In general, the best preparation for the exam is to review your homework assignments and lecture notes. You should review my lecture notes posted on this website, paying special attention to topics highlighted below.
It is also important to review key terms in the glossary. You should have a working, practical understanding of the terms that we use every day in class. Here are some especially important terms:
chromosome
chromatid
homologous chromosomes
sister chromatids
diploid
haploid
mitosis
meiosis
gamete
fertilization
crossing over
bivalent
chiasma (chiasmata)
sister_chromatid_cohesion
metaphase plate
kinetochore
metaphase checkpoint
nondisjunction
Mendelism
heredity
true-breeding (
pure-breeding)
P generation
self-fertilization
cross-fertilization
F1 generation
F2 generation
homozygous
heterozygous
trait
genotype
phenotype
dominant
recessive
First Law: Segregation
Second Law: Independent assortment
allele
gene
Punnett square
testcross
monohybrid
dihybrid
trihybrid
probability
incomplete dominance
codominance
epistasis
pleiotropy
penetrance
expressivity
coupling
repulsion
linkage analysis
We have also seen a large number of human medical terms used to describe the phenotypes of human genetic variants. In general, you are not required to be able to define any of these terms. If the term is not in the glossary, you are not required to know what it means.
Different people learn things differently. Meiosis is an elusive subject for many people. You should be able to describe and recognize the appearance of chromosomes at different stages of meiosis and in mitosis. You should understand the consequences of recombination and nondisjunction.
You might find it helpful to make chromosome models out of yarn or pipe cleaners. The models used in class are clothesline covered with duct tape. Make models of two pairs of homologous chromosomes that differ in their size and centromere position. Put your models through mitosis and meiosis.
We have discussed sex chromosome aneuploids in humans and Drosophila as a way of understanding the meiotic division at which sex chromosome nondisjunction occurred. Nondisjunction in human males is particularly informative, as we discussed in answer to a question at the beginning of lecture 7.
Monohybrid cross. You have worked a number of problems dealing with monohybrid crosses. You should understand how to recognize which allele is dominant and which is recessive. You should understand how to infer the genotype from the phenotype. You should know what types of ratios to expect from monohybrid incrosses and testcrosses. You should be able to apply this knowledge to human pedigree analysis.
Dihybrid cross. You have worked problems dealing with dihybrid crosses. You should be able to make predictions about the probability of a particular genotype given the genotype of the parents. You should be able to predict the genotype of progeny from a dihybrid cross given the results of a testcross.
Trihybrid cross. This is nothing more than a dihybrid cross with an extra pair of alleles segregating. You should be able to predict the probability of specific genotypes and phenotypes. You should be able to describe the genotype of an individual given the results of a testcross.
Remember for a dihybrid or trihybrid cross, it is faster to solve most problems if you figure the results for each allele pair separately, then multiply the frequencies, rather than trying to draw out a large Punnett square that includes all pairs of alleles. This is covered in lecture four, review question 5.
For dihybrid and trihybrid crosses, you will always be told if all of the genes in question segregate independently.
Multiple alleles. One of your homework problems dealt with multiple alleles of the tyrosinase gene, which causes albinism. We encountered multiple alleles of the mouse waltzer gene, and in an extreme case, the human CFTR gene, with over 1,000 alleles. Multiple alleles illustrate that different alleles can have produce different phenotypes, and it only makes sense to compare two alleles for a specific trait when deciding to call one allele dominant and the other recessive. Recall that it the case of the T allele in Manx cats, T is dominant to t with respect to taillessness, but T is recessive to t with respect to embryonic lethality (an example of pleiotropy).
Blood type. Human ABO blood type is controlled by two codominant and one recessive allele. One of your homework problems, and several problems that we worked in lecture 5, ask you to assess paternity based on blood types.
Epistasis. Our discussion of gene interaction in lecture 5 focused on epistasis in a trihybrid cross in mice. This is a great source of test problems.
With our understanding of sex linkage derived from Drosophila, we are in an excellent position to solve human pedigrees to determine whether a particular trait is dominant or recessive, autosomal or sex linked. While it is always possible to attempt to solve pedigrees exhaustively, there is a fast approach to pedigrees outlined in lecture 6.
We have gone over linkage analysis in considerable detail in order to give you a foundation for ideas that we will develop later.
For now, you should be able to look at the progeny from a testcross with two markers and infer the parental genotype (including whether the recessives are in coupling or repulsion). You should also be able to calculate the observed frequency of recombination between the markers.
Here is the exact text of my review points as presented in lecture on Thursday, September 12.
Vocabulary
1. Terms we use every day in your working vocabulary so that you can read the test questions
2. Medical stuff (kyphoscoliosis, gynecomastia) not required
Meiosis
1. Describe, draw, or recognize chromosomes at different stages of meiosis
2. Follow the consequences of recombination and nondisjunction
3. Correlate segregation of markers with chromosome behavior
Monohybrid Cross
1. The building block!
2. Multiple alleles (e.g. albino)
3. Predict probabilities and ratios
4. Predict genotypes
5. Human pedigrees
Dihybrid Cross
1. Two monohybrid crosses!
2. Gene interaction (e.g. albino)
3. Predict probabilities and ratios
4. Predict genotypes
5. Human pedigrees?
Trihybrid Cross
1. Three monohybrid crosses!
2. Gene interation (e.g. albino)
3. Predict probabilities and ratios
4. Predict genotypes
Linkage
1. Special case of sex linkage
2. Be able to infer genotype and coupling or repulsion from testcross results
3. Be able to calculate recombination frequency from cross with two linked genes
We opened for questions. I began by pretending that people had asked some really good questions, listed below.
1. Show me sex chromosome nondisjunction in human males again. Please see this and this. Think about various ways that we might turn this into a problem using sex-linked markers.
2. What was that trick for pedigrees? Please see this.
3. Show me blood type again. Please see this.
4. What do you mean by epistasis? Please see this.
5. What was that trick for dihybrid crosses? Please see this.
6. How do I calculate a map distance? Take the number of recombinant (nonparental) types for a pair of markers and divide by the total. Please see this.