Mendel and the Gene Idea

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Slide 53

Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain

At the organismal level, the allele is recessive

At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant

At the molecular level, the alleles are codominant

Slide 54

Frequency of Dominant Alleles

Dominant alleles are not necessarily more common in populations than recessive alleles

For example, one baby out of 400 in the United States is born with extra fingers or toes

Slide 55

The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage

In this example, the recessive allele is far more prevalent than the population’s dominant allele

Slide 56

Multiple Alleles

Most genes exist in populations in more than two allelic forms

For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

Slide 57

Fig. 14-11

IA

IB

i

A

B

none

(a) The three alleles for the ABO blood groups

and their associated carbohydrates

IAIA or IA i

A

B

IBIB or IB i

IAIB

AB

ii

O

(b) Blood group genotypes and phenotypes

Slide 58

Pleiotropy

Most genes have multiple phenotypic effects, a property called pleiotropy

For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Slide 59

Extending Mendelian Genetics for Two or More Genes

Some traits may be determined by two or more genes

Slide 60

Epistasis

In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

For example, in mice and many other mammals, coat color depends on two genes

One gene determines the pigment color (with alleles B for black and b for brown)

The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair