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

2. 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

3. Independent Assortment

4. 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

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

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

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

8. 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

9. 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

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

11. The result is more genetic variation in the offspring.

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

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

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

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

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

17. Incomplete dominance in flowers:

18. Incomplete Dominance
White allele
Red allele
FLOWER CELLS

19. Another exception to Mendelian genetics is co-dominance.

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

21. Ex: eye heterochromia.

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

23. 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

24. But in reality, there are multiple- allele traits.

25. 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

26. Categories, not a spectrum

27. Categories, not a spectrum

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

29. 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

30. 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

31. Multiple-allele trait or polygenic trait?

32. Multiple-allele trait or polygenic trait?