The experiments of Gregor Mendel, an Austrian monk who lived in the 19th Century, were groundbreaking
for the study of heredity, and earned him the title, "Father of Modern Genetics." Remember,
heredity is the transmission of traits from parent to offspring, from generation to generation.
Be sure to take a look at this light-hearted video about Mendel by visiting the link at
the bottom of this slide. We will refer to some of the lyrics from that video during
this screencast. Mendel crossed peas with a variety of traits, such as seed and pod
form and color, flower color, and stem placement and size. He started all his crosses with
seeds that breed true, which we now know means that the parents were homozygous, or had the
identical alleles for the trait he was observing. We will focus on 2 of these traits: seed form
and seed color. In his experiments Mendel placed the pollen, or male gamete of the plant
on the top of the pistil, which contains the female gamete, by hand-pollinating the plants.
In the seed form experiment, Mendel crossed plants with round seeds with plants that had
wrinkled seeds. He saw that all the plants in the first generation had round seeds. He
called these plants "hybrids" because they were the offspring of two different varieties,
or characters. From this experiment, and the others he performed over 8 years, Mendel came
up with the Principle of Dominance. The dominant trait is the only one that shows
up in the first generation, when different true-breeding organisms reproduce. He reasoned
that if the round seed form is the only one present in the F1 generation, it must be dominant
over the wrinkled seed form. The recessive trait, the wrinkled seeds reappears in the next
generation, which we call the F2. The recessive trait is thus "masked" by the presence of the dominant
trait. But, recall from the song, that Mendel said his success was due "to counting and
my green thumb." So true! Mendel counted a lot of seeds! A total of 7,324 seeds in the
F2 generation. Of those, about three-quarters were round, and the rest were wrinkled. In
the video, Mendel sang, "and when they join together my forecasting's most impressive,
betcha 3 times out of 4 I'm right, unless they're both recessive." His ratio was 2.96
to one. Essentially 3 out 4 were round. He saw the reappearance
of the recessive trait in the F2 generation. So let's talk about Mendel's results using
current genetics lingo. Mendel did not know about chromosomes and the genes they contain,
so he used a different vocabulary. We know from our cell division class that
sexually reproducing organisms have chromosomes in pairs, one from the male parent and one
from the female parent. These are homologous chromosomes, and have
the same version, or allele, of each gene when an organism is homozygous for that trait.
If an organism has unlike alleles, it is heterozygous for that trait. Mendel stated his first law
as "Equal Segregation," from examining all of his data. He reasoned that parents pass
on only half of their genetic material, or factors, as he called them, to their offspring
through the gametes. Thus the pair of alleles segregate during meiosis and haploid gametes
are produced. Remember Mendel sang, "Here's the news they come in two's, they segregate,
it's up to fate, if an egg or a sperm has a trait that will dominate." This part of
the song refers to Equal Segregation! Mendel crossed smooth yellow seeds with wrinkled
green seeds in another experiment - now he was observing 2 traits at the same time in the
offspring: seed form and seed color. In the first generation, he observed that all the
seeds were smooth and yellow. You know now, of course, that this means that not only is
smooth dominant to wrinkled, but yellow is dominant to green. But in the second generation,
he observed four seed varieties: round yellow, round green, wrinkled yellow, and wrinkled
green, two of which were new. Most of the seeds were round yellow and the least were
wrinkled green. He observed about the same amount of round green and wrinkled yellow
seeds. The ratio, he concluded was 9:3:3:1. Mendel reasoned that the "character" for seed
color and seed form were independent of each other, so he came up with his second law,
the Law of Independent Assortment: Alleles for one gene separate into gametes independently
of alleles for other genes. That is why he observed the two new varieties: round green
and wrinkled yellow. Today, Mendel is known not only for the Principle of Dominance, but
also and his two laws: Segregation and Independent Assortment. Recall that laws are predictive.
Theories, however, help to explain what we observe in nature. Today, Mendel's Particulate
Theory of Inheritance, explains how traits are passed on from parents to offspring. Because
unlike alleles exist for many genes, we have variation in the offspring. But offspring
only inherit one allele of each gene from their parents because of meiosis. A parent
has a diploid set of chromosomes, but passes on a haploid set in their gamete. An allele
can represent a dominant or a recessive trait. These copies of each gene, or alleles, segregate
into different gametes. In the case of pea plants, these gametes are pollen and egg.
Each gamete fuses randomly with a gamete of the other parent. Mendel's work is the foundation
of Modern Genetics. Sadly, he was not recognized for his work during his lifetime. As we heard
in the song "Back then no one hurrahed Gregor Mendel but his God."