Mastering 14
Two organisms with genotype AaBbCcDdEE mate. These loci are all independent. What fraction of the offspring will have the same genotype as the parents?
1/16 (Breaking the question down into individual loci makes it simpler. The offspring of Aa ×Aa will be Aa 50% (one-half) of the time. The same is true for Bb, Cc, and Dd. Two EE individuals can only have EE offspring, so that probability is 1. The chance of an offspring being identical to the parent is therefore (1/2)4, or 1/16.)
Two mice are heterozygous for albinism (Aa) . The dominant allele (A) codes for normal pigmentation, and the recessive allele (a) codes for no pigmentation. What percentage of their offspring would have an albino phenotype?
25 (The offspring would be in a 3:1 ratio of normally pigmented mice to albino mice.)
For the cross in Part B, predict the frequencies of each of the phenotypes in the F1 progeny, and determine the genotype(s) present in each phenotypic class. Complete the diagram by dragging the correct label to the appropriate location. Labels can be used once, more than once, or not at all.
3/8 - RrYY, RrYy (x2) 3/8 - rrYY, rrYy (x2) 1/8 - Rryy 1/8 - rryy The frequencies of the four phenotypic classes are determined by applying the multiplication rule. The green progeny, for example, make up ¼ of the total progeny because the probability of their formation depends on receiving a y allele from both parents (probability = ½ x ½ = ¼). Similarly, the wrinkled progeny make up ½ of the total progeny because the probability of their formation depends on receiving an r allele from both parents (probability = 1 x ½ = ½). Because the [wrinkled, green] progeny are simultaneously green and wrinkled, and these events are independent, apply the multiplication rule to obtain their frequency: ¼ x ½ = 1/8.
If a diploid cell undergoes meiosis and produces two gametes with n + 1 chromosomes and two gametes with n− 1 chromosomes, what type of error occurred?
A nondisjunction error occurred in meiosis I, in which both members of a homologous pair migrated to the same pole of the cell. (When this type of error occurs, half of the gametes receive one too many chromosomes, and the other half receive one too few. When all of the gametes are affected, the error probably occurred in meiosis I.)
If a diploid cell undergoes meiosis and produces two gametes that are normal, and one with n − 1 chromosomes, and one with n + 1 chromosomes, what type of error occurred?
A nondisjunction error occurred in meiosis II, in which both sister chromatids of a chromosome migrated to the same pole of the cell. (When this error occurs in meiosis II, only half of the gametes are affected.)
What kind of cell results when a diploid and a haploid gamete fuse during fertilization?
A triploid cell (A triploid cell has three sets of chromosomes: the two from the diploid gamete and the one from the haploid gamete.)
What is the difference between heterozygous and homozygous individuals?
All of the gametes from a homozygote carry the same version of the gene while those of a heterozygote will differ.
What is a nondisjunction?
An error in cell division that causes homologous chromosomes or sister chromatids to move to the same side of the dividing cell
How could the botanist best determine whether the genotype of the green-pod plant is homozygous or heterozygous?
Cross the green-pod plant with a yellow-pod plant. (A cross between a plant of unknown genotype and one that is known to be homozygous recessive is called a test cross because the recessive homozygote tests whether there are any recessive alleles in the unknown. Because the recessive homozygote will contribute an allele for the recessive characteristic to each offspring, the second allele (from the unknown genotype) will determine the offspring's phenotype.)
True or false? In diploid organisms, a dominant phenotype will only be expressed if the individual is homozygous dominant for that trait.
False (A dominant phenotype is indeed expressed if the individual is homozygous dominant for that trait, but the dominant phenotype is also expressed if the individual is heterozygous for the trait. In fact, heterozygous expression is the definition of dominant.)
Which of the following genotypes could the green-pod plant have? (G is the symbol for the green-pod allele; g is the symbol for the yellow-pod allele.) Select all that apply.
GG Gg
Select the correct explanation for the fact that a carrier of a recessive genetic disorder does not have the disorder.
In a recessive disorder, only a single functioning allele is necessary to determine a normal phenotype
When can nondisjunction occur? Choose the best answer.
