Mendel came up with one more important idea in genetics: the concept of independent assortment.

Independent Assortment Tan, stripe Brown, solid Brown, stripe The idea that alleles for different genes don’t stay together when gametes form. A G G

Independent Assortment

Brown eyes = B; Blue eyes = b Brown hair = H; blonde hair = h Assume there is a man who is heterozygous for both eye color and hair color. Brown eyes = B; Blue eyes = b Brown hair = H; blonde hair = h EYE COLOR: HAIR COLOR: Bb Hh

Independent Assortment BbHh 4 alleles BbHh 2 genes Brown eyes = B; Blue eyes = b Brown hair = H; blonde hair = h

When he makes sperm, the dominant alleles (brown hair and brown eyes) don’t always stay together.

Independent Assortment BbHh Brown eyes = B; Blue eyes = b Brown hair = H; blonde hair = h

Because of independent assortment, the gametes produced by living things are not identical. Eyes: Bb Hair: Hh B H B h b H b h

Brown eyes = B; Blue eyes = b Brown hair = H; blonde hair = h Let’s assume he marries a homozygous recessive woman. Brown eyes = B; Blue eyes = b Brown hair = H; blonde hair = h

Independent Assortment BbHh bbhh BH Bh bH bh bh bh bh bh BbHh Bbhh bbHh bbhh

The result is more genetic variation in the offspring.

Genetic variation ensures that at least one of the offspring will have the phenotype needed to survive. V O P S

In Mendelian genetics, alleles are always either dominant or recessive.

In the real world, however, this is not always the case.

Incomplete dominance, shown below, is one exception to Mendelian genetics.

Incomplete Dominance Hazel When alleles of equal strength blend. Ex: hazel eyes. E B H

Incomplete dominance in flowers:

Incomplete Dominance White allele Red allele FLOWER CELLS

Another exception to Mendelian genetics is co-dominance.

Co-dominance Alleles of equal strength are both expressed, but don’t blend. Ex: dalmation dog. E E B D

Ex: eye heterochromia.

Codominance White allele Red allele White allele Red allele SOME CELLS promoter White allele Red allele White allele Red allele SOME CELLS OTHER CELLS

In Mendelian genetics, there are always two alleles for each gene. Flower Color Flower Position Seed Color Seed Shape Pod Shape Pod Color Plant Height Purple Purple Axial Yellow Round Inflated Green Tall White Terminal Green Wrinkled Constricted Yellow Short

But in reality, there are multiple- allele traits.

Multiple-Allele Trait ACTTTA: White fur ACCAGA: black fur ACGAGA: Tan fur ACTAGA: Gray fur A trait with more than 2 alleles. Ex: cat fur. C

Categories, not a spectrum

Categories, not a spectrum

Finally, Mendelian genetics does not explain the spectrum of phenotypes seen in some traits, such as human skin color.

This is because human skin color is not controlled by one gene This is because human skin color is not controlled by one gene. Instead, it is a polygenic trait. Gene 1 d d D D d D d D d D d D Gene 2 Gene 3 Gene 4 Gene 5 Gene 6 Gene 7 Gene 8

Polygenic Trait Gene 1 d D d D d D d D d D d D Gene 2 Gene 3 Gene 4 Gene 5 Gene 6 Gene 7 Gene 8 A trait controlled by many genes, creating a spectrum of phenotypes. Ex: human skin color. G S S

Multiple-allele trait or polygenic trait?

Multiple-allele trait or polygenic trait?