Genetics concepts: Mendel and the gene idea

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Genetics concepts Mendel and the gene idea
Chapter 7
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Gregor Mendel used the garden pea plant Pisum
sativum in his studies because there were many
varieties that were easily obtainable He also
used peas because they can either self-pollinate
or cross-pollinate, which allowed him to create
pure-breeding lines as well as hybrids of two
different lines
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The pea plants that Mendel used differed in their
character forms For instance, flower color is a
character, and Mendels plants had either purple
or white flowers Contrasting phenotypes for a
particular character are the starting point for
any genetic analysis
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Other plant breeders had obtained results similar
to those of Mendel, but he did something that,
more than anything else, marks the birth of
modern genetics He counted the numbers of plants
with each phenotype
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Mendels application of careful statistical
analysis led to an explanation of the F2 ratios
he observed in his experiments
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Review Concepts of meiosis and the independent
assortment of chromosomes This is the foundation
for our studies in genetics and later on,
evolution
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Recall Homologous pairs of chromosomes
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And we get our four daughter cells
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Our four daughter cells again The chromosomes
carry genes, and the alleles of those genes
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Mendels pea plants
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To create hybrid plants, Mendel snipped off the
male stamens from the reproductive organs to
prevent self-fertilization. He then used a
paintbrush to transfer pollen from another plant
for fertilization.
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Mendel also did reciprocal crosses A female of
phenotype A is crossed with a male of phenotype B
and vice versa
?A ?B
?A ?B
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Purple
White
Parental PP pp
Gametes P only p only
Fusion of Gametes
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Can we make a Punnett square?
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P
F1
p
All purple flowers
In the F1 generation, we have a single phenotype
and a single genotype
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Now lets allow the F1 generation to
self-fertilize to get an F2 generation. The F1
products of meiosis assort independently
F2 Parental P p P p
Gametes P p P p
Gives us P p P p
Well use these gamete types to make a Punnett
square
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Sperm
F2
P
p
P
Eggs
p
31 Phenotypic ratio
121 Genotypic ratio
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The selfing, or intercross of identical
heterozygous F1 individuals is called a
monohybrid cross It was this type of cross that
provided the 31 progeny ratio that suggested the
principle of equal segregation of chromosomes
(plus the genes residing on them) into gametes
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Mendel then tried to follow the inheritance of
two characters at the same time He wanted to
know if two characters, such as seed shape and
color, are inherited together as a package or if
they assort independently
Will seed color and shape alleles always stay
together?
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Mendels first hypothesis Dependent Assortment
If the two characters segregate dependently (that
is, together), the F1 hybrids can only produce
the same two classes of gametes that they
received from the parents, and after selfing, the
F2 offspring will only show the parental
phenotypes in a 31 ratio
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The two characters would be inherited together
(dependent assortment) if the alleles were on the
same chromosome together This is called
linkage Fortunately, Mendels experiments were
not complicated by linkage
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Lets see how to put a dihybrid cross together
into a Punnett square
YYRR yyrr
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P Generation YYRR yyrr
Gametes YR yr
Y y R r
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This gives us all combinations for our Punnett
square
YR, Yr, yR, yr
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YR
Yr
yR
yr
YR
?
Yr
yR
yr
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YR
Yr
yR
yr
YR
Yr
yR
yr
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YR
Yr
yR
yr
YR
Yr
yR
yr
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What is a testcross?
We use a testcross to determine the genotype of
an organism that displays an ambiguous phenotype
That is, a purple-flowered pea plant can have
either the PP genotype or the Pp genotype
We can cross the purple-flowered plant with a
homozygous recessive planta white one with a
genotype that is known
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When were done with the testcross, we will
examine the phenotypes of the progeny (F1)
If we get all purple-flowered plants, we know
unambiguously that the purple parent plant is
homozygous dominantPP
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Testcross
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Testcrosses can also be done with the fruit fly,
Drosophila melanogaster
These types of crosses were originally done to
test for linkage by looking for recombinant
phenotypes
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Incomplete dominance
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The term incomplete dominance can be confusing
and misleading The three-phenotype scenario of
incomplete dominance is a perfect example of the
additive effects of alleles
In this case, additivity is a type of dosage
effect of a particular allele Each copy present
contributes a discrete amount to the phenotype
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Lets assign the variable CR to represent the
allele in snapdragons that codes for a functional
pigment, in this case, the color red
For every copy of CR present, we get one dose
of the product needed to create the red pigment
When both copies of this allele are present, we
get the red phenotype when one copy is present
we get the pink phenotype
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When there are no copies of the CR allele
present, the phenotype is white
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The characters of Mendels pea plants are called
discrete
For instance, the flower petals are either purple
or white, and the height of the plants are either
short or tall
These types of either-or characters are
actually quite uncommon Most traits are
quantitative in that they vary over a continuum
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Human height and weight are two examples of
continuous traits that vary along a continuum
Many of these kinds of characters are controlled
by the effect of many alleles of many genes
working together, and each of these alleles add a
discrete amount to the phenotype of the organism
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For instance, there is evidence that skin
pigmentation in humans is controlled by at least
three separately inherited genes, which we will
call A, B, and C Each of these genes has a
dark-pigment allele that contributes one unit
of pigment to the phenotype An AABBCC person
would be very dark, while an aabbcc person would
be very light
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What we get is a continuum of pigmentation in
humans that forms a bell-shaped curve We also
have to consider environmental effects (sunlight
in particular) that blur the line between the
different genotypes
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A simplified model for polygenic inheritance of
skin color
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Homework Pages 119 120 in your lab manual Do
questions 1 7