|Blending theory In ~1850, scientists thought
that some fluid substance in the blood of animals or in the sap of plants
was the hereditary material. The combination of the parent's characteristics
in the offspring was thought to occur by a "blending" of this fluid.
* If so, a white dog
that mated with a brown dog should produce only tan puppies;
* A tall person who
had a child with a short person should produce all "medium-size" children,
etc...clearly not the case!
* Even though people
recognized problems with this theory, it was the top theory of the day!
* Keep in mind, though,
that in the mid-1800s, very little was known about cell structure, let
alone the concepts of genes and DNA!
|A different theory was put forth by Gregor
Mendel in 1850. Mendel was an Austrian monk who was interested in plant
breeding. He performed careful experiments with the garden pea, Pisum sativum,
collected large amounts of data, and in doing so, was able to uncover the
basic principles of genetic inheritance that still hold true today!
Mendel discovered that traits are inherited
in discrete units (we now know these to be genes). Mendel's discoveries
were not understood by other scientists for over 35 years!"
Many of the RULES
of heredity (INHERITANCE) had been worked out by a monk named Gregor
Mendel. The rules are called "Mendelian genetics".
Mendel was fascinated
by the question "What is responsible for the variation in plants and animals?"
His monastery sent
him to the University of Vienna to get a teaching degree but Mendel basically
In order to EXPERIMENT
"under the abbot radar" he chose the garden pea.
For 7 years, 1856
- 1863, Mendel crossed and produced around 28,000 pea plants.
He selected seven sets of differences.
|He would plant seed
from both "sets", example: white and purple flowers
When the plants flowered
he would cover the flowers to prevent bee pollination and remove the male
parts of the flower (the stamens) of the seed plant to prevent self fertilization.
He would then pollinate by hand using a brush.
The matured seeds
from this cross are called the F1
He would plant the
F1 seeds, raise the plants and record flower color of all the plants.
Next he would remove
the stamens and hand cross pollinate the flowers from the F1 generation
to produce the F2 generation.
|Here is the cross
of the F1 generation.
It is not a "blend"
All the flowers are
|Here is the result
when two plants of the F1 generation are crossed to make the F2 generation. F1
3 plants have purple
1 plant has white
How is this explained?
|"Mendel did this
experiment with a total of 7 different traits, studying 22 strains of peas
and always using large sample sizes, and he always saw a ~3:1 ratio in
the F2 generation."
that these results were explainable if three things were true. He hypothesized
1. Every trait (like flower color, or seed shape, or seed color) is controlled
by two "heritable factors". [We know now that these are GENES
- we each have two copies of every gene].
2. If the two alleles differ, one is DOMINANT (will
be observed in the organisms appearance or physiology) and one is RECESSIVE
(cannot be observed unless the individual has two copies of
the recessive allele).
Dominant traits mask the appearance of recessive traits.
3. Alleles are randomly donated from parents to offspring - the factors
(ALLELES) separate when the GAMETES are formed by meiosis,
allowing all possible combinations of factors to occur in the gametes."
|This is how the
F1 cross is written in a "Punnett
Square" which is a "device" or method used to record the results.
PHENOTYPE VS GENOTYPE
|We are most aware ofphenotype
because we can see the differences in people, the differences in living
OTOH, the vast majority of genes are involved
in producing proteins that have no readily apparent appearance.
Genotype is all the genes an individual
has whether they can be "seen" or not.
Phenotype is what you see.
Genotype is what you get.
HETEROZYGOUS PAIRS - ALLELES
|The pairs of chromosomes inherited from
mom and pop typically contain DIFFERENT VARIATIONS in the genes.
This is called heterozygous.
These variations between the same gene
are called ALLELES, and the term used
to describe alternate forms of the same gene and is referred to
as ALLELIC VARIATION. Alleles occur when there is a difference in
the DNA sequence between pairs of genes. One example is sickle cell anemia
which is caused by a change of only one BASE PAIR. There may be 2, 3 or
more alleles or variation for any gene. Strictly speaking, alleles refer
to alternate forms of genes, but it is used when referring to alternate
forms of non-gene DNA too like in forensic DNA.
|The matching pairs of chromosomes contain
THE SAME VARIATIONS in the same gene. This is called homozygous
|This is what selection can do to an "original"
plant. In this case the selection was done by humans.