Download Lab. 11 Deviation of Mendel`s second law “Dihybrid” Part 2

Lab. 11
Deviation of Mendel’s second law
“Dihybrid” Part 2
Main topics:


What’s Epistasis?
Epistasis ratio
Deviation of Mendel’s
1. Complement “chickens comb”
second law “Dihybrid”
2. Crossing over between two gene
“Gene interaction”
3. Epistasis
Mendel’s Law II “ratio= 9:3:3:1”
3. Epistasis definition:
The genes responsible for expression of the trait in the phenotype. In the case of simple
Mendelian inheritance each gene is responsible for the expression of only one phenotypic
trait. But, in reality the situation is more complicated. For example, the same gene may
act on the expression of multiple traits or the same trait can evolve under the action of
multiple genes. Typically, the interaction of genes has a biochemical nature, i.e. it's based
on the combined action of proteins, whose synthesis is controlled by certain genes.
Interact with each other can both allelic and non-allelic genes. The interaction of several
genes that are located in different loci called non-allelic. Non-allelic gene interactions can
be divided into two groups: complementary gene interaction and epistatic gene
interaction. What is the difference between them? Perhaps, it can be explained in simple
words. Complementary genes - is non-allelic genes that act together on trait expression,
i.e. the combination of these genes in the genotype give us a new phenotype. And
epistasis it is the interaction of non-allelic genes, in which one of them suppress the action
of another. A gene that suppresses the action of another non-allelic gene called
Principles of genetics
1
suppressor or epistatic gene, and can be marked by the letters I or S. And the suppressed
gene called hypostatic. Both dominant and recessive genes can interact each other.
Therefore, there is a dominant and recessive epistasis. When a dominant allele of one
gene suppresses the action of another gene, it is called dominant epistasis. And when
the recessive allele of epistatic gene in the homozygous state suppresses the action of
another gene, it is called recessive epistasis. Often, the interaction of non-allelic genes
can be explained both in terms of complementary interaction of genes and in terms of
epistatic interactions of genes. For the complementary interaction of genes the ratio of
phenotypes depend on whether or not an independent phenotypic expression in these
genes.
What is epistasis?
A gene interaction in which the effects of an allele at one gene hide the effects of alleles
at another gene
The genes may work against each other (antagonistically) resulting in one gene masking
or suppressing the expression of another gene. Or they may work together in a
complementary fashion. 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1
Principles of genetics
2
Epistasis Gene interaction “color”
1. Recessive Epistasis
(9:3:4)
2- Dominant epistasis
(12:3:1)
The presence of a recessive allele at one One copy of a dominant epistatic allele (B)
locus prevents the expression of another of one gene masks the effect of the second
allele at a second locus (One gene masks gene, regardless of the alleles there.
the phenotypic effects of another). The Complete dominance at both gene pairs,
alleles of ‘A’ locus express themselves as but one gene, when dominant, epistatic to
a recessive allele b.(Become A-bb = the other. (Become A-B- = A-bb).
aabb) The phenotypic ratio is 9: 4: 3.
The phenotypic ratio is 12: 3: 1.
Example:
Example:

Skin color onions
Red 9/16 – yellow 3/16 – white 4/16

In Mouse coat color
 Fruit color in summer squash
12/16 white – 3/16 yellow – 1/16 Green
Agouti 9/16 – Black 3/16 – Albino 4/16
Dominant white hides the effect of yellow
or green.
Genetic anaylsis:
Genetic anaylsis:
R yellow > r white
C no effect > c white
C-R- red
W white > w green
Y yellow > y green
P: WWyy x wwYY
Principles of genetics
3
P: CCRR x ccrr
red
white
F1:
C-R- (red)
F2:C-R-: C-rr :ccR- :ccrr
9 red : 3 yellow: 4 white
white
yellow
F1:
W-Y- (white )
F2:W-Y-: W-yy: wwY-: wwyy
12 white : 3 yellow : 1 green
3. Duplicate recessive epistasis
(9:7)
4. Dominant and recessive
epistasis (13:3)
If both gene loci have homozygous Sometimes the dominant alleles of one
recessive alleles and both of them produce gene locus (A) in homozygous and
identical phenotype the F2 ratio 9:3:3:1 heterozygous (AA, Aa) condition and
would be 9:7. The genotype aaBB, aaBb, homozygous
recessive
alleles
bb
of
AAbb, Aabb and aabb produce same another gene locus (B) produce the same
phenotype. Both dominant alleles when phenotype. The F2 ratio will become 13: 3.
are present together only then they can The genotype AABB, AaBB, AAbb, Aabb
complement each other. This is known as and aabb produce one type of phenotype
complementary gene.
and genotype aaBb, aaBB will produce
another type of phenotype.
Examples:

Example:
In sweet pea flower color
9/16 purple – 7/16 white
Note
that
the
C
and
P
 Feather Color of Fowl
13/16 white : 3/16 color
genes Gene pair ‘A’: color inhibition is dominant
independently assort, and remember that to color appearance.
the presence of a recessive genotype at Gene pair ‘B’: color is dominant to white
one locus (i.e., cc; or pp) masks the effects
of the alleles at the other locus.
Principles of genetics
4
Genetic anaylsis:
Genetic anaylsis:
P: AABB (purple) x aabb (white)
F1:
A-B- (purple)
F2: A-B-:A-bb: aaB-: aabb
9 purple : 7 white
C color > c white
I “no color” > i white
P: CCII white x ccii white
F1:
CcIi (white )
F2:C-I- : C-ii :ccI- : ccii
13 white : 3 color
****Duplicate recessive epistasis
Many years after Bateson first described this 9:7 phenotypic ratio in pea plants,
researchers were finally able to determine the two genes responsible for it. These genes
control flower color by controlling pea plant biochemistry, in particular that related to
pigment compounds called anthocyanins. In peas, there is a two-step chemical reaction
that forms anthocyanins; gene C is responsible for the first step, and gene P is responsible
for the second. If either step is nonfunctional, then no purple pigment is produced, and
the affected pea plant bears only white flowers. The dominant C and P alleles code for
functional steps in anthocyanin production, whereas the recessive c and p alleles code
for nonfunctional steps. Thus, if two recessive alleles occur for either gene, white flowers
will result.
Principles of genetics
5
Epistasis Gene interaction “Novel phenotype”
4. Duplicate dominant epistasis
(15:1)
5. Duplicate interaction
(9:6:1)
If a dominant allele of both gene loci Certain phenotypic traits depend on the
produces the same phenotype without dominant alleles of two gene loci. When
cumulative effect, i.e., independently the dominant is present it will show its
ratio will be 15:1.
phenotype. The ratio will be 9: 6: 1.
Examples:
Example:

Shepherd’s purse
15/16 triangular : 1/16 ovate
 Fruit
shape
in
summer
squash
Complete dominance at both gene pairs, Disc shaped fruits 9/16
but either gene when dominant, epistatic to
the other. Seed capsule of shepherd’s
purse.
Sphere shaped fruits 6/16
Long shaped fruit colored 1/16
Gene pair ‘A’: Triangular shape dominant
over ovoid
Gene pair ‘B’: Triangular shape dominant
over ovoid (double recessive)
Principles of genetics
6
Genetic anaylsis:
Genetic anaylsis:
P: AABB (Tria.) x aabb (ovate)
F1:
A-B- (tria.)
F2:A-B-: A-bb : aaB-:aabb
15 tria.
: 1 ovate
P:AAbb (round) x aaBB (round)
F1:
A-B- (disc )
F2: A-B-:A-bb: aaB-: aabb
9 disc : 6 round : 1 long
Principles of genetics
7
Epistatic Ratios:
Ratio
Description
9:4:3
Name(s) of Relationship
Recessive epistasis
A-B- (red): A-bb (white): aaB- (yellow) : aabb (white)
9/16 (agouti) : 4/16 (albino) : 3/16 (black)
12:3:1
Dominant epistasis
A-B- = A-bb : aaB- : aabb
12 White
9:7
: 3 yellow : 1 green
Duplicate recessive epistasis
A-B- : A-bb = aaB- = aabb
9 Purple : 7 white
13:3
Dominant and recessive epistasis
A-B- = aaB- = aabb : A-bb
13 white : 3 color
15:1
Duplicate dominant epistasis
A-B- = A-bb = aaB- : aabb
15 triangular : 1 ovate
9:6:1
Duplicate interaction
A-B- : A-bb = aaB- : aabb
9 Disc : 6 round : 1 long
References:
(Dooner et al., 1991).
http://seattlecentral.edu/faculty/jwhorley/Extensions.pdf
http://www.slideshare.net/zohaibkhan404/epistatic-gene-interaction-by-biotechnology-ciit-abbottabad
http://www.roosterrooperformances.com/roo-testiments.html
http://www.slideshare.net/shainamavreenvillaroza/general-genetics-gene-segregation-and-integration-part-2
http://www.biologydiscussion.com/inheritance-in-plants/inheritance-types-top-14-types-of-inheritance-in-plants/18393
http://iweb.langara.bc.ca/biology/mario/Biol2330notes/biol2330chap13.html
Principles of genetics
8
Exercise 11
a. In chickens, most individuals have un-feathered shanks when they
are homozygous for recessive genes at two loci; the presence of a
single dominant gene at either locus causes feathers. What is the
feathered-un-feathered ratio in their offspring if chickens
heterozygous at both loci are crossed?
a. 9:7
b. 12:4
c. 13:3
d. 15:1
2. The agouti fur colour in mice actually results from alternating dark
and light bands on each hair. Given this information and what you
know about the gene interaction from the previous problem, propose
a mechanism that could explain how each of the two genes is actually
affecting the overall fur colour (agouti, black, or albino).
Principles of genetics
9
3. In summer squash, there are two pairs of alleles that determine fruit
colour. The two genes sort independently. Two white-fruited plants
are crossed. Both parents are known to be heterozygous for both
genes. The cross produces the following offspring: 20 green-fruited
plants, 58 yellow-fruited plants, and 218 white-fruited plants.
(a) Based on the observed ratio, which common type of epistasis is
operating here? That is, which kind of modified dihybrid ratio most
closely fits these data?
(b) List the four genotype classes in the offspring and give the
corresponding phenotype of each.
(c) If a doubly heterozygous white plant is crossed with a green plant,
what phenotype ratio would you expect in the progeny?
Principles of genetics
10
4. Which of the following phenotypic ratios does not indicate
epistasis in a dihybrid cross?
a.
b.
c.
d.
e.
9:3:3:1
9:3:4
12:3:1
9:7
Unknown
5. Mating between purple starfish of identical genotype produced
offspring as follows: 18 pink, 54 purple, and 24 albino.
(a) What epistatic ratio is approximated by these offspring?
(b)What are the genotypes of the parents and the offspring (use your
own symbols)?
Principles of genetics
11