1. Modified from a slide show by: Chad Caldwell for Brookings Biology
Section 15-1
POLYGENIC TRAITS
Modified from a slide show by: Chad Caldwell for Brookings Biology
Inquiry Activity

2. Section 15-1
So why are most lima beans, and other sexually-reproducing
organisms, medium-sized?

3. An individual who produces as much melanin
as humanly possible has genotype MMLLNN.
That person’s phenotype would be to have very
dark skin, so brown as to almost appear black.
MMLLNN
During meiosis, the diploid (2n) genotype, MMLLNN,
Is reduced to the haploid genotype. As the alleles are
segregated from each other, the only possibility for the
sperm or egg is MLN.
MLN
MLN

4. Section 15-1
The union of one genetic half-cell, the sperm, with the other genetic half-cell, the egg, results in one genetically whole cell, the zygote
MLN
sperm +
MLN
egg
MMLLNN
zygote
fertilization

5. Add the alleles from the sperm (MLN) and the alleles from the egg (MLN) to determine the genotype of the zygote (and the new individual)
MMLLNN
The genotype of all the offspring, (1 out of 1, or 1/1), is MMLLNN,
so all the offspring have the same phenotype—very dark skin.

6. An individual who produces as little melanin
as humanly possible has genotype mmllnn.
That person’s phenotype would be to have very
light skin, so as to appear so white that the pink
of the blood shows through.
mmllnn
During meiosis, the diploid (2n)
genotype, mmllnn, is reduced to the
haploid genotype. As the alleles are
segregated from each other, the
only possibility for the
sperm or egg is mln.
mln
mln

7. Add the alleles from the sperm (mln) and the alleles from the egg (mln) to determine the genotype of the zygote (and the new individual)
mmllnn
The genotype of all the offspring, (1 out of 1, or 1/1), is mmllnn, so all the offspring have the same phenotype —very light skin.

8. What happens if the very dark mate with the very light?
In both examples, when the very dark mate with the very darkand when the very light mate with the very light, the crosses result in no variation.
What happens if the very dark mate with the very light?

9. Add the alleles from the sperm (MLN) and the alleles from the egg (mln) to determine the genotype of the zygote
MnLlNn
The genotype of all the offspring, (1 out of 1, or 1/1), is MmLlNn, so all the offspring have the same phenotype —medium skin.

10. In all three examples, when the very extreme mate with the
very extreme, the crosses result in no variation.
What happens if the medium-skinned mate with the medium skinned?

11. What happens if the medium-skinned mate with the
During meiosis, the diploid (2n) genotype, MmLlNn
is reduced to the haploid genotype.
As the alleles
segregate and assort
independently, maximum
diversity results.
mln
MLN
MLn
mlN
MmLlNn
mLn
MlN
mLN
Mln

12. Add the alleles from the sperm (MLN) and the alleles from the egg (MLN) to determine the genotype of the zygote (and the new individual), (MMLLNN)

13. If you count up the number of alleles represented by capital letters, you get the number of alleles that code to make a person’s hair, eyes, or skin darker, because each allele represented by a capital letter causes skin cells to make more melanin (brown pigment)

14. The number of alleles coding for the cells to make melanin determine how genetically dark the individual’s hair, skin, or eyes will be. Count up the alleles and total them in the first square

15. The number of alleles coding for the cells to make melanin determine how genetically dark the individual’s hair, skin, or eyes will be. Count up the alleles and total them in the first square

16. Now, count up the alleles coding for melanin production and total them in each square of the Punnett square

17. Now, count up the alleles coding for melanin production and total them in each square of the Punnett square

18. Now, fill in the rest of the squares in the Punnett square
Now, fill in the rest of the squares in the Punnett square. Notice there is a pattern to the numbers.

19. Count how many sixes, fives, fours, threes, twos, ones, and zeroes, out of 64, and write the totals in the data table on the next slide.

20. Now, plot the data on the graph, with the numbers of alleles on the x-axis and the number of individuals on the y-axis. 1 6 15 20 15 6 1

21. Now, plot the data on the graph, with the numbers of alleles on the x-axis and the number of individuals on the y-axis.