Download slides - University of Colorado-MCDB

MCDB 1041 Class 6
How we inherit (or don t inherit)
particular genetic compositions
Quiz 1 is NEXT MONDAY
Study sheet available on website
Review session on Friday, 3:30 pm
Review:
Alleles can be dominant or recessive:
Recessive: allele’s gene product is NOT VISIBLE
phenotypically when present in one copy.
Dominant: allele’s gene product IS VISIBLE
phenotypically when present in one copy
Disease alleles are often recessive--but not always!
Remember to think of an allele simply as a variant
dominant
recessive
Polydactly -- dominant
trait (Pp)
Probability
Geneticists are always calculating the
probability of a certain genotype or phenotype.
You might want to know how to calculate the
risk of having a disease because:
•  you re a genetic counselor and need to advise
people on their chances of having a normal child
•  you (or your partner) has a history of a genetic
disease in the family
Gregor Mendel: the father of modern
genetics
1822-1884
Austria
Described a hypothesis of
transmission of traits in 1865
that we now call the laws of
inheritance
He didn t know about genes or
alleles. He described units of
inheritance, and called these
units “elementen”
We now call these “alleles”
Fig. 3.1
Mendel discovered that some plants always have
offspring that look like themselves (same traits) while
some have offspring that look different.
When all offspring always look like the parents, the plant
is said to be true breeding
A true breeding plant has (for
the trait in question):
a.  Two dominant alleles
b.  Two recessive alleles
c.  Two dominant alleles OR
two recessive alleles
d.  One dominant allele and
one recessive allele
What happens when true
breeding individuals with two
different forms of a trait
(ie, tall and short) are
crossed?
The alleles that make this plant tall
are DOMINANT
Thus, in a cross with the short plant,
the offspring are all tall
tall
tall
Why do traits disappear in one generation only
to reappear in a subsequent generation?
An organism s phenotype is
determined by the combination
of alleles it has for a gene (or
genes).
You can display the dominant trait,
but still have the recessive
allele: heterozygote or carrier
Mendel made the intellectual leap
that each individual must have
two copies of each of these
“units” (alleles)
This tall
plant
could be
Tt or TT
Mendel s First Law:
Principle of Segregation
A gene pair
(homologs)
T
Heterozygous,
tall plant
Members of a homologous pair separate during the
formation of gametes
Each gamete only contains one of the two homologous chromosomes
(and thus one of the two alleles)
An organism only contributes one of its two alleles for any given gene
to its offspring, and it has an EQUAL probability of contributing any
one allele
t
Probability
Complete the handout
Applying probability rules to genetics:
•  When two events are independent (like two rolls of a
die, or the production of a sperm and an egg)
•  you multiply individual probabilities
•  When two events are “dependent”, that means there
are multiple possible ways to get a particular outcome
from a single cross
–  for example, the chance of seeing a dominant phenotype in an
offspring could arise if they have the genotype Aa or AA
–  To calculate this, you add together the probability of each
event
Ben and his wife both have genotype Tt. The
tt genotype produces a serious disease. Why
is their chance of having an affected child
1/4?
a. He and his wife each have a 1/4 chance of
producing a gamete containing a t , so the
overall chance is 1/4
b. He and his wife each have a 1/2 chance of
producing a gamete containing a t and you
multiply these probabilities together
c. He and his wife each have a 1/8 chance of
producing a gamete containing a t and you
add these probabilities together
What s the chance that
a child of these two
individuals will be
ff (have cystic
fibrosis)?
Cystic Fibrosis
Carrier
F
gametes
f
F
FF
Ff
f
Ff
ff
Ff
Probability of
donating f
allele=50%
or .5 or 1/2
Probability of
donating f allele=
50% (.5) (1/2)
Cystic Fibrosis carrier
Ff
Since the chance of contributing
an f from one parent is .5, and
the chance from the other is .5,
you multiply them together =
25% (.25)
Drawing a pedigree
Ben and Samantha have three kids:
Pete, Kayla and Tasha
Samantha is a carrier for cystic fibrosis
Kayla is a carrier for cystic fibrosis
Pete is “normal”. What do the others look
like?
Draw the pedigree
What is the probability that
a child of these two parents (Ff and Ff)
will be a carrier (heterozygous)?
Cystic Fibrosis carrier
a.  25%
ff
b.  50%
c.  75%
gametes
F
f
d.  100%
Cystic Fibrosis
Carrier
FF
Ff
Ff
There are
2 possible ways to be
a carrier and 3 possible ways
to NOT get cystic fibrosis
F
You ADD these probabilities
together
f
Ff
ff
Thinking about two genes at once
Calculating what the offspring can look like from parents
with genotype AaBb x Aabb
What is the probability that these parents will have a child
who is Aabb?
There are two ways you can think about this:
AaBb x Aabb is the same as:
Aa x Aa
and
Bb x bb
If you calculate each one separately, you will need to multiply
the probabilities:
Chance of getting Aa x chance of getting bb
OR, remembering the rules of meiosis, you
can set up a Punnett square to represent
the alleles together
AB
aB
Ab
ab
AB aB Ab ab
Mendel s 2nd law: The Law of Independent Assortment
The inheritance of an allele of one gene does not influence which allele is
inherited of a second gene.
Possibility A
Possibility B
Take home: you always end up with ONE allele of every gene in a
gamete, but any combination of alleles is possible
and equally likely (R and Y or R and y or r and y or r and Y)
Dylan is genotype AaBB for two genes that
cause recessive diseases. If his wife is
aaBb, what is the chance his child will be a
carrier for both diseases?
a.  1 (100%)
b.  3/4 4 possible genotypes of offspring
c. 1/2 only one way to get AaBb, so ¼;
d. 1/4 Or, think about it this way:
½ chance of getting Aa x ½ chance of
e. 0
getting Bb
Inheritance: definitions
Terms you should be able to define and use:
•  Homologous chromosomes
•  Sister chromosomes (or chromatids) = replicated
chromosomes
•  Heterozygous and homozygous
•  Dominant and recessive
•  Allele
•  Gamete
•  Additive vs.mutliplicative rules for probability