Study Guide Adapted from Carley Karsten
Bio93 Discussion
TA: Alberto Lopez
e-mail: ajlopez1@uci.edu
Lecture 16-18 Study Guide
I have organized some terms and topic that I think are important. This does not mean that
other topics mentioned during lecture or in the book will not be tested. This guide is meant
to clarify and emphasize certain points, NOT to list everything you need to know. I will focus
on tying things together across lectures, and giving real-life examples of the biological
principles that we are learning. Details that I include that I think will be helpful, but that you
don’t need to know, I will write in green. Questions to think about I will write in blue.
Lecture 16: Mendel and the gene
Terminology
1. character: an observable, heritable feature. Different versions of characters are called
traits. e.g. character = eye color, traits = blue, brown, green.
2. gene: a unit of hereditary information, made up of DNA. Different versions of genes
are called alleles. e.g. gene = eye color, alleles = blue, brown, green.
3. genotype: the set of genes that an individual organism contains
4. phenotype: the physical characteristics that an individual organism expresses
5. dominant allele: has more effect on phenotype
6. recessive allele: has less effect on phenotype
7. homozygous: both inherited alleles of a gene (maternal + paternal) are the same
8. heterozygous: both inherited alleles of a gene are different
Mendelian genetics: complete dominance
1. cross a homozygous dominant with a homozygous recessive (PP x pp), and you get
offspring that all express the dominant phenotype (purple), even though their
genotypes contain both alleles (Pp, purple and white). Since P is dominant over p,
that’s the one that we see.
2. cross two heterozygotes (Pp x Pp), and you get an interesting mix:
a. genotypes: PP, Pp, Pp, pp (1:2:1)
b. phenotypes: purple, purple, purple, white (3:1)
c. notice here that in order to express the recessive phenotype (white) you need
both alleles to be p. To express the dominant phenotype, all you need is at
least one allele to be P.
3. Law of segregation: the 2 alleles for each gene segregate into different gametes. One
from the mother, one from the father.
4. Law of independent assortment: different genes are inherited separately. What is an
exception to this rule? (see lecture 18)
5. Test cross: to figure out the genotype of an individual that expresses the dominant
phenotype (it could be either PP or Pp), cross it with a homozygous recessive
individual. Why?
6. Punnett squares:
a. 1 character: Pp x Pp = 1:2:1 genotype ratio
b. 2 characters: PpRr x PpRr = 9:3:3:1 genotype ratio
Study Guide Adapted from Carley Karsten
c. 3 characters: PpRrYy x PpRrYy = complicated ratio. Don’t draw out a 3x3
Punnett square. When you have to do a 3-character question on the final, do
the math (I demonstrated this in class last week):
i. draw out individual Punnett squares for each character (e.g. Pp x Pp)
ii. calculate the ratio (fraction) of offspring in each Punnett square that
match the phenotype you want
iii. multiply all the fractions together to get the total probability of
getting offspring with the desired phenotypes
Lecture 17: Human genetic disorders
Complex patterns of inheritance
1. Incomplete dominance: the dominant allele does not completely determine
phenotype
a. Phenotypes of heterozygous individuals are like a mixture of the dominant
and recessive phenotypes
b. If the genotype ratio of offspring is 1:2:1, the phenotype ratio is also 1:2:1
2. Co-dominance: a gene can have multiple dominant alleles. When they are both
present, they both contribute to phenotype. Remember, since they’re both dominant,
neither one will mask the other.
3. Pleiotropy: a single gene affects multiple phenotypes
Note the difference between pleiotropy and polygenic inheritance
4. Epistasis: one gene alters the expression of another gene. This occurs when one gene
is dependent upon another. For example, the color of your eyes is determined by the
color of the pigment in your iris. If your iris contains no pigment, however, then you
will not express an eye color regardless of what your eye color gene says.
Note: do not confuse epistasis with epigenetic
5. Polygenic inheritance: many genes affect a single phenotype
6. Environmental effects: inherited genes can cause individuals to react differently to
the environment. For a normal range of genotypes in a normal environment,
everyone might look the same. However, if you take that same range of genotypes
and place them in a new environment, they might start to behave differently due to
genetic variation.
For each of the inheritance patterns described above, find an example NOT already listed in
the class notes. Even better, pick one of the inheritance patterns and find a few examples:
do you see any trends? What types of characters exhibit pleiotropy? Polygenic inheritance?
Pedigrees
1. display phenotypes, from which we can infer genotypes
e.g. if the parents both exhibit a dominant trait, but they have a child that expresses
a recessive trait, we can infer that the parents are both heterozygous
2. there will be a pedigree question on the final. Practice looking at these until you are
able to identify homozygous vs. heterozygous individuals
Study Guide Adapted from Carley Karsten
Lecture 18: Alterations of chromosomes
Drosophila genetics: a “+” next to an allele indicates that it is the wild-type (“normal”) allele
Thomas Hunt Morgan noted that only male flies showed a mutant trait, leading him to guess
(correctly) that the gene for that trait is located on the X chromosome.
This was super important because it provided evidence that every gene is located on a
specific part of a specific chromosome. Before this experiment, people wondered if genes
were mixed together randomly.
Gene linkage
1. traits that are located nearer to one another on a chromosome are more likely to be
inherited together
2. traits that are located near to one another but NOT inherited together were likely
separated during crossing over
3. recombination frequency = fraction of offspring that have weird combinations of
traits (e.g. traits that are located near to one another but were NOT inherited
together)
4. the maximum recombination frequency between two genes is 50%
5. knowing the recombination frequency between two genes allows us to infer how
close they are to each other
a. if they’re close together, they are UNLIKELY to cross over separately from one
another
b. if they’re far apart, they are very likely to cross over separately from one
another
Sex linkage
1. sex chromosomes DO NOT just contain sex-related genes
2. in mammals, females have 2 X chromosomes, while males have an X and a Y
3. females compensate by shutting off one of their X chromosomes (forms a Barr body,
where DNA is packed so tightly that transcriptional machinery can’t get to it)
4. study the pattern of sex-linked inheritance, and be able to identify it on a pedigree
Genetic disorders / chromosome structure
1. deletion: lose part of a chromosome
2. duplication: part of the chromosome is repeated
3. inversion: part of the DNA sequence is reversed
4. translocation: part of a chromosome is moved to a different chromosome
For each alteration listed above, the part of the chromosome being altered could be a piece
of a gene, an entire gene, or several genes. The more you lose, the worse the phenotypic
effect will be.
Epigenetics: changing gene expression by changing chromatin structure. If DNA is too
densely packed, transcriptional machinery can’t get to it, and the gene is turned off.
e.g. genomic imprinting (NOT the same as sex linkage… why?)