Professor Dave here, let’s talk about Mendelian genetics.
Through a study of chemistry, biochemistry, and biology, we can come to know a lot about
the molecular and cellular processes that generate and sustain human life.
But long before we even knew that cells and molecules existed, we were able to understand
the concept of heredity.
Children look like their parents.
Traits are passed on from generation to generation, be it hair color, skin tone, height, or anything
else.
For most of human history we had no idea how this happened, and even with the biology we
have just learned, it still isn’t entirely clear.
But a discussion of how traits are passed along from one organism to the next is a discussion
of genetics, and to do any justice to this field, we must go back to its inception, with
a man named Gregor Mendel.
Mendel grew up poor in an agricultural area of Europe, and entered an Augustinian monastery
at age 21.
After some time there, he left to study science in Vienna, and then returned to the monastery
eager to begin his research.
In Mendel’s time, it was thought that heredity had to do with a blending of attributes, like
the way two colors mix to give an intermediate color.
But this did not account for the way that traits can skip a generation.
Thus it seemed to Mendel that heredity must be based on discrete units that can be inherited,
which came to be referred to as genes.
In the gene theory, each parent has a set of genes, and these are retained intact in
the offspring.
Mendel worked for several decades with pea plants in the garden of his monastery to produce
an impressive body of data to support the gene theory, all of which was done decades
before chromosomes were observed and understood.
We will discuss the methods Mendel used to breed the pea plants, and the conclusions
he was able to derive from them.
First of all, why peas?
Well, the pea plant has a number of visible traits that can vary, so results are easy
to observe.
These are things like flower color, seed color and shape, pod color and shape, flower position,
and stem length.
Generations of the pea plant are short and the offspring are many, so data is easy to
gather.
And Mendel could easily control mating by removing the stamens, the male fertilizing
organs that produce pollen, from a particular set of plants.
Then he could deliberately cross-fertilize with pollen from other plants.
This means he had complete control over which plants were mating with which other plants.
Some plants were true-breeding with respect to a particular trait, meaning that some plants
with purple flowers, when self-fertilizing, gave generation after generation of entirely
purple flowering plants.
The same goes for the plants with white flowers.
So this meant he could breed true-breeding white flowering plants with true-breeding
purple flowering plants and see what happens.
This kind of process is called hybridization, and we can call the two true-breeding plants
the parental generation, while the offspring will be referred to as the F1 generation,
or first filial generation, from the Latin word for son.
If these F1 plants are allowed to reproduce further, we get the F2, or second filial generation.
Mendel did experiment after experiment with different kinds of pea plants, following the
traits expressed, or phenotypes, up to the F2 generation.
From this mountain of data, he developed two fundamental laws of genetics.
These are the law of segregation and the law of independent assortment.
To see how these laws were derived, let’s examine one of his experiments.
Let’s say purple flowering and white flowering plants are hybridized.
We would find that the F1 generation is entirely purple.
No white, no pink, just all purple.
This experiment alone proves that any kind of blending hypothesis must be false.
But even more perplexing is that if the F1 generation is allowed to self-pollinate, the
F2 generation gives plants that are both purple and white, in roughly a 3 to 1 ratio.
This strongly supports what we now refer to as the gene theory, because it means that
somewhere in the F1 plants there must have been some kind of information pertaining to
white flowers that was not lost, and was expressed later with the F2 plants.
This white flowering gene must simply have been hidden or silenced in the F1 plants.
We can therefore refer to purple flowers as the dominant trait, and white flowers as the
recessive trait.
This combination of dominant and recessive traits was observed for many other aspects
of the plant, and precisely the same 3 to 1 ratio was observed when examining the phenotypes
in the F2 generation for smooth vs. wrinkled seeds, yellow vs. green seeds, green vs. yellow
pods, and so forth.
The reason for this is that each trait corresponds to a particular gene, like a gene for flower
color.
But each gene has two versions, or alleles, in the genome, because the gene is found on
two homologous chromosomes, one from each parent.
These two alleles can be of a different genotype, meaning that the sequences of nucleotides
are different, and this can lead to different phenotypes when expressed, like the two different
colors that are visible on the flower.
Although Mendel didn’t know anything about DNA, he still was able to decipher this concept
of two alleles per trait.
True-breeding plants must have identical alleles for a particular trait, but when he performed
the hybridization, he realized that one allele was dominant, in that it was expressed, while
the other was recessive, and stayed dormant.
In this case, when purple and white are hybridized, the F1 generation is all purple, so purple
must be dominant.
He even realized that each gamete must have only one of these alleles such that when each
plant is fertilized, one of any of the possible combinations of alleles will result.
