Biology Now, 2e

128 ■ CHAPTER 07 Patterns of Inheritance

GENETICSGENETICS

depending on which allele of the expression

gene (E or e) is present. Dogs with a domi-

nant E allele deposit a pigment called melanin

in their fur and are therefore able to express

whatever fur color genotype is present. But a

recessive ee genotype blocks the deposition of

melanin in fur, so the dog is yellow, regardless

of the genotype at the B/b gene (BB, Bb, or bb).

If the environment affects the phenotype, it

becomes nearly impossible to predict the pheno-

type when given only the genotype of an indi-

vidual or its parents. The effects of many genes

depend on internal and external environmental

conditions, such as body temperature, carbon

dioxide levels in the blood, external tempera-

ture, and amount of sunlight.

For example, cats have a gene that codes

for an enzyme called tyrosinase, which is

involved in melanin production. Siamese cats

have a special Ct allele of the gene. The Ct allele

codes for a tyrosinase that works well at colder

temperatures (≤35°C) but does not function at

warmer temperatures (≥37°C), so the produc-

tion of melanin depends on the temperature of

the surroundings (Figure 7.10). Because a cat’s

extremities tend to be colder than the rest of its

body, melanin is produced there, and hence the

paws, nose, ears, and tail of a Siamese cat tend

to be dark. If a patch of light hair is shaved

from the body of a Siamese cat and the skin is

covered with an ice pack, the hair that grows

back will be dark. Similarly, if dark hair is

shaved from the tail and allowed to grow back

under warm conditions, it will be light-colored.

Man’s Best Friend

After describing the inheritance of size in

Portuguese water dogs, Lark and Ostrander

looked at the IGF1 gene in over 350 dogs repre-

senting 14 small breeds and 9 giant breeds. The

genotype BB was common in small dogs and

virtually nonexistent in large dogs. “All small

dog breeds had them. It didn’t matter when

they were bred or how; they all had the exact

same pattern,” says Ostrander.

“It was amazing,” adds Lark. Breeders have,

over time, been selecting for these alleles to

create smaller and smaller dogs. “What mankind

can do, without any genetic tools but just knowl-

edge of heritability, is just extraordinary,” he says.

B-, E-

Black

Genotype:

Phenotype:

--, ee

Yellow

bb, E-

Brown

Figure 7.9

Epistasis in coat color

These Labrador retrievers show complex

inheritance of coat color. The yellow dog

carries two alleles that interfere with the

deposition of melanin in hair. Both the brown

and the black dogs must carry at least one

allele that allows melanin deposition. A dash

indicates that the allele is unknown, based on

phenotype.

Q1: What are the possible genotypes (at

both genes) of the black dog? The yellow

dog? The brown dog?

Q2: Draw a Punnett square showing

possible matings between the black dog

(assuming it is heterozygous at both

genes) and the yellow dog (assuming it

is heterozygous at the B gene). List

all the possible phenotypes of their

offspring. (See Figure 7.6 for an example

of a Punnett square made with two

traits.)

Q3: If you wanted the most variable litter

possible, what colors of Labrador retrievers

would you cross?

ELAINE OSTRANDER

The “mother of all dog projects,” Elaine Ostrander is

chief and NIH distinguished investigator of the Cancer

Genetics and Comparative Genomics Branch, as

well as head of the Comparative Genetics Section at

the National Institutes of Health. She studies genes

important to growth, size variation, and cancer in dogs.