端木·宇 2008-6-19 22:15
Mendel's Experiments
Gregor Mendel lived in an Austrian monastery and tended the monasterygarden. In 1865, through his observations of the garden pea plants thatgrew there, Mendel developed three basic principles that—althoughignored at the time by his scientific colleagues—would later become thefoundation for the new science of genetics.
Every pea plant contains both male andfemale reproductive parts and will normally reproduce throughself-pollination. Mendel noticed that the self-pollinating pea plantsin his garden were true breeding: they all produced offspring withcharacteristics identical to their own. Mendel looked at sevendifferent characteristics, or [b]traits[/b], that showed up in all ofthe plants. Each of these traits had two contrasting natures, only oneof which would show up in a given true-breeding plant. For example,plant height could be either short or tall: short, true-breeding plantswould only produce short offspring, and tall plants would only producetall offspring. At some point, Mendel wondered what would happen if hemanually mated these true-breeding plants with each other—would a tallplant mated with a short plant produce a tall, medium, or shortoffspring? Focusing on only one trait at a time, Mendelcross-pollinated plants with each of the seven contrasting traits andexamined their offspring. He called the original true-breeding parentsthe P (for parental) generation and called their first set of offspringthe F1 (for “first filial,” from the Latin word [i]filius,[/i] meaning son). The F1 offspring that result from two parents with different characteristics are also called [b]hybrids[/b].
[b] Law of Dominance[/b]
When Mendel crossed a purebred tall plant with a purebred short plant, all of the offspring in the first generation (the F1generation) were tall. The same thing happened with the other pairs ofcontrasting traits he studied: hybrid offspring in the first generationalways showed just one of the two forms.
Mendel used the word [b]dominant[/b] to describe the form that dominated the phenotype, or physical appearance, in the F1 generation. The other form he called [b]recessive[/b], because the characteristic receded into the background in the F1generation. Mendel was the first to realize that hereditary informationfor two different forms of a trait can coexist in a single individual,with one form masking the expression of the other form. This principle,referred to as the law of dominance, provided the basis for Mendel’ssubsequent work.
[b] Law of Segregation[/b]
Mendel discovered that mating a tall pea and a short pea would produce an F1generation of only tall pea plants. But, he wondered, were theseoffspring tall pea plants really identical to their tall parents, ormight they still contain some element of their short parents? To answerthis question, Mendel let all seven types of hybrid F1 generation plant self-pollinate, producing what he called the F2 (second filial) generation.
Lo and behold, in each F2 generation some of the recessive forms of the traits—which had visibly disappeared in the F1 generation—reappeared! Approximately one fourth of the F2plants exhibited the recessive characteristic, and three fourthscontinued to exhibit the dominant form of the trait, like their F1 parents. This 3:1 ratio of dominant to recessive remained consistent in all of the F2 offspring.
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Mendel came up with a simple but revolutionary explanation for the results he saw in the F2generation. He concluded that within an individual, hereditaryinformation came in paired units, with one unit derived from eachparent. Each simple physical trait, such as stem height, was determinedby the combined action of a single pair of units. Each unit could comein either a dominant form, which he denoted with a capital letter “A,”or a recessive form, which he denoted with a lowercase “a.” Two unitswith two possible forms gave four possible combinations: AA, Aa, aA,and aa; since Aa and aA were equivalent, there were really only threefunctional combinations. Because “A” is dominant over “a,” both AA andAa produced plants with the same physical characteristics. Only “aa”produced a plant that showed the recessive characteristic.
Mendel realized that the results he saw in the F2generation could only be explained if, during the formation ofreproductive cells, paired units are separated at random so that eachgamete contains only one of the two units. This postulate is now knownas the law of segregation.
