Genetics Part Two

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Dihybrid Crosses looking at 2 traits
Mendel’s dihybrid crosses:
1. Mendel also performed crosses involving two pairs of traits,
e.g., seed shape (smooth vs. wrinkled) and color (yellow vs.
green).
2. If alleles sort independently, four possible phenotypes (2n)
appear in the F2 generation in a 9:3:3:1 ratio.
“Mendel’s Principle of Independent Assortment”:
Alleles for different traits assort independently of one another.
Modern formulation of independent assortment:
Genes on different chromosomes behave independently in gamete
production.
The Independent Alignment of
Different Pairs of Homologous
Chromosomes At Meiosis Accounts for
the Principle of Independent
Assortment
The alignment of one pair of homologs is
independent
of any other.Assortment: The
Principle
of Independent
assortment of one pair of genes into gametes
is independent of the assortment of another
pair of genes.
Possible Gametes:
To determine the number of different gametes a parent can have use the FOIL
method.
RREE
RE
RE
RE
RE
All gametes are RE
Possible Gametes:
To determine the number of different gametes a parent can have use the FOIL
method.
RREe
Re
RE
RE
Re
You have 2 possible gametes:
Re and RE
Possible Gametes:
To determine the number of different gametes a parent can have use the FOIL
method.
rrEe
re
rE
rE
re
You have 2 possible gametes:
rE and re
How many gametes?
To determine the number of different gametes a parent can have use
the FOIL method.
Number and kind of gametes
TTYY
=
one
TtYY
=
two
TtYy
=
four
TY
TY and ty
TY, Ty, tY, ty
Possible Gametes (sperm/egg):
If I cross:
What are the different possible gametes for these parents??
RRTT x RrTt
RT
RT, Rt, rT, rt
There are 4 total different gametes
If I cross:
What are the possible gametes?
RRTT x RrTt
RT
I have 4
possible
offspring
RT
Rt
rT
rt
RRTT
RRTt
RrTT
RrTt
If I cross:
What are the possible gametes and offspring?
rrTt x RRTt
RT
Rt
rT
RrTT
RrTt
rt
RrTt
Rrtt
I have 4 possible
offspring
If I cross:
What are the possible gametes?
RrTt x RrTt
I have 16
possible
offspring
9:3:3:1
RT
Rt
rT
rt
RT
RRTT
RRTt
RrTT
RrTt
Rt
RRTt
RRtt
RrTt
Rrtt
rT
RrTT
RrTt
rrTT
rrTt
rt
RrTt
Rrtt
rrTt
rrtt
Why Did Mendel Conclude That The
Inheritance of one Trait is Independent
of Another?
Phenotype Ratio:
9 yellow/smooth
3 yellow/wrinkled
3 green/smooth
1 green/wrinkled
Because it’s the
only way to explain
the pattern of
inheritance!
What Works
for Peas Also
Works for
Humans
Consider a cross
between parents
heterozygous for
both deafness and
albinism.
This is the same
9:3:3:1 ratio seen
for Mendel’s cross
involving pea
color and shape.
January 10, 2012
 Get
your homework out on your desk!
Warm Up #1:
A pea plant with the genotype TtWW is
crossed with a pea plant with the genotype
ttWw. How many different genotypes can be
expressed in the offspring?
(SHOW ALL WORK!)
a. 1
b. 2
c. 3
d. 4
Warm Up #2
 If
a corn plant has a genotype of Ttyy, what
are the possible genetic combinations that
could be present in a single grain of pollen
(sperm, male gamete) from this plant?
a. Ty, ty
b. TY, ty
c. TY, Ty, ty
d. Ty, ty, tY, TY
Warm Up #3
3. The pea plant produces plants of two
different sizes and seeds that are two
different shapes. Tall pea plants (T) are
dominant to dwarf pea plants (t) and round
seeds (R) are dominant to wrinkled seeds,
r. Two plants heterozygous for both traits
are mated. What fraction of their offspring
will be dwarf and have wrinkled seeds?
(SHOW ALL WORK!)
 Great
job on your quizzes!
 Hand back quizzes and work – review
most common mistakes.
 Tonight:
Talk to your parents about their
blood type. Write down each parent and
yours if you know yours. Turn this in
tomorrow for extra credit.
 What do you know about blood types?
Multiple Alleles
 More
than two allelic forms of a gene that
code for a trait
 Phenotype is still determined by a pair of
alleles! The gene pool simply contains
more than two.
 These alleles can be expressed in various
ways:



