Biology Now, 2e

126 ■ CHAPTER 07 Patterns of Inheritance

GENETICSGENETICS

to identify the genetic basis of dog size. Lark

collected skeletal measurements of 92 PWDs and

DNA samples from each dog. Ostrander used that

genotype and phenotype information to identify

a key gene for body size—IGF1, which controls

the activity of a growth factor and is known to

influence body size in mice and humans. This

gene’s two alleles are called I and B. Lark and

Ostrander discovered that PWDs homozygous

for allele I (II) were usually large dogs, and those

homozygous for allele B (BB) were always small

dogs. That single gene accounted for whether a

PWD was large or small.

Interestingly, neither IGF1 allele is dominant

or recessive. Instead, heterozygous dogs, with

an IB genotype, are medium-sized dogs. This is

an example of a trait inherited by incomplete

dominance—in which neither allele is able to

exert its full effect, so a heterozygote displays

an intermediate phenotype. Dogs with an IB

genotype aren’t large or small, but rather medi-

um-sized (Figure 7.8).

Early in the twentieth century, geneticists

identified yet another type of interaction among

alleles—codominance—that Mendel had not

observed among his pea plants. A pair of alleles

shows codominance when the effect of the two

alleles is equally visible in the phenotype of the

heterozygote. In dogs, gum color is codominant.

A dog’s gums can be pink, black, or pink with

black spots; in the latter case, both alleles are

fully on display, and neither is diminished or

diluted by the presence of the other allele (as

in incomplete dominance) or suppressed by a

dominant allele (as in the case of dominant and

recessive alleles). In humans, the blood type AB

is a codominant trait.

It’s Complicated

Many of the traits people tend to be curious

about—body weight, intelligence, athleticism,

and musical talent, to name a few—are yet

more complicated. A complex trait is a genetic

trait whose pattern of inheritance cannot be

predicted by Mendel’s laws of inheritance.

Complex traits do not fit the straightforward

single-gene, single-phenotype pattern discussed

so far.

Sometimes a single gene influences a number

of different traits. Such cases are examples of

Genotype: II

Phenotype: Large

Genotype: BB

Phenotype: Small

Genotype: IB

Phenotype: Medium

But years before Ostrander completed the dog

genome, she had begun a different pet project.

In 2001, Ostrander received a call from a scien-

tist in Utah who wanted to talk about dogs. It

was Gordon Lark, who told her he was collecting

genetic trait information about PWDs. “The day

I met Gordon was the best day of my life,” says

Ostrander. “I knew it was golden.”

In 2002, the duo published a paper pinpoint-

ing genes that control dog body shape, from

the tall, lanky look of a greyhound to the short,

stocky frame of a pit bull. In the acknowledg-

ments of the paper, they thanked Karen Miller

and all the PWD owners who had contributed

pedigree information.

In the spring of 2006, Lark and Ostrander

began their second collaboration, this time

Figure 7.8

Incomplete dominance of body size alleles

Great Danes and Chihuahuas illustrate the extreme size variation

found in domestic dogs (see the chapter-opening photo). Unlike the

case with Mendelian traits, dogs heterozygous for the main body size

gene show an intermediate size like the Cocker Spaniel pictured here

on the right.

Q1: What are the genotypes of a large and a small dog?

Q2: Is it possible to have a heterozygous large dog? Explain why or

why not.

Q3: Crossing a Great Dane and a Chihuahua is likely to be

unsuccessful, even though they are members of the same species

(and thus have compatible sperm and egg). Why is that? What are

some potential risks of such a cross?