In mitosis, when sister chromatids fail to separate In meiosis, when homologous chromosomes fail to separate In meiosis, when sister chromatids fail to separate (Nondisjunction errors can occur in meiosis I, when homologous chromosomes fail to separate, or in either mitosis or meiosis II, when sister chromatids fail to separate.)
Which of the following is true about a plant with the genotype AABbcc?
It is homozygous at two loci.
If an organism with the genotype AaBb produces gametes, what proportion of the gametes would be Bb?
None (Alleles of the same gene must separate during gamete formation; thus, the two B alleles would be distributed to different gametes.)
A white-eyed female Drosophila is crossed with a red-eyed male Drosophila. Which statement below correctly describes the results?
None of the females will have white eyes. (All of the females will have red eyes because they will inherit the dominant red-eye allele from their male parent.)
The smallest phenotypic group consists of the homozygous recessive plants, which in this case are short and white flowered.
P ... F1 ... F2
Mendel studied pea plants dihybrid for seed shape (round versus wrinkled) and seed color (yellow versus green). Recall that the round allele (R) is dominant to the wrinkled allele (r) and the yellow allele (Y) is dominant to the green allele (y). The table below shows the F1 progeny that result from selfing four different parent pea plants. Use the phenotypes of the F1 progeny to deduce the genotype and phenotype of each parent plant. Complete the table by dragging the correct label to the appropriate location. Labels can be used once, more than once, or not at all.
Parent pheno: g,y,y,wg Parent geno:Rryy,RrYy,RRYy,rry F1 progeny phenotypes from selfed parent: The ability to deduce an organism's genotype from the phenotype(s) of its progeny is an important skill in solving genetics problems. In this example, the logic was simplified because the parent plants were selfed, and therefore only one parental genotype was involved.
Use the completed Punnett square in Part B to answer the questions below about the F2 generation. Answer each question in the table by dragging the correct label to the appropriate location. Labels can be used once, more than once, or not at all.
Question 1 simply asks about the probability of forming a yellow seed in the F2, which is 3/4. Question 2 introduces the concept of conditional probability. Although there are two genotypes in the F2 that will breed true (YY and yy), only one of them is yellow. Thus, the question adds a new condition: Among yellow seeds only, 1/3 are true-breeding. Questions 3 and 4 require a full evaluation of all eight possible groupings of three randomly selected F2 seeds. Each grouping has its own probability, shown in the table below. For question 3, there are seven possible groupings that have at least one yellow seed (shown in blue in the table); the sum of their individual probabilities yields the final answer (63/64). Note that you could have also calculated this answer by subtracting the probability of the one category that doesn't fit the criterion (Green, Green, Green) from the total probability (1 - 1/64 = 63/64). For question 4, there are three possible groupings that have one green and two yellow seeds (shown in bold in the table); the sum of their individual probabilities yields the final answer (27/64). Possible grouping Probability 1. Green, Green, Green 1/4×1/4×1/4=1/64 2. Yellow, Green, Green 3/4×1/4×1/4=3/64 3. Green, Yellow, Green 1/4×3/4×1/4=3/64 4. Green, Green, Yellow 1/4×1/4×3/4=3/64 5. Yellow, Yellow, Green 3/4×3/4×1/4=9/64 6. Yellow, Green, Yellow 3/4×1/4×3/4=9/64 7. Green, Yellow, Yellow 1/4×3/4×3/4=9/64 8. Yellow, Yellow, Yellow 3/4×3/4×3/4=27/64
You cross a true-breeding red-flowered snapdragon with a true-breeding white-flowered one. All of the F1 are pink. What does this say about the parental traits?
Red shows incomplete dominance over white
A plant grown from a [round, yellow] seed is crossed with a plant grown from a [wrinkled, yellow] seed. This cross produces four progeny types in the F1: [round, yellow], [wrinkled, yellow], [round, green], and [wrinkled, green]. Use this information to deduce the genotypes of the parent plants. Indicate the genotypes by dragging the correct label to the appropriate location.
