Extension of Mendelian Genetics in health

Mendelian Genetics

1. Copyright © 2009 Pearson Education, Inc.
PowerPoint
®
Lecture Presentation for
Concepts of Genetics
Ninth Edition
Klug, Cummings, Spencer, Palladino
Chapter 4
Extensions of Mendelian Genetics
Lectures by David Kass with contributions from
John C. Osterman.
Copyright © 2009 Pearson Education, Inc.

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•Alternative forms of a gene are called
alleles.
•Mutation is the source of alleles.
•The wild-type allele is the one that occurs
most frequently in nature and is usually,
but not always, dominant.
Alleles

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Mutations
•Loss-of-function mutations
•Null alleles
•Gain-of-function mutations
•Neutral mutations

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•Phenotypic traits may be influenced by
more than one gene and the allelic forms
of each gene involved.

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•Dominant alleles are usually indicated
either by:
•an italic uppercase letter (D)
•Recessive alleles are usually indicated
either by:
•an italic lowercase letter (d)
Allelic Symbols Used

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•System Used for Drosophila melanogaster
•e+/e+ gray homozygote (wild type)
•e+/e gray heterozygote (wild type)
•e/e ebony homozygote (mutant)
•+/+ gray homozygote (wild type)
•+/e gray heterozygote (wild type)
•e/e ebony homozygote (mutant)
•Wr/Wr wrinkled-wing homozygote (mutant)
•Wr/Wr+ wrinkled-wing heterozygotes (mutant)
•Wr+/Wr+ normal wings (wild type)
Allelic Symbols Used

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•If no dominance exists, italic uppercase
letters and superscripts are used to denote
alternative alleles (R
1
, R
2
, C
W
, C
R
).
Allelic Symbols Used

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•In incomplete dominance:
•neither trait is dominant
•offspring from a cross between parents with
contrasting traits may have an intermediate
phenotype
Incomplete Dominance

9. Copyright © 2009 Pearson Education, Inc. Figure 4.1
The phenotypic ratio
is identical to the
genotypic ratio in
cases of incomplete
dominance.

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•The threshold effect comes about if
normal phenotypic expression occurs
whenever a certain level (usually 50% or
less) of gene product is attained.
•Ex. Tay-Sachs Disease
Incomplete Dominance

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•Codominance
•both alleles are expressed in the heterozygote
•One example is the MN blood group.
Codominance

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•Multiple alleles (>2) can be studied only
in populations, because any individual will
have at most two alleles of the same gene.
Multiple Allelism

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•Alleles present in population:
•A, B, O alleles
•Each individual has the A, B, AB, or O
phenotype
•I
A
and I
B
alleles are dominant to the I
O

allele
•I
A
and I
B
alleles are codominant
Multiple Allelism – ABO Blood Group

14. Copyright © 2009 Pearson Education, Inc. Figure 4.2

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Bombay Phenotype

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•Loss-of-function mutation can sometimes
be tolerated in the heterozygous state
• but may behave as a recessive lethal allele
in the homozygous state.
•In this case, homozygous recessive
individuals will not survive.
Recessive Lethal Alleles

17. Copyright © 2009 Pearson Education, Inc. Figure 4.4

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•In some cases, a mutation can be a dominant
lethal allele, in which case the heterozygote
will not survive.

•Ex. Huntington disease
•For dominant lethal alleles to exist, the
affected individual must reproduce before
dying.
Dominant Lethal Alleles

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•Mendel’s principle of independent
assortment applies to situations in which
two modes of inheritance occur
simultaneously, provided that the genes
controlling each character are not linked
on the same chromosome.
Mendel – Independent Assortment

20. Copyright © 2009 Pearson Education, Inc. Figure 4.5

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•In gene interaction, the cellular function of
numerous gene products contributes to the
development of a common phenotype.
•Epigenesis – often a phenotype occurs due
to many steps in a developmental process
that are influenced and controlled by many
genes.
•Ex. Development of organs
Phenotypes Affected by Many Genes

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•Epistasis occurs when:
•one gene masks the effect of another gene, or
•two gene pairs complement each other such
that one dominant allele is required at each
locus to express a certain phenotype
•Ex. Bombay effect
Epistasis

23. Copyright © 2009 Pearson Education, Inc. Figure 4.6

24. Copyright © 2009 Pearson Education, Inc. Figure 4.7

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•Eight cases of epistasis are described in
Figure 4.8.
•These include recessive epistasis (case
1), dominant epistasis (case 2), and
complementary gene interaction (case 3).
Section 4.8

26. Copyright © 2009 Pearson Education, Inc. Figure 4.8

27. Copyright © 2009 Pearson Education, Inc. Figure 4.9

28. Copyright © 2009 Pearson Education, Inc. Figure 4.10
Eye color in
Drosophila.
Interaction of two
gene products
result in the wild-
type eye color,
which is brick
red.

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•Two cases of mutation in Drosophila
(Figure 4.11)
•Case 1: All offspring develop normal wings
•Case 2: All offspring fail to develop normal
wings
Complementation Analysis

30. Copyright © 2009 Pearson Education, Inc. Figure 4.11

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•Pleiotropy occurs when
expression of a single
gene has multiple
phenotypic effects, and it
is quite common.
•Examples of pleiotropy
are Marfan syndrome
and porphyria variegata.
Pleiotropy
Abraham_Lincoln_standing_portr
ait_1863.jpg
Flo Hyman

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•Genes present on X chromosome exhibit
unique patterns of inheritance due to
presence of only one X chromosome in
males.
X-Linkage

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•Drosophila eye
color
•one of the first
examples of X-
linkage described
X-Linkage

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Hemizygosity
•Hemizygosity
•Occurs in males due to the inability of males
to be homozygous or heterozygous for an X-
linked gene
•Have only one copy of that gene despite
having diploid cells

35. Copyright © 2009 Pearson Education, Inc. Figure 4.14

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•Lethal X-linked recessive disorders are
observed only in males.
•Usually never reproduce
•Females can only be heterozygous
carriers that do not develop the disorders.
Lethal X-Linked Recessive Disorders

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•Sex-limited inheritance occurs in cases
where the expression of a specific
phenotype is absolutely limited to one sex.

•In sex-influenced inheritance, the sex of
an individual influences the expression of
a phenotype that is not limited to one sex
or the other.
Individual’s Sex Can Influence
Phenotype

39. Copyright © 2009 Pearson Education, Inc. Figure 4.15

40. Copyright © 2009 Pearson Education, Inc. Figure 4.16

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•Phenotypic expression of a trait may be
influenced by environment as well as by
genotype.
Epigenetics

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•In cases of genomic (parental) imprinting,
phenotypic expression may depend on the
parental origin of the chromosome.
•Imprinting is thought to occur before or
during gamete formation and may involve
DNA methylation.
Genomic (Parental) Imprinting

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The End