Biology Now, 2e

Dog Days of Science ■ 119

(see Chapter 9 for more on mutation). Genetic

diversity in a species, whether it is soybeans

or dogs or humans, comes about because the

species as a whole contains many different

alleles of its genes.

Dogs are the champions of genetic variation.

All dogs are the same species, yet a Pekingese

weighs only a couple of pounds, while a Saint

Bernard can weigh over 180 pounds. Dogs, in

fact, are reported to have more variation in the

size and shape of their species than any other

living land mammal on Earth, with the possible

exception of humans.

On a whim, Lark agreed to dabble in

dog genetics, but to do so, he would need to

compare the genotypes of individual dogs

(their genetic makeup, controlled by their

combinations of alleles) with their pheno-

types (the physical expression of their genetic

makeup). The genotype of a given trait is the

pair of alleles that codes for its phenotype. To

identify genes responsible for dog traits, Lark

would need both types of information for each

of many dogs.

Miller was already on the case. Three months

after Lark got Mopsa, Miller sent him 5,000

PWD pedigrees—detailed health and breeding

records for individual dogs. Lark was aston-

ished. It was the first of many times that the

enthusiasm and generosity of dog owners would

contribute to his research.

“That was literally how this started,” says

Lark. Today, Lark and Miller’s unlikely part-

nership has blossomed into a national research

project producing valuable knowledge about the

genetic basis of health and disease in both man

and man’s best friend. What’s more, their effort

has demonstrated how tiny genetic changes can

create huge variation in a single species.

Pet Project

The Georgie Project, as Lark fondly named it, offi-

cially began in 1996. Lark’s first task was to collect

genotypes and phenotypes from PWDs. To his

pleasant surprise, PWD owners were enthusias-

tic and began flooding him with pedigrees, blood

samples, and X-rays taken by their veterinarians.

In short order, Lark had DNA from more than

1,000 dogs and detailed body measurements for

over 500. Then the hard work began.

Using the dogs’ genotypes and phenotypes,

Lark set out to pinpoint the alleles for particular

traits. Some genes have alleles that are dominant

when paired with another allele; that is, one allele

prevents a second allele from affecting the pheno-

type when the two alleles are paired together. The

Each of the 46

chromosomes is part

of a pair, known as a

homologous pair.

Each homologous

pair consists of one

chromosome

inherited from each

parent.

Genes are segments of DNA

that encode the information

for a trait. There are two

copies, or alleles, for each

gene—one from each

parent. These alleles may be

the same, or they may be

different.

Each chromosome contains

one long DNA molecule.

Organism

(human)

Maternal

chromosome

Paternal

chromosome

Gene H

Gene A

Gene D

A human body is made up

of trillions of somatic cells,

with 46 chromosomes in

each cell.

Figure 7.2

Genes are segments of DNA that confer a genetic trait

Somatic cells (cells of the body) have two copies of most genes.

Q1: What is the physical structure of a gene?

Q2: How many copies of each gene are found in the diploid cells in a

woman’s body?

Q3: With 46 chromosomes in a human diploid cell, how many

chromosomes come from the person’s mother and how many from the

father?