Transcript Nonmendelian Genetics

Chapter 13
(Part 3)
Non-Mendelian
Genetics
Honors
Genetics
Ms. Gaynor
Extending Mendelian
Genetics for a Single Gene
 The
inheritance of
characters by a single gene
 May deviate (do NOT
follow) from simple
Mendelian patterns
 Examples
Incomplete dominance,
codominance, multiple
alleles, pleiotropy
The Spectrum of
Dominance
 Complete
dominance
 Occurs when the
phenotypes of the
heterozygote (Hh) and
dominant homozygote
(HH) are identical
 Demonstrates
or follows
“Mendelian Genetics”
inheritance pattern
“Non-Mendelian Genetics”
Incomplete (intermediate)
Dominance
 1 allele is not completely dominant
over the other, so heterozygote (Hh)
has intermediate (or mixed)
phenotype between 2 alleles
(like snapdragon flowers)
P Generation
White
CWCW

Red
CRCR
CR
Gametes
CW
Pink
CRCW
F1 Generation
1⁄
2
Gametes
Eggs
1⁄
2
CR
CR
1⁄
2
CR
1⁄
2
CR
F2 Generation
1⁄
2
Figure 14.10
1⁄
2
CR
CR CR
CR CW
CR CW
CW CW
Cw
Sperm
Let’s do some practice
problems…
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
Assume incomplete dominance…
A red gummy bear mates with a yellow gummy
bear. Red (R) is dominant. What are the
genotype/phenotype ratios of their F1
offspring?
100% Rr 100% orange
If 2 F1 gummy bears from the question above
mate. What are the genotype/phenotype ratios
of their F2 offspring?
25% RR
50% Rr
25% rr
25% Red
50% orange
25% yellow
“Non-Mendelian Genetics”
Codominance
 2 dominant alleles affect
phenotype in separate,
distinguishable ways
 BOTH phenotypes are
present
 Ex’s

of codominance
Some flowers and Roan animals
(cattle & horses)
Roan Animals Show
Codominance
Let’s do some practice
problems…



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


Assume codominance…
A blue flower mates with a yellow flower. Blue
(B) is dominant. What are the
genotype/phenotype ratios of their F1
offspring?
BB= blue
Bb= blue & yellow bb= yellow
100% Bb 100% Blue AND yellow flowers
If 2 F1 flowers from the question above mate.
What are the genotype/phenotype ratios of
their F2 offspring?
25% BB
25% Blue
50% Bb
50% blue AND yellow
25% bb
25% yellow
Multiple Alleles
A
type of codominance
 Most genes exist in
populations
 In more than two allelic
forms that influence
gene’s phenotype
Ex:
Human Blood type
The ABO
blood group
in humans
Is determined
by multiple
alleles
Multiple Alleles
(Codominance)
Blood
Type
Genotypes
A
AB
IAIA, or
I Ai
IBIB, or
I Bi
I AI B
O
ii
B
Blood Type Practice
A woman with Type O blood and a man, who is Type
AB, are expecting a child. What are the possible
blood types of their child?
ii x IAIB 
50% chance IAi (A type); 50% chance IBi (B type)
What are the possible blood types of a child who's
parents are both heterozygous for "B" blood
type?
IBi X IBi 
50% chance IBi, 25% chance IBIB, 25% chance ii
• 75% chance of B type and 25% chance of O type
More Blood Type
Practice 
What are the chances of a woman with Type AB and a man with
Type A having a child with Type O?
IA? x IAIB 
0% chance of Type O b/c mom can’t donate “i” allele
Jill is blood Type O. She has two older brothers with blood types
& B. What are the genotypes of her parents?
IAi and IBi
Jerry Springer did a test to determine the biological father of child
The child's blood Type is A and the mother's is B. Daddy Drama
#1 has a blood type of O & Daddy Drama #2 has blood type
AB. Which man is the biological father?
Dad #1 = ii and Dad #2= IAIB 
It has to be Daddy #2
Polygenic
Inheritance
 Many
genes (2+) determine one
(1) phenotype
 Many human traits
 Vary in the population along a
continuum
 Few genes actually follow a simple
Mendelian inheritance pattern
 Examples:
 Height,
eye color, intelligence, body build
and skin color
Polygenic Inheritance

