Mendel and the Gene Idea

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In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

Slide 45

Fig. 14-UN1

Slide 46

Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics

The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

Many heritable characters are not determined by only one gene with two alleles

However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Slide 47

Extending Mendelian Genetics for a Single Gene

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

When alleles are not completely dominant or recessive

When a gene has more than two alleles

When a gene produces multiple phenotypes

Slide 48

Degrees of Dominance

Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Slide 49

Fig. 14-10-1

Red

P Generation

Gametes

White

CRCR

CWCW

CR

CW

Slide 50

Fig. 14-10-2

Red

P Generation

Gametes

White

CRCR

CWCW

CR

CW

F1 Generation

Pink

CRCW

CR

CW

Gametes

1/2

1/2

Slide 51

Fig. 14-10-3

Red

P Generation

Gametes

White

CRCR

CWCW

CR

CW

F1 Generation

Pink

CRCW

CR

CW

Gametes

1/2

1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR

CRCW

CRCW

CWCW

1/2

1/2

1/2

1/2

Slide 52

A dominant allele does not subdue a recessive allele; alleles don’t interact

Alleles are simply variations in a gene’s nucleotide sequence

For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

The Relation Between Dominance and

Phenotype