This document discusses extensions of Mendelian genetics including incomplete dominance, codominance, multiple alleles, and gene interactions. It provides examples of incomplete dominance in flowers where the heterozygote has an intermediate phenotype. Codominance is explained using blood types where both alleles are expressed in the heterozygote. Multiple alleles are exemplified by the ABO blood group system which has three alleles. Gene interactions like epistasis can alter expected phenotypes when genes act together.

1. Extensions of Mendelism
(Pola Pewarisan diluar Pola
Dominan-Resesif Mendel)
By
Ummi Nur Afinni Dwi Jayanti, M.Pd.

2. Allele Variation and Gene Function
Mendel’s
Experiments
gene can exist
in alternate
forms (one
dominant, the
other recessive
Simple
functional
dichotomy
between alleles
Research in
early 20th
century genes
can exist in
more than two
allelic state
Each allele have
different effect on
the phenotype in
different ways

3. Incomplete Dominance
• An allele is dominant same phenotypic effect both in heterozygotes &
homozygotes (Mendelian crosses)
• Unlike Mendelian crosses, a cross between parents with contrasting traits may
sometimes generate offspring with an intermediate phenotype
• Ex: a four-o’clock or a snapdragon plant (Antirrhinum majus) with red flowers is
crossed with a white-flowered plant, the offspring have pink flowers. Because some
red pigment is produced in the F1 intermediate-colored plant, neither the red nor
white flower color is dominant
• White and red varieties are homozygous for different alleles of a color-
determining gen; when crossed, they produce heterozygous that have pink flowers
• The allele for red color (W) is therefore said to be incomplete, or partial,
dominance over the allele for white color (w)

4. Incomplete Dominance
• The intensity of pigmentation in
this species depends on the
amount of a product specified by
the color gene
• If the W allele specifies this
product and the w allele does
not, WW homozygotes will have
twice as much of the product as
Ww heterozygotes do and will
therefore show deeper color

5. Incomplete Dominance

6.  The most accurate way is to consider gene expression in a quantitative
way
 In the case of flower color above, the mutation causing white flowers is
most likely one where complete “loss of function” occurs. In this case, it is
likely that the gene product of the wild-type allele (R1) is an enzyme that
participates in a reaction leading to the synthesis of a red pigment.
 The mutant allele (R2) produces an enzyme that cannot catalyze the
reaction leading to pigment. The end result is that the heterozygote
produces only about half the pigment of the red-flowered plant and the
phenotype is pink
How are we interpret lack of dominance whereby an
intermediate phenotype characterizes heterozygotes?

7. Incomplete dominance in human disease
• Clear-cut cases of incomplete dominance are relatively rare
• However, even when one allele seems to have complete dominance
over the other, careful examination of the gene product, rather than the
phenotype, often reveals an intermediate level of gene exprssion
• Ex: human biochemical disorder Tay-Sachs disease, in which
homozygous recessive individuals are severely affected with a fatal
lipid-storage disorder and neonates die during their first one to three
years of life
• In afflicted individuals, there is almost no activity of the enzyme
hexosaminidase A, an enzyme normally involved in lipid metabolism.
• Heterozygotes, with only a single copy of the mutaant gene, are
phenotypically normal, but with only about 50% of the enzyme activity
found in homozygous normal individuals

8. Incomplete dominance in human disease
• Fortunately, this level of enzyme activity is adequate to
achieve normal biochemical function. This situation is not
uncommon in enzyme disorders and illustrates the concept
of the threshold effect, whereby normal phenotypic
expression occurs anytime a certain level of gene product is
attained.
• Most often, and in particular in Tay-Sachs disease the
threshold is less than 50 percent

9. Codominance
• Another exception to the principle of simple dominance arises when a heterozygote shows characteristics
found in each of the associated homozygotes
• This occurs with human blood types, which are identified by testing for special cellular products called
antigen. These factors, which are produced by the immune system, recognize antigens quite specifically
• The ability to produce the M and N antigens is determined by a gene with two alleles. One allele allows the
M antigen to be produced; the other allows the N antigen to be produced
• Homozygotes for the M allele produce only the M antigen, and homozygotes for the N allele produce only
the N antigen. However, heterozygotes for these two alleles produce both kinds of antigens
• Because the two alleles appear to contribute independently to the phenotype of the heterozygotes, they are
said to be codominant
• Codominance implies that there is an independence of allele function
• Codominant inheritance is characterized by distinc expression of the gene products of both alleles. This
characteristic distinguishes codominance from incomplete dominance, where heterozygotes express an
intermediate blended, phenotype.
• For codominance to be studied, both products must be phenotypically detectable.
• ABO blood-type system is another example of codominance when we examine the ABO blood-type system

