Transcript Human Genetics

Human
Genetics
• Problems of studying
human genetics:
• May take 75 years to
produce 3 generations
of humans (months for
peas, weeks for fruit
flies)
• Each pair of humans
only produce a few
offspring (peas and ff:
100’s)
• Ethical concerns
prevent scientists from
using the same
techniques used with
other organisms
Ways to study
Human
Genetics
• Population Sampling: scientists select a few individuals
(10’s—1000’s) that represent the population they want to
study—use formulated statistical rules to get accurate
results
• Example: to find the % of people in the US that could
taste the chemical PTC (phenylthiocarbamide) they
randomly selected a few 1000 individuals and tested
them and came up with the data that 65% are
“Tasters”—can detect a bitter taste and 35% are “Nontasters”—cannot detect a taste
Identical Twin studies
• Researchers study
identical twins (those
from 1 zygote and have
identical DNA) to
distinguish between
genetic and
environmental influences
on specific traits
• Especially are interested
in those raised apart to
study NATURE vs
NURTURE
• Some researchers feel a
baby is a blank slate
(TABULA RAZA) and
turns out a certain way
due to its environment
Pedigree studies
• Geneticists analyze
inheritance patterns
in families
• Pedigrees often
reveal a carrier—
someone who is
heterozygous for a
trait, so doesn’t have
the disease, but can
pass the rec gene on
to their children
• Many diseases can
be tracked through
families with
pedigrees
Single Allele traits
• Traits that are coded by a single
allele—you inherit that one allele
and you get that trait (disease)
• Sickle Cell Anemia is where the
dom allele A produces normal
hemoglobin (round RBC)
• A‘ is a codominant allele that
codes for abnormal hemoglobin
(sickle/crescent shaped RBC)
• AA = normal RBC
• AA‘ = normal and abnormal
RBC (live with disease)
• A‘A‘ = all abnormal RBC (death)
• Sickle cells clump together and
clog capillaries—causes great
pain due to improper flow of
oxygen
• Normal RBC live ~120 days—
sickle cells live 10-20 causing
chronic shortage of RBC-anemia
Huntington’s
Disease
• HD starts as mild forgetfulness in
30’s/40’s, leads to loss of muscle
control, severe mental illness,
death
• Since it is a dominant trait,
anyone receiving the gene will
develop HD
• Really scary because you don’t
know you have it until you have
already had children (usually)
and passed it on to them
• Gene found on the tip of the arm
of chromosome 4
• H = HD h = normal
•
H
h
• h Hh
hh
• h Hh
hh
• 50/50 chance of passing it on to
children
Multiple Allele
Traits
• Human blood types (A, B, AB, O)
are coded by several alleles
found on chromosome 9
• A and B are codominant when
they are together and both are
dominant over O
• Genotype
Blood type
• AA or AO
A
• BB or BO
B
• AB
AB
• OO
O
• Universal donors—type O
• Universal recipient—type AB
• Actually 8 blood types when you
use the Rh factor: + or –
• A type O male marries a type AB
female; what would the possible
blood types be for their children?
Sex-linked Traits
• Alleles for these traits appear
only on the X chromosome
• Males have only 1 X (females
2) so whatever is on that 1
chromosome will be expressed
• There are no complimentary
genes on the Y chromosome
to mask any rec genes
• Females have 2 X’s so the rec
allele can be masked by the
dom allele
• This is why males are more
likely to express rec sex-linked
traits/diseases
• Colorblindness and hemophilia
are 2 examples
• Since these conditions only
deal with the X chromosome,
we use the capital letter X with
a superscript to signify dom/rec
alleles
Colorblindness
• Can’t distinguish colors
• Most common: red-green
colorblindness---can’t tell the
difference between these 2 colors
• Use a superscript “c” to show the
allele for colorblindness
• XcXn carrier female
• XcY colorblind male
• XnXn normal female
• XnY normal male
• Carrier female marries a colorblind
male—what % of females will be
colorblind? Carrier?
• What % of males will have normal
vision?
• With sex-linked problems you have to
pay attention to what the problem is
asking—boy, girl or child---the % will
be different
Hemophilia
• Disease where the blood lacks
a clotting factor (Factor VIII)
• Without this clotting factor a
hemophiliac could bleed to
death from a bump or scrape
• During Victorian England, the
disease ran through the Royal
houses in Europe, due to the
way the English princesses
(carriers) were married off to
other Royal families in other
countries
• Change the superscript to a
lowercase “h” for hemophilia
• XhXn carrier female
• XhY hemophiliac male
• Normal clotting male marries a
carrier female—what are the
chances of them having a
hemophiliac CHILD?