Complex Complex InheritanceInheritance

Polygenic InheritancePolygenic 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

Polygenic InheritancePolygenic 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

Polygenic InheritancePolygenic 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 beaverage height? Let’s List them.

Extend: Can you figure out the Genotype Extend: Can you figure out the Genotype (Letters) for each person?(Letters) for each person?

1. Slightly Below Average?

2. Slightly Above Average?

3. tad shorter than extremely tall?

AaBbcc

AABBcc

aABBCC

What will the children look like? There are 4 What will the children look like? There are 4 possibilities. Fill in the chart. possibilities. Fill in the chart.

AABBCC x AaBbcc

Child’s allelle (letter) combination

What is the child’s height?

AABBCc Very tall

AaBBCc Above average

AABbCc Above average

AaBbCc Average

Tall Father Shorter Mother

Polygenic InheritancePolygenic 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

What are the Human Blood What are the Human Blood Types?Types?

BLOOD TYPE A BLOOD TYPE B

BLOOD TYPE AB BLOOD TYPE O

Multiple AllelesMultiple 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.

Blood Type in HumansBlood Type in Humans

Blood Type ProblemsBlood Type Problems

• 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?

IA i

IB

IB

IA IB

IA IB

IB i

IB i

Blood Type ProblemsBlood Type Problems

• 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?

i i

IAi IA IB

i

i

IB i

IB

IA

Sex-Linked InheritanceSex-Linked Inheritance

• Sex is determined by chromosomes

• THINK! – Who determines the sex of the baby, mom or dad?

XX XX

XYXY

X X

X

Y

Dad

Sex-Linked InheritanceSex-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.

Sex-Linked ProblemsSex-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

Y

XR Xr

XR Y

XR Xr

XR Y

XR XR

Sex-Linked ProblemsSex-Linked Problems• 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

Sex-Linked TraitsSex-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)

Examples of Vision of the Color BlindExamples of Vision of the Color Blind

http://www.vischeck.com/examples/

Color Blind Testhttp://www.toledo-bend.com/

colorblind/Ishihara.asp