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Genetics

Contents of this Section

Color Patterns in Sheep

We can follow each allele in every cross

Introduction

In many species of both plants and animals, there are color patterns. In corn, for example, different varieties produce purple pigment in the leaves, or the stem, or the whole kernel, or just part of the kernel, or any combination of these. Similarly, in animals, we often see individuals with dark markings on a lighter background color. Can we use these interesting color patterns to learn about inheritance?

What we have learned so far:

In our investigation with Labrador Retrievers, we have learned that there are heritable units of genetic information, which we call genes, and that for any particular gene, we inherit one copy from Mom and one from Dad, so we have two copies of every gene. But, we have seen that a gene may exist in different versions (which geneticists call alleles). The particular characteristics of an individual depend upon the particular alleles inherited from Mom and Dad. With respect to hair color, we have hypothesized that there is a gene that determines the structure of an enzyme that produces a brown pigment. Different versions of that gene result in more-active or less-active enzymes, and consequently more pigment (darker hair) or less pigment (lighter hair). We have also hypothesized that some alleles are dominant to other alleles--specifically, because the more-active enzyme produces enough pigment to over-shadow the effect of the less-active enzyme.

What we'd like to see:

But, we'd like to be able to follow every allele, rather than infer its presence when we can't actually "see" its effect. If we could look at the characteristics of any individual in our study, and say definitively that "this individual has these two alleles for this gene," we might be able to "see" inheritance more clearly than with alleles that show a relationship of dominant vs recessive.

Is there, or can we even imagine, any gene that we can follow in this way?

Our investigation of hair color suggests a possibility. To produce dark pigment in hair, an individual must produce the enzyme that makes the pigment. An individual with light-colored hair might carry low-activity alleles of the gene for the pigment-producing-enzyme--or--they might carry some kind of regulatory gene that controls the activation of the pigment-producing-enzyme. We actually have seen the effect of one such regulatory gene, although we probably didn't notice: the "mask" of Labrador Retrievers and many other dogs, illustrated below. The "classic" mask results from an allele of a control gene that "turns on" pigment production in the eyelids, nose, and lips. (Or we might think of it as "turning off" pigment production in other parts of the body.) The dogs on the left, below, have no mask; they may carry an allele of this gene that does not "turn on" pigment production in any parts of the body. The dogs on the right have more complex patterns, in which pigment production is activated in more places than the classic mask.

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Black Sheep

We have already studied dogs, and developed an initial understanding of inheritance. Let's look at another animal species, and see if the same kinds of rules apply. Let's choose an animal that has color patterns. We think we may be able to understand the genetic control of fur pigmentation as the production of pigment; patterns may give us some insight into the control of pigment production as well as telling us more about inheritance in general.

The traditional sheep is white. But often we see black sheep. If we talk to a sheep breeder, we may find that the inheritance of coat color is much more complicated than we are presenting it here, but this presentation should give us a reasonable understanding of both inheritance and the basis of coat color pigmentation.

In this scenario, a "novel" sheep was discovered some time ago that had black fur. This genetic factor was heritable, and eventually was named the Black gene. As more was learned about this gene, it was found that the Black gene determines whether skin cells "turn on" the enzymes that produce the pigment.

The first allele of this gene, producing sheep with black fur, led to naming the gene Black. There were, therefore, two alleles known--the allele that results in black fur, and the allele that results in white (unpigmented) fur. At some point in time, another allele was found. It became necessary to give the different alleles names that would enable people to distinguish them from one another. At first, the original Black allele was called Black-1, and the second allele was called Black-2. But then another allele was found. As more new alleles were found, they were named according to the part of the body in which they activated pigment production.

Color Pattern:

Allele Name
Black-2
Black-ears
Black-feet
Black-spotted
Shorthand symbol
Black-2
Black-e
Black-f
Black-sp

Because the Black enzyme produces a pigment, an individual carrying any of these alleles produces the pigment in the pattern determined by the allele. We can, therefore, follow inheritance easily with these alleles. For example:

x
Black-2
Black-f
  Black-2  
Black-f

With this particular cross, we have mated a Black-2 sheep with a Black-f sheep, and obtained a heterozyous offspring carrying both the Black-2 allele and the Black-f allele. We have written the allele combination of the heterozygote by writing out both allele names, separated by a horizontal line. We could use a slash, if we prefer, and write Black-2/Black-f or Bl-2/Bl-f, but it's a little easier to see the allele designations if we use a horizontal line. (Some geneticists use slashes, and some use horizontal lines.)

The other two alleles show a similar pattern:

x
Black-e
x
Black-sp
   Black-e   
Black-sp

Now...what do we find when we cross these two heterozygotes to each other?

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A Cross Between Two Different Heterozygotes

When male and female animals produce gametes, each gamete receives one allele of each gene. Gametes do not (normally) receive both alleles of the same gene. From examining the variety of offspring, we infer that for any gene, the two types of gametes are produced in equal numbers. This makes sense when we realize that gametes are produced by meiosis, which separates the maternal chromosome (which carries one allele) from the paternal chromosome (which carries the other allele).

