Mendel and the Gene Idea

Page
9

Slide 61

Fig. 14-12

BbCc

BbCc

Sperm

Eggs

BC

bC

Bc

bc

BC

bC

Bc

bc

BBCC

1/4

1/4

1/4

1/4

1/4

1/4

1/4

1/4

BbCC

BBCc

BbCc

BbCC

bbCC

BbCc

bbCc

BBCc

BbCc

BbCc

bbCc

BBcc

Bbcc

Bbcc

bbcc

9

: 3

: 4



Slide 62

Polygenic Inheritance

Quantitative characters are those that vary in the population along a continuum

Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

Skin color in humans is an example of polygenic inheritance

Slide 63

Fig. 14-13

Eggs

Sperm

Phenotypes:

Number of

dark-skin alleles:

0

1

2

3

4

5

6

1/64

6/64

15/64

20/64

15/64

6/64

1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

AaBbCc

AaBbCc



Slide 64

Nature and Nurture: The Environmental Impact on Phenotype

Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

The norm of reaction is the phenotypic range of a genotype influenced by the environment

For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

Slide 65

Fig. 14-14

Slide 66

Norms of reaction are generally broadest for polygenic characters

Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

Slide 67

Integrating a Mendelian View of Heredity and Variation

An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior

An organism’s phenotype reflects its overall genotype and unique environmental history

Slide 68

Concept 14.4: Many human traits follow Mendelian patterns of inheritance

Humans are not good subjects for genetic research

– Generation time is too long

– Parents produce relatively few offspring

– Breeding experiments are unacceptable

However, basic Mendelian genetics endures as the foundation of human genetics