Transcript MENDEL AND THE GENE IDEA

Mendel and the Gene Idea
Chapter 14
• Gregor Mendel - monk - studied pea
plants, looked at traits.
• Pea plants many varieties with
distinct heritable features
(characters) with different
variants (traits).
• Pea plants self-fertilize; Mendel
cross-fertilized to study traits.
• Mendel cross-pollinated (hybridize)
2 contrasting, true-breeding pea
varieties.
• True-breeding parents - P
generation; hybrid offspring - F1
generation.
• F1 hybrids then self-pollinate to
produce F2 generation.
http://nitro.biosci.arizona.edu/courses/EEB320-2005/Lecture02/pics/pea.jpeg
• Thought genes blended - purple
flower crossed with white flower
result would be light purple flowers.
• All the flowers purple.
• When flowers self-fertilized, white
flower reappeared in next
generation.
• Ratio of purple to white in F2
generation was 3:1.
• Developed hypothesis to explain
process.
• 1Alternative versions of genes
(alleles) account for variations in
inherited characters.
• 2For each character organism
inherits 2 alleles, 1 from each
parent.
• Alleles can be same or different.
• 32 alleles differ - 1 (dominant
allele) fully expressed in organism.
• Other (recessive allele) no
noticeable effect on organism’s
appearance.
• 42 alleles for each character
segregate (separate) during gamete
production.
http://discover.edventures.com/images/termlib/d/dominant_allele/support.gif
• Found that alleles not linked due to
inheritance patterns.
• Independent assortment of each
pair of alleles during gamete
formation - law of independent
assortment.
• Therefore, Mendel had three different
laws.
• The law of dominance and recessiveness
states that one gene is dominant over
the more recessive gene.
• The law of segregation states that
alleles separate during meiosis.
• The law of independent assortment
states that alleles organize in the
gametes regardless of other alleles.
• An organism with two identical alleles
for a character is homozygous for that
character.
• Organisms with two different alleles
for a character is heterozygous for
that character.
• A description of an organism’s traits is
its phenotype.
• A description of its genetic makeup is
its genotype.
• A Punnett square predicts the results
of a genetic cross between individuals
of known genotype.
• A testcross, breeding a homozygous
recessive with dominant phenotype,
but unknown geneotype, can determine
the identity of the unknown allele.
• Mendel’s experiments focused on
monohybrid crosses meaning that he
looked at only one trait at a time.
• Later on he started looking at dihybrid
crosses involving probabilities of two
different traits.
• Mendel’s ideas are based on probability.
• If you were to toss a coin 4 times, the
coin has a ½ chance of coming up heads
every time.
• Each toss is independent of the one
done before.
• The probability of it coming up heads all
four times is: ½ * ½ * ½ * ½ = 1/8.
• This is known as the rule of
multiplication.
12.2
• There is also a law of addition that
determines the chances of an event
happening in different ways.
• For example, there are two ways that F1
gametes can combine to form a
heterozygote.
• The dominant allele could come from the
sperm and the recessive from the ovum
(probability = 1/4).
• Or, the dominant allele could come from
the ovum and the recessive from the
sperm (probability = 1/4).
• The probability of a heterozygote is 1/4
+ 1/4 = 1/2.
• Incomplete dominance can also occur in
offspring.
• In incomplete dominance, heterozygotes
have a completely different phenotype
than homozygotes.
• This happens in snapdragons.
• Homozygous recessive flowers are
white; homozygous dominant flowers are
red; heterozygotes are pink.
• Another inheritance pattern is
codominance in which two alleles affect
the phenotype in separate,
distinguishable ways.
• An example of this is blood type.
• If you inherit an A allele and a B allele,
your blood type will be AB; if it is AA or
AO, your blood type will be A.
• This means that A is dominant to O, B is
dominant to O, but A is codominant to B.
• Blood type is important because type A
has anti-B antibodies.
• If exposed to B blood, it will clump
together causing a transfusion reaction.
• People with blood type O have both
antibodies and therefore can donate to
any other blood type.
• On the other hand, AB has neither
antibodies and therefore can receive
from any blood type.
• Dominant genes do not mean that they
are more popular in a given population.
• Also, most genes do not control only one
trait but are pleiotropic, affecting more
than one phenotypic character.
• In epistasis, a gene at one locus alters
the phenotypic expression of a gene at a
second locus.
• In mice, one gene determines whether
or not there will be a coat color.
• If that gene is turned off, the mouse
will be white; if it is turned on, another
locus will determine what the color is
(brown or black).
• Quantitative characters vary in a
population along a continuum.
• This is because of polygenic inheritance
which is when more than one gene
controls a single trait.
• An example of this is skin color which is
controlled by at least three different
genes and is responsible for the variety
of skin colors.
• Phenotype also depends on environment.
• For humans, nutrition influences height,
exercise alters build, sun-tanning
darkens the skin, and experience
improves performance on intelligence
tests.
• Genetic experiments cannot ethically be
performed on humans, so geneticists use
pedigrees to look at traits found in
families.
• A family tree is then created showing
the absence or presence of a specific
trait to determine how it is passed.
• Ethnicity plays a role in genetic disease
patterns.
• For example, sickle-cell anemia is found
predominately in African-Americans.
• This disease causes the red blood cells
to be sickle shaped instead of the
normal disk shape causing the cells to
get stuck in the vessels.
• Cystic fibrosis affects mostly
Caucasians.
• Cystic fibrosis is a multi-system disease
that causes mucous to build up in
various organs, especially the lungs.
• Tay-Sachs affects people of Jewish
descent.
• Tay-Sachs affects the brains of small
children, ultimately causing the death of
the child prior to 5 years old.
• Some genetic diseases, such as
dwarfism, are dominant diseases.
• This means that a child has a 50%
chance of inheriting the disease
because one of the parents has the
disease.
• Huntington’s disease is also a dominant
disease that affects the nervous
system.
• Most dominant diseases are not lethal
(Huntington’s disease is).
• Genetic counseling is a field of study
that works with people that have a
history of genetic disease in the family.
• A child with a recessive disease can be
born to phenotypically normal parents.
• There are several tests that can be
performed to determine a couple’s risk.
• One technique is amniocentesis.
• Cells are extracted from the amniotic
fluid surrounding the fetus and then
analyzed to search for potential
problems in a technique called
karyotyping.
• Karyotyping is essentially mapping out
the chromosomes of an individual.
• A second technique, chorionic villus
sampling (CVS) can allow faster
karyotyping and extracts a sample of
fetal tissue from the chorionic villi of
the placenta.
• A more routine test is an ultrasound
which detects only physical
abnormalities that are present.