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Genetics

Contents of this Section

Labrador Retrievers as a Model System to Study Inheritance of Hair Color

Unlike humans, who usually have only one child at a time, and rarely manage more than a dozen children over their lifetime, dogs produce litters of puppies. A single litter can have a dozen puppies, and a single dog can have numerous litters in her lifetime. Therefore, dogs offer a greater number of individuals per family than do humans, which is useful in the study of inheritance.

A second advantage of dogs over humans, for the study of inheritance, is that the breeder can choose the parents at each generation. In a human family, as generations progress, each individual chooses her, or his, mate from the general population. This makes it hard to follow the genetic characteristics within families. However, if the breeder can choose the male/female pairings, then it is possible to ensure that we know the genetic history of each individual.

Many dog breeders are particularly proud of their favorite breed of dogs. Some breeders specialize in breeding Labrador Retrievers; some specialize in breeding particular colors of Labs. We can therefore begin our study with the knowledge, based on these breeders' extensive records, that one family of dogs, over many generations, "breeds true" and always produces Yellow Labs (i.e. dogs with blonde hair). Similarly, another family of dogs, over many generations, also "breeds true" and always produces Chocolate Labs (i.e. dogs with brown hair).

This "true breeding" characteristic of these families is important for our study of inheritance. It tells us that however hair color is determined, the Yellow Lab family carries only the "determinants" of yellow (blonde) hair color. Similarly, the Chocolate Lab family carries only the determinants of chocolate (brown) hair color.

The First Cross: Chocolate x Yellow

Definition of Terms: For this study, we will create a term to use when referring to a mating that we have specifically set up: we'll call such a mating a "cross." We can represent this by using an "x" in our records of our study.

We have two true-breeding families of Labrador Retrievers, chocolate and yellow. What if we cross them? That is, mate a female Chocolate Lab with a male Yellow Lab, or a female Yellow Lab with a male Chocolate Lab? What do we find, and does it matter which color is the male and which color is the female?

Let's make a table to keep track of what we find. To make sure we remember that we are studying Labrador Retrievers to learn about inheritance of hair color in humans, we'll refer to Chocolate as "brown" and Yellow as "blonde."

Number of Crosses
Female Parent
x
Male Parent
Offspring
 
brown
blonde
 
number
%
number
%
1
Brown
x
Blonde
11
100
0
0
1 Blonde x Brown 9 100 0 0

In the first litter from each cross, all the puppies have brown hair. In my family, there were some generations in which most of the children from brown-haired parents had brown hair, but an occasional child had blonde hair. Is one litter enough for us to conclude that this does, or does not, happen with Labrador Retrievers? Let's ask our breeders to give us this kind of information for addtional matings like these. We'll add the new numbers to the table.

Our table now looks like this:

Number of Crosses
Female Parent
x
Male Parent
Offspring
 
brown
blonde
 
number
%
number
%
17
Brown
x
Blonde
187
100
0
0
21 Blonde x Brown 203 100 0 0

Now that's interesting. Regardless of which way we do the cross (brown x blonde or blonde x brown), the offspring are all brown. How can we explain this observation?

There could be a number of explanations. Maybe brown is "more powerful" than blonde, or maybe blondeness is simply lost when a blonde parent has offspring with a brown parent. Or maybe the heritable "determinant for blonde hair" is present in the offspring, but is masked by the "determinant for brown hair" (maybe because brown is darker than blonde?). It's hard to tell from the information we have. We don't have enough information to distinugish among multiple explanations.

We'll need to perform additional investigations.

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Cross 2: Test Crosses with the Parental Types

Definition of terms: Sometimes, in studies of inheritance, it is useful to set up matings that test specific possibilities. In particular, when a particular cross produces offspring in which one of the parental characteristics seems to have disappeared (as did the blonde characteristic in the crosses described above), it may be informative to set up a mating between one or more of the offspring and the parental type (in this case, the true-breeding blonde family). Such a mating is called a test cross.

What we have learned so far, in our investigation with Labrador Retrievers, is that when we mate a Chocolate Lab (brown) with a Yellow Lab (blonde), the offspring are all brown [that is, if the parents are from "true-breeding" families]. From this information alone, we really can't tell how hair color is inherited.

