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.
