Genetics can be fun (Chs 14 and 15)

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Genetics can be fun (Chs 14 and 15)
Principles of Biology II, M. Marshall Shippensburg
University Biology Dept. Fall 2011
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Figure 14.0x Mendel
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Figure 14.1 A genetic cross
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Figure 14.2 Mendel tracked heritable characters
for three generations
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Figure 14.x1 Sweet pea flowers
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Figure 14.3 Alleles, alternative versions of a
gene
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Table 14.1 The Results of Mendels F1 Crosses
for Seven Characters in Pea Plants
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Figure 14.x2 Round and wrinkled peas
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Figure 14.4 Mendels law of segregation (Layer 2)
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Figure 14.5 Genotype versus phenotype
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Figure 14.6 A testcross
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Figure 14.7 Testing two hypotheses for
segregation in a dihybrid cross
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Segregation of homologs at Metaphase One
determines the allele combinations of gametes
when the genes for the traits of interest are on
separate chromosomes
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Figure 14.8 Segregation of alleles and
fertilization as chance events
We discussed this topic extensively in lab. Look
over the lab hand-out sheet to review this topic,
including the product and sum rules.
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Non-Mendelian trait type 1., Incomplete dominance
in snapdragon color
Non-Mendelian traits are any whose inheritance
pattern does not result in standard Mendelian
ratios in the F1 and F2 generations. Both
Mendelian and non-Mendelian traits can and do
exist within the same species.
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Incomplete dominance in carnations
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Non-Mendelian trait type 2., Multiple alleles for
the ABO blood groups
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Human blood type differences involve different
glycoprotein (with some glycolipid) types on the
red blood cell membrane.
Technically type O is NOT the complete absence
of such material, but the lack of the specific
galactose sugars that constitute the type A B
molecules. The O gene cannot code for the
glycosyltransferase enzyme to add these sugars,
while the A and B genes code for different enzyme
specificities.
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ABO blood types as detected by the use of anti -
A and anti - B protein antisera.
People who have never been exposed to other blood
types still may have anti-A or Anti-B antibodies
in their blood due to the fact that similar
sugars are found elsewhere in nature, on food, on
bacteria, etc. and these are similar enough that
most people naturally have cross-reacting
antibodies. A transfusion mismatch, if it
occurs, is likely to be mild on the first
exposure. Subsequent mistakes, however can be
life threatening as the first is likely to
greatly increase the antibody concentration
(titer). The RH factor ( / -) is a different
gene altogether and involves a surface protein
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Non-Mendelian trait type 3., Epistasis, where one
gene modifies the expression of another.
In this example B Black, which is
dominant b brown, which is recessive But
neither color can be expressed at all in the
absence of at least one C allele C color
conferred by B gene is expressed c color
conferred by B gene is not expressed The
individual gene alleles actually operate in a
Mendelian fashion, but their interaction makes
this difficult to see at first.
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Non-Mendelian trait type 4., Polygenic
inheritance of skin color
Many traits in nature are expressed as a result
of several to many genes working together.
This can create difficulties when the trait
involved is one that we desire to modify or
manipulate. Resistance to disease organisms
works this way in many plant species.
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Multi-gene inheritance, a more detailed view
Again, the alleles of the individual genes
behave in a Mendelian fashion, but the fact
that they all affect the same phenotype makes
this difficult to see.
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Non-Mendelian trait type 5., Environmentally
variable phenotype
Hydrangeas produce blue-violet flowers when the
soil pH is acidic, as this allows Al to be taken
up from the soil. Alkaline pH is obviously not
acidic, and it prevents Al uptake and the flowers
have a pink coloration
Anthocyanins are flavonoid pigments that change
color with pH and the ions that they are
complexed with, as shown by this in-vitro demo
done with rose pigments..
See http//www.demochem.de/p26_anth-e.htm
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Non-Mendelian trait type 6., Pleiotropic effects
of the sickle-cell allele.
Homozygous individuals suffer from sickle cell
RBC damage, but heterozygous individuals have a
milder form of the disease, AND are more
resistant to the malaria parasite which completes
much of its life cycle within the human host
within the red blood cell. So in regions where
malaria is endemic the sickle cell trait confers
an advantage.
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Pedigree analysis
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Large families provide excellent case studies of
human genetics
Dr Nancy Wexlar pursued her quest to map the
location of the gene for Huntington's chorea
using pedigree analysis connected to DNA finger
printing. Huntingtons disease has some
similarities with muscular dystrophy in that it
involves a deterioration of muscle control, in
this case due to neurodegeneration. It is
different, however, in that it is an autosomal
dominant trait (most mutations are recessive)
that usually is not detected until the person is
in their 30s, in many cases after they have had
children of their own. Wexlers work involved
analyzing a large effected population living in
villages on Lake Maracaibo, Venezuala, and met
with success in 1983. since then the gene has
been located and characterized, although the
disease is still not totally understood.
