1. Complex Inheritance

2. Polygenic Inheritance
Many genes working together to control a single phenotype
Just break down the word:
“poly” means many
“genic” means genes
Height and skin color are traits that are controlled by more than one gene

3. Polygenic Inheritance
Here’s how it works. Let’s say 3 genes control height
A, B, & C are “tall” alleles each contributing a unit of tallness
a, b, & c are “short” alleles each contributing a unit of shortness

4. Polygenic Inheritance
THINK! – What is the phenotype for each of the genotypes below?
AABBCC =
aabbcc =
AaBbCc =
Extremely Tall
Extremely Short
Average Height
THINK! - Can you think of other genetic combinations that would still turn out to be
average height? Let’s List them.

5. Extend: Can you figure out the Genotype (Letters) for each person?
1. Slightly Below Average?
AaBbcc
3. tad shorter than
extremely tall?
aABBCC
2. Slightly Above Average?
AABBcc

6. AABBCc Very tall AaBBCc Above average AABbCc AaBbCc Average
What will the children look like? There are 4 possibilities. Fill in the chart.
AABBCC x AaBbcc
Tall Father
Shorter Mother
Child’s allelle (letter) combination
What is the child’s height?
AABBCc
Very tall
AaBBCc
Above average
AABbCc
AaBbCc
Average

7. Polygenic Inheritance
THINK! – Can you think of more examples for polygenic inheritance?
HINT: Think of traits in which the phenotype has a broad range of variation.
Weight
Hair Color
Eye Color

8. What are the Human Blood Types?
BLOOD TYPE A
BLOOD TYPE B
BLOOD TYPE AB
BLOOD TYPE O

9. Multiple Alleles
Example: Blood Type in Humans
There are 3 alleles floating in the population: IA, IB, i; but a person will only get 2 of them.
The IA & IB alleles are said to be codominant.
Codominant means both alleles are expressed. Thus, the person has type AB blood.

10. Blood Type in Humans

11. Blood Type Problems IA i IB IA IB IB i IA IB IB i
THINK! – Amy is heterozygous for type A blood. Jim is homozygous for type B blood. What is the chance they will have a child with type B blood? IB
IA IB
IB i
IA IB
IB i

12. Blood Type Problems i IB IA IA IB IAi i IB i i i
THINK! – Jill is blood type O. She has two older brothers. One has type A blood and one has type B blood. What are the genotypes of the parents? IA
IA IB
IAi i
IB i
i i

13. Sex-Linked Inheritance
Sex is determined by chromosomes
THINK! – Who determines the sex of the baby, mom or dad?
X X X Y XX XX XY XY
Dad

14. Sex-Linked Inheritance
Any gene located on the X chromosome is considered to be sex-linked.
Sex-linked inheritance was discovered by Thomas Hunt Morgan in fruit flies.

15. Sex-Linked Problems
THINK! – Morgan took a pure red-eyed female and crossed it with a pure white-eyed male. What did the Punnett Square look like?
THINK! – What did the F1 generation look like?
All red eyed flies
XR XR Xr Y
XR Xr
XR Xr
XR Y
XR Y

16. Sex-Linked Problems XR Xr XR Y XR Xr XR XR XR Y Xr Y
THINK! – Next, Morgan took a red-eyed female from the F1 generation and a red-eyed male from the F1 generation and mated them. What did the Punnett Square look like?
THINK! - What did the F2 generation look like?
Only the males had the recessive trait of white eyes
XR Xr XR Y
XR Xr
XR XR
XR Y
Xr Y

17. Sex-Linked Traits In sex linked traits:
Females need 2 copies of a recessive allele to show the trait
Males only need 1 copy of a recessive allele to show the trait
Mothers pass sex-linked traits to their sons via the X chromosome
Other sex-linked traits include:
Colorblindness
Male pattern baldness
Hemophilia (blood clotting disorder)

18. Examples of Vision of the Color Blind
Color Blind Test – Ishihara Test