BIOL 1202; Chapter 23 HW

In Part A, you looked at a single genetic cross involving two parents of genotype Rr. Imagine now that instead of a single mating, you consider all the matings that occur in a population, and all the offspring that are produced. The figure at right shows a population of flowers with two alleles for color, a red allele CR and a white allele CW . The allele frequencies across the entire population are 80% CR and 20% CW . In other words: The frequency of the CR allele, called p, is 0.8. The frequency of the CW allele, called q, is 0.2. If the population is not evolving, then the population is said to be in Hardy-Weinberg equilibrium. In this case, the Hardy-Weinberg principle tells us that offspring inherit alleles as if they were drawn from the gene pool at random. This means that: The proportion of individuals with genotype CRCR is expected to be p2. The proportion of individuals with genotype CRCW is expected to be 2pq. The proportion of individuals with genotype CWCW is expected to be q2. Furthermore, if a population is in Hardy-Weinberg equilibrium, the allele frequencies (p and q) and the genotype frequencies stay the same from one generation to the next. Parents 1. What is the probability that a gamete (egg or sperm) from this population carries a CR allele? 2. That a gamete from this population carries a CW allele? Offspring 3. Of all the offspring resulting from all the matings in this population, what percentage should have the genotype CRCR? 4. What percentage should have the genotype CWCW? 5. What percentage should have the genotype CRCW? Comparing p and q in parents and offspring 6. In the offspring generation, what is the frequency of the CR allele? 7. In the offspring generation, what is the frequency of the CW allele?

1. 80% 2. 20% 3. 64% 4. 4% 5. 32% 6. 80% 7. 20% If a population is in Hardy-Weinberg equilibrium, then the next generation of offspring should inherit alleles as if they were drawn from the gene pool at random. This also means that the allele and genotype frequencies should not change from one generation to the next. In the flower population explored here, if the frequency of the CR allele is p (0.8) and the frequency of the CW allele is q (0.2): The proportion of individuals with genotype CRCR is expected to be p2 , or 64%. The proportion of individuals with genotype CRCW is expected to be 2pq, or 32%. The proportion of individuals with genotype CWCW is expected to be q2 , or 4%. Note that the genotype frequencies always add up to 1. p2 + 2pq + q2 = 1 0.64 + 0.32 + 0.04 = 1 (in this example) In addition, in the offspring, the frequency of the CR allele is p = 0.8, just like in the parental generation. And the frequency of the CW allele is q = 0.2, just like in the parental generation.

True or false? Heterozygote advantage refers to the tendency for heterozygous individuals to have better fitness than homozygous individuals. This higher fitness results in less genetic variation in the population.

False Heterozygote advantage results in more genetic variation in the population.

Women often have complications during labor while giving birth to very large babies, whereas very small babies tend to be underdeveloped. Which kind of selection is most likely at work regarding the birth weight of babies? Disruptive selection Directional selection Stabilizing selection

Stabilizing selection Stabilizing selection causes no change in the average of the population; extreme phenotypes (in this case, large and small babies) become less common.

Genetic drift is a process based on _____. mutation emigration differential reproductive success correlated to the relationship between a phenotype and the environment immigration the role of chance

the role of chance

A mutation occurs when _____. population sizes are small some individuals leave more offspring than other individuals individuals leave a population individuals enter a population there is a change in the DNA sequence of a gene

there is a change in the DNA sequence of a gene

Every few years a giant axe chops off the head of every person who is over 6 feet tall. How will this affect the human population? It will increase in number since shorter people use fewer resources than taller people. Genetic drift will play less of a role in the evolution of humans. Alleles that promote "tallness" will decrease in frequency. Gene flow will increase. The mutation rate will increase.

Alleles that promote "tallness" will decrease in frequency.

Consider a population of wildflowers in which the frequency of the red allele CR is p = 0.7. What is the frequency of the white allele (CW ) in this population? 0 0.3 0.49 0.7

0.3 If the frequency of the red allele is p, and the frequency of the white allele is q, we know that p + q = 1. Since p = 0.7, we know that q = 0.3.

