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Complex Traits

Author(s): Nancy Moreno, PhD.

Categorizing Hair Types in Dogs

Activity 5: Slides 27–40


Students explore phenotypic differences in hair types found in the coats of different dog breeds, as they begin to learn about complex traits. Based on information provided, students also predict the genotypes of different dog breeds.

Most characteristics, such as height, body shape and disease susceptibility, in living organisms are controlled by more than one gene. Coat type in dogs is an example. Variations in three genes determine the patterns of hair length, curl and presence of “furnishing” (beard and bushy eyebrows, combined with wiry hair) that are observable in about 95% of all dog breeds. In this activity, students will categorize the coat phenotypes of eight different dog breeds, and then will connect the phenotypes to possible genotypes.

The dog breeds that are included in this activity are the Bassett Hound, Bichon Frise, Border Terrier, English Cocker Spaniel, Golden Retriever, Havanese, Irish Water Spaniel, and Kerry Blue Terrier. These breeds display the complete range and combinations of coat types: long vs. short hair, curly vs. straight hair, and presence or absence of furnishings. The ancestors of modern dogs all had coats with short, straight hair without furnishings. The other characteristics have arisen because of mutations in genes related to hair growth and development. Mutations are changes in the DNA sequence, usually as a result of mistakes when DNA is copied. If the mutation is present in the egg or sperm cell of a parent, it is passed onto the offspring. In dogs, selective breeding has consolidated and increased the frequency of many mutations that affect appearance or behavior.

Three different genes regulate hair type in dogs. Hair length (L), as described previously, can be long or short. The ancestral condition in dogs is short hair. Long-haired dogs have inherited a mutation involving the substitution of a single nucleotide in the gene responsible for terminating hair growth. Dogs with one or two copies of the mutation have long hair. Thus, dogs with the genotype “Ll” and “LL” have short hair. Dogs with the genotype “ll” have long hair.

Curl (C) is governed by a change in one of the genes responsible for a structural protein in hair (keratin). The mutation again involves the substitution of a single nucleotide—this time, a T (thymine) is substituted for a C (cytosine). The substitution is believed to affect folding of the completed protein, leading to curled or wavy hair. Dogs with straight hair have two copies of the straight form of the gene (cc); dogs with wavy hair have one copy of the straight form of the gene and one copy of the curly form (Cc); dogs with curly hair are homozygous for the curly mutation (CC).

Wiry hair is an interesting characteristic in dogs. Dogs that possess hair that is coarse and bristly like wire, always also have a beard (longer hair on the chin and muzzle and bushy eyebrows. Dog experts refer to the additional facial hair features as “furnishings.” All of these physical changes are due to the same mutation in a single gene. The gene is responsible for producing a signaling protein important for keratin development and initiation of hair growth. Importantly, in this case, one mutation in a single gene leads to multiple changes in the phenotype. The Irish terrier, shown at right, provides a good example of furnishing and wiry coat. The mutation leading to wiry hair and furnishings consists of the insertion of 167 base pairs within the signaling protein gene called RSPO2. This means that extra DNA was added into the existing sequence that comprised the gene. The extra section of DNA appears to change the levels and actions of the proteins produced by the gene.

Only one copy of the “furnishings” (F) mutation is necessary for the characteristic to be present. A dog with the genotype “ff” will not have furnishings. However, a dog with Ff or FF will present the furnishings phenotype.

When only one copy of an allele is necessary for a trait to be present, inheritance of the trait is described as “dominant.” Furnishings is an example of a dominant trait. Dogs with only one copy of the furnishings allele (genotype of Ff) will have wiry hair and a bearded face. Conversely, absence of furnishings is a recessive trait, because two copies of the allele (ff) are necessary for furnishings to be absent. 


Allele: One member of a pair (or any of the series) of genes occupying a specific spot on a chromosome (called locus) that controls the same trait. 

Gene: A gene is a segment of DNA (on a specific site on a chromosome) that is responsible for the physical and inheritable characteristics or phenotype of an organism. 

Homozygote: A nucleus, cell or organism where the alleles for a particular gene on each chromosome are identical. 

Heterozygote: A nucleus, cell or organism possessing two different alleles for a particular gene. Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive allele).

This activity is based on original research published in the following paper: Cadieu E, et al., 2009. Coat Variation in the Domestic Dog Is Governed by Variants in Three Genes. Science 326: 150–153.

Part 1. Hair Phenotypes

1. Remind students about the activity “Genotypes and Phenotypes.” Ask, Is short or long hair the only distinguishing characteristic of dog’s coats? Students might mention curly vs. straight hair or coat color. Explain, Coat color and patterns of color, for example, are governed by interactions among many different genes. Show students this slide of dogs with two different coat patterns. Mention, for example, that several different genes can be responsible for something that appears simple, such as a black coat.

Funded by the following grant(s)

Science Education Partnership Award, NIH

Gene U: Inquiry-based Genomics Learning Experiences for Teachers and Students
Grant Number: 5R25OD011134

Robert Wood Johnson Foundation

Using Learning Technology to Build Human Capital
Grant Number: 57363