Although we certainly don't know the explanation here, we can write down some possible explanations. Let's write down a few. Doing so will give us some ideas about what investigations we might do to distinguish among them.

  • possible explanation 1: whatever causes brown hair changes blonde to brown
  • possible explanation 2: only brown hair was inherited in the crosses we've studied so far
  • possible explanation 3: whatever causes blonde hair, and what ever causes brown hair, are both present in the offspring we've studied, but brown is somehow "stronger" than blonde
  • there are probably many more ideas we can come up with, but these may be enough to get us started.

Both #1 and #2 share the possibility that the brown-haired offspring might not be able to have blonde-haired offspring (if blondeness has been changed, or lost, somehow). Perhaps we can test this idea by mating some of the brown-haired offspring with blonde-haired dogs from among the original true-breeding families we started with. Then, we can examine the offspring from these crosses.

Definition of Terms: as we perform more and more crosses, and look at more and more offspring, we will need to know which offspring from which cross we are talking about. In the early years of the study of genetics, scientists recognized this potential problem, and developed terminology that would serve the purpose. At that time, most scientists worked and published their findings in the language of their country, whether Germany, France, Spain, England, etc. The common language was Latin. Therefore, geneticists used the Latin for son (filius) or daughter (filia) to refer to the offspring of a cross, and simply shortened it to F (for "filial generation"). The first generation of offspring are then the F1 generation; the second generation are the F2 generation, etc. The initial parents are referred to as the parental, or P generation.

Using the P, F1, and F2 terminology, we can keep track of which generation we're talking about. Let's think about those three possible explanations mentioned above, and consider how we might test them. Our original Parental cross, brown x blonde, produces F1 offspring that are all brown; the several possible explanations suggest that:

    1. If brown somehow converts blonde to brown, then the brown F1 dogs should not be able to produce blonde-haired offspring, even when mated with blonde-haired dogs. We can test this easily by performing a test-cross, of brown (F1) x blonde (of the Parental type). [We'll say "of the Parental type" to indicate that the blonde parent is from a true-breeding family, but not one of the actual parents of the F1 dog.]
    2. If the brown F1 dogs inherited only the brown-hair-color trait from their parents, then again, they should be unable to produce blonde-haired offspring. We can test this with the same cross: brown (F1) x blonde (Parental type).
    3. If the brown F1 dogs carry the "genetic factors" for blonde-hair (from one parent) and for brown-hair (from the other parent), then these dogs should be able to produce blonde-haired offspring. Again, we can test this with the same test-cross: brown (F1) x blonde (Parental type).

What do we find when we do this cross? Let's do many such crosses, so that our numbers of offspring are reasonably large. Let's also remind ourselves what the original Parental cross was.

Perhaps, to obtain the most information, we should perform test-crosses to the blonde parental type and to the brown parental type.

Cross
Female Parent
x
Male Parent
Offspring
   
brown
blonde
   
number
%
number
%
a P
Brown
x
Blonde
187
100
0
0
b F1 back-cross Brown (F1) x Blonde (P) 92 48 98 52
c F1 back-cross Blonde (P) x Brown (F1) 73 51 70 49
d F1 back-cross Brown (F1) x Brown (P) 126 100 0 0
e F1 back-cross Brown (P) x Brown (F1) 195 100 0 0

The observation is fairly clear: the brown-haired F1 dogs can certainly have blonde-haired offspring. We see this result in crosses b and c, the test-crosses with the blonde-haired parental type. This result argues that the ideas we listed as #1 and #2 above cannot be the explanation for our first crosses. The results support our third possibility, that the genetic factor for blonde-hair is, indeed, present in the F1 brown-haired dogs, even if we don't see it in their overall characteristics. Crosses d and e, however, produce entirely brown-haired offspring; this is consistent with the results of the first cross, but doesn't provide much insight into the possible explanations.

The numbers in crosses b and c may be important. It looks as if the ratio of brown-haired offspring to blonde-haired offspring is very close to 1:1. A statistical analysis would tell us how firmly we can justify this conclusion, but for now it looks pretty good.

Now what explanations can we develop for our findings?

