Non-Mendelian Genetics
Inheritance patterns that don’t
follow the rules
A.
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Linkage
1. Image of Bateson
and Punnett
2. tried to replicate
work of Mendel using
other traits of garden
pea
3. traits investigated
-flower color P (purple)
and p (red flowers)
-pollen size L (long) and
l (short pollen)
4.
Parental cross
• a. pure breeding purple flowers with long pollen
grains were crossed with pure breeding red
flowered plants which possessed short pollen
• b. write down the genotypes of the parental
generation
• c. write down the genotypes of the F-1
generation
• d. the F-1 generation were then test crossed
• e. expected outcome?
• f. actual outcome 7P_L_:1ppL_:1P_ll:7ppll
5.
Explanation
• a. results far from anticipated 1:1:1:1
• b. showed that alleles for purple color and long pollen
traveled together and that red color and short pollen
traveled together usually
• c. create punnett square for the test cross
• d. which parent determined the outcome of the
offspring
• e. expected that equal numbers of each gamete type
would be produced
• f. appears that alleles traveled as a unit most of the
time-why
g.
linkage
If crossing over happens, four different gamete types are
produced, but in very different numbers.
Sample Problem
• In Drosophila melanogaster there is a dominant gene for gray
body color and another dominant gene for normal wings.
• The recessive alleles of these two genes result in black body
color and vestigial wings respectively.
Flies homozygous for gray body and normal wings were crossed
with flies that had black bodies and vestigial wings.
The F1 progeny were then test-crossed, with the following results:
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Gray body, normal wings 236
Black body, vestigial wings 253
Gray body, vestigial wings 50
Black body, normal wings 61
• Would you say that these two genes are linked? If so, how
• many units apart are they on the chromosome?
B.
Mapping
• 1. in the example above crossing over occurred 2/16
times or 12.5% of the time
• 2. if the above two genes had been found further apart
on the chromosomes, would they have been more or
less likely to rearrange themselves by crossing over?
• 3. further apart two genes are on a chromosome, the
more likely they are to recombine by the process of
crossing over
• 4. by convention, a 1% crossing over frequencey
occurs when the two genes are 1 map unit apart from
each other
5.
Sample problem-determine the order of the
following genes
• a. the frequency of crossing over between the
following genes is listed below
• b. A and B
40%
B and C
20%
C and D 10%
C and A
20%
D and B
10%
6.
Other points about mapping
a. a map unit previously was called a Dalton
b. the physical length of a map unit is not
constant
c. some regions of a chromosome experience
crossing over more frequently than others
d. more money was spent on fruit fly genetics
determining order of genes on their
chromosomes than was getting men to the
moon
C.
Multiple allele inheritance-blood type
• 1. definitions of antigens and antibodies
• 2. agglutination
• 3. three alleles involved in blood type
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IA produces antigen A on the surface of the RBC
IB produces antigen B on the surface of the RBC
i produces neither of the antigens A nor B
notice that the first two alleles are codominant
An individual who doesn’t possess the antigen does
possess a circulating antibody against the antigen
4.
Table of genotypes/phenotypes
Genotype
IA IA
Phenotype
Antigens
present
Antibodies
present
5.
Differing percentages by race
Blood type
WASP
Hawaiian
Japanese
O
45
37
30
A
41
61
38
B
10
2
22
AB
4
1
10
6.
Practice genetic’s problem
• If a man with blood type B, one of whose
parents had blood type O, marries a
woman with blood type AB, what will be
the theoretical percentage of their children
with blood type B?
7.
Rh factor
D.
Gene Interactions-don’t appear as typical
9:3:3:1 ratios
1. Collaboration
a. Two dominant alleles
produce a phenotype
that neither can
produce alone
b. Mutually dependent
upon one another to
produce the new
phenotypes
c. Comb types in
chickens good
example
d. Single comb shown
here
e.
