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Genetics

Contents of this Section

Inferences from Labrador Retriever Study

Initial Reasoning from the Data

The data from the study with Labrador Retrievers 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 (using the names for the colors as we apply them to human hair color; with Labrador Retrievers, they are called chocolate and yellow)
  • 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, (one from each parent) or
    • two blonde factors, (one from each parent) or
    • one blonde and one brown factor (one from each parent)
  • 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.
  • There are also other genetic factors that influence hair color; Labrador Retrievers come in many colors, ranging from nearly white to pure black. In appropriate genetic studies,
    • black is dominant to brown
    • black is dominant to blonde
    • brown is dominant to blonde

What are these genetic factors, and what determines which one is dominant? Let us begin with dominance.

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Dominance

How might we explain the relationships outlined above? Perhaps we can build a first-level explanation based on the observation that a darker hair color tends to be dominant to a lighter hair color. Perhaps dominance reflects something as simple as the amount of pigment that each genetic factor controls. Let's write this out as a working model:

  1. Each of these genetic variations appears to influence the total amount of pigment that is put into the hair of the individual. For example:
    • the black factor puts a great deal of pigment into hair, so the hair is very dark.
    • the brown factor puts somewhat less pigment into hair, so the hair is much less dark.
    • the blonde factor puts much less pigment into hair, so the hair is very pale.
  2. Each genetic factor, one from each parent, appears to function independently of the other.
    • an individual with a black factor should be expected to have a great deal of pigment in her hair, even if she also carries a blonde factor from her other parent. A "great deal of pigment" plus "a small amount of pigment" is still a great deal of pigment
    • an individual with a blonde factor from each parent should be expected to have only as much pigment in his hair as these two factors can produce, which is "a small amount."
  3. Dominance, then, may simply reflect the overall "activity" of the heritable genetic factors; a factor with more activity should be dominant to a factor with less activity.

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The Nature of Heritable Genetic Factors

The studies of inheritance that we have described here are insufficient to reveal precisely what the "heritable genetic factors" might be. However, a very large number of genetic studies have been performed over the years by a very large number of researchers.

The Early Days

Some of the basic rules of inheritance, such as the fact that there are "heritable factors" that determine an individual's traits, were worked out in the 1860's by Gregor Mendel, who studied inheritance in peas. He discovered dominance, showing that green seed color is dominant to yellow seed color. We can understand this example according to the same working model that we have proposed for hair color above. The "heritable factor" responsible for pigment production appears to exist in at least two versions, one of which has high activity and produces plenty of green pigment, and one of which has rather low activity, and produces very little green pigment.

Mendel also observed that round pea shape is dominant to wrinkled pea shape. This is not a characteristic involving a colored pigment, so it is not so easy to relate to our working model, but let's try to do so. We can suggest that the relevant genetic factors produce the material inside the seed--the nutrients that are stored in the seed to support the growth of the tiny seedling. With less material inside the seed, then the seed coat would wrinkle as the seed matures. By this logic, we can see that the same working model can also apply to dominance of round over wrinkled seed shape. The "heritable factor" responsible for producing this particular seed component appears to exist in at least two versions, one of which produces much less of the seed component than does the other.

Gregor Mendel did not name these "heritable factors." It wasn't until 1909 that Wilhelm Johannsen suggested the term "gene" for these factors. But having a name did not necessarily reveal what a gene actually is, or how a gene works. It was thought that peas might have "a gene for seed color" and another "gene for seed shape," and yet another for flower color. Dogs were thought to have "a gene for hair color," and so on.

It was clear that each gene can exist in different versions--such as the high-activity and low-activity versions of "the seed-color gene," that produce green and yellow seeds respectively. Similarly, there are the high-activity, moderate-activity, and low-activity versions of "the gene for hair color" in dogs, that produce black, brown, or lighter-colored fur.

It was also clear that an individual can carry only two copies of any particular gene, one inherited from the mother and the other inherited from the father. It was clear that during gamete production, the two copies of a gene are independently packaged into gametes, as we have illustrated in our consideration of hair color in Labrador Retrievers. These fundamental insights, initially derived from Mendel's data, now form the basis of Mendel's Laws:

Mendel's Laws

  1. Mendel's First Law: the Law of Segregation
    • Alternative versions of genes account for variations in inherited characteristics. These different versions of genes are called alleles. It is important to remember that one gene may have many alternate versions. For example, a gene responsible for producing hair pigment might have many different versions, producing such variations as very dark hair, dark hair, somewhat dark hair, somewhat light hair, light hair, and very light hair.
    • For each gene, an organism inherits two alleles, one from each parent. This is true whether the alleles happen to be identical or different.
    • The two alleles for each gene segregate during gamete production. This, too, is true whether the two alleles happen to be identical or different. Gametes carry only one set of genes while the "whole" plant or animal carries two sets (one from each parent).
  2. Mendel's Second Law: the Law of Independent Assortment
    • For the genes that Mendel studied, the inheritance pattern of one gene did not correlate with the inheritance pattern of another gene. For example, inheritance of round seeds or wrinkled seeds was entirely independent of the inheritance of green seed color or yellow seed color.
    • We now know, based on vastly more information, that independent assortment occurs for genes that are located on different chromosomes. Genes located on the same chromosome may be inherited in the same pattern, as if they are physically linked. Now that we know that a chromosome is a DNA molecule, and that a gene is a segment of a chromosome, it is easy to explain this observation: genes that show similar inheritance, as if they are physically linked, really are physically linked because they are part of the same DNA molecule--they are on the same chromosome.

