Notes

Every living thing plant or animal, microbe or human being has a set of characteristics inherited from its parent or parents. Since the beginning of recorded history, people have wanted to understand how that inheritance is passed from generation to generation. The delivery
of characteristics from parent to offspring is called heredity. The scientific study of heredity, known as genetics, is the key to understanding
what makes each organism unique.
The modern science of genetics was founded by an Austrian monk named Gregor Mendel. Mendel was born in 1822 in what is now the Czech Republic. After becoming a priest, Mendel spent several years studying science and mathematics at the University of Vienna. He spent the next 14 years working in a monastery and teaching high school. In addition to his teaching duties, Mendel was in charge of the monastery garden. In this simple garden, he was to do the work that changed biology forever.
Mendel carried out his work with ordinary garden peas, partly because peas are small and easy to grow. A single pea plant can produce
hundreds of offspring. Today we call peas a “model system.” Scientists use model systems because they are convenient to study and may tell
us how other organisms, including humans, actually function. By using peas, Mendel was able to carry out, in just one or two growing
seasons, experiments that would have been impossible to do with humans and that would have taken decades if not centuries to do
with pigs, horses, or other large animals.
When Mendel began his experiments, he knew that the male part of each flower makes pollen, which contains the plant’s male reproductive cells, called sperm. Similarly, Mendel knew that the female portion of each flower produces reproductive cells called eggs. During sexual reproduction, male and female reproductive cells join in a process known as fertilization to produce a new cell. In peas, this new cell develops into a tiny embryo encased within a seed.
Pea flowers are normally self-pollinating, which means that sperm cells fertilize egg cells from within the same flower. A plant grown
from a seed produced by self-pollination inherits all of its characteristics from the single plant that bore it; it has a single parent.
Mendel’s monastery garden had several stocks of pea plants. These plants were “true-breeding,” meaning that they were self-pollinating, and would produce offspring identical to themselves. In other words, the traits of each successive generation would be the same. A trait is a specific characteristic, such as seed color or plant height, of an individual. Many traits vary from one individual to another. For instance, one stock of Mendel’s seeds produced only tall plants, while another produced only short ones. One line produced only green seeds, another produced only yellow seeds.
To learn how these traits were determined, Mendel decided to “cross” his stocks of true-breeding plants that is, he caused one plant to reproduce with another plant. To do this, he had to prevent self-pollination. He did so by cutting away the pollen-bearing male parts of a flower. He then dusted the pollen from a different plant onto the female part of that flower. This process, known as cross-pollination, produces a plant that has two different parents. Cross-pollination allowed Mendel to breed plants with traits different from those of their parents and then study the results.
Mendel studied seven different traits of pea plants. Each of these seven traits had two contrasting characteristics, such as green seed
color or yellow seed color. Mendel crossed plants with each of the seven contrasting characteristics and then studied their offspring.
The offspring of crosses between parents with different traits are called hybrids.
When doing genetic crosses, we call each original pair of plants the P, or parental, generation. Their offspring are called
the F1, or first filial, generation. (Filius and ilia are the Latin words for “son” and “daughter.”)
What were Mendel’s F1 hybrid plants like? To his surprise, for each trait studied, all the offspring had the characteristics of only one of its parents. In each cross, the nature of the other parent, with regard to each trait, seemed to have disappeared. From these results, Mendel drew two conclusions. His first conclusion formed the basis of our current understanding of inheritance.
Today, scientists call the factors that are passed from parent to offspring genes. Each of the traits Mendel studied was controlled by a single gene that occurred in two contrasting varieties. These variations produced different expressions, or forms, of each trait. For example, the gene for plant height occurred in one form that produced tall plants and in another form that produced short plants. The different forms of a gene are called alleles.
Mendel’s second conclusion is called the principle of dominance. This principle states that some alleles are dominant and others are recessive. An organism with at least one dominant allele for a particular form of a trait will exhibit that form of the trait. An organism with a recessive allele for a particular form of a trait will exhibit that form only when the dominant allele for the trait is not present. In Mendel’s experiments, the allele for tall plants was dominant and the allele for short plants was recessive. Likewise, the allele for yellow seeds was dominant over the recessive allele for green seeds.
Mendel didn’t just stop after crossing the parent plants, because he had another question: Had the recessive alleles simply disappeared, or were they still present in the new plants? To find out, he allowed all seven kinds of F1 hybrids to self -pollinate. The offspring of an F1 cross are called the F2 (second filial) generation. In effect, Mendel crossed the F1 generation with itself to produce the F2 offspring.
When Mendel compared the F2 plants, he made a remarkable discovery: The traits controlled by the recessive alleles reappeared in the second generation. Roughly one fourth of the F2 plants showed the trait controlled by the recessive allele. Why, then, did the recessive alleles seem to disappear in the F1 generation, only to reappear in the F2 generation?
To begin with, Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation. However,
the trait controlled by the recessive allele did show up in some of the F2 plants. This reappearance indicated that, at some point, the allele for shortness had separated from the allele for tallness. How did this separation, or segregation, of alleles occur? Mendel suggested that the alleles for tallness and shortness in the F1 plants must have segregated from each other during the formation of the sex cells, or gametes Did that suggestion make sense?
Let’s assume, as Mendel might have, that all the F1 plants inherited an allele for tallness from the tall parent and one for shortness from the short parent. Because the allele for tallness is dominant, all the F1 plants are tall. Thus, each F1 plant produces two kinds of gametes those with the tall allele and those with the short allele. Alleles separate during gamete formation and then pair up again in the F2 generation. A capital letter represents a dominant allele. A lowercase letter represents a recessive allele. Now we can see why the recessive trait for height, t, reappeared in Mendel’s F2 generation. Each F1 plant in Mendel’s cross produced two kinds of gametes those with the allele for tallness and those with the allele for shortness. Whenever a gamete that carried the t allele paired with the other gamete that carried the t allele to produce an F2 plant, that plant was short. Every time one or both gametes of the pairing carried the T allele, a tall plant was produced. In other words, the F2 generation had new combinations of alleles.