This gives us two types of eggs, and two types of sperm. Since either type of sperm can fertilize either type of egg, four different allele combinations can be produced when eggs are fertilized.

Therefore, we end up with four different color patterns among the offspring. Of course, since sheep tend to have no more than one or two offspring at a time, we need to perform many crosses like this with many sheep if we want to be certain that we have recovered all of the possible types of offspring. One thing we find as we do this is that no matter how many offspring we get, and no matter how many times we do this cross, there are only four different patterns among the offspring.

We also see that none of the offspring have the same color pattern as their parents--each has inherited part of their color pattern from each parent. Apparently, these different alleles of the Black gene always segregate into different eggs (or sperm) during gamete production. In fact, this behavior is how we determine whether different genetic factors are alleles of the same gene:

Operational Definition of Alleles: Alleles of the same gene segregate from each other during gamete production (i.e. during meiosis).

In this case, it's hard to tell by looking at the sheep whether Black-feet might be due to the same gene as Black-ears or to a different gene. The only way to find out is to do the crosses: construct heterozygous individuals (Black-feet / Black-ears) , then examine their offspring. If none of the offspring have both black feet and black ears, then it appears that the two genetic factors segregate from each other, and may actually be alleles of the same gene.

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Dominance and Co-Dominance

The pattern of inheritance that we have seen here is different from that which we observed with Labrador Retrievers. There, black was dominant to brown and to blonde, and brown was dominant to blonde. Here, each of the patterns (black ears, feet, face, or spots) seems to be "visible" regardless of the other allele carried by any individual sheep. Is this difference due to fundamentally different inheritance, or can the difference be explained easily?

Remember the biochemical model we inferred from the study of Labrador Retrievers:

  • genes code for enzymes
  • enzymes involed in hair color produce pigments
  • an enzyme (encoded by a gene) that has high activity will produce more pigment than an enzyme with low activity--and will therefore be dominant.

Dominance, in that study, referred to different variations of the same characteristic, or "trait," of the same part of the individual organism. In this study, the different variations affect different parts of the individual. That is, the Black gene determines which part of the animal produces pigment, so we can see different parts producing pigment at the same time.

The underlying biochemistry is much the same, however. If the cells produce pigment, then the fur contains pigment, and we see it. If one allele turns on pigment production in the ears, we see black ears; if another allele turns on pigment production in the feet, we see black feet. If an individual has both alleles, then pigment production is turned on in the ears (due to one allele) and in the feet (due to the other allele).

We can create a term for this kind of relationship--co-dominance. We will use this to refer to allele combinations in which we see the contributions of both alleles, rather than seeing that one allele appears to be "masked" by the other.

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A Simpler Diagram to Use When Thinking About Crosses

Looking back at the cross between two heterozygotes (above), it seems that we used fairly cumbersome reasoning to figure it out. Can we simplify this reasoning somehow? We probably can, using a strategy first suggested for this purpose by Reginald C. Punnett, and described in his book, Mendelism, in 1905.

Our reasoning was to think of the male producing two alternative types of gametes, one type carrying one of his Black alleles, the other type carrying the other of his Black alleles. We illustrated this (above) with arrows and images of sperm cells. Similarly, we illustrate the female's gametes with images of egg cells. Then, in a separate illustration, we considered the possible combinations of egg and sperm.

Punnett's idea was to write the possible types of gametes on the top and left sides of a table, like this:

Punnett didn't draw in the images of egg cells and sperm cells, but it may help us to do this at first.

Now, having identified the gametes that can be contributed by each parent, we can fill in the table, like this:

This gives us, very conveniently and easily, a good view of the allele combinations that are possible in the offspring. Needless to say, of course, these are only the possible combinations; any single offspring could be any one of these four possibilities. Unless there is something peculiar about the influence of one of the Black alleles on gamete survival, on fertilization itself, or on the embryonic development of the embryo, there seems to be no reason to imagine that any of these four possibilities would be any more or less likely than any other. If we were to count the numbers of each type from a large number of matings, we would expect to find that we get about 25% of each.

This type of table proved to be very helpful in Reginald Punnett's work. It helped him solve many genetic puzzles without resorting to the sort of drawings we used here. It was, in fact, fascinating to the general populace of England. His book, Mendelism, was the first genetics text published, and by 1908 was tied for #1 (with a novel) on the Westminster Gazette's best-seller list.

The Punnett Square, as this type of table is now called, provides an easy way to identify the possible allele combinations in the offspring of a cross. It also helps identify--or predict--the possible traits that the offspring may have. Each allele combination ("genotype") confers a particular set of characteristics on the individuals that carry that allele combination. Therefore, when we prepare a Punnett Square like this, we can also "see" the characteristics of the offspring (the "phenotype"). The table below illustrates this, without the images of egg and sperm cells; roll your mouse over the boxes in the table to call up the images of the individuals--that is, to see how the genotype listed in the table shows up in the phenotype, or traits, of individual sheep.