RrYy, rrYy Answering this question requires two logical steps: First, eliminate those genotypes that are inconsistent with the phenotypes of the parents. Then examine the remaining possibilities: Which are consistent with the phenotypes of the progeny? Because the cross produces green progeny (and both parents are yellow), both parents must be Yy: If either parent were YY, only yellow progeny would result. The cross also produces wrinkled progeny, so the [round, yellow] parent must be heterozygous (Rr): If it were RR, all of the progeny would be Rr and have a round phenotype
Gregor Mendel set up a dihybrid cross with one pea plant from the parental generation (P) producing round yellow peas and the other pea plant producing wrinkled green peas. The F2 generation included 315 plants producing round yellow peas, 108 with round green peas, 101 with wrinkled yellow peas, and 32 with wrinkled green peas. How would these results have differed if pea shape and pea color had been linked genes, located close together on the same chromosome?
The F2 generation would have included a higher percentage of pea plants producing round, yellow peas. (If pea shape and pea color were determined by closely linked genes, they would not show independent assortment. Chromosomes would carry the combination of alleles seen in the P generation: yellow + round or green + wrinkled. Approximately ¾ or 75% of the F2 generation would inherit at least one chromosome carrying the yellow + round alleles. That is higher than Mendel's observed percentage of 315/556 or 56%.)
Which of these descriptions of the behavior of chromosomes during meiosis explains Mendel's law of independent assortment?
The arrangement of each pair of homologous chromosomes on the metaphase plate during metaphase I is random with respect to the arrangements of other pairs.
Look at the Punnett square, which shows the predicted offspring of the F2 generation from a cross between a plant with yellow-round seeds (YYRR) and a plant with green-wrinkled seeds (yyrr). Select the correct statement about wrinkled yellow seeds in the F2 generation.
The chance that an individual taken at random from the F2 generation produces wrinkled seeds is 25% and the chance that the same individual produces yellow seeds is 75%.
Which of the following statements best explains the outcome of this cross?
The forked mutation and the twist mutation are codominant alleles of the same locus. (The absence of wild-type progeny in the F2 of a cross indicates a monohybrid cross: The two lines crossed are mutant at the same locus. Any other explanation that involves two loci (for example, recombination) fails because any cross with two loci will produce at least some F2 individuals with a wild-type genotype for both loci and therefore a wild-type phenotype. In this case, the F1 expresses both mutant phenotypes, indicating that the forked and twist alleles are codominant.)
Which of the following is true of an X-linked gene, but not of a Y-linked gene?
The gene is present in both males and females. (All cells have at least one functional X chromosome.)
In humans, what determines the sex of offspring and why?
The male gamete determines sex because each male gamete can contribute either an X or a Y chromosome. (In humans, the male produces both X- and Y-bearing sperm, whereas females only produce X-bearing eggs.)
Select the correct statement(s) about sex determination in animals. Select all that apply.
The mechanism of sex determination varies with different animal species. (There are various chromosomal systems for sex determination found in animals.)
Each chromosome in this homologous pair possesses a different allele for flower color. Which statement about this homologous pair of chromosomes is correct?
These homologous chromosomes represent a maternal and a paternal chromosome. (Homologous chromosomes initially come together during fertilization with the contribution of a set of chromosomes from the sperm and the egg.)
What defines a short sleeper?
They can function well on 6 hours of sleep or less.
When a dominant allele coexists with a recessive allele in a heterozygote individual, how do they interact with each other?
They do not interact at all. (An allele is called dominant because it appears in the phenotype of a heterozygote, not because it subdues a recessive allele. Read about dominance.)
Of the following chromosomal abnormalities, which type is most likely to be viable in humans?
Trisomy (Some trisomies are viable in humans, but they result in developmental abnormalities, such as Down syndrome (trisomy 21). Only humans with trisomies of the small chromosomes (13, 18, or 21) or the sex chromosomes survive past birth.)
True or false? The same phenotype can be produced by more than one genotype.
True (Since there exist dominant and recessive versions of many genes, a phenotype that is based upon the dominant version will be expressed in both homozygous (AA) and heterozygous (Aa) genotypes.)
Which syndrome is characterized by the XO chromosome abnormality?
Turner syndrome
Consider the following family history: Bob has a genetic condition that affects his skin. Bob's wife, Eleanor, has normal skin. No one in Eleanor's family has ever had the skin condition. Bob and Eleanor have a large family. Of their eleven children, all six of their sons have normal skin, but all five of their daughters have the same skin condition as Bob. Based on Bob and Eleanor's family history, what inheritance pattern does the skin condition most likely follow?
