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In this lesson, you will review Gregor Mendel’s work on the inheritance of traits. Scientists, regardless of their specialization, need to possess a basic level of science literacy across all disciplines. This lesson provides an overview of the major concepts pertaining to Mendelian genetics that all students of the sciences should know. The chapters that follow will focus in greater detail on these ideas.
Although Gregor Mendel knew nothing of DNA, mutations, or chromosomes when he did his work during the mid-1800s, he did observe the variation in organisms around him. He recognized that variation was the result of characteristics passed from parents to offspring. Humans had been practicing artificial selection in plants and animals for many years, selectively breeding for desirable traits, but no one knew how traits were passed. Mendel set out to ascertain how traits were passed from parent to offspring by experimenting with pea plants in his garden.Mendel worked with garden peas. Pea plants were a great organism to work with for several reasons. They usually self pollinate, produce many offspring quickly, are easy to cross pollinate, and have traits that could clearly be distinguished in two distinct forms: dominant and recessive. So Mendel was able to work with true breeding strains and had plenty of numbers to crunch.
Which definition is correct for “true breeding” in classical genetics?
If you chose B, you are correct. Choice A describes independent assortment, choice C describes alternation of generations, and choice D describes natural selection.
Mendel started his experiments by cross pollinating true breeding plants, which he called the P generation. In the offspring produced (F1 generation) he found that only one form of each trait was visible. He called it the dominant form. The form that did not appear in the F1 generation he called the recessive form. Mendel was careful to rule out factors, such as traits being carried only on pollen or in ovules. For example, all F1 seeds, whether they developed from white or purple flowers, produced plants with purple flowers.
P Generation (true breeding parents)
Purple X White —> All offspring had purple flowers.
Purple is the dominant form, white is recessive.
Mendel then allowed the F1 generation to self pollinate. He found that 75% of their offspring, termed the F2 generation, showed the dominant form of the trait. The other 25% showed the recessive form, which was not observed in the F1 generation!
F1 Generation (self pollination)
Purple X Purple —> 3/4 offspring were purple, 1/4 were white.
Recessive form reappeared.
Mendel carefully counted numbers of offspring in each generation, and observed that a pattern began to emerge. By analyzing the numerical patterns he observed, Mendel drew the conclusion that the traits seen in the pea plants were carried on certain factors (now call genes), and that offspring got one copy of a factor from each parent. Even though one copy of the factor showed dominance in the F1 generation, Mendel saw that the other was still present and could be passed to the next generation. These findings were confirmed in subsequent generations, and the ratios predicted by the one factor per parent transfer of information were confirmed.
F2 Generation (self pollination)
1/3 of purple flowers were true breeding.
2/3 of purple flowers produced 3/4 purple and 1/4 white offspring.
All white flowers were true breeding.
By conducting dihybrid crosses, Mendel also established the process of independent assortment, even though scientists knew nothing about meiosis at the time. One of the things that Mendel noted in his experiments was that flower color was passed from one generation to the next independently of pea color and both were passed independently of pea shape. Flower position was passed independently of flower color. Once again, by looking at the numbers, Mendel observed a pattern. This pattern is demonstrated with the Punnett Square, which was developed by Reginald Punnett during the early 1900s.
This incomplete Punnett Square shows the possible trait combinations of an F1 generation. What was the dihybrid cross?
| ?? | ?? | ?? | ?? | |
| ?? | TTPP | TTPp | TtPP | TtPp |
| ?? | TTPp | TTpp | TtPp | Ttpp |
| ?? | TtPP | TtPp | ttPP | ttPp |
| ?? | TtPp | Ttpp | ttPp | ttpp |
The correct answer is B. Understanding the meaning of dihybrid cross (e.g., AaBb x AaBb) would prevent having to look at the Punnett Square. The other choices are not dihybrid crosses and would result in different combination ratios.
Perhaps Mendel’s greatest contribution to science was the use of quantitative analysis to draw conclusions from his data. By using pea plants as a model system and observing numerical patterns, Mendel was able to infer a lot about inheritance without the benefit of knowledge about chromosomes and meiosis. Scientists studying genetics after Mendel have often used fruit flies (Drosophila sp.) as a model organism because they also produce many offspring quickly. Scientists look at numerical patterns to decide whether observations are significant and meaningful or if they are due to random chance.
During his lifetime, Mendel’s work was ignored. It wasn’t until after cell division had been observed that Walter Sutton reviewed and incorporated Mendel’s patterns of inheritance into his chromosome theory. Yet, we now know that not all of Mendel’s rules apply all the time. Genes that are close to one another on the same chromosome are called linked genes, because they are usually passed together. Sex-linked genes show distinct patterns of inheritance. Not all genes show complete dominance, and in some cases more than two versions of a gene may exist.
From Mendel and his peas to Dolly the sheep and cloning, genetics has come a long way.
What was Mendel’s greatest contribution to genetics?
Hope you answered B… all that other great stuff came from scientists after Mendel!