Unit 4

18) In the cross AaBbCc × AaBbCc, what is the probability of producing the genotype AABBCC? A) 1/4 B) 1/8 C) 1/16 D) 1/32 E) 1/64

Answer: E

heterozygous

having two different alleles for a given gene on the homologous chromosome

homozygous

having two identical alleles for a given gene on the homologous chromosome

adaptation

heritable trait or behavior in an organism that aids in its survival and reproduction in its present environment

blending theory of inheritance

hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance

codominance

in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic

incomplete dominance

in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype

law of dominance

in a heterozygote, one trait will conceal the presence of another trait for the same characteristic

inbreeding depression

increase in abnormalities and disease in inbreeding populations

adaptive evolution

increase in frequency of beneficial alleles and decrease in deleterious alleles due to selection

relative fitness

individual's ability to survive and reproduce relative to the rest of the population

continuous variation

inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them

dominant lethal

inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age

recessive lethal

inheritance pattern in which an allele is only lethal in the homozygous form; the heterozygote may be normal or have some altered, non-lethal phenotype

discontinuous variation

inheritance pattern in which traits are distinct and are transmitted independently of one another

linkage

phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together

sexual dimorphism

phenotypic difference between the males and females of a population

vestigial structure

physical structure present in an organism but that has no apparent function and appears to be from a functional structure in a distant ancestor

model system

species or biological system used to study a specific biological phenomenon to be applied to other different species

population genetics

study of how selective forces change the allele frequencies in a population over time

17) In certain plants, tall is dominant to short. If a heterozygous plant is crossed with a homozygous tall plant, what is the probability that the offspring will be short? A) 1 B) 1/2 C) 1/4 D) 1/6 E) 0

Answer: E

evolutionary fitness

(also, Darwinian fitness) individual's ability to survive and reproduce

allele frequency

(also, gene frequency) rate at which a specific allele appears within a population

centimorgan (cM)

(also, map unit) relative distance that corresponds to a recombination frequency of 0.01

7) Which of the following differentiates between independent assortment and segregation? A) The law of independent assortment requires describing two or more genes relative to one another. B) The law of segregation requires describing two or more genes relative to one another. C) The law of segregation requires having two or more generations to describe. D) The law of independent assortment is accounted for by observations of prophase I. E) The law of segregation is accounted for by anaphase of mitosis.

Answer: A

3) What was the most significant conclusion that Gregor Mendel drew from his experiments with pea plants? A) There is considerable genetic variation in garden peas. B) Traits are inherited in discrete units, and are not the results of "blending." C) Recessive genes occur more frequently in the F₁ generation than do dominant ones. D) Genes are composed of DNA. E) An organism that is homozygous for many recessive traits is at a disadvantage.

Answer: B

4) How many unique gametes could be produced through independent assortment by an individual with the genotype AaBbCCDdEE? A) 4 B) 8 C) 16 D) 32 E) 64

Answer: B

6) Why did Mendel continue some of his experiments to the F₂ or F₃ generation? A) to obtain a larger number of offspring on which to base statistics B) to observe whether or not a recessive trait would reappear C) to observe whether or not the dominant trait would reappear D) to distinguish which alleles were segregating E) to be able to describe the frequency of recombination

Answer: B

1) What do we mean when we use the terms monohybrid cross and dihybrid cross? A) A monohybrid cross involves a single parent, whereas a dihybrid cross involves two parents. B) A monohybrid cross produces a single progeny, whereas a dihybrid cross produces two progeny. C) A dihybrid cross involves organisms that are heterozygous for two characters and a monohybrid cross involves only one. D) A monohybrid cross is performed for one generation, whereas a dihybrid cross is performed for two generations. E) A monohybrid cross results in a 9:3:3:1 ratio whereas a dihybrid cross gives a 3:1 ratio.

Answer: C

10) When crossing an organism that is homozygous recessive for a single trait with a heterozygote, what is the chance of producing an offspring with the homozygous recessive phenotype? A) 0% B) 25% C) 50% D) 75% E) 100%

Answer: C

11) Mendel accounted for the observation that traits which had disappeared in the F₁ generation reappeared in the F₂ generation by proposing that A) new mutations were frequently generated in the F₂ progeny, "reinventing" traits that had been lost in the F₁. B) the mechanism controlling the appearance of traits was different between the F₁ and the F₂ plants. C) traits can be dominant or recessive, and the recessive traits were obscured by the dominant ones in the F₁. D) the traits were lost in the F₁ due to dominance of the parental traits. E) members of the F₁ generation had only one allele for each trait, but members of the F₂ had two alleles for each trait.

Answer: C

15) Mendel's second law of independent assortment has its basis in which of the following events of meiosis I? A) synapsis of homologous chromosomes B) crossing over C) alignment of tetrads at the equator D) separation of homologs at anaphase E) separation of cells at telophase

Answer: C

19) Given the parents AABBCc × AabbCc, assume simple dominance for each trait and independent assortment. What proportion of the progeny will be expected to phenotypically resemble the first parent?A) 1/4 B) 1/8 C) 3/4 D) 3/8 E) 1

Answer: C

12) The fact that all seven of the pea plant traits studied by Mendel obeyed the principle of independent assortment most probably indicates which of the following? A) None of the traits obeyed the law of segregation. B) The diploid number of chromosomes in the pea plants was 7. C) All of the genes controlling the traits were located on the same chromosome. D) All of the genes controlling the traits behaved as if they were on different chromosomes. E) The formation of gametes in plants occurs by mitosis only.