Complete dominance
Co-dominance
Incomplete dominance
Multiple Alleles: Human Blood Type
Blood Type is Coded by Multiple Alleles A, B, and O.
*A and B are co-dominant.
*A and B are completely dominant over O.
Genotypes
AA
AO
BB
BO
AB
OO
Phenotypes
Type A
Type A
Type B
Type B
Type AB
Type O
*The genes cause the production of a cell surface protein A or B.
Type O cells have neither protein, so Type O individuals are
Universal DONORS. Which individuals are Universal RECEPIENTS
(have both proteins)?
Type A
Type AB
No antibodies
Type B
Type O
Multiple Alleles
Many genes are present
in 3 or more versions
(alleles) – this is known
as multiple alleles.
The human ABO blood group
is determined by three alleles
(IA, IB, and i) of a single gene.
The AB phenotype
(genotype IA IB) is an
example of
codominance!!
Codominance
The human ABO blood group
illustrates another genetic
phenomenon – codominance.
Codominance occurs when
the phenotype associated with
each allele is expressed in the
heterozygote – both are
expressed!
Problem 1
1. In humans, the blood types A, B, AB and
O are determined by three alleles. A man
who has AB blood marries a woman with
O blood. What are the genotypes and
phenotypes of the offspring?
Problem 2
2. A man who is heterozygous type A
marries a woman who is heterozygous
type B. What percentage of their offspring
will be Type 0?
Problem 3
3. If a man with blood type B, one of whose
parents had blood type O, marries a
woman with blood type AB. What
percentage of their children should have
blood type B?
Problem 4
4. A man with type O blood and a woman
with type AB blood get married. One of
their children needs an operation. This
child has type B blood. Can the child
safely receive a blood transfusion from
either parent? If not, why?
Multiple Alleles: Whose Baby ?
A major mix-up occurred. Have the babies been returned
to the correct set of parents?
1
2
Baby A - Type O Blood
Baby B - Type B Blood
Smiths - Types O and AB
Jones - Types A and B
Baby C - Type AB Blood
Baby D - Type A Blood
Squares -Types AB and AB
Angles -Types AB and O
1/13/11: Warm Up
Both Mrs. Smith and Mrs. Jones had babies the
same day in the same hospital. Mrs. Smith took
home a baby girl, whom she named Shirley. Mrs.
Jones took home a baby girl, whom she named
Jane. However, Mrs. Jones began to suspect that
her child had been accidentally switched with
another baby in the nursery.
Mr. Smith – type A
Mrs. Smith – type B
Shirley – type O
Mr. Jones – type A
Mrs. Jones – type A
Jane – type B
**Did a mix-up occur? SHOW YOUR WORK!!!
Sex – Linked Genes
 Review:





What are the two sex chromosomes?
What is a woman’s genotype?
What is a man’s genotype?
Which do you think there are more of – xlinked genes or y-linked genes?
Who do you think inherits sex-linked
disorders more – males or females?
X-linked Genetics
•Genes that are located on the
X chromosome but not the Y
•MALES receive only ONE copy
•FEMALES receive TWO copies
Color Blind Test
 Are
you red-green colorblind?
 Lets find out…
Sex- Linked Inheritance
A
male always receives a sex-linked
condition from his mother, from whom he
inherits an X chromosome.
A
female must receive 2 alleles, one from
each parent, for the trait to be expressed.
X-linked disorders are often recessive.
Sex-linked ( X-linked) disorders
–blindness
 Hemophilia – protein for blood clotting is
missing
 Duchenne Muscular dystrophyprogressive wasting of muscles
 Color
(There are about 205 X-linked recessive disorders)
Color-blindness
Father
(normal vision)
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Mother
(carrier)
Daughter
(carrier)
Son
(colorblind)
More males than females are affected by this type of disorder.
WHY?
True or False?

The X chromosome carries alleles that are not on
the Y chromosome. Therefore, a recessive allele
on the X chromosome is expressed in males.

Certain traits that have nothing to do with the
gender of the individual are controlled by genes
on X chromosomes.

Males have 2 X chromosomes, and therefore, Xlinked recessive alleles are expressed.
Solving X-linked Problems
 How

do you know if x-linked?
Know color-blindness and hemophilia are!
 Make
Punnet Square using XX and XY
 Attach alleles to X’s ONLY
 To help determine percentages, circle the
male offspring
Let’s Practice
1. Both the mother and the father of a male
hemophiliac appear to be normal. From
whom did the son inherit the allele for
hemophilia? What are the genotypes of
the mother, the father, and the son?
2. A woman is color blind. What are the
chances that her sons will be color blind?
If she is married to a man with a normal
vision, what are the chances that her
daughters will be color blind? Will be
carriers?
Cont…
3. A husband and a wife give birth to a baby
girl. Mom’s vision was normal, so she
assumed the baby’s would be normal too.
However, she gave birth to a color-blind
daughter. What can you deduce about the
girl’s parentage?
Cont…
4. A man with normal color vision marries a
woman who carries the recessive gene,
although her vision is normal. What
percentage of male children will be
colorblind? What about females?
Cont…
5. If a hybrid female for the colorblind trait
marries a colorblind male, what
percentages of offspring will be colorblind?
Paternity Suit
6. One of the sons from question #5 is
involved in a paternity suit. The child in
question is a cb female. The judge rules
that the child could not possibly belong to
this man. Which sons, of problem #5, was
involved in this case? Give his genotype
and phenotype and tell how you know.
Sex-Linked Problems
 Finish
for homework!!!!
Article Anticipation Guide
 Read
the statements on your guide and
mark your educated guesses in the boxes
Skin Color and Genetics Article
Polygenic Inheritance – when a Single Trait is
influenced by many genes
Examples:
1. Height
2. Hair texture
3. Skin color
4. eye color
Types of Traits – Single Gene
•Some traits are
controlled by single gene
•Expressed as “either/or”
Discontinuous
Variation
Types of Traits – Polygenic Traits
•Polygenic Traits are
controlled by the
additive effects of 2 or
more genes
•Shows a range of
phenotypes
Continuous Variation
Types of Traits
Example: A Biology Class
Single Gene Trait
Polygenic Trait
Discontinuous Variation
Continuous Variation
Types of Traits – Polygenic Traits
Human Population - Skin Color

