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Chapter 12
Inheritance Patterns
and
Human Genetics
Walter Sutton - American
Geneticist
Developed
the
Chromosome
Theory of
Heredity
The Chromosome Theory of
Heredity
Chromosomes are located in the
nucleus
Factors (genes) are found on
chromosomes
Sutton discovered that genes are on
chromosomes in 1902
Chromosome Theory of
Heredity
States that genes are located on
chromosomes and each gene occupies
a specific place on a chromosome
Only one allele is on a chromosome
Finally explains what Mendel was
talking about
Thomas Hunt Morgan
Studied fruit flies – Drosophila
melanogaster
Fruit Flies are excellent for
genetic studies because:
Reproduce quickly
Easy to raise
Many mutations
Have 8 chromosomes (n=4)
Morgan looked at TWO traits
Gray bodies – G
Normal Wings - W
Black bodies – g
Small wings – w
P1
F1
GGWW
x
GgWw
100%
ggww
Morgan then mated the F1 back
to the recessive parent
GgWw
x
ggww
Expected ratio – 1:1:1:1
25% GgWw
25% ggWw
25% Ggww
25% ggww
Morgan’s Actual Results
41.5%
41.5%
8.5 %
8.5%
gray normal
black small
black normal
gray small
Conclusion
Gene for body size and wing color were
somehow connected or linked
Can’t undergo independent assortment
Gene Linkage
Genes on the same chromosome are
linked together
Inherited together – THEREFORE they
do not undergo independent
assortment
Linkage Groups
Package of genes that are always
inherited together
Genes on the same chromosome
One linkage group for each homologous
pair
Fruit flies – 4 linkage groups
Humans – 23 linkage groups
Corn – 10 linkage groups
So linkage groups explain the
high percentages (41.5%) but
What about the
8.5%??????
17% had new combinations
The combinations that were
expected would be:
Gray normal – GW
or
Black small - gw
When they are lined up they
can become twisted and
switch genes
Crossing
Over
The 17% that had new
combinations are known as
Recombinants – individuals with
new combinations of genes
Crossing Over – gives rise to new
combinations – Prophase I
Chromosome Maps
The likelihood a crossover will occur
that will result in the separation of
two genes depends on the distance
between the two genes
Chromosome maps are diagrams that
show the location of genes on a
chromosome
Two genes separated by crossingover 1% of the time = one map unit
apart
Alfred Sturtevant (Morgan’s student)
constructed the 1st chromosome map
of fruit flies
NETTIE STEVENS
WORKED WITH MEAL
WORMS
DISCOVERED THAT THEY
HAD 20 CHROMOS. MALES
HAD 19 REG. SIZE AND 1
SMALL
SHE FOUND THE SAME
THING IN FRUIT FLIES
SHE SAID THE 19 THAT
WERE THE SAME WERE
AUTOSOMES, & THE
MISMATCHED WERE SEX
CHROMOS
MALES HAVE 1 X AND 1 Y
Sex Chromosomes
Nettie Stevens – made observations of
meal worm chromosomes
Sex Chromosomes
One pair
Female – XX
Male – XY
Sex Determination 50/50
Genes on Sex Chromosomes
Sex chromosomes determine a
person’s sex
Sex chromosomes also contain other
genes
Sex Linked
A gene located on a sex chromosome
Usually X
Example – Fruit Fly Eye Color
The gene for
eye color is on
the X chromosome
- not the Y
Fruit Fly Sex Chromosomes
X
X
X
Y
Males
Females
XRXR
XRY
Red Eyed
XRXr
XrXr
White Eyed
XrY
Sex-Linked Genetic Disorders
Gene for the trait is on the X or Y
X has many genes – Y has few
Defects easy to spot – appear more in
males
Carrier
A heterozygous female – has the gene
but does not express it – can pass it on
to her children
Colorblindness
Recessive X linked disorder
Cannot distinguish colors
C
Dominant Gene – X
c
Recessive Gene - X
Hemophilia
Recessive X linked disorder
Blood does not clot
H
X – good gene
h
X – hemophilia gene
Muscular Dystrophy
Results in the progressive wasting away
of muscle
Mutations
-A change in the DNA of an organism
-Can involve an entire chromosome or a
single DNA nucleotide
-May take place in any cell
Mutations
Germ Cell Mutations - Occur in an
organism’s germ cells (gametes)- only
affect offspring
Somatic Mutations - Take place in an
organisms body cells and only affect the
organism
Mutations
Lethal Mutation:
Can cause death
Often before birth
Good Mutations:
organisms have a
better chance to
reproduce
have an evolutionary
advantage
Provide the variation
on which natural
selection acts
Chromosome Mutations
Changes in the structure of a
chromosome
Loss or addition of an entire
chromosome
Four Types:
duplication
deletion
inversion
translocation
Gene Mutations
Point mutation-single
nitrogen base is
changed
Substitutions may
not be fatal - there is
redundancy in the
amino acid
codons
Frameshift Mutation
When a nucleotide is lost or added so
that the remaining codons are grouped
incorrectly
This can code for the wrong amino acid
and create an incorrect protein
Insertions and deletions are frameshift
mutations
THE FAT CAT ATE THE RAT
Deletion causes a frameshift:
If you delete the “E” in the
THF ATC ATA TET HER AT
Nondisjunction
Some chromosome mutations alter the
number of chromosomes found in a cell
Nondisjunction – the failure of a
chromosome to separate from its
homologue during meiosis
Nondisjunction
Example: Trisomy 21
Nondisjunction also affects sex
chromosomes
Examples: Klinefelters and Turner’s
syndrome
Polyploidy
Condition in which an organism has an
extra set of chromosomes
3N, 4N
Usually fatal in animals
Plants – usually more robust
Caused by - Nondisjunction
Mutations
http://learn.genetics.utah.edu/content/
begin/traits/predictdisorder/
Pedigree Chart
A family record that shows how a trait
is inherited over several generations.
