Transcript B O

NONMENDELIAN
GENETICS
INCOMPLETE DOMINANCE
DEFINITION: When neither allele is dominant over
the other. When the two alleles are found together,
they result in an intermediate trait.
LET’S TRY A PROBLEM:
In Mountain Boomers, the genes for length of tail
exhibit “incomplete” dominance. Use a Punnett Square
to predict the result of a cross between a homozygous
long-tailed and a homozygous short-tailed Mountain
Boomer. What do the offspring look like?
DETERMINE THE LETTERS YOU WILL USE TO
REPRESENT THE TRAITS:
L = long-tail
S = short tail
LS = medium-tail
DETERMINE THE GENOTYPES OF THE
PARENTS THAT ARE BEING CROSSED:
Long-tailed Parent = LL Short-tailed parent = SS
DETERMINE THE POSSIBLE GAMETES THAT EACH CAN
PASS ON:
LL = GAMETE L
SS = GAMETE S
DRAW A PUNNET SQUARE AND INSERT THE
GAMETES ON EACH SIDE.
S
L
LS
ANSWER: OFFSPRING WILL ALL BE
MEDIUM-TAILED
How many offspring will be medium-tailed if you
cross two medium-tailed lizards?
MULTIPLE ALLELES
DEFINITION: When not just two but THREE or
more alleles of the same gene code for a single trait.
EXAMPLE: ABO BLOOD TYPE
In humans, the A allele for blood type and the B allele for
blood type are both dominant over the O allele for blood
type. Neither A nor B blood types is dominant over the
other, and when they mix you get AB blood type.
What are the four types of human blood?
Type A
Type B
Type AB
Type O
What are the possible genotypes for human blood?
AA
AO
BB
BO
AB
OO
PROBLEM: If a man with type A blood marries
a woman with type B blood, can they have a child
with type O blood?
What are the possible genotypes of the parents?
Man = AA or AO
Woman
A
O
= BB or BO
B
Draw Punnett Squares to represent
AB
BO
the different Possibilities:
AO
OO
O
A
A
A
A
A
B
AB
AB
B
B
AB
AB
AB
AB
AB
B
AB
AO
AO
O
B
O
BO
BO
ANSWER: ONLY IF BOTH PARENTS ARE
HETEROZYGOUS FOR THEIR BLOOD TYPE CAN THEY
HAVE A CHILD WITH TYPE O BLOOD.
PROBLEM: CAN A MALE HETEROZYGOUS FOR
BLOOD TYPE B, BE THE FATHER OF A CHILD WHO
HAS TYPE A BLOOD IF THE MOTHER IS TYPE O?
1) Determine the possible genotypes of the parents and
child..
DAD
Dad = BO Mom = OO
B
O
Child = AA or AO
2) Place possible gametes in
BO
OO
Punnett Square and multiply. M O
Dad = B or O; Mom = O O
ANSWER = NO, he can
M
only have a type B or O
child.
Some genes alter the affects of other genes
EPISTASIS occurs when the phenotype of one gene is
affected by another gene.
For example: The dominant coat color in mice is
gray(B); the recessive black (b). However, another gene
on a different chromosome allows for color. A = normal
color pigment, a = no color. In order for a gray mouse to
be produced there must be both a dominant gray gene
(B) and a dominant color gene (A). If a mouse is
recessive for color pigment (aa), it will be albino.
Try this problem:
If a mouse with the genotype AaBb (B = gray color, b = black color;
A = color pigment, a= no color pigment) mates with a mouse with
the genotype aaBb. What are the phenotypes of their offspring?
1) Determine the cross.
AaBb x aaBb
2) Determine the possible gametes Parent 1 =AB or Ab or aB or ab
for each parent.
Parent 2 = aB or ab
3) Place each parents gametes in the Punnett square and multiply.
AB
Ab
aB
ab
aB AaBB AaBb aaBB aaBb
ab
AaBb Aabb aaBb aabb
4) Look at all the genotypes and determine the phenotypes.
All the A_B_ = ?
All the aa B_ = ?
All the A_ bb = ?
All the aabb = ?
Results :
1/8
1/2
3/8
_____gray;
_____ black: _____ albino
MULTIPLE GENES =
POLYGENIC TRAITS
DEFINITION: When several genes work together to
control the expression of a trait, causing the trait to
appear in a wide variety of forms.
Examples: Hair color, eye color, skin color, height
SEX DETERMINATION
In humans the 23rd pair of chromosomes = Sex chromosomes
These chromosomes determine if the child will be a male or
female.
In MALES the 23rd pair are not HOMOLOGOUS = X and Y allele.
In FEMALES the 23rd pair are HOMOLOGOUS = X and X allele.
The FATHER determines the sex of the baby.
LET’S SEE HOW THIS WORKS:
X
1. Each parent can pass on only
ONE of their alleles. Females only
X
an X; Males an X or Y. Place them in
XX
the square and see the results.
XY
Y
2. 50% chance of male or female
X
XX
XY
SEX LINKED GENES
Sex Linked Genes are genes located on the X or Y chromosome. There
are very few traits on the Y chromosome, so most males get only ONE
allele for these traits because they get only 1 X. Females get two
alleles, one on each X. THUS, males have a greater chance of getting a
disease that is found on the sex chromosomes.
EXAMPLE: Color Blindness
Color Blindness (red-green) is a recessive trait carried on the X
chromosome. Only females can be carriers. Males either have the
disease or they don’t.
SAMPLE PROBLEM:
If a red-green colorblind man marries a woman with normal
color vision whose father was colorblind, what will be the
expected phenotypic results of their children?
