1 wrinkled, green
Mendel's second principle states ...
that, during gamete formation, the alleles at one locus segregate into the gametes independently of the pair of alleles found at a different locus.
If we set up a Punnett square, we can see why this is so (figure 3.11).
The genotypes of the two F1s are given below.
RrYy x RrYy
Given independent assortment of the alleles in an F1 plant, we would expect
4 types of gametes to be produced by each of the F1s.
- RY, rY, Ry, ry
Each gamete receives one allele for seed color and one allele for seed
shape (figure 3.11).
Genotypic ratio Phenotypic ratio
RrYY 2 round, yellow 9
RRyy 1 round, green 3
rrYY 1 wrinkled, yellow 3
rryy 1 wrinkled, green 1
Extending these results to more than 2 loci yields the following table.
| monohybrid|| dihybrid ||trihybrid || n-hybrid|
|F1 gametic genotypes|| 2 || 4 || 8 || 2n|
|proportion of homozygous recessives in F2|| 1/4 || 1/16 || 1/64 || (1/2n)2|
|number of different F2 phenotypes given complete
dominance|| 2 || 4 || 8 || 2n|
|number of different genotypes|| 3 || 9 || 27 || 3n
In the crosses that we have examined so far, the alleles have either been
dominant or recessive. For example, yellow peas are dominant to green
peas, tall plants are dominant to short plants
However, alleles can also be codominant in that the alleles are expressed
equally and completely in the phenotype (table 5.1, page 103, as in the heterozygote who
expresses both M and N antigens (for example, MN blood group in humans).
1) there are two alleles Lm, Ln
2) genotypes phenotypes
LmLm M blood
LnLn N blood
LmLn MN blood
e.g. sickle cell anemia
1) two alleles Hba (normal), Hbs
2) genotypes phenotypes
HbsHbs sickle cell anemia
At the molecular level, the traits are codominant as both types of
hemoglobin are produced in equal amounts by a heterozygous
person. However, at the organismal level, Hba is dominant to
Hbs, HbaHba is normal, HbaHbs is essentially normal,
and HbsHbs is sickle cell anemia.
Incomplete dominance means one allele is incompletely dominant over
another allele (figure 5.2, page 103).
P RR (red) x rr (white)
F1 Rr x Rr (pink)
F2 RR Rr rr
red pink white
1 2 1
Thus dominance and recessiveness are due to the relative expression of the
alleles at the organismal level, but at the molecular level most allelic pairs are likely to be
- at the organismal level, the red allele is incompletely dominant
over the white allele
- However, at the molecular level the alleles are codominant in that pink is caused by 1 allele (red) coding for a functional protein that produces red pigment and by 1 allele (white) coding for a non-functional protein that does not produce any red pigment. Thus only half the amount of red pigment is produced and a pink flower is the result.
Another important point: Mendel also demonstrated that the alleles are unchanged in the passage from one generation to the next. At the time of Mendel's discoveries, the general feeling was that traits were blended in the offspring and the modified allele (blended alleles) were then passed to subsequent generations. Mendel showed that the traits are passed as discrete particles (round or wrinkled) and are not changed when passed from generation to the next. This particulate nature of inheritance also supported Darwin's theory of natural selection in that now selection could operate to change the frequency of alleles while not changing the allele itself.
Epistasis means to stand on.
The expression of one allele prevents or interferes with the expression
of alleles at another locus (chapter 5, pages 108 - 112).
In mice, as in many mammals, there are actually 5 loci that control
coat color. We will look at two of these loci.
- agouti pattern (gray) results from the banding pattern of
pigments deposited in each hair
A is dominant to a and causes banding
AA, Aa are agouti
aa produces no bands and the individual is black
- another locus controls the expression of the black pigment
C is dominant to c and causes production of the black pigment
CC, Cc produces black pigment
cc produces no black pigment and is white (albino)
P homozygous black CCaa x ccAA homozygous white
F1 CcAa all agouti
dihybrid cross of F1 x F1
F2 9 agouti: 3 black: 4 albino
This phenotypic ratio is similar to the 9:3:3:1 ratio expected for the
F2 generation in a dihybrid cross with alleles exhibiting complete
dominance, except that the last two categories (3+1) have been combined.
The recessive genotype of the color gene (cc) is epistatic to,
or it interferes with, the dominant allele of the agouti pattern.
- one locus whose dominant allele (C) is necessary for the
development of color and
- another locus whose dominant allele (A) is necessary for the
banding pattern to produce the agouti color
precursor -----> black pigment ------> agouti pattern
(allele C) (allele A)
Duplicate recessive epistasis
Epistasis in corn
P homozygous purple AABB x aabb homozygous white
F1 AaBb all purple
dihybrid cross of F1 x F1
F2 9 purple:7 white
9:3+3+1 = 9:7
A-, B- purple 9
A-, bb white 3
aa, B- white 3
aa, bb white 3
"-" equals a wild card
The biochemical pathway is . . .
precursor (colorless)----->intermediate (colorless)----->purple pigment
(allele A) (allele B)
Thus aa is epistatic to B and bb is epistatic to A.
Review table 5.2 (page 112) to get a picture of the variety of ratios that can be produced depending upon the type of loci and alleles involved. The key to recognizing epistasis is to recognize that the F2 phenotypic ration will be some permutation of 9:3:3:1.
Links on the Web
2) Variation and Mendel's Laws -- Discussion of dominance, co-dominance, and multiple alleles.
3) Diverse gene expression patterns are possible -- The notes on extensions to Mendel's principles from a faculty member at Washington State University.
Last updated on 23 January 2007
Provide comments to Dwight Moore at email@example.com
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