Circles are females, squares are males
Blank = Normal
Half = Carrier – not shown, pass on
Filled = Displays trait
Pedigree chart
Female – circle
Male – square
Colored - recessive
Empty - dominant
Genetic Traits and Disorders
Genetic disorders – diseases or debilitating condition
that has a genetic basis
Single Allele Traits
Controlled by a signal allele
Usually recessive
Example: Hitchhiker’s thumb
Multiple Allele Traits
Controlled by 3 or more alleles
Example: Blood types
Polygenic Traits
A trait that is controlled by two or more genes
Example: Skin color
Huntington’s Disease
Caused by a single
dominant allele
Appears in 30’s or 40’s
Progressive loss of
muscle control and
mental function  death
Gene on chromosome 4
Sickle Cell Anemia
Caused by a change in one of the
polypeptides found in hemoglobin
(carries oxygen in RBC)
One nucleotide difference
Codominant inheritance
A S
pattern – H H
A
H - normal allele
S
H – Sickle Cell allele
Sickle Cell Anemia
Sickle Cell Anemia – common in people
of African ancestry and from tropical
regions
Carriers (heterozygous) of Sickle Cell
A S
trait (H H ) resistant to malaria
Human Blood Groups
Multiple alleles – genes with more than
two forms
Example – ABO and Rh blood groups
* Remember – an organism can have two
alleles only!
Blood Type
Blood Type – determined by the
presence or absence of certain things in
the blood
RBC (Red blood cell) – can carry two
different antigens
Antigens – molecules that can be
recognized by the immune system
Genotypes and Phenotypes
ii
Type O
A A
A
I I or I i
B B
II
A B
I I
or
Type A
B
Ii
Type B
Type AB
II. BLOOD GROUPS-TYPING
DONORS
A’S CAN GIVE TO A’S,
AB’S
B’S CAN GIVE TO B’S,
AB’S
AB’S CAN GIVE TO
AB’S
O’S CAN GIVE TO
ANYONE!
THEY ARE A
UNIVERSAL DONOR
RECEIVERS
A’S CAN RECEIVE
FROM A’S & O’S
B’S CAN RECEIVE
FROM B’S & O’S
O’S CAN RECEIVE
FROM O’S ONLY!
AB’S CAN RECEIVE
FROM AB’S, A’S, B’S,
& O’S (UNIVERSAL
RECIPIENTS)
Rh Blood Groups
Rh antigen also on RBC
+
Rh - have antigen (Dominant)
_
Rh - no antigen (Recessive)
IF A PREGNANT WOMAN HAS AN Rh+ BABY & SHE
IS Rh- THEN THIS COULD BE A PROBLEM IF THE
CORD BLOOD MIXES W/THE MOM’S
SHE COULD HAVE ANTIBODIES AGAINST THE
BABIES BLOOD AND CAUSE DEATH TO THE BABY
Blood Typing Activity
http://nobelprize.org/educational_games/medicine/landstein
er/
Polygenic Inheritance
Traits controlled by two or more genes
Examples – height, skin color, animal
coat patterns
Phenotypes are seen in a range
POLYGENIC INHERITANCE
INHERITANCE
OF SEVERAL
GENES TO
EXPRESS A
SINGLE TRAIT
LIKE SKIN COLOR
Sex Influenced Traits
A trait that is caused by a gene whose
expression differs in males and females
Baldness:
A sex influenced trait
Single gene – two alleles
Gene expression is influenced by male
hormones
Baldness
Genotype
Male
Female
BB
Normal
Normal
bb
Bald
Bald
Bb
Bald
Normal
Diagnosis of Genetic Disorders
Down’s
Syndrome –
Trisomy 21
Extra copy of the
21st chromosome
Can be detected
by microscopic
examinations of
chromosomes
karyotype
Prenatal Diagnosis
Amniocentesis – removes fluid from the
sac around the baby
The fluid can be used to grow cells and
make a karyotype
Chorionic Villus Biopsy – cells are
removed from the embryo
Faster results than amnio
Prenatal Diagnosis
CVB and Amnio make it possible to
detect chromosomal abnormalities
Test for: biochemical abnormalities
Presence of certain DNA sequences
Ethics
We can detect over 100 disorders
Knowledge leads to choices and
decisions
Ethical Considerations:
Do you consider a developing embryo a
person?
Are you pro-life or pro-choice?
Should a person knowingly bring a baby
with a deadly disease into the world?