In a bell-shaped curve, the x-axis (horizontal direction) of the graph represents which of the following? The number of individuals Time The value of a particular characteristic; characteristics of an organism can include such traits as size and color.

The value of a particular characteristic; characteristics of an organism can include such traits as size and color. The value of the characteristic increases from left to right.

Homologous pairs of chromosomes are lined up independently of other such pairs during _____. telophase II prophase II metaphase I metaphase II anaphase I

metaphase II

Crossing over, resulting in an increase in genetic variation, occurs between _____. sister chromatids of nonhomologous chromosomes nonsister chromatids of nonhomologous chromosomes nonsister chromatids of homologous chromosomes sister chromatids of homologous chromosomes sex cells and somatic cells

nonsister chromatids of homologous chromosomes

Long necks make it easier for giraffes to reach leaves high on trees, while also making them better fighters in "neck wrestling" contests. In both cases, which kind of selection appears to have made giraffes the long-necked creatures they are today? Directional selection Stabilizing selection Disruptive selection

Directional selection Directional selection drives the average of the population in one direction, in this case, toward longer necks.

Black-bellied seedcrackers have either small beaks (better for eating soft seeds) or large beaks (better for hard seeds). There are no seeds of intermediate hardness; therefore, which kind of selection acts on beak size in seedcrackers? Disruptive selection Directional selection Stabilizing selection

Disruptive selection Disruptive selection causes both extreme phenotypes (large and small beaks) to be favored over the intermediate phenotypes.

Use the following information to answer the question below. In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β hemoglobin alleles in the human gene pool. In the United States, the parasite that causes malaria is not present, but it is present in African-Americans whose ancestors were from equatorial Africa. What should be happening to the sickle-cell allele in the United States, and what should be happening to it in equatorial Africa? disruptive selection; stabilizing selection stabilizing selection; disruptive selection directional selection; stabilizing selection directional selection; disruptive selection

directional selection; stabilizing selection

Which type of selection tends to increase genetic variation? Stabilizing selection Disruptive selection Directional selection

disruptive selection Disruptive selection eliminates phenotypes near the average and favors the extreme phenotypes, resulting in increased genetic variation in a population.

In human gamete production there is an average of _____ crossover events per chromosome pair. 9-10 5-6 10+ 2-3 0-1

2-3

The ease with which humans travel across the globe is likely to increase _____. genetic drift natural selection mutation gene flow all of these

gene flow

Small Aristelliger lizards have difficulty defending territories, but large lizards are more likely to be preyed upon by owls. Which kind of selection acts on the adult body size of these lizards? Directional selection Disruptive selection Stabilizing selection

Stabilizing selection Stabilizing selection causes no change in the average of the population; extreme phenotypes (large and small lizards) become less common.

Imagine that you are studying a gene with two alleles, R and r. What genotypes (sets of alleles) would you expect to find in the offspring of two Rr parents? What is the probability of producing an offspring with each of the possible genotypes? The figure shows how these probabilities can be calculated. Parents 1. According to Mendel's law of segregation, what is the probability that a gamete (egg or sperm) from an Rr parent carries an R allele? 2. What is the probability that a gamete from an Rr parent carries an r allele? Offspring 3. When an Rr female is crossed with an Rr male, what is the probability of producing a homozygous dominant (RR) offspring? 4. What is the probability of producing a homozygous recessive (rr) offspring? 5. What is the probability of producing a heterozygous (Rr) offspring?

1. 50% 2. 50% 3. 25% 4. 25% 5. 50% In a single genetic cross between two parents of genotype Rr, each parent has a 50% chance of passing on an R allele and a 50% chance of passing on an r allele. By multiplying these probabilities in the Punnett square, you see that: 25% of the offspring are expected to inherit an R allele from the egg and an R allele from the sperm, producing RR offspring. 25% of the offspring are expected to inherit an R allele from the egg and an r allele from the sperm, producing Rr offspring. 25% of the offspring are expected to inherit an r allele from the egg and an R allele from the sperm, producing Rr offspring. 25% of the offspring are expected to inherit an r allele from the egg and an r allele from the sperm, producing rr offspring. Therefore, the proportions of the different genotypes expected in the offspring are 25% RR, 50% Rr, and 25% rr.