AaBbCc
aabbcc
20⁄
15⁄
64
64
6⁄
64
1⁄
64
Aabbcc
AaBbcc
AaBbCc
AaBbCc
AABbCc
AABBCc
AABBCC
Nature and Nurture:
The Environmental Impact on
Phenotype

Departs from simple Mendelian genetics
 phenotype depends on environment
as well as on genotype
Called multifactorial inheritance
 Ex: human fingerprints
hydrangea flowers
Al in soil; need
LOW pH
Add P to
soil; need
HIGHERpH
Chapter 11
(Part 4)
Human Genetics
Honors Genetics
Ms. Gaynor
Many human traits follow
Mendelian patterns of
inheritance
 Humans
are not
convenient subjects for
genetic research
 However, the study of
human genetics
continues to advance
 We use pedigrees!
Pedigree Analysis
pedigree
Is a family tree that
describes the
interrelationships of
parents and children
across generations
A
Inheritance patterns of particular traits
can be traced and described using
pedigrees
Ww
ww
Ww ww ww Ww
WW
or
Ww
ww
Ww
First generation
(grandparents)
Ww
ww
Second generation
(parents plus aunts
and uncles)
FF or Ff
Ff
Ff
Third
generation
(two sisters)
ww
Widow’s peak
Ff
No Widow’s peak
Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
Figure 14.14 A, B
(a) Dominant trait (widow’s peak)
(b) Recessive trait (attached earlobe)
Pedigrees
 Can
also be used to
make predictions
about future offspring
Recessively Inherited
Disorders
Many genetic disorders are
inherited in recessive manner
 Show up only in individuals
homozygous for the alleles
 Carriers
 Are heterozygous
individuals, who carry
recessive allele but are show
“normal” phenotype

Cystic Fibrosis
Example of recessive
disorder
 Affect mostly people of
European descent
 Symptoms
 Mucus buildup in the
some internal organs
 Abnormal absorption
of nutrients in the
small intestine

Sickle-Cell Disease


Another recessive disorder
 Affects one out of 400
African-Americans
 Is caused by the
substitution of a single
amino acid in the
hemoglobin protein in
red blood cells
Symptoms
 Physical weakness, pain,
organ damage, and even
paralysis
Dominantly Inherited
Disorders
Some human disorders
 Are due to dominant
alleles
 Example is
achondroplasia
 Form of dwarfism
lethal when
homozygous for the
dominant allele

Another Dominant
Disorder

Huntington’s disease (HD)
 degenerative disease of nervous
system
 No obvious phenotypic effects until
about 35 to 40 years of age
HD
Normal
Down Syndrome
 Down
syndrome
 Is usually the
result of an
extra
chromosome
21
trisomy
21
Genetic Testing and
Counseling
 Genetic
counselors
 Can provide
information to
prospective parents
concerned about a
family history for a
specific disease
Tests for
Identifying Carriers
 For
a growing number of
diseases
 Tests are available that
identify carriers and
help define the odds
more accurately
 Examples
Tay Sachs & CF
Fetal Testing
 In
amniocentesis
 The liquid that bathes
fetus is removed & tested
 In chorionic villus
sampling (CVS)
 A sample of the placenta
is removed and tested
Can make karyotypes, too!
Newborn Screening
 Some
genetic disorders can
be detected at birth
 Simple tests are now
routinely performed in
most hospitals in the
United States
 Example- PKU test
Chapter 13
(PART 5)
The Chromosomal Basis of
Inheritance
Introduction to Sex Linkage
Honors Genetics
Ms. Gaynor
Gene Linkage

Linked genes
 Usually inherited together
because located near each other
on the SAME chromosome

Genes closer together on the same chromosome
are more often inherited together
Each chromosome
 Has 100’s or 1000’s of genes
 Sex-linked genes exhibit unique
patterns of inheritance; genes on
the X or Y chromosome

Morgan’s Experimental
Evidence
 Thomas
Hunt Morgan
 Provided convincing
evidence that
chromosomes are
the location of
Mendel’s heritable
alleles
Sex linkage
explained
http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html