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The Middle Years

In the 1940's and 1950's, genetic inheritance was studied intensely in many laboratories around the world. So were physiology, biochemistry, embryology, medicine, and agriculture. Genetic analysis became a favorite investigational approach to learning how living things work, and was increasingly applied to these other fields.

For many years, biochemists had been studying the chemical reactions that occur in living things. They had discovered that nearly all such reactions require cellular catalysts--molecules that are produced by cells and that make the reactions occur. All of the cellular catalysts they analyzed turned out to be of a single class of chemical compound. Since there were so many different kinds of these catalysts, yet they were all chemically similar, they were named proteins, after the Greek God, Proteus. Proteus could take any shape he wanted; similarly, it seemed that this kind of chemical catalyst could take on any of hundreds of different functions. To make matters worse (or at least more curious), many proteins had structural roles in building cells--such as the protein, keratin, of which skin and hair are made. What could these be, these strange molecules named proteins?

A beautiful analysis of proteins, biochemistry, and genetics published by George W. Beadle and Edward L. Tatum in 1941showed clearly that a protein catalyst of this kind, an enzyme, is produced under the control of a gene. More specifically, it was clear from their studies that one gene "codes for" one enzyme. This was (and still is) a profoundly-significant finding.

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The Current Time

So now we know that a gene--a unit of heritable information--functions by directing the production of a particular protein. Genes, we now know, are portions of DNA molecules, which are long strings of smaller chemical units (nucleotides) strung together by chemical bonds. Proteins, whether enzymes (catalysts) or structural proteins, are also long strings of smaller chemical units (amino acids) strung together by chemical bonds. A set of cellular enzymes function to "read" the sequence of nucleotides in DNA, and eventually build a protein with the amino acid sequence determined by the DNA.

There are a number of details involved in this cellular process. However, it is enough at present to know as much as we have outlined above.

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Implications, and a Biochemical Model for Dominance and Hair Color

Our analysis of Labrador Retrievers, Mendel's analysis of peas, and hundreds of other analyses of genetic inheritance compel us to recognize that every gene comes in multiple variations. In fact, new variations of genes can be created with ease. A new variation--called a mutation--can arise spontaneously, or in response to DNA damage caused by X-rays, cosmic rays, radioactivity, chemicals, and oxygen radicals. Virtually all of these are encountered daily at some level. Many of the mutagenic chemicals, for example, are produced by plants to deter animals from eating them; we use many of these plants as spices. But that's OK, really. Mutations are what produce genetic diversity, and account for all of the variation we see among individuals. They are the reason we can tell one another apart.

Above, we proposed that the "heritable factors" for hair color might be responsible for producing a brown pigment. Can we give this model more substance, in the light of our knowledge of genes, proteins, and mutations? Let's see:

  • Perhaps there is a gene that "codes for" an enzyme that is responsible for producing a brown pigment, like this:

a common cellular chemical ----[enzyme action]----> brown pigment

  • Perhaps mutations, accumulated over the generations, have resulted in slightly different versions of this gene, with slightly different DNA sequences. Let's tentatively name this gene "haircolor." Perhaps, we have something like this:
    • one version of the gene, haircolor-black, produces an ezyme that has very high activity, and thus puts a great deal of pigment into hair, so the hair is very dark.
    • another version of the gene, haircolor-brown produces an ezyme that has moderate activity, and thus puts somewhat less pigment into hair, so the hair is much less dark.
    • another version of the gene, haircolor-blonde produces an ezyme that has rather lowactivity, and thus puts much less pigment into hair, so the hair is very pale.

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A General Statement of the Model

  1. genes code for proteins
  2. genes are subject to mutation, which creates multiple variations of any particular gene
    • let's call these different variations alleles
  3. proteins produced from different variations of a gene (different alleles) may have different amounts of activity, ranging from high to moderate to low, and probably to no activity at all.

Implications of the Model

  1. alleles that produce more activity should be dominant to alleles that produce less activity.
    • this "rule of thumb" may have exceptions, but seems likely to cover many situations
  2. there are likely to be many alleles of a particular gene in a population of organisms
    • but any particular individual can carry only two--one from Mom and one from Dad
    • we must remember this if we use a form of shorthand to represent alleles in our discussions, or ensure that our shorthand takes this into account

    For example, we might consider a brownhair gene, as described above, and use bh as a shorthand symbol for this gene. To keep track of three versions of these gene, we might want to use bh-black, bh-brown, and bh-blonde.

    If we were considering only inheritance of black-hair and blonde-hair, we might consider using Bh for black hair, and bh for blonde hair... but it might become confusing when we consider brown hair, which is recessive to black, and dominant to blonde. It might actually be more straightforward to use symbols that allow more options than just uppercase and lowercase.

 

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Sheep Color Patterns