This document discusses various ways that alleles can alter phenotypes and modify Mendelian ratios. It describes different types of mutations that can cause loss or gain of function. It also explains genetic symbols used to represent alleles and different inheritance patterns such as incomplete dominance, codominance, lethal alleles, and epistasis. Multiple examples are provided to illustrate these concepts, such as coat color in mice, fruit color in squash, and flower color in peas. The key point is that while principles of segregation and independent assortment still hold, gene interactions can produce novel phenotypes and modified dihybrid ratios expressed in sixteenths.

1. MODIFICATION OF
MENDELIAN RATIO

2. ALLELES ALTER PHENOTYPES IN
DIFFERENT WAYS
Allele – alternative forms of the same gene.
Wild-type Allele – the one that we arbitrarily designated
as normal and occurs most frequently in the population. It
was often, but not always, dominant.
Mutant Allele – an allele that contains modified genetic
information and often specifies an altered gene product.

3. ALLELES ALTER PHENOTYPES IN
DIFFERENT WAYS
Loss- of-function Mutation – mutation that causes
dimunition or the loss the specific wild-type function.
Null Allele – resulting allele in the complete loss in mutation.
Gain-of-function Mutation – mutation that enhance the
function of the wild-type product wherein it results an
increase in quantity of the gene product.
Neutral Mutation – mutation which create no change in
function can be detected.

4. GENETICISTS USE A VARIETY OF
SYMBOLS FOR ALLELES
• The initial letter of the name of the Recessive trait is
lowercased and italicized.
• The initial letter ot the name of the Dominant trait is
uppercased.
Example:
In the case of tall and dwarf, where dwarf is recessive, D and
d represent the alleles responsible for these respective traits.

5. GENETICISTS USE A VARIETY OF
SYMBOLS FOR ALLELES
• In differentiating wild-type and dominant trait, the study uses
the initial letter or the combination of two or three letters of the
name trait. If the trait is recessive, lowercase is used while
uppercase is used if it us dominant. The constructing wild-type
trait is denoted by the same letter, but with superscript (+).
Example:
Ebony (e) is a recessive body color mutation in drosophila.
The normal wild-type body color is gray (e+).

6. GENETICISTS USE A VARIETY OF SYMBOLS
FOR ALLELES
e+/e+ gray homozygote (wild-type)
e+/e gray heterozygote (wild-type)
e/e ebony homozygote (mutant)
• The slash between the letters indicates that the two allele
designation represents the same locus on two homologous
chromosomes.

7. GENETICISTS USE A VARIETY OF SYMBOLS
FOR ALLELES
• Another variation is utilized when no dominance
exist between alleles. We simply use uppercase
italic letters and superscripts to denote alternative
alleles.

8. INCOMPLETE OR PARTIAL DOMINANCE
•In incomplete or partial dominance, neither allele is
dominant.
•It requires careful examination of the gene product
rather than the phenotype, often reveals an
intermediate level of gene expression.
Tay-Sachs Disease – a disease wherein
homozygous recessive individuals are severely
affected with a fatal lipid disorder. There is almost no
activity of the enzyme. hexosaminidase.

9. CODOMINANCE
• In Codominance, the influence of both alleles in a
heterozygote is clearly evident.
• Codominance is the joint expression of both alleles
in a heterozygote
• Codominant inheritance is characterized by distinct
expression of the gene products of both alleles.
MN blood group – characterized by an atigen called
glycoprotein, found on the surface of red blood cells.

11. MULITIPLE ALLELES
• Multiple alleles is ther term used when three or more
alleles of the same genes are present which creates
a unique mode of inheritance.
• It can only be studied only in populations.
The ABO Blood Type – The simplest case of
multiple alleles which three alternative alleles of one
gene exist. It was discovered by Karl Landsteiner in
the early 1900s.
The Bombay Phenotype – a term used to describe

15. LETHAL ALLELE
• A term used when one wild-type allele may be sufficient
to produce enough of the essential product to allow
survival. However, such mutation behaves as a
recessive lethal allele, and homozygous recessive
individuals will not survive.
• In other cases, mutation may behave as a dominant
lethal allele wherein the presence of just one copy of the
allele results in the death of individual.

16. LETHAL ALLELE
Huntington Disease – A disorder due to a dominant
autosomal allele H, where the onset of the disease in
heterozygous (Hh) is delayed, usually well into
adulthood.
Dominant lethal allele are rarely observed. For these
alleles to exist in population, the affected individual
must reproduce before the lethal allele is expressed.