X-linked dominant (If the skin condition is caused by an X-linked dominant allele, a father would pass the allele on to all of his daughters, who would all have the skin condition. In contrast, the father would not pass the allele on to any of his sons because the sons would receive the father's Y chromosome, not his X chromosome. As a result, none of the sons would inherit the skin condition.)
This means that the F1 generation makes gametes with the following genotypes:
YR, Yr, yR, yr
The resulting offspring--the F1 generation--have genotype ____________.
YyRr
SRY is best described in which of the following ways?
a gene region present on the Y chromosome that triggers male development
When an F1 plant undergoes meiosis, what gamete types will it produce, and in what proportions? Use your understanding of Mendel's law (or principle) of segregation to label each gamete possibility as "correct" or "incorrect."
a) Incorrect b) Correct c) Incorrect d) Incorrect e) Incorrect f) Incorrect
According to Mendel's law of independent assortment, the alleles for seed color and seed shape are transmitted into gametes in ________________________.
all possible combinations
According to Mendel's law of segregation,
alleles segregate into gametes with equal frequency.
According to Mendel's law of segregation, a gamete
always receives only one of the parent's alleles for each gene
What is an allele?
an alternative version of a gene (A diploid organism carries two alleles for each autosomal gene. The two alleles are found at comparable locations (loci) on homologous chromosomes. The alleles may be identical or slightly different, but they affect the same genetic character.)
While it is estimated that less than 5% of the population are short sleepers,
an estimated 30% of the population sleep only 6 hours per night or less.
Genes are located on chromosomes; the behavior of chromosomes during meiosis accounts for inheritance patterns.
chromosome theory of inheritance
Consider pea plants with the genotypes GgTt and ggtt . These plants can each produce how many type(s) of gametes?
four ... one
At fertilization, two gametes (egg and sperm) come together. The resulting F2 generation exhibits ________ different phenotypes in the ratio of __________________
four; 9 to 3 to 3 to 1
When constructing a Punnett square, the symbols on the outside of the boxes represent _______, while those inside the boxes represent _______.
gametes, progeny
Heterozygous
having two different alleles for a given gene
Homozygous
having two identical alleles for a given gene
What can we observe in order to visualize Mendel's Law of Segregation?
homologous chromosomes separating during meiosis I (Homologous chromosomes each carry an allele for every gene. Those alleles may be the same or different, but they are segregated during meiosis I and distributed to each gamete.)
During gamete formation, the alleles for one gene separate independently of the alleles for all other unlinked genes.
law of independent assortment
During gamete formation, the two alleles for each gene separate so that each gamete receives only one allele.
law of segregation
Suppose that the botanist carried out the test cross described in Parts A and B and determined that the original green-pod plant was heterozygous (Gg). Which of Mendel's findings does her test cross illustrate?
law of segregation
Genes that are located on the same chromosome tend to be inherited together.
linkage
In __________________, the major events are the pairing of homologous chromosomes during prophase and their separation during anaphase.
meiosis I
During which part of meiosis (meiosis I or meiosis II) do the two alleles of a gene separate? During which phase does the separation occur?
meiosis I, anaphase (Alleles separate from one another during anaphase of meiosis I, when the homologous pairs of chromosomes separate.)
In __________________, the major event is the separation of sister chromatids during anaphase.
meiosis II
The risks of sleep deprivation include which of the following?
obesity memory problems cardiovascular disease
Since there is a genetic explanation for short sleepers, you would expect
that this trait appears more frequently among family members.
Genotype
the genetic makeup (allele composition) of one or more genes in an organism
The discovery of a gene associated with short sleepers means that
there is potential to manipulate sleep pathways with medication.
A tall, purple-flowered pea plant (TtPp) is allowed to self-pollinate. (The recessive alleles code for short plants and white flowers.) The phenotypic ratio of the resulting offspring is 9:3:3:1. What is the genotype of the plant whose phenotype appeared once out of every 16 offspring (the "1" in the 9:3:3:1 ratio)?
ttpp (The smallest phenotypic group consists of the homozygous recessive plants, which in this case are short and white flowered.)
What name is given to the most common phenotype in a natural population?
wild type
In the P generation, a true-breeding pea plant with genotype YYRR is crossed with a true-breeding plant with genotype _________.
yyrr
