Mendel's Genetics
For thousands of years farmers and herders have been selectively breeding their plants and
animals to produce desired traits. Selective breeding in the 1800s was somewhat of a hit or miss
process since the actual mechanisms governing inheritance were unknown. Knowledge of these
genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out
over the last century and a half.
By the 1890's, the invention of better microscopes allowed biologists to discover the basic facts of cell
division and sexual reproduction (chromosomes). The focus of genetics research then shifted to
understanding what really happens in the transmission of hereditary traits from parents to children.
A number of hypotheses were suggested to explain heredity, but Gregor Mendel, a little known
Central European monk, was the only one who got it more or less right. His ideas had been
published in 1866 but largely went unrecognized until 1900, which was long after his death. His early
adult life was spent in relative obscurity doing basic genetics research and teaching high school
mathematics, physics, and Greek in Brno (now in the Czech Republic). In his later years, he became
the abbot of his monastery and put aside his scientific work.
While Mendel's research was with plants, the basic underlying principles of heredity that Mendel
discovered also apply to people and other animals because the mechanisms of heredity are
essentially the same for all complex life forms.
Mendel’s Traits
The traits that were the focus on Mendel’s studies followed certain rules. While not all traits follow
these rules, it still provided our basis of understanding how heredity works.
RULE 1: While selectively breeding his pea plants, Mendel observed many offspring whose traits
were not a blend of the parents’ traits. This means that all of the traits that Mendel used were traits
where the effects of one allele completely overpowered the effects of the other allele. For instance,
the offspring of pea plants with either purple or white flower had flowers that were either purple or
white. None of the offspring had light purple flowers.
RULE 2: Mendel observed seven traits that are easily recognized and apparently only occur in one of
two forms. In other words, each gene had a maximum of two alleles:
1.
2.
3.
4.
flower color is purple or white
flower position is axial or terminal
stem length is long or short
seed shape is round or wrinkled
5. seed color is yellow or green
6. pod shape is inflated or constricted
7. pod color is yellow or green
WHY IS SINGLE GENE, TWO-FORMS IMPORTANT?
This observation that these traits do not show up in offspring plants with intermediate forms was
critically important because the leading theory in biology at the time was that inherited traits blend
from generation to generation. Most of the leading scientists in the 19th century accepted this
"blending theory" including Charles Darwin and Jean Baptiste Lamarck, both contributors to our early
understanding of evolution.
WHY PEAS?
Mendel picked common garden pea plants for the focus of his research because they can be grown
easily in large numbers and their reproduction can be manipulated. Pea plants have both male and
female reproductive organs. As a result, they can either self-pollinate themselves or crosspollinate with another plant. In his experiments, Mendel was able to selectively cross-pollinate
purebred plants with particular traits and observe the outcome over many generations. This meant
having a collection of flowers with pistils, but no stamens (stamens were removed) and another
collection with stamens and no pistils. This way he avoided self-pollination. To ensure pollen transfer,
Mendel used a paint brush and manually moved pollen from stamen to pistil and then harvested the
seeds.
Reproductive
structures of
flowers
CONSISTENT RATIOS (and REPEATABILITY)  ID MECHANISM OF INHERITANCE
Mendel established a group of true breeding plants for each trait. A true breeding plant is one that,
when self-pollinated, creates only offspring with the same trait as the parent. Mendel found that,
when a true breeding green seed plant was cross-pollinated with a true breeding yellow seed plant,
the first generation of offspring (f1) always has yellow seeds. However, the following generation (f2)
consistently has a 3:1 ratio of yellow to green.
This 3:1 ratio occurs in later generations as well. Mendel realized that this underlying regularity was
the key to understanding the basic mechanisms of inheritance.
He came to three important conclusions from these experimental results:
1. that the inheritance of each trait is determined by "factors" that are
passed on to descendents unchanged
2. that an individual inherits one such factor from each parent for each trait
3. that a trait may not show up in an individual but can still be passed on to
the next generation.
WHAT DID MENDEL CONCLUDE?
Mendel's observations from these experiments were summarized in three principles or laws.
1. Principle of segregation: for any particular trait, the pair of alleles that each parent has for a
particular gene separates and only one allele is passed from each parent to an
offspring. Which of the two alleles in each parent is passed on is a matter of chance.
2. Principle of independent assortment: alleles from different genes are passed to offspring
independently of each other. The result is that new combinations of genes present in neither
parent are possible. For example, if a purple flowered, tall pea plant is crossed with a white
flowered, short pea plant, the flower color inherited by the offspring has no impact on the
height that the offspring inherits.
3. Principle of complete dominance: when two alleles are combined, the effect of one will
always over power the effect of the other. The “stronger” allele is referred to as dominant while
the masked allele is referred to as recessive.
