Biol Spring 2023
Week 3 (1/31)
Mendelian Inheritance
I. An Introduction to Mendelian Genetics:
a. When Darwin published his work on the origin, diversity, and changing of species
through time, he acknowledged that he did not know the mechanism by which traits
were passed from parent to offspring. Although he knew they were passed. Practical
geneticists (i.e., farmers) had been demonstrating that for millennia!
b. In 1866 Gregor Mendel, an Austrian monk, biologist, mathematician, and gardener,
published a paper announcing his findings about pea plant inheritance patterns and a
couple of rules that made inheritance itself predictable. Mendel’s findings provided
an exact explanation of how traits are inherited, thus filling in the gaps of Darwin’s
work.
c. This link between evolution and inheritance largely went unnoticed by the scientific
community (and even by Mendel himself) for a few reasons. The first was that the
world was still reeling from Darwin’s book. Darwin’s message had far-reaching
impacts that were being dealt with on a scientific, religious, and social scale, the likes
of which the world had never seen. Second, Mendel’s work was mathematical in
nature, and even this relatively simple math (by today’s standards) was considered
too complex and out of place for most scientists who were interested in Biology.
Third, in the middle 1800’s information did not spread like it does today. Today you
can quickly do an internet search to get information on your point of interest. A web
search on Google for the word “inheritance patterns” today yields over 2 million hits.
A search for inheritance in BIOSIS (a common search engine for primary biological
literature) yields over 40,000 articles. The scientists of Mendel’s and Darwin’s time
did not have this kind of access to information. If a scientific discovery was not
Earth-shattering (like evolutionary theory), it went unnoticed by most.
d. Mendel’s work went unnoticed until around 1900 when 3 research teams, working
independently, formed the same conclusions that Mendel did. During their literature
searches, they discovered that Mendel had already published this information. The
three teams re-published Mendel’s work, using their own data as support to his
findings. Mendel finally got the credit he deserved. He had been dead for sixteen
years.
II. Before Mendel:
a. Practical genetics- For millennia, farmers knew that you could select certain
livestock/plants that provided some benefit, and breed them. The resulting offspring
would frequently provide the same benefits their parents did.
i.
Ex. Jacob’s herds in Genesis 29 and 30.
b. Blending- For over a century prior to Mendel’s work scientists believed that
offspring traits should be a blend of their parents.
i.
Ex. A black cow mated with a white bull should result in a gray calf (or at
least one with black and white patches in its fur).
c. Acquired characteristics- See previous notes about Lamarck.
III. Mendel’s Experiments:
a. Mendel’s 2 guiding questions:
i.
Why do offspring resemble parents?
ii. Why are most offspring not ‘blends’ of parents?
b. Mendel worked with pea plants (Pisum spp.). Pea plants are self-fertilizing if
undisturbed. Petals on pea flowers enclose the pistil and stamens, but can be pulled
apart to cross pollinate plants using an artist’s brush to transfer pollen from the anther
of one plant to the pistil of the other. Cutting off the anthers from the recipient plant
prevents self -pollination. Pea plants readily produce pure lines (homozygous
condition at each locus) as a result of self-pollination. This was known and exploited
by Mendel to produce pure lines of various traits, such as wrinkled and round peas.
c. Question: Are offspring blends of parents?:
i.
Prediction: If blending occurs, then a cross of pure-breeding wrinkled and
round peas (P generation) would produce all mildly wrinkled F1 offspring.
And self- fertilizing of the F1 offspring would produce all mildly wrinkled
offspring in the F2 generation.
ii. Results: All F1 peas were round and indistinguishable from those of the
round parent, discounting blending. F2 offspring had a ratio of 3:1 round to
wrinkled, further evidence that there was no blending.
iii. Interpretation 1: No blending. Mendel proposed that both the round and
wrinkled traits were present in the F1 generation and that the wrinkled trait
was not expressed in the F1 generation because the round trait dominated it.
Hence dominant and recessive traits.
iv.
Interpretation 2: Particles occur in pairs, and in order to occur in the 3:1 ratio
in F2, the particles must segregate into the gametes (egg and sperm) before
they leave the reproductive structures.
v.
Mendel’s Law of Segregation: Particles (alleles) separate, without dilution,
into gametes where each gamete contains one particle (allele) of each trait
(gene).
Monohybrid Punnett Square for a cross of Round (R) and Wrinkled (r) peas.
(Use this space to create your own Punnett Square map of the monohybrid cross.)
d. Question: Are different traits inherited together or separately?
Mendel chose 2 traits: seed color and shape. He knew that yellow is dominant over
green pea color and round is dominant over wrinkled. He crossed parents with both
traits. He took pure parental plants that produced round, yellow peas and crossed
with pure bred green, wrinkled peas.
i.
Prediction: If the 2 traits sorted dependently (ie, occurred together only), he
would get one set of results in F2 (round yellow OR wrinkled green). If they
assorted independently, he would get an entirely different result in the F2.
ii. Results: Round, yellow 315 (9/16), wrinkled, yellow 101 (3/16), Round,
green 108 (3/16), and wrinkled, green 32 (1/16).
iii. Interpretation: Particles (alleles) of one particular trait (gene) are passed on
to offspring independently of each other- Mendel’s Law of Independent
Assortment.
1. Only works for genes occurring on different chromosomes.
2. Remember, Mendel still didn’t know about chromosomes. He thought
about his “particles” as independent little bodies floating around in the
cells. Almost as if each gene had its own chromosome, not many,
many genes on a few chromosomes.
3. Those genes on the same chromosome we’ll talk about later.
Dihybrid Punnett Square for a cross of Round (R), Yellow (Y) and Wrinkled (r), Green (y) peas.
(Use this space to create your own Punnett Square map of the dihybrid cross.)
e. The Test Cross was used to determine the genotype of an individual showing
dominant traits. If an individual displays a dominant trait, like yellow peas, how do
you know whether it is homozygous dominant (having identical dominant alleles at
the locus), or heterozygous (having different alleles at the locus)? A Test Cross is
when this individual is crossed with a homozygous recessive individual. The
resulting offspring phenotype distribution tells you what the unknown parent is.