Genetics and Evolution (HY)

Question 1

what is a hemizygous genotype?

Answer 1

a genotype where only one allele of a gene is present (ex. some alleles are parts of X chromosome in males)

Question 2

what is complete dominance, codominance, and incomplete dominance?

Answer 2

complete dominance: one dominant and one recessive allele for a gene are present, and dominant allele masks recessive allele

codominance: more than one dominant allele is present (ex. presence of both alleles for A and B blood antigens)

incomplete dominance: heterozygote genotype but presents a phenotype that is intermediate of homozygous dominant and homozygous recessive genotypes

Question 3

what is penetrance?

Answer 3

proportion of the population with a given genotype who actually express the phenotype
(100% penetrance - if you have the genotype, you will definitely present the phenotype)

Question 4

what is the difference between constant and variable expressivity?

Answer 4

constant expressivity: all individuals with same genotype have same phenotype
variable expressivity: individuals with same genotype have different phenotypes

Question 5

What are the four tenets of mendel’s first law of segregation?

Answer 5

1. genes exist in differing forms (alleles)
2. organisms have two alleles for each gene (one from each parents)
3. two alleles segregate during meiosis, forming gametes with one allele for inherited traits
4. when two alleles for a gene are different, one allele is fully expressed (dominant) while other is silent (recessive)

Question 6

what forms of dominance are exceptions to mendel’s first law of segregation?

Answer 6

codominance and incomplete dominance

both are cases when two different alleles are both expressed (neither is silenced)

Question 7

what cellular process corresponds to mendel’s first law of segregation?

Answer 7

anaphase I of meiosis: separation/segregation of homologous chromosomes

Question 8

what is mendel’s second law of independent assortment?

Answer 8

states that inheritance of one gene does not affect inheritance of another

Question 9

what cellular process allows for independent assortment?

Answer 9

recombination through tetrads

Question 10

what types of genes are exceptions to the law of independent assortment?

Answer 10

linked genes

Question 11

what is the filial or F generation?

Answer 11

offspring generation

Question 12

what is the genotype and phenotype ratios of offspring after crossing two heterozygotes for a trait with complete dominance?

Answer 12

genotype- 1:2:1

phenotype- 3:1

Question 13

what is a test cross or back cross?

Answer 13

• used to determine an unknown genotype
• cross of an unknown genotype with a known homozygous recessive, and use ratio of offspring phenotypes to determine unknown parental genotype

Question 14

what is the phenotypic ratio of offspring for a dihybrid cross between two heterozygotes with complete dominance?

Answer 14

9:3:3:1

Question 15

On what chromosome are sex linked traits?

Answer 15

X chromosome

assume also that they are recessive, unless told otherwise

Question 16

For a man who has a sex-linked trait, how will his daughter be affected? Can his sons receive a sex-linked trait from him?

Answer 16

all his daughter will either be carriers or express the trait (if mother also has the trait).

He can never transfer a sex-linked trait to a son. To have a son, a sperm with the Y chromosome must fertilize the egg and sex-linked traits are on the X chromosome.

The reasoning for both situations arises from the fact that it is the sperm that determines offspring sex.

Question 17

what is recombination frequency (theta)?

Answer 17

likelihood that two alleles are separated from each other during synapsis in prophase I of meiosis

Question 18

For genetic maps, what does 1 map unit or 1 centimorgan represent in terms of recombination frequency?

Answer 18

1 centimorgan = 1% chance of recombination occurring between two genes

Question 19

what are the 5 conditions for Hardy-Weinberg equilibrium (gene pool to be stable and for evolution not to occur)?

Answer 19

1. population is large (no genetic drift)
2. no migration into/out of population
3. all genes are equally successful at reproducing
4. random mate selection (no sexual selection)
5. no mutations that affect gene pool

Question 20

what are the two equations for hardy-Weinberg equilibrium?

Answer 20

p+q=1
p^2 + 2pq + q^2 = 1

p^2 = frequency of TT 
q^2 = frequency of tt 
2pq = frequency of Tt

Question 21

what part of the hardy-weinburg equilibrium equations correspond to the frequency of the dominant phenotype?

Answer 21

p^2 + 2pq = 1 - q^2

where p is frequency of T and q is frequency of t

Question 22

what is natural selection?

Answer 22

survival of the fittest; a mechanism for evolution

chance variation exists between individuals, and variations that are favorable increase an individual’s fitness (reproductive success) –> individual fitness affects relative genetic contribution to next generation

Question 23

What is the modern synthesis model for natural selection, also termed neo-Darwinism?

Answer 23

accounts for mutation and recombination as mechanisms of variation that affect differential reproduction (likelihood of reproductive success)

Question 24

what is inclusive fitness? give an example of a behavior that may evolve to increase inclusive fitness?

Answer 24

measure of an organism’s success in a population, beyond just fitness of an individual for reproduction

ie: # of offspring, success in supporting offspring, ability of offspring to support others

ex behavior: altruism

Question 25

what is the theory of punctuated equilibrium?

Answer 25

idea that changes in species occurs in rapid bursts as opposed to gradual, even change over time

Question 26

what are the three types of natural selection? define each

Answer 26

1. stabilizing selection: keeps phenotypes in a range, selects against extremes (ex. birth weight)
2. directional selection: selection towards one extreme phenotype (antibiotic resistance)
3. disruptive selection: selection towards two extreme phenotypes (finches)

Question 27

Disruptive selection is facilitated by the existence of what in populations?

Answer 27

polymorphisms (naturally occurring differences in form in a population)

Question 28

What is adaptive radiation?

Answer 28

increase in # of species from a common ancestor

contributes towards disruptive selection –> species take on different niches

Question 29

what is a species?

Answer 29

largest group of organisms able to breed to produce fertile offspring

Question 30

what are the two category of mechanisms for reproductive isolation?

Answer 30

1. pre-zygotic mechanisms: prevent zygote formation

2. post-zygotic mechanisms: yield nonviable/sterile offspring

Question 31

What are a few examples of prezygotic mechanisms of reproductive isolation?

Answer 31

1. temporal isolation
2. ecological isolation (different niches)
3. behavioral isolation (lack of attraction)
4. incompatible anatomy
5. gametic isolation (gametes don’t meet)

Question 32

What are a few examples of post-zygotic mechanisms of reproductive isolation?

Answer 32

1. hybrid inviability
2. hybrid sterility (ex. mules)
3. hybrid breakdown (first-generation of hybrids is viable and fertile, but second-generation hybrid offspring are inviable and infertile)

Question 33

what are the three types of evolution?

Answer 33

1. divergent evolution (common ancestor, development of dissimilar characteristics)
2. parallel evolution (related but separate species, evolve in similar ways)
3. convergent evolution (not related, development of similar characteristics)

Question 34

what is the molecular clock model?

Answer 34

idea that species that diverged more recently have more similar genomes