17. COMBINATION OF TWO GENE PAIRS WITH
TWO MODES OF INHERITANCE
• 3:1 – Mendel’s F2 monohybrid ratio
• 9:3:3:1 – Classical ratio in dihybrid cross
Principle of Independent Assortment – Mendel’s
principle stating that the genes controlling each
character are not linked on the same chromosome –
in other words, they do not demostrate what is called
genetic linkage.

18. PHENOTYPES ARE OFTEN AFFECTED BY
ONE GENE
Gene Intraction – a term often use to describe the
idea that several genes influence a particular
characteristic.
Epigenesis – a developmental concept whereby
each step of development increases the complexity of
sensory organ and is under the control and influence
of one or more gene. An enlightening example of
epigenesis and multiple gene interaction involves the

19. PHENOTYPES ARE OFTEN AFFECTED
BY ONE GENE
Epistasis – the expression of one gene or gene pair
masks or modifies the expression of another gene or
gene pair.
Epistatic – a term used to describe the allele at the
first locus compared to those at the second locus
Hypostatic – a term used to describe the allele at
the second locus compared to those at the first locus.

20. PHENOTYPES ARE OFTEN AFFECTED
BY ONE GENE
It is important to note the following points when examining the
cross and the predicted phenotypic ratio:
1. A key distinction exists in this cross compared to the modified
dihybrid cross in figure 4 where blood type and skin
pigmentation are followed as separate phenotypic
characteristics.
2. Even though only a single character was followed, the
phenotypic ratio is expressed in sixteenths. If we knew
nothing about H substance and the genes controlling it, we
would still be confident that a second gene pair, other than

21. PHENOTYPES ARE OFTEN AFFECTED
BY ONE GENE
In case 1 is the inheritance of coat in mice. Normal wild-type
coat color is agouti, a grayish pattern formed by alternating
bands of pigment on each hair. Agouti us dominant to black
which is caused by recessive mutation, a. Thus, A- results in
agouti, while aa yields black coat color. When it is
homozygous, a recessive mutation, b, at a separate locus
eliminates pigmentation altogether, yielding albino mice (bb)
regardless of the genotype at the locus. In the cross between
agouti (AABB) and albino (aabb), members of the F1 are all
AaBb and have agouti coat color.

22. PHENOTYPES ARE OFTEN AFFECTED
BY ONE GENE
• Recessive Epistasis – a recessive allele at one genetic locus
masks the expression of the alleles at the first locus.
Example:
bb genotype being masks or supresses the expression of the A
gene.
• Dominant Epistasis – a dominant allele at one genetic locus
masks the expression of the of the alleles at the second locus.
Example:
A- genotype being masks or supresses the expression of the B
gene.

23. PHENOTYPES ARE OFTEN AFFECTED
BY ONE GENE
• In case 2, inheritance of color in summer squash,
the dominant allele A results in white fruit color
regardless of the genotype at the second locus B. In
the absence of the dominant A allele (the aa
genotype), BB or bb results in yellow color, while bb
results in green color. Therefore, if two white colored
double heterozygous (AaBb) are crossed, this type
of epistasis generates an interesting phenotypic

24. PHENOTYPES ARE OFTEN AFFECTED
BY ONE GENE
The third type of gene interaction was first discovered by
William Bateson and Reginald Punnet (punnet square
frame). In case 3, It is demonstrated in a cross between two
true-breeding strains of white flowered sweet peas.
Unexpectedly, the results of this cross yields all purple F1
plants and the F2 plants occur in a ratio of 9/16 purple to 7/16
white.
Complementary gene interaction – the presence of atleast
one dominant allele of each of two gene pairs is essential for
flowers to be purple.

25. PHENOTYPES ARE OFTEN AFFECTED BY
ONE GENE
Novel Phenotype – the production of new phenotype in the F2
generation during gene interaction, im addition to producing
modified dihybrid ratio.
Example:
In case 4, when plants with disc-shaped fruit (AABB) are
crossed to plants with long fruit (aabb). The F1 generation all
have disc fruit. However, in the F2 progeny, fruit with a novel
shape-spear appear, along with fruit exhibiting the parental
phenotypes.

26. OTHER MODIFIED DIHYBRID RATIOS
The remaining cases (5 – 8) show additional
modifications of the dihybrid ratio and provide still
other examples of gene interactions. However all
eight cases have two things in common.
First, we have not violated the principles of
segregation and independent assortment to explain
the inheritance pattern of each case.
Second, the F2 phenotypic ratio in each expresaion
are all expressed in sixteents.