Genetics
Fall 2002

Behavior is determined by both nature and nurture. That is, your genes predispose you to certain behaviors (Nature), and the environment influences whether these behaviors are realized (Nurture). Genes influence both physical traits (e.g., height, eye color) and complex behavioral traits such as personality, intelligence, and mood. The goal of genetic neuroscientists is to separate out the effects of environment and genes.
 

Important Terminology:

DNA is a chain of molecules that hold the genetic code. These molecules form a double helix. The goal of the human genome project was to map the sequence of these molecules.
Chromosomes are composed of an extremely long strand of DNA. Humans have 23 pairs of chromosomes.
Genes are relatively short segments of DNA that code for specific traits. There are many genes on each chromosome. Chromosomes are paired (called homologs) because you get one from each parent. Thus, your traits are determined by the genes passed to you by your mother and your father. These gene pairs are called alleles.
heterozygous alleles code for different traits (2 different genes).
homozygous alleles code for the same trait (2 identical genes).


Single gene trait: Some traits are coded by a single gene. These traits form bimodal distributions. That is, you either have the trait or you don't (e.g., color blindness).
Quantitative trait: Some traits are coded by multiple genes. These traits form continuous or normal distributions (e.g., height).
 

Examples of gene transfer are easier to understand using single gene traits. However, the same principles apply to quantitative traits. An example of the offspring produced when two people with heterozygous alleles mate is shown below.

Father's genes are 1 & 2
Mother's genes are 1 & 2 Offspring get one gene from each parent and can end up with homozygous or heterozygous alleles (11 or 12 or 21 or 22)


Linking Genes to Specific Behaviors:

Inbred strains of animals are created to produce animals that are genetically identical to each other, yet different from other inbred strains. Any differences in behavior between the strains must be caused by the difference in genetics (differences within a group are caused by environmental influences). The goal is to make a strain with homozygous alleles.

Step 1: Mate a heterozygous male (1 2) with a heterozygous female (1 2) will produce offsping with the following alleles: 11 12 21 22
Step 2: Mate sister 11 with brother 12 produces offspring with alleles: 11 11 12 12
Step 3: Mate sister 11 with brother 11 produces offspring with alleles: 11 11 11 11
    Repeated making will also produce a strain that is all 22. Inbreeding eliminates genes from the population. That is, it removes trait variability because related animals share many of the same genes. In mice, 8-10 generations will produce a 99% inbred strain. Although the allele selected is random, it can becomes fixed in either state (e.g., 11 or 22).
 

The two approaches to link genes to specific behaviors are:

1. Top-down approach

Test two inbred strains on a particular task (e.g., locomotor activity).

a) If this behavior is coded by a single gene trait, then two distinct responses will result (e.g., active and inactive).

b) If this behavior is coded by a quantitative trait (i.e., multiple genes), then a range of responses will result depending on the combination of genes. If three alleles code the trait, then there are six possible combinations, and thus, there should be six different responses on the task.
 

Allele A 11 11 11 22 22 22
Allele B 11 11 22 11 22 22
Allele C 11 22 22 11 11 22
Rat Strain Bob Sam Joe Cal Doc Skeeter
2. Bottom-up approach

This makes use of transgenic animals. That is, models in which specific genes of interest are inserted or removed (called a knock-out model). A "bad" piece of DNA is inserted in place of a gene (i.e., the gene is knocked out), then the animal is inbred for the homozygous blank allele. The animals are then tested to determine what behavioral deficits are caused by loss of that gene.
 

Human Genetics

How do you do genetic studies in humans? You can't control human breeding and you can't knock out genes in humans so you need to think of another solution. Hint: Are there any humans who have nearly identical genes?
 
 

The studies described above demonstrate that genes influence behavior. Genes do this by the proteins they make.

Step 1: DNA splits so messenger RNA can copy the sequence of base pairs (called transcription). The mRNA moves out of the nucleus (Figure 2.8) to the endoplasmic reticulum.
Step 2: Each of three base pairs on the mRNA strand bind to a different amino acid creating a chain of amino acids (called a protein) to form.
Step 3: These proteins move to various places within the cell and perform various jobs (e.g., enzymes, receptors). The genes that drive human behavior today are the genes that have been passed down from one generation to the next. Only genes that help you reproduce will get passed on. If an individual does not reproduce then that individual's genes will disappear. The fact that this happens is based on three principles: 1. Everyone has different genes. That is, there is variability.
2. Some genes allow for greater reproduction. That is, there is unequal reproduction within a species.
3. Those who reproduce will pass on their genes. That is, traits are inherited. Because of these three principles, evolution of the species is inevitable. It is surprising that evolution is such a controversial topic in America given that these three principles are so simple. This controversy has more to do with politics than with science because the definition of evolution, a change in gene frequencies in a population over generations, is not a controversial idea. Dog breeders take advantage of these principles to breed dogs with very specific traits. Gene selection also occurs naturally as is evident by the evolution of antibiotic-resistant strains of bacteria.

A small increase in the number of offspring in one strain of animal will have a huge effect on the gene frequency in the population in just a few generations. For example, compare two mothers one of whom always has triplets and another who always has twins. How many relatives will each mother have in five generations? Do the math and you will see that the gene frequency in the population has changed from 1:1 to 7.6:1 in favor of the relatives who have triplets. As stated in the previous paragraph, this is the definition of evolution.
 

Sociobiology

The study of how genes influence behavior is called sociobiology. Sociobiology predicts that humans will engage in behaviors that enhance their ability to pass their genes to future generations (i.e., survive and have children who survive).

Sociobiology can even account for complicated behaviors like altruism. Altruism seems unlikely because doing things for others at a cost to yourself does not seem to help you pass on your genes. However, most altruism can be explained in ways that help the altruist:

1. Kin selection. That is, you are more likely to help relatives who share your genes than strangers who don't.
2. Reciprocal altruism. That is, you are more likely to help those who help you. These are complex human behaviors mediated by the nervous system. Genes have effects on behavior by influencing the anatomy, physiology, and pharmacology of the nervous system.
 

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