Brief Lecture Notes for Unit 3
Genotype refers to the hereditary characteristics inherited from one's ancestors. Phenotype refers to the outward, observable (behavioral or physiological) expression of the genotype. Note that phenotypes are almost always (to a greater or lesser extent, see below) strongly influenced by environmental as well as genetic considerations. In the simplest case, a phenotypic characteristic or trait is controlled by a single pair of genes. However, most characteristics of interest to psychologists, such as intelligence or temperament, are pleiotropic (determined by multiple gene pairs acting interactively or in concert).
Except for the germ cells (ova and sperm), all normal human cells contain 23 pairs of chromosomes (= 46 chromosomes total), the so-called diploid number. These somatic cells reproduce exactly through the process of mitosis. Germ cells, however, are produced through a different process of meiosis, which ensures that the resulting cells have only one chromosome from each pair (the haploid number). Do you see why this is necessary? (What would happen if germ cells were produced through the process of mitosis?)
The different gene possibilities that might be found at a given gene location (all of which potentially govern the same trait) are called alleles. In the simplest situation, the genotype for a given trait consists of two alleles, one from each parent. (Exceptions are noted below.)
In the simplest, conventional situation, there are two possible alleles, one dominant and the other recessive. The dominant allele will be expressed phenotypically if even one allele is present; the recessive allele is expressed phenotypically even if no dominant alleles are present. For instance, the dominant gene for the ability to curl your tongue might be denoted C (capital letters conventionally indicate dominant alleles), and the recessive gene for the inability to do so might be denoted c. There are thus three possible genotypes: CC or homozygous dominant, Cc or heterozygous, and cc or homozygous recessive. (If the two alleles were equally prevalent in the gene pool, we would expect 75% of humans to be able to curl their tongue. Do you see why? Construct a genetic transmission diagram as discussed in lecture.)
In some cases, there are more than two alleles. Usually in such cases, one allele is recessive, but the other two may be co-dominant (neither is dominant over the other, so that both are phenotypically expressed to some extent when one of each is present). For instance (simplifying the real case to some extent), eye color is controlled by three possible alleles: B = brown, G = green, b = blue. The BG genotype results in the "hazel" eye color. (What other possible genotypes exist, and what phenotype would be associated with each? See if you can figure that out.)
Of the 23 chromosome pairs, the first 22 are autosomes (same gene locations are found on the two sister chromosomes of the pair). The 23rd pair are the sex chromosomes, because they control the sex of the individual. The two types are called X and Y chromosomes: XX means a female, XY means a male. (Do you see why the YY pattern is impossible?) The X chromosome contains some gene locations that are not duplicated anywhere on the Y chromosome; thus, some (not all) of the traits governed by gene locations on the sex chromosomes are sex linked (are transmitted differently for male vs. female children, since males and females have different genotypes for these traits). For these traits, females have two alleles (one on each X chromosome), while males have only one allele (on their X chromosome), with a nonexistent gene location or blank on the Y chromosome. For instance, pattern baldness is controlled by two alleles, B = bald and b = nonbald. Possible female genotypes are BB, Bb, bb, but possible male genotypes are B0, b0. (Do you see why this explains the greater prevalence of pattern baldness in men vs. women?) A female with the heterogeneous genotype is known as a carrier: she does not express the recessive trait phenotypically but can pass it on to her offspring. (For more about the determination of gender, see Unit 4.)
Click on this hyperlink to see some practice genetics problems of the type that will be presented on the exam.
Heredity does not rigidly fix behavior or other phenotypic traits, but may establish a range of possible responses or expressions (the range of reaction). In some cases, the range of reaction is wide (environment accounts for most of the variance in the trait), and we then say that the heritability of the trait is low. In other cases, the range of reaction is narrow (heredity accounts for most of the variance in the trait), and we then say that the heritability of the trait is high. The relative role of heredity vs. environment can be quantitatively expressed as a heritability quotient that can range from a low of 0 (heredity plays no discernable role) to a high of 1 (heredity completely determines the trait). A trait with an extremely narrow range of reaction is sometimes said to be canalized. Some representative heritability quotients (if these terms don't mean anything to you, check out this hyperlink on the Big Five model of personality):
Since genotypes themselves can influence the environment in a number of ways, the two kinds of influences can interact and are not wholly separate. Three ways in which this influence can take place are:
Research methods used to study heritability include adoption studies and twin studies. In the former, the trait similarity between a child and her/his adoptive parents (representing environmental influences) in compared to that between the child and her/his biological parents (representing hereditary influences). To avoid confounds, children adopted at birth are studied, but since adoptions are not made at random, confounds can rarely be eliminated entirely. In the latter, similarities between identical (monozygotic) twins are compared to those between fraternal (dizygotic) twins. Hybrid designs can also be used (e.g., studies of identical twins separated at birth). Potential confounds include niche picking and the fact that two children raised in the same home experience many nonshared environmental factors.
See the library reserve article on Abby and Britty Hensel for an interesting sidelight on these and related questions.
Study Guide
1. How do genotype and phenotype differ? What, other than genotype, has an influence on phenotype? What are pleiotropic characteristics?
2. How do the diploid and the haploid number differ? What kinds of cells have each, respectively? How do mitosis and meiosis differ?
3. How do gene locations and alleles differ? How are alleles related to genotype?
4. How do dominant, co-dominant, and recessive alleles differ? How do homozygous and heretozygous genotypes differ? Give examples, and be able to work simple genetics problems such as those provided in the hyperlinked material.
5. How do autosomes and sex chromosomes differ? What are two types of sex chromosomes, and how do they differ? How does this fact determine the transmission of sex-linked traits? What is a carrier?
6. What is the relationship between heritability, range of reaction, and canalization? How is heritability quantified by means of the heritability quotient?
7. Describe there different forms of heredity-environment interaction. Give (or be able to recognize) examples of each. What is niche picking?
8. Describe how adoption studies and twin studies are used to estimate heritability. What are some possible confounds in such studies? What is meant by the notion of nonshared environment?
9. Summarize the case study information relating to Abby and Britty Hensel (paper handout).