Inheritance Patterns

Incomplete dominance

Co dominance

Incomplete Dominance What happens when a

true breeding red snapdragon (RR)

crosses with a

True breeding white snapdragon (R'R')?

Snapdragon plants

Result?

p. 143

The F1 generation are all heterozygous (RR') Pink

What happened to these traits? Did they blend?

F2 generation yielded…

F1 F2

1 red: 2 pink: 1 white (different than that of Mendel’s F2 monohybrid cross ratio of 3:1)

Conclusion: traits do not blend Explanation: Genes code for specific

proteins Most are enzymes with

different degrees of catalytic activity

In this case, One allele (R) codes for

an enzyme that produces red pigment

Conclusion: traits do not blend The other allele (R') codes

for an enzyme that does not produce pigment

So, heterozygous plants (RR'), with only one allele for the functional enzyme produces just enough red pigment to display pink. (not completely dominant!)

2 alleles with functional enzymes (RR) produce enough pigment to show red

Incomplete dominance

A condition in which the heterozygous phenotype is intermediate between the 2 homozygous phenotypes

We use RR' (not Rr) to indicate incomplete dominance

Sample problem In four O’clock plants,

red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of

1. The F1 generation?

Solution What is required?

The flower colour of F1 generation?

What is given? Red flowers (RR) incompletely dominant White flowers (R'R') Heterozygous flowers (RR') are pink

Carry out solution…

RR' RR'

RR' RR'

R' R'

R

R

The F1 generation is all pink

Your turn… In four O’clock plants,

red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of

The F1 generation crossed with its red parent?

The F1 generation crossed with its white parent?

Solution What is required?

F1 (RR') X red parent (RR) F1 (RR') X white parent (R'R')

What is given? Red flowers (RR) incompletely dominant

White flowers (R'R')

Heterozygous flowers (RR') are pink

Results:

RR RR'

RR RR'

RR' R'R'

RR' R'R'

R R'

R

R

2 red, 2 pink

R R'

R'

R'

2 white, 2 pink

Co dominance Sometimes, both alleles of a given

characteristic are dominant Therefore, in heterozygotes, both alleles

are expressed.

I.e. Black rooster is crossed with a white hen

Result?

Black and white plumage Both colours were

expressed

Multiple alleles In addition to co dominance, Many genes may have multiple alleles:

More than two alleles available to determine the phenotype of an organism

I.e. co dominance & multiple alleles

I.e. ABO blood group There are 3 different

alleles for blood type: IA IB

i Each type indicates a

specific glycoprotein present on blood cells (People with type O have

no glycoproteins on their blood cells)

(People with AB have both types present)

A is dominant to O B is dominant to O AB are both co

dominant

Sample problem If a woman has blood type AB, and a man

has blood type A, what possible blood types will their children have?

What is required? Possible blood types

of children?

What is given? Woman AB IAIB

Man A IAIA or IAi

Solution:

IAIA IAIB

IAIA IAIB

IAIA IAIB

IAi IBi

IA IB

IA

i

2 type A, 2 type AB 2 type A, 1 type AB, 1 type B

IA IB

IA

IA

Your turn… Suppose a man with

blood type B marries a woman with type AB blood. What blood types would you expect to find among their children?

What is required? Possible blood types

of children

What is given? Man – IBIBor IBi Woman - IAIB

Solution:

IAIB IAIB

IBIB IBIB

2 type AB, 2 type B

IAIB IAi

IBIB IBi

1 type AB, 2 type B, 1 type A

IB IB IB i

IA

IB

IA

IB