Updated: Jan 9, 2022
Author: Ellie Wang
Editors: Vincent Chang
Artist: Daisy Zheng
Gregor Johann Mendel is known as the father of genetics. In 1854, he developed the mathematical basis of genetics by cross-pollinating pea plants for a hybridization experiment at his monastery. These pea plants had seven traits that each consisted of two distinct forms: seed color (yellow or green), seed texture (smooth or wrinkled), plant height (tall or short), flower color (purple or white), flower position (axial or terminal), pod shape (inflated or constricted), and pod color (green or yellow). Of these traits, Mendel discovered that the pairs of forms each had a dominant and recessive trait due to the allele pairing of the genes linked to the traits’ inheritances.
An allele is one instance of the same gene on separate chromosomes of the same type since each parent contributes one copy of their DNA. In Mendel’s experiments, the type of genetic dominance was complete dominance, where the appearance of one dominant allele in an allele pairing will cause the dominant trait to appear. For recessive traits, both of the alleles need to be recessive. This explains why certain inherited traits will “skip” generations.
For an example of Mendel’s experiment, let’s cross-pollinate two pea plants and observe the heights. First, there’s a tall homozygous dominant pea plant, which has two dominant alleles represented by two uppercase letters–TT. Then, there is a short homozygous recessive pea plant, which has two recessive alleles represented by two lowercase letters—tt. These are homozygous plants because they only have one type of allele. This group will be the first generation, F0. The allele pairs separate into individual alleles–an occurrence known as the law of segregation–and pair up with the alleles of the other plant. Every possible combination produces only tall heterozygous plants, as the unique combination is Tt. These are heterozygous because they have one dominant and one recessive allele. If you take four of these plants for the second generation, or F1, and cross-pollinate them, you will get combinations of TT, Tt, Tt, and tt. Thus, you will theoretically get a 1:2:1 ratio of tall homozygous dominant to tall heterozygous to short homozygous recessive upon the third generation, F2, of breeding the descendants of a homozygous dominant organism with a homozygous recessive organism. Upon experimenting on the largest scale of its time, Mendel also discovered that all the traits were inherited separately from each other, which was then dubbed as the law of independent assortment.
These discoveries and many more by Mendel paved the way for others to expand the field of genetics. By laying the mathematical groundwork for the inheritance of DNA, countless discoveries have been made since then– all on this revolutionary foundation.
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