1) To begin the problem – DETERMINE the GENOTYPES of the P1
generation (parents). Because the trait is on the X chromosome we write
it a little differently. To write the genotype we must include the X and Y
allele and a superscript for the colorblindness allele above each X allele.
The Y allele does not carry the colorblindness allele. You include a 0
superscript above the Y.
Male Genotype = XcY0 Female = XCXc
2) Next determine the possible gametes each parent can pass on to their
offspring.
Male gametes = Xc or Y0
Female = XC or Xc
3) Place the possible gametes for each parent in the Punnett Square
and multiply to get the possible offspring from this cross.
Male
Xc
Y0
Female
XC
XCXc
XCY0
Carrier
Normal male
XcXc
Xc
colorblind
XcY0
colorblind male
Female
4) Answer the question:
½ the males and ½ the females will be colorblind
½ the females will be carriers of the disease
ANOTHER SAMPLE PROBLEM
CALICO CATS
One pair of genes for coat color in cats is sex-linked. The
gene B produces yellow coat, b produces black coat, and
the heterozygous (carrier) Bb produces tortoise-shell coat
[CALICO]. What kind of offspring will result from the
mating of a black male and a calico female?
BXb
XBY0
X
Gametes = Male _________ Female _________
XB
Yo
Offspring =
Can you
BXB
BY0
½ females
get a
X
X
B
X
calico, ½
calico
black; ½
male?
B
b
b
0
X X
XY
males black,
Xb
½ yellow
Linked Genes
When two or more genes are on the same chromosome, we say
they are linked.
Linked genes are not inherited by independent assortment. They
usually split together during meiosis (not randomly) because they
are on the same chromosome.
You can tell if two or more genes are linked by looking at
the results of a particular cross.
• Sample Problem
If the genes G or g and W or w are on the same gene, what type of
genotypic results would you get from the following cross:
GgWw x GgWw
In unlinked genes, the gametes an individual could pass on with these
genotypes would be GW or Gw or gW or gw
With linked genes the possible gametes is different because they pull
together during meiosis. The gametes would be: GW or gw.
Draw the Chromosomes
Here: See white board.
Compare the two Punnett Squares – Linked and Unlinked
UNLINKED
GAMETES = GW OR Gw OR gW OR gw
Punnett Square
GW
Gw
GGWW
Gw
gW
GW
gw
LINKED
gW
gw
GGWw
GgWW
Ggww
GGWw
GGww
GgWw
Ggww
GW
GgWW
GgWw
ggWW
ggWw
gw
GgWw
Ggww
ggWw
GAMETES = GW or gw
GW
gw
GGWW GgWw
GgWw
ggww
ggww
Classic phenotypic numbers =
9 dominant for both traits; 3 dominant for
first, recessive for second; 3 recessive for
first, dominant for second; and 1 recessive
for both traits
Classic phenotypic
numbers = 3 dominant for
both traits; 1 recessive for
both traits
If the genes on the same chromosome cross over with their
homologous pair the number results may even appear more
unusual (not typical 3:1).
AS A GENERAL RULE: The CLOSER together two genes are
the less likely they will crossover. The FURTHER APART two
genes are the more they are likely to cross over.
SEX INFLUENCED TRAITS
DEFINITION: Certain traits that are dominant in
one sex and recessive in the other. Certain
chemicals cause these differences.
EXAMPLE: BALDNESS
Baldness is dominant in males and recessive in
females.
SAMPLE PROBLEM: If a man who is not
bald mates with a female that is not bald but
whose father was bald, what % of their
children will be bald?
Determine the genotypes:
Male Genotype: bb
Cross the parents: bb x
b
B
b
Bb
bb
Female Genotype: Bb
Bb using a Punnett Square
Determine the possible
offspring:
How many boys will be
bald?
How many girls will be
bald?
Can women be bald?
Who determines if male
offspring will be bald?
MUTATIONS
DEFINITION: A mistake during DNA replication causing a change in
either an individual gene or the whole chromosome.
TYPES OF MUTATIONS:
1) CHROMOSOME – PART OF MANY GENES OR AN
ENTIRE CHROMOSOME IS AFFECTED.
A) Deletion: piece of chromosome breaks off
Example: Cri du chat – Mental retardation, small head,
unusual face features, cat like cry
B) Duplication: Segment of chromosome
repeats itself.
Example: Fragile X syndrome – Mental
retardation
C) Inversion: Piece breaks and rejoins in
reverse order on the same chromosome
D) Translocation: When a piece breaks and rejoins to different
chromosome
E) Nondisjunction: A chromosome pair fails to separate during meiosis;
leads to polyploidy (extra numbers of chromosomes) or aneuploidy (too
few chromosomes)
Types of Diseases Caused by Nondisjunction
DOWN’S SYNDROME = Extra 21st Chromosome; 2n = 47
TURNER’S SYNDROME = XO
SUPER MALE = XYY
Female whose sex organs don’t
develop, sterile, no mental
retardation, short stature
Found in many criminals,
fertile.
KLINEFELTER’S – XXY
Large breasts, high voice,
sterile, retarded male
TRISOMY X =
XXX
NORMAL
2) GENE MUTATIONS = Change in the DNA at one
gene site
A) POINT MUTATION – Change in just one base in a
single gene. May be addition, deletion or substitution
of a base.
1)Frameshift = Type of point mutation - Only addition or
deletion (more disastrous)
Example = Sickle Cell Anemia
Mutations can occur in either somatic or sex cells. Those that
occur in somatic cells affect only the individual. Those that
occur in sex cells affect the offspring too.
CAUSES OF MUTATIONS:
1) Unknown 2) Age 3) Mutagens – viruses,
radiation, air pollution, foods and their additives,
chemicals (thalidimide, DDT)