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Variations on Mendel’s Laws
CHAPTER 9
• Incomplete Dominance
• Codominance
• Multiple Alleles
• Pleiotropy ("Multiple Phenotypes")
• Polygenic Inheritance
• Following Genes on Chromosomes
• Sex Chromosomes and Sex-Linked Traits
Variations On Mendel’s Laws
• Some patterns of genetic inheritance are not
explained by Mendel’s Laws
• Some of these non-Mendelian patterns are:
– Incomplete dominance
– Multiple allelism
– Pleiotropy
– Polygenic inheritance
Incomplete Dominance in Plants and People
• In incomplete dominance, F1
hybrids have an appearance
in between the phenotypes of
the two parents.
• The allele R is incompletely
dominant over the allele r.
• Some geneticists prefer to
use different symbology for
incompletely dominant
genes, e.g. CR for red color,
CW for white color in
snapdragon flowers.
Multiple Allelism and Codominance: The ABO blood
groups in humans
Three alleles are
responsible for
greating a twogene genotype for
ABO blood typing:
IA = Type A blood
IB = Type B blood
i = Type O blood
where IA and IB
are codominant to
each other and
dominant over i.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Variations on Mendel’s Laws
CHAPTER 9
• Incomplete Dominance
• Codominance
• Multiple Alleles
• Pleiotropy ("Multiple Phenotypes")
• Polygenic Inheritance
• Following Genes on Chromosomes
• Sex Chromosomes and Sex-Linked Traits
Pleiotropy and Sickle-Cell Disease
• Pleiotropy is the
impact of a single
gene on more than
one characteristic.
• Pleiotropy means
“many tendencies”
or “many
phenotypic effects”
• The sickle-cell
hemoglobin gene
(HbS) is an
example. Normal
hemoglobin is HbA.
Polygenic Inheritance
• Polygenic inheritance is
the additive effects of
two or more genes on a
single phenotype.
• Examples include
– Human height
– Human intelligence
– Human skin color
The Role of Environment
• Many human
characteristics
result from a
combination of
heredity and
environment.
• Identical twin
studies have
revealed role of
the environment
on phenotype
Variations on Mendel’s Laws
CHAPTER 9
• Incomplete Dominance
• Codominance
• Multiple Alleles
• Pleiotropy ("Multiple Phenotypes")
• Polygenic Inheritance
• Following Genes on Chromosomes
• Sex Chromosomes and Sex-Linked Traits
Linked Genes
• Linked genes
– Are located close together on a chromosome.
– May be inherited together.
• Genes on the same chromosome do not sort
independently as Mendel would predict
• An example of linked genes are blonde hair and
blue eyes. This combination tends to be inherited
together.
Sex Determination in Humans and Fruit Flies
• Sex chromosomes
– Influence the inheritance of certain traits.
• Sex chromosomes
– Are designated X and Y.
– Determine an individual’s sex.
father
mother
XY x XX
X
Y
X
X
Y
X
XX
XY
X
XX
XY
X
2 XX: 2 XY
1 male: 1 female
Figure 9.28
Sex-linked Genetic Disorders
• Sex-linked (X-linked) disorders are
caused by recessive genes for traits
carried on the X chromosome
•
Red-green color blindness is
X linked
•
Characterized by a
malfunction of lightsensitive cells in the eyes.
•
Color vision genes are
written attached to the X
chromosome: XC (normal
vision) and Xc (color blind)
Color blind
Normal
father
mother
XcY x XCXC
Xc Y
Xc
XC XC
Y
XC
XCXc XCY
XC
XCXc XCY
2 XCXc: 2 XCY
1 normal female (carrier):
1 normal male
Figure 9.28
Hemophilia is a Sex-Linked Disease
• Blood-clotting
genes are also Xlinked
• Genes are also
written attached to
the X
chromosome: XH
(normal blood
clotting), Xh
(abnormal clotting:
hemophilia)
Biology And Society: Testing Before Birth
• Genetic testing
– Allows expectant
parents to test for
possibilities in their
unborn child.
– Includes
amniocentesis and
CVS.
– Has risks associated
with it.
Ultrasound of Human Fetus 1
Variations on Mendel’s Laws
CHAPTER 9
• Incomplete Dominance
• Codominance
• Multiple Alleles
• Pleiotropy ("Multiple Phenotypes")
• Polygenic Inheritance
• Following Genes on Chromosomes
• Sex Chromosomes and Sex-Linked Traits