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Figure 14.17 Testing a fetus for genetic
disorders
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Chromosomal structural effects. Some traits have
their inheritance influenced by
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Figure 15.1 The chomosomal basis of Mendels laws
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Non-Mendelian trait type 7 Sex-linked inheritance
T.H. Morgan was a geneticist who pioneered the
use of fruit flies as a genetic model system
which could be easily cared for and would rapidly
breed in the lab One of the first mutations
thathe discovered was for eye color. This is a
perfectly good example of how a sex-linked trait
is inherited, but it involves fruit flies
(Drosophila) and Drosophila genetics has its own
(complicated ) convention for labeling
alleles. So lets look at a more straight-forward
example human color blindness.
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Morgans first mutant was in a sex-linked trait
eye color
T.H. Morgan in his lab at Columbia U., circa 1910.
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Sex-linked inheritance, another view Color
Blindness
The XN allele confers normal color vision, the Xn
allele confers abnormal color vision XNXN and
XNXn women have normal vision XnXn women and
XnY men do not. An XN Y man would be normal
Color blind
A carrier
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The transmission of sex-linked recessive traits
Your textbooks version
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Color vision test
- - means youd see nothing in particular no
obvious number
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Sex-linked inheritance patterns
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Non-Mendelian trait type 8 Linkage - Evidence
for linked genes in Drosophila
You should get a 1111 ratio from a standard
test cross, but in this case you do not. How can
you explain the relatively small number of
recombinant phenotypes?
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Drosophila testcross
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Linkage Linkage with Recombination due to
crossing over can explain the seemingly odd
ratio.
Recombinant numbers are small, as the chance of
it occurring between these two loci is small.
The closer together the loci are, the lower the
recombinant numbers will be.
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Linkage Recombination due to crossing over
Crossing over at any given location is a rare
event. The only crossovers that will be detected
as recombinant progeny are those occurring
between the two loci involved. The higher the
recombinant numbers, the greater the distance
between the loci . The number of recombinants
seen are a composite (a sum total) of many
different cross-overs that all occurred at some
point between the two loci.
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Linkage Using recombination frequencies to
construct a genetic map
If the cross-over frequency between locus b and
vg is as shown, and that between b and cn and cn
and vg (calculated as total recombinants / total
progeny 100) are also as shown (as arrived at
from data from the three crosses involved), then
the relative positions of the loci MUST be as
shown also. The can be converted to map units
which imply no real physical distance, but do
accurately indicate relative positioning and
relative spacing.
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Linkage A partial genetic map of a Drosophila
chromosome
Long before DNA sequencing was possible, loci
positions for hundreds of traits were worked out
by painstakingly doing crosses involving linked
genes. This was done for many of the model
systems used by geneticists, fruit flies, certain
fungi, corn, tomatoes, etc. Aberrations from
the expected results also indicated that certain
DNA sequences could actually move their location
over time. This was first discovered in corn.
Today these jumping genes are known as mobile
or transposable elements, similar to the PV 92
Alu sequence that we used in lab.
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Figure 15.x1 Translocation
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X inactivation and the tortoiseshell cat
Because female cells have two X chromosomes, one
is inactivated through condensation and its genes
are not available for transcription. As the locus
for tortoise shell coat color is located on the
X, and different skin cells differ in which X is
inactivated, a mottled appearance can result if
the cat is heterozygous.
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The Calico cat also has white areas where neither
XO nor XB are expressed.
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Figure 15.11 Meiotic nondisjunction
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Figure 15.14 Down syndrome
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Alterations of chromosome structure occur during
DNA replication.
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Genomic imprinting
The alleles of certain traits can be silenced if
they come from one sex or the other. In a given
species only certain genes behave this way, but
the behavior is fairly uniform with certain genes
being silenced only if they come from the male or
female on a case by case basis. So these traits
behave almost as if they were an example of a
sex-linked inactivation, but they are almost
always found on autosomal (non-sex)chromosomes.
So unlike standard mendelian traits that are
inherited the same in reciprocal crosses (where
male and female are switched) imprinted gene
traits would not be. The imprinting effect is
erased in the next go-round of gamete
production.
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Cytoplasmic inheritance in tomato leaves
The small amount of DNA in mitochondria and
chloroplasts can contain genes that code for
detectable traits these are inherited through
the maternal line only as the progeny organelles
come from the egg.