Transcript Ch 14 & 15, Genetics, FALL 2011

Genetics can be fun (Chs 14 and 15)
Principles of Biology II,
M. Marshall
Shippensburg University Biology Dept.
Fall 2011
Figure 14.0x Mendel
Figure 14.1 A genetic cross
Figure 14.2 Mendel tracked heritable characters for three generations
Figure 14.x1 Sweet pea flowers
Figure 14.3 Alleles, alternative versions of a gene
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Figure 14.x2 Round and wrinkled peas
Figure 14.4 Mendel’s law of segregation (Layer 2)
Figure 14.5 Genotype versus phenotype
Figure 14.6 A testcross
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Segregation of homologs at Metaphase One determines the allele combinations of
gametes when the genes for the traits of interest are on separate chromosomes
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.
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 nonMendelian traits can and do
exist within the same
species.
Incomplete dominance in carnations
Non-Mendelian trait type 2., Multiple alleles for the ABO blood groups
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.
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 antiA 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
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.
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.
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.
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
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.
Pedigree analysis
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. Huntington’s 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. Wexler’s 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.
Figure 14.17 Testing a fetus for genetic disorders
Chromosomal structural effects. Some traits have their
inheritance influenced by
Figure 15.1 The chomosomal basis of Mendel’s laws
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.
Morgan’s first mutant was in a sex-linked trait – eye color
T.H. Morgan in his lab at Columbia U., circa 1910.
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”
The transmission of sex-linked recessive traits – Your
textbook’s version
Color vision test
- - means you’d see nothing in particular; no obvious number
Sex-linked inheritance patterns
Non-Mendelian trait type 8: Linkage - Evidence for linked
genes in Drosophila
You should get a 1:1:1:1 ratio
from a standard test cross, but in
this case you do not.
How can you explain the
relatively small number of
“recombinant” phenotypes?
Drosophila testcross
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.
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.
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.
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.
Figure 15.x1 Translocation
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.
The Calico cat also has white areas where neither XO nor
XB are expressed.
Figure 15.11 Meiotic nondisjunction
Figure 15.14 Down syndrome
Alterations of chromosome structure occur during DNA
replication.
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 sexlinked inactivation, but they are almost
always found on autosomal (nonsex)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.
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.