Download Genetics: Beyond Mendel

Genetics:
Beyond Mendel
IB Biology
Mendelian Genetics
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This is the term used to describe the
basic principles of inheritance for traits
that are not inherited in complicated
ways.
There are many exceptions to the
principles we have learned with basic
Mendelian genetics.
Incomplete Dominance
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Sometimes, neither allele for a trait is
dominant, so there is a blending of
phenotypes
Example:
– Homozygous red carnations are crossed
with Homozygous white carnations and
produce 100% pink carnation offspring
Red
White
Pink
Codominance
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Sometimes, both alleles for a trait are
dominant, resulting in offspring with
both phenotypes.
Example:
Crossing a homozygous white
horse with a homozygous red
horse produces a roan horse
with a coat of red AND white
hairs.
Multiple Alleles
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This term describes traits for which
there are more than 2 alleles
Example
– There are three alleles for blood type: A
(IA), B (IB) (codominant), and O (i)
(recessive), allowing for 6 possible
genotypes:
– IA IA, IAi (type A phenotype)
– IBIB, IBi (type B phenotype)
– IA IB (type AB phenotype)
– ii (type O phenotype)
Blood Types
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Type A blood produces the A type
glycoprotein on blood cell membranes (Type
B produces B glycoprotein, type O produces
a carbohydrate with no effect)
Giving someone with type A blood a type B
transfusion will cause their immune system
to recognize the type A glycoproteins as
foreign antigens, attack them, and cause
clotting and usually death for the patient
Blood Types
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Those with type AB blood produce both
glycoproteins (codominant), so they can
receive A or B transfusions without an
immune response
Those with type O blood are universal
donors because the carbohydrate on their
cell surfaces do not trigger an immune
response, however, those with type O
cannot receive any other form of blood
Epistasis
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This occurs when one gene affects the
phenotypic expression of a second
gene.
Frequently occurs in the expression of
pigmentation
– One gene turns on (or off) the production
of pigment, while a second gene controls
either the amount of pigment produced,
or the color of the pigment
Epistasis
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Example: in mice, one gene codes for
pigmentation, and another for the
color of the pigment…
– CC or Cc genotypes produce pigments, cc
produces no pigments
– BB or Bb makes the pigments black, bb
makes the pigments brown (if present)
– What color would a ccBb mouse be?
Pleiotropy
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This occurs when a single gene has
more than one phenotypic expression
Example:
– The gene in pea plants that expresses the
round or wrinkled texture of seeds also
influences the phenotypic expressions of
starch metabolism and water absorption
– This is like killing 2 (or more) birds with
one stone
Pleiotropy Example
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Sickle-cell anemia is an example of a
pleiotropic human blood disease.
It is caused by an allele that incorrectly
codes for hemoglobin, causing normally
round red blood cells to become sickleshaped – leading to a painful death when
homozygous recessive with that allele
Heterozygous individuals with that allele are
more resistant to the mosquito-born
pathogen malaria
Polygenic Inheritance
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Many traits are not expressed in just 2
or 3 varieties, such as yellow and
green pea seeds or A, B, O blood
types
Your height, for example, is usually
not just short or tall, but can be one of
a nearly infinite continuum of
possibilities within a certain range
Many genes are required to shape
single complex phenotypes like height.
Linked Genes
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The law of independent assortment only
works for genes on different chromosomes
Linked genes are genes that reside on the
same chromosome and cannot therefore
segregate independently
Example:
– Body color and wing structure genes in fruit flies
are linked
Linked Genes
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If the normal fruit fly body color is gray (B),
while the mutant allele is expressed as black
(b); and normal wings are full (V), while the
mutant shriveled wings are vestigial (v)…
A dihybrid cross would typically reveal the
following cross between this gray, normalwinged male (BbVv) and a black, vestigialwinged female (bbvv):
Normally: B’s and V’s sort independently
Male
Female
BV
bv BbVv
Bv
bV
bv
Bbvv
bbVv
bbvv
Probabilities: ¼ BbVv, ¼ Bbvv, ¼ bbVv, and ¼ bbvv
Linked: B’s and V’s do not sort so if B is on
the same chromosome as V, they do not
mix with b or v
Male
Female
Probabilities: ½ BbVv,
½ bbvv
BV
bv BbVv
bv
bbvv
Experimental Probabilities: 41/100 BbVv,
41/100 bbvv, 9/100 Bbvv, 9/100 bbVv
(41:41:9:9 ratio) . . . How? Crossing Over:
Since genes cross over homologous
chromosomes in prophase I of
meiosis, in this case, about 18% of
the time, we do see some of the
unexpected combinations above (9%
for each = 18% crossover rate). 82%
of the time, normal linked
combinations are revealed (41% for
each expected result = 82%).
Linked Genes
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The greater the distance between two
genes on a chromosome, the more
places between the genes that the
chromosomes can break and thus the
more likely the two genes will cross over
during prophase I of meiosis.
So, we can think of every 1% of
crossover rate as 1 map unit of distance
separating the genes on a chromosome
This can help us to visualize the
arrangement on a chromosome:
Linked Genes
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Suppose you knew that for a fly with a
genotype BBVVAA (where A is the
apterous, or wingless mutant) the
crossover frequency between B and V
was 18%, between A and V was 12%,
and between B and A was 6%. In
what order do the genes lie on the
chromosome, and how far apart are
they?
Linked Genes
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Hint: think of the 18% crossover rate
between B and V as 18 map units
apart (making these two the farthest
apart from each other)
Draw out a possible solution:
Sex-Linkage
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The one pair of homologous
chromosomes in animals that does not
have exactly the same genes are X
and Y (sex chromosomes)
Traits whose genes are located on X
(usually) or Y are determined, in part,
by the sex of the offspring
Sex-Linkage
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Example: red-green colorblindness (bb) is
due to a gene on the X chromosome, normal
sight will be represented by (BB or Bb):
Normal
Female
Carrier
Female
(normal)
XB
Xb
XB
Y
XBXB
XBY
XBXb
XbY
Normal
Male
Colorblind
Male
Because their Y doesn’t have the colorblindness gene at all, males
only need one copy to have the recessive phenotype (disorder); so
they are much more likely to inherit sex-linked traits.
In the color slideshow, can you see a number
hidden in the circle of dots?
X-Inactivation
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During fetal development in females,
one of the two X chromosomes will be
randomly inactivated
– It is formed into a Barr body and its
genes are not expressed
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Daughter cells from a cell which has
inactivated one X will also inactivate
the same X.
X-Inactivation
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When X-inactivation begins, some cells
will inactivate one X, and others will
inactive the other X
It is unlikely that the same X will be
inactivated in all initial embryo cells,
but if it happens, then females can be
subject to sex-linked disorders like
hemophilia with only one copy of the
mutated gene (like men)
Nondisjunction
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If chromosomes do not separate
correctly in meiosis, a parent can
donate too many or too few
chromosomes to their offspring:
Nondisjunction
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Examples:
– Down Syndrome: caused by an extra 21st
chromosome (trisomy 21). Causes
mental retardation, heart defects,
respiratory problems, deformities, etc…
– Turner Syndrome: caused by a missing X
chromosome (X0). Results in a female
with some physical abnormalities and
sterility