Let's list some of the things we now know, and think about what these things tell us:

  • each individual has 2 parents
    • this suggests that an individual may receive one "genetic factor" from each parent
  • there are true-breeding brown-haired families, and true-breeding blonde-haired families
    • this suggests that an individual can receive a "genetic factor" from one parent that is the same as the "genetic factor" received from the other parent (for example, in the blonde family, each parent contributes a "blonde" genetic factor to the offspring)
  • the test-cross (brown F1 x blonde parental) gives two classes of offspring
    • this suggests that the "blonde" genetic factor is present in the brown-haired F1 individuals, and can be passed on to offspring
  • the two classes of offspring from the test-cross (brown F1 x blonde parental occur in roughly equal numbers
    • this suggests that the brown-haired F1 individuals have only two genetic factors for this particular trait (blonde vs brown hair)--one of which they inherited from one parent, the other of which they inherited from the other parent
  • the F1 offspring from the original Parental cross carried two genetic factors for hair color, but showed only one of the two characteristics--brown, not blonde
    • this suggests that some genetic factors (e.g. the brown-hair factor) can have a greater effect on an individual's characteristics than do other genetic factors (e.g. the blonde-hair factor)
  • the test-cross of the brown F1 x brown parental offspring produced only brown-haired offspring
    • this suggests that the blonde-hair genetic factor, whether inherited from a blonde parent or from a brown F1 parent, is "masked" by the brown-hair genetic factor; it seems to depend on the genetic factors themselves, and not the characteristics of the parents who provide the genetic factors.

On the basis of this information, can we now develop a possible explanation of the results?

Definition of Terms: in all fields of science, the goal is to obtain enough information (whether we call it observations, measurements, data, etc.) to propose a plausible explanation. In some fields, the explanation is called "an explanation." In other fields, the explanation is called "a model." In some fields, the explanation is called "a working model," with the term "working" added to indicate that the model is tentative, and is currently being used to guide the investigation. In yet other fields, the explanation is call "an hypothesis." Whatever term is used, the function is the same: provide a description, or statement, of our current understanding of how the system works. The field of Genetics is at the interface of so many different sub-fields in biology that all of these terms may be used, depending on the background of the investigator. Here, we have chosen to avoid all of these terms, and refer to "a possible explanation" of the data.

As indicated in the bulleted list above, the data (so far) indicate that there appear to be heritable genetic factors that influence the characteristics of individuals. The data suggest that there is one such factor for blonde hair color, and one such factor for brown hair color. The data further suggest that a single individual can have both of these at once, one from each parent. In addition, the data suggest that these genetic factors contribute unequally to the characteristics of the individual--the "brown" factor has a more dominant role than the "blonde" factor. Our possible explanation, then, is as follows:

  • Hair color in Labrador Retrievers depends on genetic factors. Each individual inherits one factor from each parent, and therefore carries two factors at once. In the case of blonde hair color, it is necessary to inherit a blonde factor from each parent. A brown factor, inherited from either parent, is enough to "over-rule" the blonde factor, resulting in brown hair color.

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Cross 3: inter se crosses among F1 individuals

According to the explanation of inheritance (or model, or hypothesis) outlined above, the F1 offspring from the initial cross carry one genetic factor for blonde hair color and one for brown hair color. If we were to cross F1 individuals among each other (not necessarily matings of siblings, but certainly matings of F1's from the Parental cross), we should find some brown-haired offspring, and some blonde-haired offspring. The blonde-haired offspring would occur if an individual inherits the blonde genetic factor from each parent.

Definition of Terms: as noted above, Latin was the common scientific language for many decades. To describe crosses among F1 offspring such as we are considering here, they used the Latin term, inter se, which means, in English, "among themselves."

What do we find with such crosses? Again, let's perform many such crosses to obtain reasonably large numbers.

Female Parent
x
Male Parent
Offspring
brown
blonde
number
%
number
%
Brown F1
x
Brown F1
112
73
40
27

At first glance, the data appear to support our model. There are blonde offspring among from this cross. This doesn't prove that our model is correct, but it is encouraging support for the model.

Can we learn more from the data?

Is there any significance to the observation that there are roughly 3 times as many brown-haired offspring than there are blonde-haired offspring in this cross? It might...or it might not. Perhaps we can figure this out if we make our model more concrete.

