Question

When a tall homozygous plant was crossed with a short homozygous plant, the F1 offspring were all tall. No short plants were present in the F1 generation. Which principle does this situation represent?



The principle of unit characters



The principle of dominance



The principle of segregation

Answer 1

Gregor Mendel, an Austrian monk, revealed through numerous experiments with pea plants that offspring are simply not "blends" of their parents. Rather, he clearly demonstrated that traits tend be passed to offspring in a "particulate" fashion. Indeed, if the blending theory were true, then everyone would eventually look about the same. Who knows; perhaps the most favorable phenotypes would be similar to Rodney Dangerfield and Phyllis Diller.

Mendel decided to perform some crosses with his plants to test the blending theory. First, he crossed tall plants (i.e. long stemmed) which had parents and grandparents that had all been tall, with short plants (short stemmed) which had parents and grandparents that had all been short. The plants he crossed were termed the P (parental) generation. He found that the offspring, or F1 (filial) generation, were 100% tall. However, when he crossed these F1 plants together he found that the F2 generation represented a mixture: 3/4 were tall, and 1/4 were short. Whenever he performed such crosses, he always came out with these mathematical ratios. These mathematical ratios are very important because changes in ratios can sometimes be indicators that evolution is occurring.

How can this "particulate" nature of heredity be explained? Well, Mendel had a background in mathematics and through years of crossing and eating peas he came up with the following hypothesis: that there must be 2 factors (now called alleles) for each trait (now called genes), and that these factors (alleles) behave as distinct "particles" when passed to the offspring. Some of these traits are dominant (i.e. when present they are expressed), whereas others are recessive and their expression can be "masked" by dominant alleles. Offspring receive one of the factors (an allele) from one parent, and receive the other factor from the other parent.

Lets assign letters to each trait and perform some genetic crosses using a single gene. This is termed a monohybrid cross (simply a cross between two individuals where we look at a single gene). In pea plants, tall is dominant and is represented by a big T. Any time a big T is present, the plant is tall. When no big T is present, the plant will appear short. Since Mendel's tall parent plants were all true breeding (i.e. only produced tall plants) both of their factors must have been big so they are represented as "TT." If any small factors were present, they would have certainly have shown up at some time during his observations of the parent and grandparent plants. This "TT" is the genotype (i.e., the genetic makeup representing the 2 alleles that are present). Because the short plants were true breeding (i.e. all offspring from crosses yielded 100% short plants), their genotype must have been "tt." We can visually show such a cross between two parents by using a Punnett's square:

Answer 2

^^ you know what you call that? Plagiarism and Copying and pasting