Answer: D

14) Mendel's observation of the segregation of alleles in gamete formation has its basis in which of the following phases of cell division? A) prophase I of meiosis B) anaphase II of meiosis C) metaphase I of meiosis D) anaphase I of meiosis E) anaphase of mitosis

Answer: D

16) Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). What fraction of the progeny of crosses BbTt × BBtt will be expected to have black fur and long tails? A) 1/16 B) 3/16 C) 3/8 D) 1/2 E) 9/16

Answer: D

2) Why did the F₁ offspring of Mendel's classic pea cross always look like one of the two parental varieties? A) No genes interacted to produce the parental phenotype. B) Each allele affected phenotypic expression. C) The traits blended together during fertilization. D) One phenotype was completely dominant over another. E) Different genes interacted to produce the parental phenotype.

Answer: D

5) The individual with genotype AaBbCCDdEE can make many kinds of gametes. Which of the following is the major reason? A) segregation of maternal and paternal alleles B) recurrent mutations forming new alleles C) crossing over during prophase I D) different possible alignments of chromosomes E) the tendency for dominant alleles to segregate together

Answer: D

8) Two plants are crossed, resulting in offspring with a 3:1 ratio for a particular trait. What does this suggest? A) that the parents were true-breeding for contrasting traits B) that the trait shows incomplete dominance C) that a blending of traits has occurred D) that the parents were both heterozygous for a single trait E) that each offspring has the same alleles for each of two traits

Answer: D

13) Mendel was able to draw his ideas of segregation and independent assortment because of the influence of which of the following? A) His reading and discussion of Darwin's Origin of Species. B) The understanding of particulate inheritance he learned from renowned scientists of his time. C) His discussions of heredity with his colleagues at major universities. D) His experiments with the breeding of plants such as peas and fuchsia. E) His reading of the scientific literature current in the field.

Answer: E

9) A sexually reproducing animal has two unlinked genes, one for head shape (H) and one for tail length (T). Its genotype is HhTt. Which of the following genotypes is possible in a gamete from this organism? A) tt B) Hh C) HhTt D) T E) HT

Answer: E

nonrandom mating

changes in a population's gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others

microevolution

changes in a population's genetic structure

gene pool

all of the alleles carried by all of the individuals in the population

epistasis

antagonistic interaction between genes such that one gene masks or interferes with the expression of another

sex-linked

any gene on a sex chromosome

autosomes

any of the non-sex chromosomes

recombination frequency

average number of crossovers between two alleles; observed as the number of nonparental types in a population of progeny

macroevolution

broader scale evolutionary changes seen over paleontological time

test cross

cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait

geographical variation

differences in the phenotypic variation between populations that are separated geographically

population variation

distribution of phenotypes in a population

genetic structure

distribution of the different possible genotypes in a population

genetic variance

diversity of alleles and genotypes in a population

genetic drift

effect of chance on a population's gene pool

selective pressure

environmental factor that causes one phenotype to be better than another

founder effect

event that initiates an allele frequency change in part of the population, which is not typical of the original population

F1

first filial generation in a cross; the offspring of the parental generation

gene flow

flow of alleles in and out of a population due to the migration of individuals or gametes

heritability

fraction of population variation that can be attributed to its genetic variance

X-linked

gene present on the X, but not the Y chromosome

allele

gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes

law of independent assortment

genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur

variation

genetic differences among individuals in a population

cline

gradual geographic variation across an ecological gradient

bottleneck effect

magnification of genetic drift as a result of natural events or catastrophes

inbreeding

mating of closely related individuals

phenotype

observable traits expressed by an organism

modern synthesis

overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today

reciprocal cross

paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

law of segregation

paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors

homologous structures

parallel structures in diverse organisms that have a common ancestor

P0

parental generation in a cross

hemizygous

presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant

sum rule

probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities

product rule

probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone

divergent evolution

process by which groups of organisms evolve in diverse directions from a common point

convergent evolution

process by which groups of organisms independently evolve to similar forms

homologous recombination

process by which homologous chromosomes undergo reciprocal physical exchanges at their arms, also known as crossing over

hybridization

process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring

nonparental (recombinant) type

progeny resulting from homologous recombination that exhibits a different allele combination compared with its parents

parental types

progeny that exhibits the same allelic combination as its parents

natural selection

reproduction of individuals with favorable genetic traits that survive environmental change because of those traits, leading to evolutionary change

monohybrid

result of a cross between two true-breeding parents that express different traits for only one characteristic

dihybrid

result of a cross between two true-breeding parents that express different traits for two characteristics

F2

second filial generation produced when F1 individuals are self-crossed or fertilized with each other

stabilizing selection

selection that favors average phenotypes

directional selection

selection that favors phenotypes at one end of the spectrum of existing variation

frequency-dependent selection

selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection)

diversifying selection

selection that favors two or more distinct phenotypes

trait

variation in the physical appearance of a heritable characteristic

good genes hypothesis

theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease

handicap principle

theory of sexual selection that argues only the fittest individuals can afford costly traits

Chromosomal Theory of Inheritance

theory proposing that chromosomes are the vehicles of genes and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed

recessive

trait that appears "latent" or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed

honest signal

trait that gives a truthful impression of an individual's fitness

dominant

trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait

genotype

underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism

Punnett square

visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid

assortative mating

when individuals tend to mate with those who are phenotypically similar to themselves