The Cross

 
Bl-e
Bl-sp
Bl-2
Bl-f

Because of the strict correlation between the genotype (the allele combinations, or "type of genes") of an individual and that individual's characteristics (or traits, collectively called the phenotype), a Punnett Square enables us to predict both the distribution of alleles among the offspring and the distribution of characteristics. In this particular example, each of the alleles produces a unique color pattern that is not influenced by the other alleles; we can identify the phenotype conferred by each allele quite easily. We can, therefore, follow the inheritance of each allele.

We see that the Black-2 allele, which leads to pigmented hair on the face, is inherited by half of the offspring, and the Black-f allele, which leads to pigmented feet, is inherited by the other half of the offspring. These two alleles were carried by the mother; apparently, they segregate from each other during gamete production, and end up in different offspring. Similarly, the father's two alleles, Black-e and Black-sp, segregate from each other. We are unlikely to see this pattern of inheritance if we look at only two or three offspring; we would need to study a very large number before we could be reasonably certain that this is the pattern of inheritance. But once we have a working model for inheritance--once we have studied inheritance enough to have discovered that genes have multiple alleles--then we can use Reginald Punnett's method to analyze the genes behind the traits. We can use his Square to identify the possible allele combinations among the offspring, and even predict the frequency with which any particular allele combination should occur.

Here, where we can follow every allele independently, because each produces its own characteristic color pattern, the inheritance and the Punnett Square are reasonably straightforward. But what if we look at alleles that show a dominance/recessiveness relationship, rather than this pattern of co-dominance?

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Analysis of Dominant and Recessive Alleles

Just as with Labrador Retrievers and humans, there are some individuals with brown hair rather than black or blonde. One of the genes responsible for brown hair is an allele of Black, that--as we inferred from Labrador Retrievers--produces less of the pigment. Therefore, we expect that the black-hair trait should be dominant to the brown-hair trait; that is, the Black-1 allele should be dominant to the Black-br allele. In a cross of a true-breeding black breed of sheep to a true-breeding brown breed, we see exactly that: all of the offspring have black hair:

What might we see if we were to cross these offspring among each other? Let's say this using "genetics language:" if we consider the original parents to be the Parental Generation (the P cross), then their offspring are the F1 generation. If we cross two of the F1 individuals, what do we see in the F2 generation? We can use Punnett's invention to figure this out.

 
Bl-1
Bl-br
Bl-1
Bl-br

Again, you can move your mouse over the table to see the phenotypes of the sheep in each category.

What do we see? It looks like both parents will produce Black-1 gametes and Black-br gametes, since both were heterozygous for these alleles. Among the possible egg/sperm fertilization events, there are four possible combinations, which we have written in each of the boxes of the table. What's interesting, and easy to see when we use this diagram to figure out the possible allele combinations, is that heterozygous animals (Bl-1 / Bl-br) can be produced by two of these combinations. A a Black-br sperm can fertilize a Black-1 egg, or a Black-1 sperm can fertilize a Black-br egg to produce this same allele combination. (We didn't see this before, because each of the alleles was unique.)

Therefore, we expect that if we were to examine a large number of offspring, we might find that 1/4 of them carry two copies of the Black-1 allele (i.e. they are homozygous for this allele), 1/4 of them carry two copies of the Black-br allele (i.e. are homozygous), and 2/4 (i.e. 1/2) of them carry one copy of each allele (i.e. they are heterozygous for these alleles).

This ratio of 1/4 : 1/2 : 1/4 (or 1:2:1) is what we expect for the possible combinations of alleles--the genotypes of the offspring. What about the coat color? Roll your mouse over the table, and you'll see that only the Black-br homozygous animals have brown hair. Brown is recessive to black, so the individuals that have both alleles have black hair. So do the individuals that have only the Black-1 allele. So, in terms of the hair color trait (the phenotype), we would expect a ratio of 3:1 among the offspring of this cross.

Will we actually see these ratios among the offspring? Probably not, unless we work with sheep breeders who have large flocks. Like humans, sheep have only one or two offspring at a time, so it will not be easy to obtain very large numbers in any single family. The smaller the number of offspring, the less accurate our ratios will be--a fact that is obvious when we have a very small number such as "only one."

Although the actual numbers of offspring may be small for humans or sheep, we can use the principles we've seen here to think about the likelihood that any particular new-born individual will have a particular allele combination, or a particular hair color. The chance of having brown hair, rather than black, is 1 out of 4, or 25%. The chance of having black hair is 75%--but the chance of having two copies of the Black-1 allele is only 25%. We can determine the probabilities simply by counting the boxes in the Punnett Square, and dividing by the total number. We can also use more sophisticated mathematics, but Reginald Punnett created such a useful tool with this Square that we often don't need to use more sophisticated math.

 

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Re-Examining My Family