Download 2. Principles of Mendelian Genetics I

Bio 105/Summer 2013
2. Principles of Mendelian Genetics I
Assigned Reading: Chapter 2
7/24/13
Sections 2.1 p. 14
2.2 p. 19
2.3 p. 30
FAST FORWARD: Genes Encode proteins p. 20.
Suggested problems: Solved problems I, II, and III; 1, 7, 9, 10, 16, 18, 20, 22, 25, 26, 27,
28, 29, 30, 31
2.1
Background: The Historical Puzzle of Inheritance
p. 14
The cultivated pea, Pisum sativum, is a simultaneous hermaphrodite with bisexual flowers that is
self-fertile (no physiological self-incompatibility mechanism) and which is also capable of
facultative outcrossing. Hundreds of generations of self-fertilization led to the development of
many true-breeding, homozygous lines (or varieties) under cultivation.
Characters Studied by Mendel in Pisum
You are not responsible for memorizing the list of seven traits Mendel studied (Fig. 2.8) or for
knowing the allele symbols. It is interesting to note that Mendel worked most extensively with
the Seed Color and Seed Shape Characters because these are expressed in the embryonic plant
rather than in the adults. This allowed him to observe the result of a genetic cross at the end of
the summer, rather than waiting until the following year.
The physiological basis for the seed shape character is described in the "Fast Forward" section of
the text.
The product of the dominant Y allele inhibits the synthesis of chlorophyll in the embryo, so that
you see the yellow carotenoid pigments instead. In yy individuals chlorophyll synthesis masks the
yellow carotenoids, which are nonetheless present. The wild ancestors of the cultivated peas had
yellow seeds. (What good is chlorophyll underground?) Presumably green varieties have been
selected under cultivation to satisfy the preferences of consumers. Note that the situation is
reversed for color of the pod; here green is dominant to yellow.
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Bio 105/Summer 2013
2.2
2. Principles of Mendelian Genetics I
7/24/13
Genetic Analysis According to Mendel
p. 19
STANDARD MENDELIAN MONOHYBRID CROSS
STANDARD MENDELIAN DIHYBRID CROSS
LINE 1!
True-Breeding!
(Homozygous)!
!
A A B B!
a a b b!
A A b b!
aa Bb
Switch for RECIPROCAL Cross
LINE 1!
True-Breeding!
(Homozygous)!
!
AA
A
LINE 2!
True-Breeding!
(Homozygous)!
!
aa
a
LINE 2!
True-Breeding!
(Homozygous)!
!
a a b b!
A A B B!
a a B B!
AA bb
"CROSS"
"CROSS"
F 1!
"Hybrid"!
(Heterozygous)!
!
Aa
F 1!
"Hybrid"!
(Heterozygous)!
!
Aa Bb
"SELFED or INBRED"
"SELFED or INBRED"
F 2!
!
9:3:3:1
F 2!
!
3:1
TESTCROSS
UNKNOWN!
!
A A!
A a!
aa
KNOWN!
Homozygous Recessive!
!
aa
A
a
"CROSS"
Testcross Progeny!
!
The phenotypic ratio reveals the genotype!
of the "UNKNOWN" parent.
Be sure that you are comfortable with applying the "Product Rule" and the "Sum Rule" for
probability calculations.
2.3
Mendelian Inheritance in Humans
p. 30
The most vexing aspect of human pedigree analysis is that "sampling error" in small human
families often distorts Mendelian ratios. In Fig. 2.22 for example we see ratios of 1/4 in (a), 1/2
in (b), and 2/5 in (c), all for the offspring of three different CF+ CF X CF+ CF matings.
Another source of confusion is introduced by "exogamous" individuals (individuals from outside the
original lineage). To deal with these we adopt the simplifying assumption that the allele
responsible for the trait in question is RARE in the human population. Therefore we can assume
that exogamous individuals do not carry the allele UNLESS THERE IS EVIDENCE TO THE
CONTRARY. Fig 2.22 (a) provides a good example of this.
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Bio 105/Summer 2013
2. Principles of Mendelian Genetics I
FAST FORWARD: Genes Encode proteins
7/24/13
p. 20.
The R allele of SBE1 has been interrupted by insertion of a transposable element to create the
non-functional ("null") allele r. rr lines of pea plants have apparently been favored under
domestication by selection for the sweetness associated with higher sucrose content.
REFERENCES
1.
A Monk in the Garden
Robin Marantz Henig
Houghton Mifflin 2001.
A recent biography of Gregor Mendel written for a popular audience.
2.
Gregor Mendel's Experiments on Plant Hybrids
Alain F. Corcos And Floyd V. Monaghan
Rutgers University Press, New Brunswick, NJ, 1993.
A more technical historical analysis of Mendel’s experimental work. For a review of this
book see Science 261: 370.
3.
A wealth of information on Mendel's life and work, including an English translation of his
publication, is available at the "MendelWeb" site < http://www.mendelweb.org/>.
4.
Bhattacharyya, M. K. et al. (1990)
The Wrinkled-Seed Character of Pea Described by Mendel is Caused by a Transposon-like
Insertion in a Gene Encoding Starch-Branching Enzyme
Cell 60, 115.
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