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A More Concrete Model of Inheritance

We have proposed, based on the data from the crosses, that the brown F1 dogs carry one brown genetic factor and one blonde genetic factor (one from each parent). Might it be possible for dogs to choose which of these genetic factors they pass on to their offspring? This is a difficult idea to test. In these crosses, different litters had different numbers of blonde-haired puppies, so if there is any kind of choice, different individuals choose differently. But, what if there is no choice involved? What if each dog produces gametes (eggs or sperm, depending on whether the dog is female or male) with roughly equal numbers for each of these two genetic factors?

If half of the gametes carry the blonde factor, and half carry the brown factor, then what are the possible combinations that can occur upon fertilization of the egg? We'd better draw some pictures to try to figure this out. First, let's think about the parents in this cross:

If each parent in the F1 cross carries a blonde genetic factor and a brown genetic factor, then each parent should be able to produce gametes with one factor or the other.

Now let's think about the possible combinations that might occur upon fertilization:

It seems unlikely that any particular sperm cell can choose what particular egg cell it fertilizes, or vice versa. Therefore, sperm carrying the brown genetic factor should be able to fertilize eggs carrying either the brown or blonde genetic factor. Similarly, sperm carrying the blonde genetic factor should be able to fertilize both types of eggs.

If, as our previous reasoning suggests, the brown factor has a greater influence on the characteristics of the offspring, then three of these combinations should produce brown-haired puppies, and one should produce blonde-haired puppies.

In any particular litter of puppies from these crosses of the F1 dogs, among themselves (inter se), we usually find more brown-haired puppies than blonde-haired puppies. We usually don't see exactly three browns for each blonde, as we might predict from the reasoning we've just been through. But what we have considered here is the possibilities for the inheritance of these genetic factors that control hair color. At best, we can say that any particular puppy has a one-out-of-four chance of inheriting a blonde factor from each parent, a one-out-of-four chance of inheriting a brown factor from each parent, and a two-out-of-four chance of inheriting a blonde factor from one parent and a brown factor from the other.

Studying a large number of such crosses might give us enough individual puppies to see these possibilities work themselves out in the actual numbers of individuals. The table above, showing the data provided by our dog breeders, shows that the actual numbers are pretty close to the predictions we might make.

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A Tentative Statement of the Rules of Genetic Inheritance of Hair Color in Labrador Retrievers

Overall, the data from these crosses lead us to suggest that hair color may be inherited as follows:

  • The characteristic, or trait of hair color depends upon heritable genetic factors
  • There are at least two such factors, brown and blonde
  • Each individual inherits one of these factors (not both) from each parent
  • Each parent can pass one of these factors (not both) to each offspring
  • An individual can carry two brown factors, or two blonde factors, or one blonde and one brown factor; only if an individual carries two blonde factors does the individual have blonde hair.
  • The brown factor is somehow "dominant" to the blonde factor, since it determines hair color in individuals with both versions of the hair color genetic factors.

But...What are these genetic factors, and what determines which one is dominant?


Inferences From This Study

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Additional Information Concerning Inheritance of Hair Color

There are many colors of Labrador Retrievers, ranging from white to black:

The inheritance investigation told above for Yellow Labs and Chocolate Labs can be carried out for any and all of these. Some of the rules of inheritance seem straightforward:

  • black is dominant to brown
  • black is dominant to blonde
  • brown is dominant to blonde

But it isn't quite that simple. The dog on the left of the series shown above has a black nose, black lips, and black eyelids. In addition to carrying the black genetic factor, she also carries a genetic factor that affects the distribution of black pigmentation, so she produces the black pigment only in her nose, lips, and eyelids.

It is likely that the left-most four dogs in this series all carry this distribution characteristic. The difference in color among them could be due to yet another genetic factor, or to their suppressing the production of black pigmentation to different degrees.

There are, in fact, a number of different genetic factors that contribute to hair color. The same genetic factors (i.e. genes) affect hair color in humans and in other mammals. For further investigation, here are a couple of more detailed discussions:

http://www.vetgen.com/canine-coat-color.html

http://www.ratbehavior.org/CoatColorMutations.htm

 

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Inferences From the Study of Labrador Retrievers