Labrador retrievers may be black, brown, or golden in color...

Labrador retrievers may be black, brown, or golden in color (see the chapter opening photograph on $\mathrm{p} .53$ ). Although each color may breed true, many different outcomes occur if numerous litters are examined from a variety of matings, where the parents are not necessarily true-breeding. The following results show some of the possibilities. Propose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color. (a) black $\times$ brown $\longrightarrow$ all black (b) black $\times$ brown $\longrightarrow \quad 1 / 2$ black $1 / 2$ brown (c) black $\times$ brown $\longrightarrow \quad 3 / 4$ black $1 / 4$ golden (d) black $\quad \times$ golden $\longrightarrow \quad$ all black (e) black $\times$ golden $\longrightarrow \quad 4 / 8$ golden 318 black $1 / 8$ brown (f) black $\times$ golden $\longrightarrow \quad 2 / 4$ golden $1 / 4$ black $1 / 4$ brown (8) brown $\times$ brown $\longrightarrow \quad 3 / 4$ brown $1 / 4$ golden (h) black $\times$ black $\longrightarrow 9 / 16$ black $4 / 16$ golden $3 / 16$ brown

Labrador retrievers may be black, brown, or golden in color (see the chapter opening photograph on $\mathrm{p} .53$ ). Although each color may breed true, many different outcomes occur if numerous litters are examined from a variety of matings, where the parents are not necessarily true-breeding. The following results show some of the possibilities. Propose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color.
(a) black $\times$ brown $\longrightarrow$ all black
(b) black $\times$ brown $\longrightarrow \quad 1 / 2$ black
$1 / 2$ brown
(c) black $\times$ brown $\longrightarrow \quad 3 / 4$ black
$1 / 4$ golden
(d) black $\quad \times$ golden $\longrightarrow \quad$ all black
(e) black $\times$ golden $\longrightarrow \quad 4 / 8$ golden 318 black
$1 / 8$ brown
(f) black $\times$ golden $\longrightarrow \quad 2 / 4$ golden $1 / 4$ black
$1 / 4$ brown
(8) brown $\times$ brown $\longrightarrow \quad 3 / 4$ brown $1 / 4$ golden
(h) black
$\times$ black $\longrightarrow 9 / 16$ black
$4 / 16$ golden $3 / 16$ brown

Key Concepts

Breeding True

Breeding true refers to organisms that, when mated with another of the same type, consistently produce offspring with the same phenotype. This often indicates that the organism is homozygous for the allele(s) responsible for that trait. Recognizing which animals breed true is essential for establishing which genotypes correspond to particular phenotypes and for confirming the proposed mode of inheritance.

Mendelian Inheritance

Mendelian inheritance refers to the set of principles that govern how alleles are transmitted from parents to offspring. It involves concepts like the segregation of alleles during gamete formation and the independent assortment of genes. These principles help in interpreting patterns in genetic crosses and explaining predictable ratios of phenotypes, which is essential when proposing a mode of inheritance for traits such as coat color.

Dominance Hierarchies

In genetics, dominance hierarchies describe the relationship between different alleles at a locus, where one allele can mask the expression of another in a heterozygote. In cases where multiple alleles exist, such as those potentially involved in coat color, the dominant allele determines the phenotype unless the organism is homozygous for a recessive allele. This concept is crucial when determining why certain crosses produce only one visible phenotype while others result in a mix.

Genotype-Phenotype Correlation

This concept involves linking the genetic makeup (genotype) of an organism to its observable characteristics (phenotype). Understanding which combinations of alleles produce which colors is key to analyzing breeding results. For example, individuals that 'breed true' for a trait are typically homozygous for the allele responsible for that phenotype, while heterozygous individuals might show a dominant phenotype but can produce different outcomes in their progeny.

Test Cross Analysis

Test cross analysis is a method used to determine the genotype of an organism by crossing it with a homozygous recessive individual. The progeny ratios help infer the genotype of the parent displaying the dominant trait. This technique is instrumental in experiments where the genetic basis of a trait is being deduced from offspring phenotypes, as seen in the breeding experiments described.

Allelic Interaction and Multiple Alleles

Allelic interaction refers to how different alleles at the same locus interact to produce a phenotype, which can include complete dominance, incomplete dominance, or codominance. In cases where more than two alleles are present in a population, establishing a clear dominance hierarchy becomes necessary to explain the observed phenotypic ratios. This is particularly relevant in traits that exhibit more than two distinct phenotypes, such as the varied coat colors in some dog breeds.

Transcript

00:01 The color of the coats in a particular breed of dog called labrador retrievers.

00:07 And the coats can be black, they can be yellow, or they can be chocolate, which is really just a brown color, but they're called chocolate.

00:17 So i will write chocolate.

00:20 And in this breeding experiment, we started off with a cross between black labs.

00:31 And we crossed them to yellow.

00:38 And we got all black in the f1.

00:42 So yellow disappeared.

00:45 That's our f1 generation.

00:49 And then when blacks were made it to each other, we got the f2 generation.

00:54 And then things got interesting because we had 91 black puppies, 39 yellow, and then 30 chocolate or brown.

01:13 That's 160 puppies, which is an awful lot of puppies.

01:17 But you'll notice that that comes out to be a 9 to 7 ratio.

01:21 So 9 .16th of them were black, and 716s were either yellow or chocolate.

01:36 And when you see that kind of ratio, it's often indicative of the idea that we have two genes and they're having some kind of epistasis effect in between them.

01:47 So epistasis.

01:50 So let's just answer the question, which are the, what are the genotypes of black chocolate and yellow here? so the blacks are nine.

02:02 So by the way, let me just say that typically when dealing with coat colors and labs, the genes are b and e.

02:11 So that's where i'm going with this.

02:14 So capital b and capital e.

02:19 So if you have a dominant leal for both of those, then you will end up with a black coat.

02:29 If you have a homocygous little b and at least one capital e allel, then you will get chocolate.

02:48 And one reason for thinking that that is the chocolate and not the yellow is note that there are fewer chocolates than there are yellows.

02:57 And so there has to be a little extra way to get to the yellows.

03:01 And then for the yellows, so let me write that this would be 316s in parentheses to sort of stress what i'm going for.

03:09 And then the 416s would be a capital b and then homozygous recessive for e or homozygous recessive for both of them...

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