Thomas Hunt Morgan
(Columbia University 1910)
Fruit Flies (Drosophila)
melanogaster)
© 2007 Paul Billiet ODWS
Morgan’s Choice of
Experimental Organism
 Morgan
worked with
fruit flies
 Lots of offspring
 A new generation
can be bred every
two weeks
 They have only 5
pairs of
chromosomes
Morgan and Fruit Flies
Morgan first observed and noted
 Wild type (most common)
phenotypes that were common in
the fly populations
 Traits alternative to the wild type are
called mutant phenotypes

w+
WILDTYPE
w
MUTANT
The case of the whiteeyed mutant
Character
Eye color
type)
Traits
Red eye (wild
White eye
(mutant)
P Phenotypes
Wild type (red-eyed) female x White-eyed male
F1 Phenotypes All red-eyed
Red eye is dominant to white eye
Hypothesis
A cross between the F1 flies
should give us: 3 red eye : 1
white eye
F2
Phenotypes
Numbers
So far so good
Red eye
White eye
3470
82%
782
18%
An interesting
observation
The F2 generation showed the 3:1
red: white eye ratio, but only
males had white eyes
Phenotypes
F2
Numbers
Redeyed
males
Redeyed
females
Whiteeyed
males
Whiteeyed
1011
2459
782
0
24%
58%
18%
0%
females
A reciprocal cross
Morgan tried the cross the other way
round
white-eyed female x red-eyed
male
Result
All red-eyed females and all whiteeyed males
This confirmed what Morgan
suspected
The gene for eye color is linked to the
Morgan’s Discovery:
Sex Linked Traits
 Eye
color is linked on X
Chromosome
 Females carry 2 copies of gene;
males have only 1 copy
 If mutant allele is recessive,
white eyed female has the trait
on both X’s
 White eyed male can not hide
the trait since he has only one X.
The Chromosomal Basis
of Sex
 An
organism’s sex
 Is
an inherited phenotype
determined by the
presence or absence of
certain chromosomes
 XX = girl
 XY = boy
Inheritance of SexLinked Genes
sex chromosomes
 Have genes for many
characters unrelated to sex
(especially the X
chromosome)
 A gene located on either sex
chromosome
 Is called a sex-linked gene
 The
(Usually on X chromosome)
What genes are on the X
chromosome?
carries a couple
thousand genes but
few, if any, of these
have anything to do
directly with sex
determination
 Larger and more
active than Y
chromosome

What genes are on the Y
chromosome?



Gene called SRY
triggers testis
development, which
determines male sex
characteristics
This gene is turned “on”
~6 weeks into the
development of a male
embryo
Y-Chromosome-linked
diseases are rare
Sex-linked genes follow
specific patterns of inheritance
Fathers  pass sex-linked alleles
to ALL their daughters but NONE
to their sons
 XY (Father)  XX (daughter)
 XY (Father)  XY (son)
 Mothers  can pass sex-linked
alleles to BOTH sons and
daughters
 XX (Mother)  XX (daughter)
 XX (Mother)  XY (son)

Sex Linkage

If sex-linked recessive on Xn
 Females have to be Xn Xn to show
sex-linked trait
 Xn X Females do NOT show sexlinked trait
 Males have to be Xn Y to show sexlinked trait
**Most sex-linked disorders affect
males; sometimes females
Sex-Linked
Disorders
 Some
recessive alleles found
on the X chromosome in
humans cause certain types of
disorders
 Color blindness
 Duchenne muscular
dystrophy
 Hemophilia
 Male pattern baldness
X-Linked Trait =
Male Pattern Baldness
Baldness
Another X-Linked Trait =
Hemophilia

About 85% of
hemophiliacs suffer from
classic hemophilia
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1 male in 10 000
cannot produce factor VIII
The rest show Christmas
disease where they can’t
make factor IX
The genes for both forms
of hemophilia are sex
linked
Hemophiliacs have trouble
clotting their blood
Another X-Linked Trait =
Red-Green Colorblindness
Normal vision
Color blind simulation
http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm
Another X-Linked Trait =
Duchenne Muscular Dystrophy