Dihybrid (or greater) Crosses: Review
For either genotype or phenotype, the expected
outcomes of a particular cross can be calculated by
multiplying the probabilities of the individual
outcomes.
Crosses: Review
For example, the ratios of an F1 resulting from the
mating of a homozygous dominant (AA) to a
homozygous recessive (aa) are
Phenotype = 3:1 dominant : recessive
Genotype = 1:2:1 AA:Aa:aa
Crosses: Review
The probabilities of all expected outcomes must add
up to 1.
So, if the ratio is 3:1, the probabilities of the two
phenotypes are 0.75 or ¾ and 0.25 or ¼
For the genotypes, when the ratio is 1:2:1, the
probabilities are 0.25 or ¼ AA, 0.5 or 2/4 Aa and
0.25 or ¼ aa
Crosses: Review
If two traits are both inherited according to
Mendelian principles, one can calculate the
probabilities of all the individual outcomes for
either phenotype or genotype.
If two heterozygotes are crossed in a dihybrid cross
AaBb x AaBb
Crosses: Review
You can either make a Punnett Square and count
them all up, or you can multiply the probabilities
of the individual probabilities.
Punnett Square of Dihybrid Cross: F2
AB
Ab
aB
ab
AB AABB AABb AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Crosses: Review
You can calculate the expected phenotype ratio
Dominant at both: ¾ x ¾ = 9/16
1st locus dominant, 2nd recessive: ¾ x ¼ = 3/16
1st recessive, 2nd dominant: ¼ x ¾ = 3/16
Recessive at both: ¼ x ¼ = 1/16
So the phenotype ratio is 9:3:3:1
Crosses: Review
How are the genotypes of the 9 dominant phenotypes
assorted among heterozygous and homozygous?
Crosses: Review
Of the 9 dominant phenotypes,
¼ x ¼ = 1/16 will be homozygous dominant (AABB)
2/4 x 2/4 = 4/16 will be heterozygous dominant
(AaBb)
¼ x 2/4 = 2/16 will be AABb
2/4 x ¼ = 2/16 will be AaBB
Extensions of Mendelian Genetics
Designations
Genotype is determined by the presence of alleles at
a given locus.
Each individual can have only two alleles, one on
one homologous chromosome and a second at the
same locus on the homologous chromosome.
Designations
If an allele is dominant, the second allele may be indicated
by a dash, A-, which indicates that the second allele may
be another dominant allele or a recessive allele that does
not contribute to the phenotype.
So, going back to our original dihybrid cross, the results
would be
9 A-/B3 A-/bb
3 aa/B1 aa/bb
Designations
Wild-type is used to indicate the normal state, and is
often designated as a superscript wt or +.
A loss of function mutation may be designated with a
superscript -.
Remember the Ames test? The strain of Salmonella
used was his-, indicating a mutation in the histidine
biosynthesis pathway that resulted in the inability
to synthesize histidine. The Ames test looks for
reverse mutation to the wild type, his+.
Designations
By convention, genes are referred to in italics, while
gene products (proteins) are written in plain text.
For example, a gene that has been associated with
breast cancer in women is BRCA1, and the protein
it encodes is BRCA1.
Mendel
Mendel was careful to focus on traits that were
influenced by a single locus and for which there
were only two alleles possible at that locus, e.g.,
plants either inherited the tall phenotype (DD or
Dd) or the dwarf phenotype (dd).
Only one locus influenced plant height, and the only
two alleles D and d, were possible.
Gene Interaction
Several genes may influence a particular
characteristic.
1. One gene may affect another.
2. Cellular function of numerous gene products
contribute to the development of the common
phenotype (e.g. combine to form a structure of
specific size, shape, color, texture, activity)
Think of development as a process of many steps, or
a cascade of events
Epigenesis
Epigenesis is the idea that development occurs as
new structures appear and grow.
This is in contrast to the idea of preformation, which
holds that development is simply a continuation of
growth of structures that were already present in
the egg (nothing new is formed in the offspring)
Gene Interaction
Epistasis: Expression of genes at one locus may
affect expression of genes at another locus.
For example, one gene or gene pair may modify or
mask the expression of a second gene or gene pair
at another locus.
Epistasis is derived from the Greek word “to stop”
Epistasis
The homozygous presence of a recessive allele may
prevent or override the expression of other alleles
at a second locus.
In this case, the alleles at the first locus are
epistatic to those at the second locus, and the
alleles at the second locus are hypostatic to
those at the first allele.
(To help you remember, the locus that is epistatic does the stopping.)
Coat color in mice
Coat color options:
1. Agouti: a grayish coat characterized by bands of
black and white pigment on individual hairs
2. Black
3. White or albino
Coat color in mice
Agouti is the dominant to black (non-agouti), which
results from homozygous expression of a mutation,
a.
Therefore, if a mouse is A- (AA or Aa), the color will
be agouti, while an aa mouse is black.
Coat color in mice
Homozygous inheritance of a recessive mutation bb,
at a second locus eliminates pigment formation
altogether and results in albino mice, regardless of
genotype at the a locus.
The presence of at least one B allele allows pigment
to form.
Therefore, the B locus is epistatic to the A locus, and
the A locus is hypostatic to the B locus.
Coat color in mice
Pigment formation follows a pathway like this:
Gene B
Gene A
Precursor  Black pigment  Agouti
(colorless) BA- pattern
Incomplete Dominance
Incomplete or partial dominance exists
when one allele does not have clear dominance
over the other.
The phenotype of the heterozygote is intermediate
between the homozygous of either allele.
For example, mating homozygous white snapdragons
to homozygous red snapdragons yields F1
offspring with pink flowers.
Incomplete Dominance
In this example, the gene dosage for red in the F1
plants is half that of the homozygous red parent,
and half as much gene product is made in the
offspring.
In the F1, the red gene is present, so some red
pigment is made (but not as much as in the red
parent, which has two red genes) and the flowers
are pink.
Codominance
When to alleles of a single gene encode two distinct
and detectable gene products, the joint expression
of both alleles in the heterozygote is called
codominance.
Multiple Alleles
There may be several alleles possible at a given
locus.
An individual only inherits two, one from each
parent.
When three or more alleles are availaible, they can
only be studied in populations.