11. Multiple Alleles of a Gene May Exist in a Population
• The information stored in any gene is extensive, and mutations can modify this
information in many ways. Each change produces a different allele. Therefore, for
any gene, the number of alleles within members of a population need not to be
restricted to two.
• When three or more alleles of the same gene-which we designate as multiple
alleles-are present in a population, the resulting mode of inheritance may be
unique.
• It is important to realize that multiple alleles can be studied only in populations.
• Any individual diploid organism has, at most, two homologous gene loci that may
be occupied by different alleles of the same gene. However, among members of
a species, numerous alternative forms of the same gene can exist

12. Multiple Alleles of a Gene May Exist in a Population
• Example:
1. The ABO Blood Groups
• the simplest case of multiple alleles occurs when three alternative alleles of one
gene exist. This situation is illustrated in the inheritance of the ABO blood groups
in humans
• The ABO system, like the MN blood types, is characterized by the presence of
antigens on the surface of red blood cells
• The A and B antigens are distinct from the MN antigens and are under the control
of a different gene located on chromosome 9
• One combination of alleles in the ABO system exhibits a codominant mode of
inheritance

13. Multiple Alleles of a Gene May Exist in a Population
• Example:
1. The ABO Blood Groups
• The ABO phenotype of any individual is scertained by mixing a blood sample with
an antiserum containing type A or type B antibodies. If an antigen is present on
the surface of the person’s red blood cells, it will react with the corresponding
antibody and cause clumping, or agglutination, of the red blood cells.
• When an individual is tested in this way, one of four phenotypes may be revealed.
Each individual has either the A antigen (A phenotype), the B antigen (B
phenotype), the A and B antigen (B phenotype), the A and B antigens (AB
phenotype), or neither antigen (O phenotype). These phenotypes were inherited
as the result of three alleles of a single gene.
• To distinguish these three alleles, we’ll use the symbols IA, IB and i. The I
designation stands for isoagglutinogen, another term for antigen.

14. Multiple Alleles of a Gene May Exist in a Population
• Example:
1.The ABO Blood Groups
In these assignments, the IA and IB, alleles are
dominant to the i allele, but codominant to each other

15. Multiple Alleles of a Gene May Exist in a Population
• Example:
2. The white Locus in
Drosophila
• many other
phenotypes in plants
and animals are
influenced by multiple
allelic inheritance

16. Multiple Alleles of a Gene May Exist in a Population
• Example:
3. The gene that controls
coat color in rabbits
• the color-determining
gene, denoted by a c
(albino), cb (himalayan),
ccb (chinchilla), and c+
(wild-type). z

17. GENE INTERACTIONS (ATAVISME)
Two independently assorting genes can affect a trait (due to the interactions between gene products at
biochemical/ cellular levels based on Bateson & Punnett’s breeding experiments with chickens (domestic
breeds of chickens have different comb shapes)
Rose,
Wyandottes
(RRpp)
Pea, Brahmas
(rrPP)
Walnut (hybrid
crosses
between Rose
& pea
(RrPp)
Single,
Leghorns
(rrpp)
If F1 hybrids RrPp intercrossed with
each other all 4 types of combs
appear in the progeny
9/16 walnut (R-P-)
3/16 rose (R-pp)
3/16 pea (rr P-)
1/16 single (rr pp)

18. GENE INTERACTION
Bateson & Punnett’s
experiment

19. GENE INTERACTION (Polimeri interaksi
gen bersifat kumulatif, saling menambah)
Triticum vulgare

20. GENE INTERACTION (Kriptomeri faktor dominan yang baru tampak pengaruhnya
apabila bertemu dengan faktor dominan lain yang bukan alelnya )
Linaria maroccana.
A+B  bunga ungu
A tanpa B  bunga merah
Rasio fenotip Ungu: Merah
: Putih = 9:3:4

21. EPISTASIS-HIPOSTASIS gen dominan lain yang bukan alelnya menutupi gen dominan
lainnya )
H epistasis terhadap K
K hipostasis terhadap H
12:3:1

22. GEN KOMPLEMENTER interaksi antara dua gen dominan, jika terdapat bersama-sama akan saling
melengkapi sehingga muncul suatu fenotip )
Interaksi antara dua gen dominan, jika
terdapat bersama-sama akan saling
melengkapi sehingga muncul suatu fenotip.
Jika salah satu gennya tidak ada, maka
pemunculan sifat terhalang.
9:7