Transcript Genetics 1 - MaxSkyFan

How are traits inherited?
Darwin's dilemma
• Darwin found a lot of
evidence supporting his
theory of evolution by
natural selection, but one
thing troubled him.
• He knew that species
occasionally would
appear with
advantageous traits, but
feared those traits would
get blended away.
• Consider a giraffe that is born
with a longer neck.
• It would eat better than other
giraffes, and would have a
better than average chance
to mate.
• But who would it mate with?
• He thought the kids would be
half-way between both
parents.
• Who would the kids mate
with?
• Darwin died wondering about
this paradox.
Blending vs. Particulate Inheritance
• Ironically, the answer was
available in a journal in
Darwin's library, but he
never read that article.
• Gregor Mendel, an
Austrian monk, published
his data on pea plants
providing evidence that
blending inheritance was
incorrect.
Mendel’s particulates factors were
genes.
• Mendel determined
that hereditary factors
occurred in discrete
pairs, with one of
each coming from
each parent.
• 40 years later, we
discovered that these
‘factors’ (now called
genes) were carried
on chromosomes.
Once the connection was made
between genes and inheritance,
Darwin's theory of evolution by
natural selection was greatly
strengthened.
Incidentally, the combination of
Darwinian evolution and population
genetics is called the New Synthesis.
Mendelian Genetics
• Mendel knew that his
'factors' were discrete
and non-blending.
• He also knew much
more about the
behavior of these
units of inheritance.
• So let’s revisit his
peas!
Law of Segregation
• Mendel's First Law (Law
of Segregation): Mendel
determined that each
individual has two copies
of each gene (e.g., Pp).
• These copies are called
alleles. If both alleles are
the same, then the
individual is homozygous
(e.g., PP or pp).
• If the two alleles are
different, then the
individual is heterozygous
(e.g., Pp).
• When an individual
creates gametes (sex
cells: egg or sperm in
humans, egg or pollen
grain in plants), only one
of each allele is packaged
in the gamete.
• Mendel determined that
which allele appears in
the gamete is random,
with each allele having a
50% chance. This rule is
the Law of Segregation.
Flower color
• Pea flowers are either
purple or white.
• Peas fertilize
themselves, so
• white  white and
purple  purple.
• called true-breeding
• But…
• …if you cross a truebreeding purple with
a true-breeding
white…
• …all of the offspring
have purple flowers.
• Hence Mendel said
that purple was
dominant to white.
• PP: purple
• pp: white
• Pp: purple!
Terms to understand
• gene: stretch of DNA that
codes for a particular
trait. (e.g., flower color)
• allele: a particular variant
of a gene (e.g., purple)
• genotype: what alleles
an individual has for a
particular trait or set of
traits (e.g., Pp)
• phenotype: the
expression of the genes;
what the individual looks
like (e.g., purple)
• dominant trait: an allele
that is expressed no
matter what the other
allele is (e.g., purple
flower color being
dominant to white flower
color in pea plants)
• recessive trait: an allele
that is only expressed if it
is the only allele present
(i.e., both alleles are the
same) (e.g., white flower
being recessive to purple
flower color)
Terms to understand
• homozygous: has 2
copies of the same allele
for a given trait (e.g., PP
or pp)
• heterozygous: has 1
copy of each of two
alleles for a given trait
(e.g., Pp)
• F1 generation: the kids
of the parents
• F2 generation: the
grandkids of the parents
(kids of F1)
• gamete: sex cell (egg or
sperm); only has ONE
allele for each gene since
it only has one
homologous chromosome
(either the one you
received from Mom or the
one you received from
Dad)
• True-breeding:
homozygous for the trait.
Forming gametes
• How many different
gametes can PP
make?
• 1
• P
• How many different
gametes can Pp
make?
• 2
• P or p
• When forming
gametes, you always
need one allele for
each gene.
• How many different
gametes can PPTt
make?
• 2
• PT or Pt
Determining the number of different
gametes possible
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•
AaBBCc?
2x1x2=4
AaBbCC?
2x2x1=4
AaBbCcDd?
2 x 2 x 2 x 2 = 16
AAbbCCddEE?
1x1x1x1x1=1
What is it?
AbCdE
• Which of the following
gametes can this parent
(AABbCCDdeeFf) make?
a. AAbCEf
b. ABCDEF
c. abcdef
d. ABCdef
• d is the answer.
• What is the chance of
that parent producing that
gamete?
• 1/8 Why?
Determining the number of different
alleles
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•
AaBBCc?
2 + 1 + 2 = 5 alleles
AaBbCC?
2 + 2 + 1 = 5 alleles
AaBbCcDd?
2+2+2+2=8
AAbbCCddEE?
1+1+1+1+1=5
How many different
genes are shown at
right?
• 3, 3, 4, and 5 (top to
bottom)