POLYGENIC TRAITS and the Bell curve Modified from

POLYGENIC TRAITS and the Bell curve Modified from a slide show by: Chad Caldwell for Brookings Biology

Section 15 -1

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

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 MLN During meiosis the 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.

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 + egg MLN fertilization MMLLNN zygote

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.

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 mln During meiosis, the diploid 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.

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.

In both examples, when the very dark mate with the very dark and 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?

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

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?

What happens if the medium-skinned mate with the medium skinned? MLN mln MLn ml. N Mm. Ll. Nn Ml. N m. Ln m. LN Mln During meiosis, the diploid (2 n) genotype, Mm. Ll. Nn is reduced to the haploid genotype. As the alleles segregate and assort independently, maximum diversity results.

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)

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)

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 each square.

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

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

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.

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

• PRINT NEXT TWO SLIDES AS HANDOUTS

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.

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)