Introduction to Genetics - Mendel
Study the picture of the mother, father and their daughter.
Do you notice any resemblance?
What about differences?
What do you think might cause a child to resemble his/her parents?
What might cause a child not to resemble his/her parents?
Think about your family - what physical traits do you and your family members share?
Questions like the ones above have been on the minds of humans for many generations. It was in 1856 when a man named Gregor Mendel began to investigate these happenings. He didn't study humans, he studied peas. Studying humans can be complicated. Peas, Mendel noticed, have seven very clear traits. In peas, the flowers are either purple or white. The stems are either shorter than 8 inches or taller than 20 inches. The peas are either round of wrinkled. See below.
Mendel noticed these variations and wondered if there was any rhyme or reason as to why the plants displayed these traits. He decided to investigate the appearance of traits by cross-pollinating tall pea plants with short pea plants. He made sure to use pollen from tall plants that had come from a generational line of tall plants. These, he considered purebred tall. For his data chart, he used the symbol, T to mean tall. His short pea plants also came from a generational line of short plants and were considered purebred short. For his data chart, he used the symbol, t to mean short.
Mendel's 1st Cross
Mendel cross-pollinated purebred tall pea plants with purebred
short pea plants.
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What would you expect from this cross?
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Mendel was surprised, and you may be too, to find that all of the offspring pea plants were tall!
Mendel wondered why!
He knew that each of these tall pea plants had one purebred tall parent and one purebred short parent,
but the short "factor" as he called it, was completely hidden.
The purebred tall plants, have a tall mother (T) and a tall father (T), so the purebred tall plants are symbolized as TT since they have one factor each from mother and father. The purebred short plants were symbolized as tt.
TT x tt = all tall plants
Since the offspring have one tall factor and one short they are symbolized as Tt = tall.
Mendel's 2nd Cross
He decided to do one more cross.
He cross-pollinated the tall pea plants that he knew were hybrid because they each had one tall parent and one short parent.
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What do you think happened?
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To Mendel's surprise, and possibly yours too, for every 3 tall plants there was
1 short pea plant.
In other words, 3/4 of the pea plants were tall and 1/4 were short.
The tall pea plants with one tall parent (hybrid) and one short parent were symbolized as Tt.
Tt x Tt = 3/4 tall and 1/4 short
- Purebred - an organism having parents with the same genes for a trait. Another term for purebred is homozygous. Symbolized by two letters of the same case - TT or tt, etc.
- Factors - Mendel used the term factor, but we will call these genes or alleles.
- Hybrid - an organism having parents with different genes or alleles for a trait. Another term for hybrid is heterozygous.
Part 2: Punnet Squares
Mendel did not know it, but his investigations, his observations, and conclusions laid the ground work for the study of genetics. Mendel is now know as the Father of Genetics. A few years following Mendel's work, an English scientist, Reginald Punnett, developed a way to sketch out a genetic cross and make predictions. Please continue reading to learn about Punnett Squares.
American scientist, Thomas Hunt Morgan, set out to study how species changed over many generations. For his studies he chose to use Drosophila melanogaster, fruit flies. Fruit flies, like pea plants, have a few easy to observe traits, but an added benefit is that they reproduce very quickly. In about a week, fruit flies grow from egg to adult, so he could study many generations very quickly. While he confirmed some of Mendel's findings, he learned some new information as well. It was a