Which of these individuals is a homozygous genotype? Aa AG Gg aG AA

AA

Which statement about variation is true? All nucleotide variability results in neutral variation. All new alleles are the result of nucleotide variability. All phenotypic variation is the result of genotypic variation. All genetic variation produces phenotypic variation.

All new alleles are the result of nucleotide variability.

Which statement about the beak size of finches on the island of Daphne Major during prolonged drought is true? Each bird evolved a deeper, stronger beak as the drought persisted. The frequency of the strong-beak alleles increased in each bird as the drought persisted. Each bird's survival was strongly influenced by the depth and strength of its beak as the drought persisted. Each bird that survived the drought produced only offspring with deeper, stronger beaks than seen in the previous generation.

Each bird's survival was strongly influenced by the depth and strength of its beak as the drought persisted.

Consider a wildflower population with the following allele and genotype frequencies. Frequency of the CR allele: p = 0.6 Frequency of the CW allele: q = 0.4 Frequency of CRCR : 50% Frequency of CRCW : 20% Frequency of CWCW : 30% Is this population in Hardy-Weinberg equilibrium? No, the frequency of genotype CRCW is too low. No, the frequency of genotype CWCW is too low. Yes, the genotype frequencies are what we would expect for a population in Hardy-Weinberg equilibrium. No, the frequency of genotype CRCR is too low.

No, the frequency of genotype CRCW is too low. To tell if this population is in Hardy-Weinberg equilibrium, you should calculate the expected genotype frequencies based on the allele frequencies. The frequency of genotype CRCR (red flowers) in the offspring generation is expected to be p 2 = (0.6)2 = 0.36. The frequency of genotype CRCW (pink flowers) is expected to be 2pq = 2(0.6)(0.4) = 0.48. The frequency of genotype CWCW (white flowers) in the offspring generation is expected to be q 2 = (0.4)2 = 0.16. Comparing these expected values with the actual genotype frequencies in the population, you can see that the frequency of genotype CRCW is too low while the frequencies of the other two genotypes are too high.

In Part B, you learned that if a population is in Hardy-Weinberg equilibrium, the allele and genotype frequencies stay the same from one generation to the next. A population in Hardy-Weinberg equilibrium is not evolving. Five conditions have to be satisfied in order for a population to be in Hardy-Weinberg equilibrium: no mutations random mating no natural selection extremely large population size no gene flow If any of these conditions are violated, the population does not stay in Hardy-Weinberg equilibrium, and allele frequencies and genotype frequencies may change from one generation to the next. Explore some potential departures from Hardy-Weinberg equilibrium by completing the table below. Drag the labels to the table below to describe how allele frequencies would be affected under different conditions. Remember that p is the frequency of the CR allele, and q is the frequency of the CW allele. Labels may be used once, more than once, or not at all. a)? b)? c)? d)?

a) p will decrease and q will increase b) p will increase and q will decrease c) changes in p and q cannot be predicted d) p will decrease and q will increase If a population is in Hardy-Weinberg equilibrium, allele frequencies and genotype frequencies stay the same from one generation to the next. But if any of the Hardy-Weinberg conditions are not met, then allele frequencies and/or genotype frequencies will change from one generation to the next. That is, when a population is not in Hardy-Weinberg equilibrium, p and q may change over time. In addition, p2 and q2 may no longer accurately predict the frequency of homozygous individuals, and 2pq may no longer accurately predict the frequency of heterozygous individuals.

All the genes in a population are that population's _____. phenotype fitness Hardy-Weinberg gene pool genotype

gene pool

Modern travel along with migration reduces the probability of _____ having an effect on the evolution of humans. gene flow disease mutation genetic drift natural selection

genetic drift