Transcript Day 12: Genetics Part 2 Powerpoint

Mendelian Genetics
By the end of this class you should understand:
• The Mendelian model of genetics and Punnett
squares
• How the structure and function of genes
influences phenotypes
• The different modes of inheritance and
expression of genes
• The concept of a pedigree chart and how to
read one
Gregor Mendel
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Austrian Monk (18221884)
Discovered the modern
principles of genetics
working in a pea garden
His work was not widely
acknowledged until the
20th century
Why Pea Plants?
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All animals and plants use
the same DNA and
chromosome structure
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Plants complain much less
when you force them to
mate with particular
individuals and take their
children away for a
breeding program
Many traits with different
alleles at a given locus
Allele
• An allele is a particular version of a gene
– How do different versions come about?
• There are many alleles for almost all genes
– Many of them are functionally identical
– Sometimes the function is different for different
alleles
– Some alleles are defective!
Alleles
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In Mendelian
genetics (simplified
case) there are only
two alleles, one is
capitalized and the
other is lowercase
In reality there are
many alleles and any
symbol can stand for
any of them
Terminology
• The matching genes at the
same locus are on
homologous chromosomes
• Having two of the same
allele for a gene is called
homozygous
• Having two different alleles
for one gene is called
heterozygous
Mendel's Findings
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Mendel studied seven traits and started with
true-breeding specimens
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Are true-breeding individuals homozygous or
heterozygous?
Mendel's Discoveries
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Mendel discovered that when
he crossed true-breeding peas
for opposite traits (P
generation), all the offspring
had only one of the traits (F1
generation)
When these offspring were selfcrossed or crossed with other
F1 plants, the F2 plants had a
3:1 ratio
One trait was dominant over
the other
Pattern of Inheritance
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All seven of the listed
traits follow the same
pattern
Dihybrid Crossing
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What if a truebreeding yellow
smooth pea is crossed
with a true-breeding
green wrinkled pea?
According to Mendel's
work the traits are not
linked but sort
randomly
Dihybrid Cross Analysis
Chromosomal Inheritance
• Half your chromosomes come
from each parent (via meiosis)
• Each parent randomly passes
on 1 of the 2 chromosomes
s/he has
• This means each gene your
parent has, you have a 50%
chance of having
–
This fits Mendel's model!
Probability Table
• A table of which genes
get passed on is called
a Punnett Square
• Mother's possible
genes go on one axis,
father's possible genes
go on the other
Phenotype
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When an organism has
two different alleles, how
are they expressed?
–
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The phenotype is
what is actually
expressed
What if two different
organisms look the same
but have different alleles?
–
They have different
genotypes
One of these green peas is not
like the other....
Dominant/Recessive
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“Dominant” and “Recessive” are relative terms
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Much like “taller” and “shorter”
One allele may dominate over another
Sometimes two alleles do not have a
dominant/recessive relationship
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If both are equally expressed they are called
codominant
If the phenotype is a blend between the two
this is called incomplete dominance
Incomplete Dominance
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The phenotype is a
blending of the two
This means there are
red proteins and
white proteins here
This is very common
in more complex
traits
Height, etc
“Blending theory”
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Mendel's work disproved earlier ideas of
“blending theory”
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Blending theory states you are a literal mix of
your parents
Blending theory gained support because many
of your genes are codominant or incompletely
dominant alleles from your parents
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Developmental genes control your height,
body type, facial structure, etc
Dominant/Recessive
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Many genetic disorders and diseases are
recessive
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Only found in people who are homozygous for
the allele that causes the disease
Albinism, cystic fibrosis, phenylketonuria, Taysachs disease, etc
These disease come from a lack of a specific
enzyme or other protein
“Recessive”
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Typically a “recessive” gene is a defective gene
Blue eyes are a defect in an allele for coloring
the eye
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This means blue eyes are recessive and
parents with brown eyes can be carriers for
blue eyes
Example: Cystic Fibrosis
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A gene called CFTR produces a
protein channel that pumps
chloride ions onto the surface of
mucus membranes
Through osmosis, water
follows the chloride ions out
Failure to produce this protein
or have it be expressed means
mucus builds up in the
respiratory tract and can
become fatal
Cystic Fibrosis
Example: Albinism
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The protein melanin is the
pigment for our skin and is
present in our hair and eyes as
well
If one allele is defective and one
is normal, what is the genotype?
What is the phenotype?
If both alleles are defective,
what is the genotype? What is
the phenotype?
Carriers
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How many copies
of a working
blueprint do you
need to make the
enzyme?
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Just 1!
Having 1 working
and 1 defective
allele means you
are healthy
Pedigree Chart
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A pedigree chart, or just
pedigree, shows family
history for a particular
condition
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Can be for hair color,
eye color, etc
Most commonly for
a genetic disorder
Can be used to
determine the nature of
the inheritance
Example: Recessive Disorder
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Often “skips”
generations
When both parents are
carriers, about 1 in 4
offspring are affected
When one parent has
the condition:
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1 in 2 offspring are
affected and other half
are carriers
OR all are carriers
Multi-Allele Genes
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There are three alleles for a marker on our red
blood cells:
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A and B markers are large and can be detected
by the immune system
O marker is small and cannot be detected
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A, B, and O
As though it weren't there
What allele is recessive?
ABO blood type
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Six possibilities for
genotype:
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AA
AO
BB
BO
AB
OO
Codominance
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Since A and B are
both fully expressed,
they are codominant
O is recessive
because it is only
expressed when
there are no other
alleles present
Why does this matter?
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We have an immune system!
Your immune system will attack
any markers that were not in
your body when you were a fetus
This includes A and B markers
O type blood is the universal
donor!
See you in lab!
• Coming soon to a lab near you: more genetics!