From Mendel’s studies he established these three rules which, still today, provided our basis of
understanding regarding mechanisms of heredity. HOWEVER, limitations in Mendel’s work and the
choice of traits he examined prevented him from seeing some subtleties, or exceptions, to these rules
that subsequent research exposed.
What we’ve learned since:
1. Principle of segregation: This one still hold true, but we have a greater understanding of
how and why it happens. Now that we know the detailed steps of meiosis, we understand
how each pair of homologous chromosomes is separated during the process and each
gamete ends up with only one of each pair. That means that each gamete is getting only
one allele for each gene. The pair of alleles we inherit (abbreviated by letters) is called our
genotype.
2. Principle of independent assortment: While the discovery of meiosis also helped shed
light on how this works, we also discovered some instances where gene pairs DO tend to
travel together. For example: Mom has a chromosome that has the allele for both blonde
hair AND the allele for blue eyes. If this chromosome is the one that gets passed on, then
the offspring will likely inherit these alleles as a pair. In this case, hair color and eye color
are NOT independent of each other, because they are physically attached on the same
chromosome. Genes on the same chromosome are said to be linked.
Notice in the image above how A&B always travel together and a&b always travel together. We never see
A&b or a&B. They are NOT independently assorting because they are linked. D, however, can be paired with
either A&B or a&b and d can be paired with either A&B or a&b. D IS independently assorting with each of the
other two genes.
More research revealed that, while less likely, it is still possible to separate linked genes and pass on
only one. (ex: pass on the blonde hair but NOT the blue eyes) Once again it comes back to meiosis.
When homologous chromosomes pair up in prophase I, they swap some genes in crossing over. So
even though blonde/blue might have been physically attached in the original cell, it is possible for a
swap to happen and for the blue to get swapped with the brown from the other chromosomes creating
a recombinant chromosome with a new combination of alleles. That would lead to the independent
assortment effect that Mendel saw.
The likelihood of linked genes to be passed on together or separate during crossing over is related to
the physical distance between the genes on the chromosome. The closer the two genes are, the
more likely it is that they will be passed on together.
3. The principle of complete dominance still holds true as well, BUT we now know that not
every gene follows this inheritance pattern. While, yes, SOME genes have a pair of
alleles one of which over powers the other, other genes have alleles that create a blending
effect. When the effects of two alleles blend together to create a phenotype that is
intermediate to either allele by itself this is called incomplete dominance. Still other
genes have alleles that, when both present, are both visible. They don’t blend and one
doesn’t overpower the other. Instead we clearly see the phenotype (physical effect) of
each allele at the same time. This type of inheritance pattern is called co-dominance.
Directions: Complete the following on a separate piece of lined paper. ALL answers must be in your
own words. Credit will not be given for any passages, sentences or phrases copied from this, or any
other, reading.
1. The vocabulary in this reading is grouped into terms with related (but not identical) meanings. For each
group, write a few sentences (1-3) that not only tells us what each means, but uses them in a way to
show how the terms are related.
a. Selective breeding, phenotypes,
b. Heredity, Genetics
c. Inheritance pattern, co-dominance, incomplete dominance, complete dominance
d. True breeding, self-pollination, cross-pollination
e. Independent assortment, linked genes, recombinant chromosomes
2. Considering what you know about sexual and asexual reproduction, is self-pollination an example of
sexual or asexual? Explain citing events that happen during meiosis to support your argument.
3. For this question you will pretend that you are Mendel doing your experiment. For one trait of your
choice you will write a procedure for how he did his experiment. Be explicit and detailed.
a. What trait did you pick?
b. What is the independent variable?
c. What is the dependent variable?
d. What was Mendel’s hypothesis?
e. Write a procedure for the experiment done to evaluate this trait.
f.
Summarize the data Mendel would have gotten.
g. Did his data support his hypothesis? Explain.
4. Use the chromosome models to demonstrate examples of each
a. Demonstrate the stages of meiosis that lead to segregation.
b. Which alleles are linked?
c. Which linked alleles are MOST likely to assort independently despite being linked? Why?
(demonstrate with your chromosomes)
d. Show a gamete in which independent assortment happened only with unlinked genes
e. Show a gamete in which independent assortment happened in unlinked genes and genes that
are far apart
f.
Show a gamete in which segregation failed. What name do we have for this?
5. Examine the gene information below and then answer the questions:
Characteristic
Allele 1
Allele 2
Inheritance pattern
Flower color
White (W)
Red (w)
Co-dominance
Height
Tall (T)
Short (t)
Incomplete dom.
Leaf shape
Round (R)
Oval (r)
Complete dom.
a. What genotype(s) would give you a plant with round leaves?
b. If a plant inherits a W allele and a w allele, what will its phenotype be?
c. What alleles would a plant have to inherit to be solid white, medium height and oval leaves?
d. How do we determine the inheritance pattern of a gene? (What allele combination would we have to
look at?