variations of inheritance patterns ch. 10.3. objectives 1.describe intermediate inheritance...

Variations of Inheritance Patterns

Ch. 10.3

Objectives

1. Describe intermediate inheritance

2. Multiple alleles

3. Understand polygenic inheritance

4. Effect of environment on phenotypes

Variations of Inheritance Patterns

• With Mendel’s pea plants, alleles were quite simple:– If a dominant allele present in genotype -->

always present in phenotype– If two recessive alleles present in genotype -->

present in phenotype

• What about genes where NEITHER gene is dominant?

“Blue” Chickens?

• A black chicken and a white chicken can produce blue offspring in the F1 generation

• Neither black nor white allele is dominant

• But, this isn’t blending• Both black and white

phenotypes can reappear in F2

generation

• This is Intermediate Inheritance: – offspring have phenotype that

is intermediate between phenotypes of parents

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P

F1

F2

CB CB CW CW

CB CW

CB CB CB CW CB CW CW CW

Since no dominant alleles, C is used for “color” and superscripts used for alleles.

Make the Punnett squares of the F1 generation, then the F2

Multiple Alleles with Blood Types• 4 options for blood types• Each parent gives one allele --> 6 possible combinations• Blood type AB represents codominance:

– A heterozygote expresses both traits

Blood Type Possible Genotypes

A AA or AO

B BB or BO

AB AB

O OO

Blood

GroupAntigens Antibodies Can give

Blood to

Can receive blood from

AB A and B None AB AB, A, B, O

A A B A & AB A & O

B B A B & AB B & O

O None A & B AB, A, B, O

O

A bit more on blood types . . .

Polygenic Inheritance

• Intermediate Inheritance and codominance involve multiple alleles.

• It’s also possible for multiple genes to affect a character. This is polygenic inheritance:

• Polygenic inheritance:– Two or more genes affect a single character

• Height and skin color are two examples• Let’s look at height as an example . . .

Polygenic Inheritance• Let’s assume 3 genes produce height in humans

– Assume inherited separately (like Mendel’s pea genes)

• A, B, & C are tall alleles• X, Y, & Z are short alleles• Parents each give 3 alleles each• AABBCC would be a tall individual• AXBBCC would be a little shorter• AXBYCC would be even shorter, and so on . . . until• XXYYZZ would be the shortest individual• The potential combinations of alleles (and phenotypes)

for a character increases with the # of genes that affect that character --> huge range in heights!

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AABBCC

XXYYZZ

Importance of Environment

Genotype never changes, but leaves vary in size, shape, and color depending on wind and sunlight Enzyme

responsible for black fur in Siamese cat is only active in cooler temperatures

Identical twins with exact same genotype accumulate differences in phenotype due to different diets, activity level, exposure to sunlight, etc.