Beyond Mendel
Beyond Mendel

Despite the importance of
Mendel’s work, it would be
a mistake to characterize
the principles he
discovered as “laws”
because there are
important exceptions to
most of them.
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Not all genes show simple
patterns of dominant and
recessive alleles.
In most organisms, genetics
is more complicated, because
the majority of genes have
more than two alleles.
In addition, many important
traits are controlled by more
than one gene.
Incomplete Dominance

A form of intermediate
inheritance in which
heterozygous alleles
are both expressed,
resulting in a
combined phenotype.
Incomplete Dominance

For example, in
cross-pollination
experiments
between red and
white snapdragon
plants the resulting
offspring are pink.
Codominance


A number of
human traits are
the result of 2
types of alleles
that are equally
dominant.
Such traits are said
to be codominant
for that trait.
Codominance

When an individual
is heterozygous for
such traits, the
resulting
phenotype or
expression of
these two traits is
a blending,
because both
traits are
expressed equally.
Codominance

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The alleles for curly hair and
straight hair are examples of
alleles for a trait that are
codominant.
Individuals with curly hair are
homozygous for curly hair
alleles.
Individuals with straight hair
are homozygous for straight
hair alleles.
Individuals who are
heterozygous, with one of each
allele have wavy hair, which is a
blend of the expressions of the
curly and straight hair alleles.
Multiple Alleles

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Diploid organisms naturally
have a maximum of 2 alleles
for each gene expressing a
particular characteristic, one
deriving from each parent.
In some cases, however,
more than two types of allele
can code for a particular
characteristic, as is the case
of genetic coding for blood
type in humans or coat color
in rabbits.
Multiple alleles result from
different mutations of the
same gene.
Multiple Alleles



Coat color in rabbits is
determined by four
alleles.
Human ABO blood
types are determined
by alleles A, B, and O.
A and B are
codominants which
are both dominant
over O.
Multiple Alleles

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The only possible
genotype for a type O
person is ii.
Type A people have
either IAIA or IAi
genotypes.
Type B people have
either IBIB or IBi
genotypes.
Type AB have only the
IAIB (heterozygous)
genotype.
Multiple Alleles


The A and B alleles of gene
I produce slightly different
glycoproteins (antigens)
that are on the surface of
each cell.
Homozygous A individuals
have only the A antigen,
homozygous B individuals
have only the B antigen,
homozygous O individuals
produce neither antigen,
while a fourth phenotype
(AB) produces both A and
B antigens.
Rh Blood type system

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When we are told our
blood type, it is usually
expressed as a letter
followed by either a
positive (+) or negative ().
This + or - refers to the Rh
system of blood testing
and is usually quoted in
combination with the ABO
system described earlier in
this lecture:
e.g. AB+ or O-
Rh Blood type system


The Rh system is actually much more
complicated than the ABO system
because there are more than 30
combinations possible when
inherited, however for general usage,
the Rh proteins are grouped into two
families:
either positive (+) or negative (-).
Rh Blood type system
As with the ABO system, there is a
dominant allele which happens to be the
positive family.
 This means that the genetic pairs that
can exist in humans are as follows:
Genotype
Blood type
++
Rh +
+Rh +
-Rh 
Who can donate to who?
Polygenic Traits

Traits controlled by
two or more genes
are called
polygenic traits.
Polygenic traits

Usually polygenic traits
are distinguished by:
1.
Two or more gene
pairs contribute to the
phenotype.
2.
Traits are usually
quantified by
measurement rather
than counting.
3.
Phenotypic expression
of polygenic traits
varies over a wide
range.
Polygenic Traits

Human polygenic
traits include:
1.
2.
3.
4.
5.
6.
Height
Weight
Eye Color
Intelligence
Skin Color
Many forms of
behavior