
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
a.
b.
c.
d.
Heterozygous and homozygous
Dominant and recessive alleles
Incomplete dominance and codominance
Polygenic traits- phenotypes determined by the interaction of
more than one gene. Many traits are polygenic! For
example, see book for skin color determination and the next
slide for eye color!
Example of polygenic trait: eye color
Eye color is determined by the interaction of at least 2 genes:
Brown-blue and Yellow-absent. Yellow coloration (Y) is
dominant to absent or no coloration (y).
If genotype at
Blue-brown is
And genotype at
Yellow-absent is
Then phenotype
will be
Bb or BB
Yy or YY
Hazel
Bb or BB
yy
Brown
bb
Yy or YY
Green
bb
yy
Blue
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
a.
b.
c.
d.
e.
Heterozygous and homozygous
Dominant and recessive alleles
Incomplete dominance and codominance
Polygenic traits
Epistasis- when one gene has a permissive effect on another
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
a.
b.
c.
d.
e.
f.
Heterozygous and homozygous
Dominant and recessive alleles
Incomplete dominance and codominance
Polygenic traits
Epistasis- when one gene has a permissive effect on another
Pleiotropy- when one gene affects many aspects of phenotype
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
3. Predicting genotypes and phenotypes: punnet squares (laboratory
activity).
a. Monohybrid crosses only consider one gene at a time
• If mom is Aa for a particular gene, each of her eggs
will have EITHER the A or a allele.
• Same for dad’s sperm if he’s Aa
• What allele will each egg carry if mom is AA? aa?
P generation: the
parents
F1 generation:
the offspring (in
this example, all F1 are
Pp, and we are mating
two “siblings” to
produce the:
F2 generation:
the
grandchildren
(offspring of
the F1)
• Here’s an example
using alleles that
show incomplete
dominance
II. The basics of how genetic inheritance
works (using humans as the example)
A.
In the individual
1.
2.
3.
Genotype and Phenotype
Some more definitions, hooray!
Predicting genotypes and phenotypes: punnet squares.
a. Monohybrid crosses only consider one gene at a time
b. Test crosses allow you to determine the genotype of an individual with the
dominant phenotype of a gene. So, in this case you are not trying to
predict the outcome of an F1 generation. Instead, you are using the F1
generation to figure out the genotype of a parent with a dominant
phenotype. To do so, mate the “unknown” parent with another parent of
known genotype: a homozygous recessive.
Example: test cross
In laborador retrievers, brown (chocolate) color (b) is
recessive to black color (B)
You own a male black lab, Rocky (short for RockEater). What
are Rocky’s possible genotypes?
Let’s say you mate Rocky with your neighbor’s chocolate lab,
Lucinda. What is Lucinda’s genotype?
If Lucinda has 20 puppies, and 10 are chocolate, do you know
what Rocky’s genotype is?
What if all 20 are black?
What if only 2 are chocolate?
So, in a test cross, you are trying to figure out the genotype of
a dominant phenotype parent by mating that parent with a
recessive phenotype parent. The outcome of the offspring
“Rocky”
“Lucinda”
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
1. Genotype and Phenotype
2. Some more definitions, hooray!
3. Predicting genotypes and phenotypes: punnet squares.
a. Monohybrid crosses only consider one gene at a time
b. Test crosses allow you to determine the genotype of an
individual with the dominant phenotype of a gene.
c. Dihybrid crosses allow you to consider two genes at a time
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
B. In a population- there a typically many alleles for each
gene. Each individual can only get 2.
In the human population,
there are 3 major alleles for
the ABO Blood surface
protein gene:
A
I
codominant
B
I
i
Recessive to both
If your genotype is
AA
I I
B B
I I
A B
I I
ii
or
or
A
I i
B
I i
Then your phenotype
(blood type) will be
A
B
AB
O
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
B. In a population- there a typically many alleles for
each gene. Each individual can only get 2.
C. Mendel
-described two principles which can now be explained by the
mechanics of meiosis:
-Principle of segregation- alleles are separated from each other
II. The basics of how genetic inheritance
works (using humans as the example)
A. In the individual
B. In a population- there a typically many alleles for
each gene. Each individual can only get 2.
C. Mendel
-described two principles which can now be explained by the
mechanics of meiosis:
-Principle of segregation
-Independent assortment- genes that are on separate
chromosomes separate from each other
III. Disorders caused by “faulty” alleles
A. Deleterious dominant alleles- will only survive in a
population if they allow their holders to survive to
reproductive age
III. Disorders caused by “faulty” alleles
A. Deleterious dominant alleles- will only survive in a
population if they allow their holders to survive to
reproductive age
Ex, Huntington’s disease, doesn’t strike until 40’s.
III. Disorders caused by “faulty” alleles
A. Deleterious dominant alleles
Ex, Huntington’s disease, doesn’t strike until 40’s.
B. Deleterious recessive alleles- survive in a
population better than dominant alleles because
they can “hide” in heterozygotes. Heterozygotes
are called “carriers.”
III. Disorders caused by “faulty” alleles
A. Deleterious dominant alleles
Ex, Huntington’s disease, doesn’t strike until 40’s.
B. Deleterious recessive alleles- survive in a
population better than dominant alleles because
they can “hide” in heterozygotes. Heterozygotes
are called “carriers.”
Ex, cystic fibrosis
III. Disorders caused by “faulty” alleles
A. Deleterious dominant alleles
Ex, Huntington’s disease, doesn’t strike until 40’s.
B. Deleterious recessive alleles- survive in a
population better than dominant alleles because
they can “hide” in heterozygotes. Heterozygotes
are called “carriers.”
Ex, cystic fibrosis
*When deleterious recessive alleles are X-linked, the
disease condition is more common in males
Ex, hemophilia
*Inbreeding tends to “bring out” recessive traits; why
would that be?
IV. Tracking alleles through
generations: pedigrees
• Laboratory activity