Other comb types
• R produces rose combs
• P produces pea combs
• R_P_ produces walnut
• f. RRpp (Wyandotte chicken) X rrPP (Brahma rooster)
• g. list the F1
• h. free range breeding to produce the F2
Sample Problem
• A dominant gene, A, causes yellow color in rats.
• The dominant allele of another independent
gene, R, produces black coat color.
• When the two dominants occur together (A/-R/-),
they interact to produce gray.
• Rats of the genotype a/a r/r are cream-colored.
• If a gray male and a yellow female, when mated,
produce offspring approximately 3/8 of which are
yellow, 3/8 gray, 1/8 cream, and 1/8 black,
• What are the genotypes of the two parents?
2. Epistasis
• a. the dominant allele of one gene
overshadows the expression of the dominant
allele of a second gene
• b. C is the color gene in fowl
• c. cc produces a white fowl
• d. I is epistatic over the C allele of the color
gene
• e. any bird who possesses “I” in his/her
genotype is going to be white
• f. ii will be colored if the dominant allele C is
present
• g. IICC (white leghorn
chicken) X iicc (white
Wyandotte rooster)
• h. show the F1 and F2
results
Sample Problem
• In Leghorn chickens colored feathers are due to
a dominant gene, C;
• White feathers are due to its recessive allele, c.
• Another dominant gene, I, inhibits expression of
color in birds with genotypes CC or Cc.
Consequently both C-I- and cc-- are white.
• A colored cock is mated with a white hen and
produces many offspring, all colored.
• Give the genotypes of both parents and
offspring.
3. Multiple gene inheritance
a. so far been discussing traits that are governed only by one gene
b. far from the truth
c. most phenotypes that we are aware of are distributed in a bell-shaped curve
like human height
d. often multiple genes affect such traits
e. height in plants might be affected by three genes each possessing two
alleles
f. the dominant allele of each gene might add 1 cm to basic height of plant
g. the recessive allele of each gene would not affect the basic 10 cm height
h. aabbcc
X AABBCC
i. F1 generation selfs itself
j. 1/64 6/64
15/64
20/64
15/64
6/64
1/64
k. the more genes affecting a trait, the smoother is the bell curve
l. the environment also affects phenotype smoothing off the curve even more
Human Pigmentation
E. Sex Determination in humans
1.human condition there
are 23 pairs of
homologous
chromosomes
• 2. 22 pairs of
autosomes-nonsex
chromosomes
• 3. one pair of sex
chromosomes that exist
in two different
configurations
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4. X and Y
XX female and XY is male
5. Concept of Barr Body and determination of genetic sex
6. Females are mosaics when it comes to traits carried on the X
chromosome as either one or the other X chromosome can be active
7. XO Turner’s
syndrome
-a. appear as a female phenotypically
-b. the female pattern is the default pattern
set to develop as female unless
something triggers a change-c. but doesn’t develop into the adult pattern
-d. hips and breasts remain immature
-e. no Barr bodies
-f. mental function is not impaired
-g. 1/5000 affected
-h. approximately 15-20% of aborted fetuses
F. Sex Linkage
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1.
genes that are carried on the X chromosome are said to be sex-linked
2.
since the male only has one X chromosome, he only has one allele
present for that trait
3.
if he has the allele, it is expressed no matter what
4.
males are genetically weaker than females for this reason
5.
the female has a back up copy of the allele if the first one is defective
6.
another interesting point is that a male inherits his sex-linked traits
from his mother ie male pattern baldness
7.
example is hemophilia
a. the trait for hemophilia is inherited as a sex-lined recessive trait
b. express the alleles as
XH for normal and Xh as the allele that
represents the recessive state
c. example
Practice Problems
• Red-green color blindness is inherited as a sexlinked recessive. If a color-blind woman marries
a man who has normal vision, what would be the
expected phenotypes of their children with
reference to this character?
• Suppose that gene b is sex-linked, recessive,
and lethal. A man marries a woman who is
heterozygous for this gene. If this couple had
many normal children, what would be the
predicted sex ratio of these children?