This is the law of segregation.
We now understand this is true because we have learned about meiosis and the haploid
daughter cells that result.
In this way, Mendel was able to explain the phenotypes in the F2 generation by rationalizing
that true-breeding purples have two purple alleles, and true-breeding whites have two
white alleles, so when the sperm from one fertilizes the egg from the other, all of
the resulting plants in the F1 generation must have one of each allele.
Because purple is dominant, they are all purple.
But when the F1 generation produces its own gametes, some will be purple and some will
be white, so when they self-fertilize, there are four possible combinations, and those
are purple and purple, purple and white, white and purple, or white and white.
Since purple is present in three of those, that explains the three to one ratio, given
that only a plant with two recessive white alleles will appear white.
This kind of logic can be displayed using Punnett squares.
For these, we make a grid, and along the top boxes we place the alleles for one plant.
If we are allowing the F1 generation to reproduce, all of those have one purple allele, which
we can symbolize with a capital P, since we capitalize the dominant allele, and the white
allele will be a lower case p, as it is recessive.
The other plant will have its alleles to the left of the boxes.
Then, we just populate the boxes to form all the possible pairing combinations.
Dominant alleles are listed first, and there is the distribution just as we described before,
with a three to one ratio of purple to white in the F2 generation.
When an organism has identical alleles for a particular gene it is said to be homozygous
for that gene.
If it has one of each allele, we call it heterozygous.
As we can see, homozygous dominant and heterozygous both result in the dominant phenotype.
Only homozygous recessive results in the recessive phenotype being visible in the plant.
For this reason, we must understand that observing the phenotype that is visible for a particular
trait in an organism does not tell us with certainty what genotype is present, as multiple
genotypes result in the same phenotype.
However, we can breed this organism with another organism that has a known genotype for that
trait and analyze the phenotypic distribution in the resulting generation to decipher what
the unknown genotype must be.
When Mendel discovered the law of segregation, he was looking at one trait at a time, which
we would call a monohybrid cross.
But when he started looking at two traits at the same time, he discovered the law of
independent assortment.
He knew that the seeds of the pea plant could be either yellow or green, with yellow being
dominant, and they could be either or round or wrinkled, with round being dominant.
He took true-breeding plants with round yellow seeds and crossed them with true-breeding
plants with wrinkled green seeds, performing a dihybrid cross.
As we would expect, all the plants in the F1 generation end up heterozygous for both
traits, thus exhibiting the dominant phenotype for both traits.
But because we are examining two traits at once, there are now four possible combinations
of alleles produced in the gametes of the F1 plants.
If these plants reproduce, we end up with a variety of possible genotypes for the F2
plants, because the Punnett square must involve all possible combinations of all possible
gametes from two different plants.
The phenotypic distribution for a dihybrid cross will always be 9 to 3 to 3 to 1, where
the 9 represents the proportion of the F2 generation that will exhibit the dominant
phenotype for both traits, 3 represents the proportion that will show dominant for one
and recessive for the other, the other 3 is for the reverse situation, and the 1 is for
the ones that will be recessive for both.
These results are significant, because they show that each trait is determined individually.
Just because the F1 generation was heterozygous, it doesn’t mean that the F2 will be.
The alleles for each trait separate at random during gamete formation, thus demonstrating
the law of independent assortment, and also demonstrating that the combination of alleles
that occur in any given organism is determined entirely by probability.
Sixteen possible combinations, one in sixteen chance for each one, so roughly one in every
sixteen plants will exhibit a particular genotype.
Because of this mathematical reliability, we can use the rules of probability to predict
phenotypic distributions for more complicated combinations.
Not all patterns of inheritance are this simple, unfortunately, and as impressive as Mendel’s
experiments were, he could not explain certain observations.
But since his time we have extended Mendelian genetics to explain these observations as
well, as we now understand that certain alleles are not completely dominant or recessive,
some genes have more than two alleles, and a single gene can sometimes produce multiple
phenotypes.
For example, there are degrees of dominance.
With everything so far, we have been discussing complete dominance, where the dominant allele
is expressed whether the genotype is homozygous or heterozygous, and there is no difference
in the resulting phenotype.
But there is also incomplete dominance.
When red snapdragons are crossed with white ones, the F1 generation is all pink.
This is a third, intermediate phenotype.
But this does not mean that the red and white alleles are gone.
If we look at the F2 generation, we get a 1 to 2 to 1 ratio of red to pink to white.
The alleles maintain their identities.
Codominance is also possible, where two different phenotypes are both expressed simultaneously.
But this more advanced understanding of genetics required knowledge about chromosomes that
Mendel never had, so let’s learn about chromosomes.