Investigating different patterns of inheritance

Codominance and Incomplete Dominance

Simple Mendelian Genetics

Codominance

Both alleles of a gene pair are equally dominant.

Neither allele is dominant or recessive to the other and as a result both alleles are expressed in a heterozygous offspring.

Codominance

The heterozygous offspring will express (show) both phenotypes

Punnett Square for Codominance

X

100% mixed color carnations (RW)

RR WW RW

R R

W

W

RW RW

RW RW

Punnett Square for Codominance

X

25% red carnations (RR)50% mixed color carnations (RW)25% white carnations (WW)

RW RW

R

R

W

W

RR RW

RW WW

Incomplete Dominance

One allele of a gene pair is only partially dominant over the other.

In these cases an intermediate form of the trait is expressed in heterozygous organisms.

Often seen in flowers.

Incomplete Dominance

The heterozygous offspring will express (show) a blend of the Parent phenotypes

Punnett Square for incomplete dominance

X

100% pink snapdragons (Rr)

RR rr

R R

r

r

Rr Rr

Rr Rr

Rr

Punnett Square for incomplete dominance

X

25% red snapdragons (RR) 50% pink snapdragons (Rr)25% white snapdragons (rr)

Rr Rr

R r

R

r

RR Rr

Rr rr

Investigating different patterns of inheritance

Sex Linked Traits

Sex Linked Traits

• Traits that are controlled by genes located on the sex chromosomes (X or Y)

• These traits are more common in males than females.

• Why???

Sex Linked TraitsX ChromosomeLarger and contains multiple genes

Y ChromosomeSmaller and contains no genes

Girls = XXBoys = XY

Sex Linked TraitsGirls = XX Boys = XY

XB XB - Normal

Color blindness is a recessive condition inherited on the X chromosome:BB = Individual has normal visionBb = Individual has normal vision (carries the colorblind

gene)bb = Individual is colorblind

XB Xb – Normal (carrier)

Xb Xb – Color Blind

XB YØ - Normal

Xb YØ – Color Blind

http://colorvisiontesting.com/ishihara.htm

Are you colorblind?

Sex Linked ExampleGirls = XX Boys = XY

Consider a case where a mother is color blind and the father has normal vision. What is the probability that this couple’s children will be colorblind?

• All Daughters are carriers (XBXb ) with normal vision

• All Sons are color blind (XbYØ)

Xb

XB

Xb

YØ

XBXb XBXb

XbYØ XbYØ

Sex Linked ExampleGirls = XX Boys = XY

Consider a case where a mother is a carrier and the father has normal vision. What is the probability that this couple’s children will be colorblind?

• 50% the daughters will be carriers (XBXb ) but all will have normal vision.

• 50% of the sons will be color blind (XbYØ) and 50% will have normal vision (XBYØ).

XB

XB

Xb

YØ

XBXB XBXb

XBYØ XbYØ

Sex Linked Traits are more common in boysGirls = XX Boys = XY

What is the only way that a female can inherit colorblindness?

XB

Xb

Xb

YØ

XBXb XbXb

XBYØ XbYØ

Dad has to be color blind and Mom has to at least be a carrier of the trait!

Investigating different patterns of inheritance

Multiple Alleles

Multiple Alleles

• Traits that are controlled by three or more alleles.

• Even though more that two alleles exist for a particular trait, humans only carry two at a time.

• ABO Blood Type is a good example!

Blood Type - ABO

• Three alleles exist in the general population – A-B-O

• Humans only carry two • A and B are co-dominant• O is recessive• Let’s see how this works!

Blood Type - ABOA and B are Co-Dominant and O is recessive

What allele combinations are possible for A blood type?

AA or AO

What allele combinations are possible for B blood type?

BB or BO

Blood Type - ABOA and B are Co-Dominant and O is recessive

What allele combination is possible for AB blood type?

AB

What allele combination is possible for O blood type?

OO

ABO Blood Type

Consider the following genotypes:

Father genotype = AO Father’s Blood Type = ________ Mother Genotype = OO Mother’s Blood Type = ________

A

O

O

O

AO OO

AO OO

What are the possible blood types for the children? What are the genotypes?

50% chance of blood type A (AO)

50% chance of blood type O (OO)

A

O

ABO Blood Type

Consider a family where the father is blood type B, the Mother is blood type O, and their daughter is blood type O. What is the possibility that future children will have the following blood types?

B

O

O

O

BO OO

BO OO

Blood type A _____%

Blood type B _____%

Blood type O _____%

Blood type AB _____%

0

50

50

0

Why not BB?

Most Common Blood types in the United States