Controlled by many
genes
 A given individual can
have as many as six
alleles (possibly
more!) for
pigmentation
Polygenics
What are other traits that are coded for by
MANY genes????
Multifactorial Traits
 Influenced
by genetic and environmental
factors
*Even with a the same genetic make-up, environment can
affect the phenotype, as seen in the Hydrangea above. The
color variation is a reaction to acid in the soil. Similar
environmental influence is observed in humans such as skin
color and body build, giving even identical twins slight
differences.
Review Problems
 Gametes:




For each of the diploid genotypes presented
below, determine all of the possible haploid
gametes.
a. Rr
b. RrYy
c. RrYyBb
Example 1

In turkeys a R produces a bronze color and is
dominant over red color. Another dominant gene
H, results in normal feathers; its recessive allele h
produces feathers without webbing. Two
heterozygous bronze turkeys with normal
feathers were mated.




What kind of problem is this? ___________________
Give the genotypes of P:_________________
Give the genotypes of F1:________________
Give the phenotypes of F1:_______________
Example 2
 In
pea plants purple flowers are
dominant to white flowers.



If two white flowered plants are cross, what
percentage of their offspring will be white
flowered? ______________
What kind of inheritance is this? __________
Monohybrid or dihybrid? ________________
Example 3
A
cross between a blue-flowered plant and
a white-flowered plant gives all pale-blueflowered plants.



What kind of inheritance is this?___________
What are the genotypes of the
offspring?_______
Monohybrid or dihybrid?_________________
Example 4
A
father and mother claim they have been
given the wrong baby at the hospital! Both
parents are blood type A. The baby they
have been given is blood type O. Could
they have the right baby?


What kind of inheritance is this?___________
P genotypes: ______________
Example 5
 In
a certain cactus, prickly spines can be
two-pronged or one-pronged. If a onepronged cactus is crossed with a twopronged cactus, the F1 generation has a
both types of spines, some are twopronged, some are one-pronged.




What kind of inheritance is this?___________
Monohybrid or dihybrid?_________________
Give the genotypes of P:_________________
Give the genotypes of F1:________________
Example 6
 Located
on the X chromosome of a cat is
a gene that codes for deafness. This gene
is recessive. A black female cat that is is
heterozygous for deafness (Dd) is crossed
with a yellow male cat that is not deaf.


What type of problem is this?_____________
P genotypes:__________________________
Human Traits
 Genetic
counselors analyze how the trait
is passed from one generation to the next.
They construct pedigree charts that show
the pattern of inheritance for a particular
condition, and show if a disorder is
dominant or recessive.
Pedigree Chart
Genetic Disorders
 Autosomal
( non-sex linked) Dominantindividuals with alleles AA or Aa will have
the disorder.
Recessive – individuals with
alleles aa with have the disorder.
 Autosomal
Autosomal Disorders
 Do
you remember what an autosome
was?
Autosomal Dominant Disorders:
- Dwarfism known as achondroplasia
- Huntington’s disease
Autosomal Recessive Disorders:
- Phenylketonuria ( PKU)
- Tay-Sachs disease
- Cystic Fibrosis
- Albinism
Incompletely dominant disorders:
- Sickle-Cell disease
Ways to recognize autosomal
dominant disorders:

Affected children usually have an affected
parent.
 Two affected parents can produce an
unaffected child.
 Two unaffected parents will not have
affected children.
Inheritance Pattern I:
Ways to recognize autosomal
recessive disorders:

Most affected children have normal parents.
 Two affected parents will always have
affected children.
 Affected individuals with homozygous normal
mates will have normal children.
Inheritance Pattern II:
Dominant or recessive?
Dominant or recessive?
Dominant or recessive?
Dominant or recessive?
Sex – Linked Pedigrees
Y

X



– linked (ONLY males will inherit)
Male infertility
– linked
Hemophilia
Red/Green Color Blindness
MS
Pedigree Analysis
http://www.knowledgene.com/public/view.php3?db=gene_school&ui
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