Our Botany Block offered the perfect opportunity for an introduction to genetics with the work of Friar Gregor Mendel. Mendel is the person that bred several generations of pea plants to discover the basic understanding of genetics. In this book Gregor Mendel: The Friar Who Grew Peas, the story of Gregor Mendel is told and a basic understanding of genes and how they work is presented.
We worked through this introduction in three days. The first day, we read the book and went over the information about the pea plants and Mendel’s discoveries. On the second day, we drew a simplified version of a Punnett’s square. This square charts the information in a visual way. The third day we worked on a modified project from Book 3 of E.O. Wilson’s Life on Earth Series, a high school biology text, available for free on Apple. (I highly recommend it.)
Punnett Square
For the Punnett Square we drew both the first and second generation, but instead of calling them ‘F1’ and ‘F2’ as traditional genetics does, we labeled them ‘children’ and ‘grandchildren.’ As the book reminded us, Mendel cross bred pea upon pea until he arrived at a ‘pure’ breed, meaning both alleles of the gene were the same. For instance, in our case, both alleles for the gene for pea color were either both the dominant yellow (y,y) or both the recessive green (g,g).
Our first square was a crossing from the ‘pure’ parents with one parent offering two dominant yellow alleles in their gene, and the other parent offering two recessive green alleles in their gene. We discussed how this was similar to area model multiplication, and then marked each of the four squares with their corresponding allele cross from each parent. As expected, and a little unexcitingly, all the children had both one yellow allele and one green allele, giving us with 4 children that all presented as yellow peas.
The second square was a cross of two of the children. So each of the parents offered on dominant yellow allele and one recessive green allele. When we ‘multiplied’ this group, we had at a little more varied results. We found that one of the ‘grandchildren’ had two yellow alleles (yy), two of the grandchildren had a mix of alleles with one yellow and one green (yg), and one grandchild had two green alleles (gg). This resulted in three of the grandchildren being yellow, as yellow was dominant. Only one of the grandchildren was green. The ratios of alleles were 1/4 yy, 1/2 yg, 1/4 gg. They expressed this as 3/4 being yellow and 1/4 being green.
Genetics Demonstration
The following day, we did our adaptation of E.O. Wilson’s genetic project by simplifying to one trait. Again, we crossed the parents, and then the children to go through two generations. To represent the alleles, we used beans. Red beans represented the recessive allele, and the modeled one was determined to be dominant. We limited ourselves to 20 children each time.
The parents were again ‘pure’ bred, so I took 20 red beans, and she took 20 what we were calling ‘white’ beans, and we placed them into our prospective cups. From here we each pulled one ‘allele’ and matched them to give that ‘individual’ their genes. We repeated this until all 20 of the beans were used. As expected, all 20 of our individuals were mixed alleles with one red and one white, expressing as white.
For the second generation, she took the first half of the pairs, and I took the remaining 10 pairs, placing our beans in our prospective cups. Again, one by one, we pulled a bean out of our cups at random and paired them. We noted the paired alleles, and how they would be expressed. Then we sorted them into their similar groups.
When we were finished, we counted how many we had of each combination. We had 5 pairs of two white beans, 5 pairs of two red beans, and 10 pairs of a mixed combination. When we looked at the fraction of each group, we found the 5/20 or 1/4 of the group had two white alleles, 5/20 or 1/4 of the group had two red alleles, and 10/20 or 1/2 of the group had a mix of the two alleles. We then followed up with noticing that our results were the same as the Punnett square showed.
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