Download Genetics Part II

Exceptions to
Mendelian Genetics
&
Pedigrees
Mendel’s work led to the
Chromosome Theory
•
Genes are located on
chromosomes
• Separation of genes during
meiosis account for the
inheritance patterns discovered
by Mendel
However…
Inheritance of traits by a single
gene may deviate from simple
Mendelian patterns
• The first solid evidence
associating a specific gene with a
specific chromosome came from
Thomas Hunt Morgan
• Morgan’s experiments with fruit
flies provided convincing
evidence that chromosomes are
the location of Mendel’s heritable
factors
• Several characteristics make fruit flies
a convenient organism for genetic
studies:
–They breed at a high rate
–A generation can be bred every two
weeks
–They have only four pairs of
chromosomes
–They are easy to keep alive
–They don’t need a lot of space
• In one experiment, Morgan mated
male flies with white eyes (mutant)
with female flies with red eyes (wild
type)
–The F1 generation all had red eyes
–The F2 generation showed the 3:1
red:white eye ratio, but only males
had white eyes
• Morgan determined that the whiteeyed mutant allele must be located on
the X chromosome
Fig. 15-3
Fig. 15-4
EXPERIMENT
P
Generation

F1
Generation
All offspring
had red eyes
RESULTS
F2
Generation
CONCLUSION
P
Generation
w+
X
X

w+
X
Y
w
Eggs
F1
Generation
w+
Sperm
w+
w+
w
w+
Eggs
F2
Generation
w
w+
w
Sperm
w+
w+
w+
w
w
w+
• Morgan discovered sex-linked
genes
• Genes found on the sex
chromosomes
• Usually it’s found on the X
chromosome
Difference in Human X and Y
Chromosomes
• Females are XX, and males are
XY
• Each ovum contains an X
chromosome, while a sperm may
contain either an X or a Y
chromosome
• For example, a gene for making a
blood clotting protein is found on
the X chromosome
• Having a mutation of this gene
causes hemophilia
• Males that inherit a mutation of
the gene found on their X
chromosome, lack another gene
on their Y chromosome and have
hemophilia
Swelling of the joints in
hemophiliac patient
• Duchenne (pseudohypertropic)
muscular dystrophy is also sexlinked
Classic signs of Duchenne MD
• 8% of males have some type of
color blindness
• 0.4% of females are color blind
• The most common type is
distinguishing red from green
Complete dominance occurs
when phenotypes of the
heterozygote and dominant
homozygote are identical
For example: Aa genotype gives
the dominant phenotype and the
“a” gene is completely hidden
There are exceptions!
In incomplete dominance, the
phenotype of heterozygotes is
somewhere between the phenotypes
of the two parental varieties
NEITHER the dominant or recessive
trait shows up – there’s a third
phenotype
Fig. 14-10-3
P Generation
Red
CRCR
Such as
inheritance
of petal
F
color
in
Japanese
4 o’clocks
1
White
CWCW
CR
Gametes
CW
Pink
CRCW
Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
• The sickle cell gene is also
incompletely dominant.
• A person heterozygous has sickle
cell trait
In codominance, if two different
alleles are present, they will both
show their trait
For example, ABO human blood
types
• Sometimes there are more than 2
alleles for a trait and are called
multiple alleles
• Example: the A, B, and O genes
in human blood types
• Polygenic traits are controlled by
more than one pair of genes and
accounts for a wide variety of a
trait
• Skin color in humans is an
example of polygenic inheritance
• Alfred Sturtevant, one of
Morgan’s students, constructed a
genetic map, an ordered list of
the genes along a particular
chromosome
• He did this through studying how
frequently two genes located on
the same chromosome separate
during crossing over
• Sturtevant discovered that some
genes are found on the same
chromosome very close together
and, if present, are often inherited
together
• For example, the genes for hair
color and presence of freckles are
both found on the same
chromosome close to each other
and are often inherited together
• This deviates from Mendel’s
principle of independent
assortment
Even though there are exceptions
to Mendel’s principles, basic
Mendelian genetics endures as
the foundation of genetics
• A pedigree is a family tree that
describes the interrelationships of
parents and children across
generations
• Inheritance patterns of particular
traits can be traced and described
using pedigrees
Fig. 14-15a
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
ww
Ww ww ww Ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Fig. 14-15c
1st generation
(grandparents)
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?