Download Chapter 2: Genes and Medical Genetics

Introduction
• So far, it’s been all about the cell.
• Last lecture we examined how cells divide.
A process which facilitates life.
• This time we’ll consider genetic and there
impact on how we “look,” and what
potential pitfalls may occur when cell
division and replication don’t “give us what
we wanted.”
Introduction
• As always we have several new terms.
• Genotype: genes of the individual. Your
genetic makeup, if you will.
• Phenotype: What you look like, or more
precisely, what physical manifestations are
observed.
• Allele: Alternate forms of a gene which are
located in the same position on a pair of
chromosomes.
• Three paths:
Two results??
• Not really!
• Again, new terms…
• Dominant allele
(capital letter)
• Recessive Allele
(lower case letter)
Fig. 02-01
• As you can see during
meiotic cell division, alleles
are isolated within each
gamete.
• How do alleles (genes) get mixed
up??
• Rem: Meiosis? Crossing over??
• Alleles are spread around in an
amazing way during sexual
reproduction.
• As we will see, this leads us to
some interesting results which don’t
always seem logical.
• Alleles on autosomes
• Basic Mendelian Genetics
and the Punnett Square
• If we “cross multiply,” it
becomes evident that not
all genotypes result in the
same phenotype.
• The example at the left is a
single-trait cross.
• Notice the expected
phenotypic ratio (3:1).
This will become important
later.
• What happens if we
mix it up a bit??
• Same method, different
results.
• Now the phenotypic
ratio is 50:50 (Better odds
than you’d get in Vegas!)
• Okay, Emiril, let’s…
“Kick it up a notch!!”
• Dihybrid crosses
(and beyond) are
handled similarly.
• You must be careful
to transfer all your
alleles though!!
• Once this is done,
figuring out what
you have can be
challenging.
Genetic Disorders
• Most of the time, genes are expressed and
individual live quiet lives.
• Sometimes two alleles get together that
shouldn’t!
• Many are autosomal dominant and
expressed with alarming frequency.
• In the case of autosomal dominance, even
heterozygotes express the phentype.
Genetic Disorders:
Autosomal Dominant
Disease
Occurence
Symptoms
Neurofibromatosis
1/3500
Dark Spots
Huntington
??
Neuopathy
Polycystic Kidney
Nephretic cysts/
hypertension
renal failure
Genetic Disorders:
Autosomal Recessive
Disease
Cystic Fibrosis
Occurence
1/2500
Symptoms
bronchial mucus
clogged pancreatic ducts
Phenylketonuria
1/5000
high urinary phenylalanine
concentration
Tay-Sachs
??
Blindness, paralysis, death
Polygenic Inheritance:
• Height, skin color, etc.
• The absolute reason for
expression of some traits
such as a persons height
can be nebulous.
• Many alleles dictate the
height of an individual, so
just because your dad
was 7’ doesn’t mean you
will be (though it doesn’t
hurt either).
Some traits share an additional genetic component,
despite differing environmental factors
• Multiple Allelic Traits
• Blood types are coded
for by three different
alleles.
• Codominance exists
because A and B are
dominant over O.
• Incomplete dominance
• Paul Mitchell aside,
hair curls, or lack of
them are expressed
as a mixture of straight
vs. curly locks.
• What can result is a
continuum of hair curls,
waves and looks.
• Incomplete dominance is also observed in
Sickle-cell Disease. Here homozygotes (HH) are normal,
heterozygotes (Hhs)are carriers, and recessives (hshs)
have the disease. Oddly enough, carriers are superior in
certain environments due to Malaria resistance conferred
by the Sickle-cell trait.
• Sex-linked traits
• Occur when alleles are
located on sex chromosomes.
• Sex-linked traits can be harsh.
Ex.: Muscular distrophy &
Hemophilia
Fig. 02-14
Muscular Distrophy
Sex-Influence Traits
Fig. 02-15
Fig. 2D
Fig. 2C