Module 1 MG
Genetic distances within a given linkage group
-are dependent on crossover frequencies between paired, nonsister chromatids. -can be measured in centiMorgans or map units.
How many Barr bodies would one expect to see in cells of: Turner syndrome females? Klinefelter syndrome males?
0,1
If a couple with Sickle cell disease genotypes Ss and Ss mate and have one child with the disease, what is the chance the next child has the disease as well?
0.25
Males with an X-linked gene are said to be ______________ for these X-linked genes.
hemizygous
What is the probability of obtaining 3 heads in three flips of a fair coin
1/8
Mitosis ends with _____, while meiosis ends with _____
2 diploid cells, 4 haploid cells.
Which are abnormalities involving numbers of X chromosomes?
XXXX syndrome Turner syndrome Klinefelter syndrome
A cross was made between homozygous wild-type female Drosophila and yellow-bodied male Drosophila. All of the resulting offspring were phenotypically wild type. Offspring of the F2 generation had the following phenotypes: Sex Phenotype Number malewild 96 maleyellow 99femalewild1 97 Based on this information, fill in each of the blanks below. Is the mutant gene for yellow body behaving as a recessive or dominant? Is the yellowlocus on an autosome or on the X-chromosome?
recessive x chromosome
How does genetic recombination facilitate evolution
Increases Variation
Genes come in different versions called:
alleles
During the cell cycle, chromosomal replication occurs in
interphase
Mendel's law of independent assortment has its physical basis in the:
separation of alleles into haploid cells.
What does it mean if a human female oocyte is no longer arrested in metaphase II of meiosis?
she is pregnant
Which of the following parental genotypes will produce only heterozygous F1 offspring?
AA x aa
Linked genes ___________
co-segregate.
What is the probability of flipping a penny and a nickel and obtaining one head and one tail?
1/2
Genes can be located in which organelles? a. ribosome b. nucleus c. golgi d. chloroplast e. vacuole f. mitochondria
b, d, f
If a crossover event happened in one half of all tetrads, how many recombinant gametes would be produced?
25%
During interphase of the cell cycle,
DNA content essentially doubles
Genetic information can be carried in which of the following biomolecules?
Either DNA or RNA
When two genes fail to assort independently, the term normally applied is:
Linkage
Crossing over occurs during
prophase
During prophase I of meiosis, crossing over is indicated by what microscopically visible structure? The ______
synaptonemal complex
In the G0 stage of the cell cycle, which of the following is true:
the cell exits the cell cycle
Assume that the genes for tan body and bare wings are 15 map units apart on chromosome II in Drosophila. Assume also that a tan-bodied, bare-winged female was mated to a wild-type male and that the resulting F1 phenotypically wild-type females were mated to tan-bodied, bare-winged males. Of 1000 offspring, what would be the expected phenotypes, and in what numbers would they be expected?
wild type = 425; tan-bare = 425; tan = 75; bare = 75
Which modes of inheritance are influenced by the sex of individuals. Select all that apply.
x-linked y-linked sex linked
Assuming independent assortment, what proportion of the offspring of the cross AaBbCcDd X AabbCCdd will have the aabbccdd genotype?
zero
In a Chi-square test, as the value of the (chi)2 increases, the likelihood of rejecting the null hypothesis ________.
increases
According to Mendel's postulate of ________, all possible combinations of gametes will be formed in equal frequency.
independent assortment
Homologous chromosomes move toward opposite poles of the dividing cell during
meiosis I
In order to be functional, a chromosome requires all of the following except:
nucleomeres
How many Barr bodies would you expect to see in an XXY individual?
1
In eukaryotes, chromosomes do not contain:
proteases
Describe two evolutionarily significant characteristics of meiosis that are not present in mitosis. How do these characteristics increase genetic diversity relative to mitosis?
1) Reshuffling of chromosomes through independent assortment 2) Crossing over exchanges alleles between maternal and paternal chromosomes.
At what stage of the meiotic cell cycle and during what chromosomal configuration does crossing over occur?
1. before the end of prophase I 2. at the four strand stage of meiosis 3. after synapsis of homologous chromosome
One form of hemophilia is caused by a sex-linked recessive gene. Assume that a man with hemophilia marries a phenotypically normal woman whose father had hemophilia. Given that their first child is a son, what is the probability that their first son will have hemophilia?
1/2
Two forms of hemophilia are determined by genes on the X chromosome in humans. Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man. What is the probability that their first daughter will have hemophilia?
1/2
Two organisms, AABBCCDDEE and aabbccddee, are mated to produce an F1 that is self-fertilized. If the capital letters represent dominant, independently assorting alleles, what proportion of the F2 genotypes will be recessive for all five loci?
1/243
In the following tetrahybrid cross AaBbCcDd x AaBbCcDd what proportion of the F2 offspring would be expected to be phenotypically recessive?
1/256
A cross is made between a female calico cat and a male cat having the gene for black fur on his X chromosome. What fraction of the offspring would one expect to be calico?
1/4
Albinism, lack of pigmentation in humans, results from an autosomal recessive gene (a). Two parents with normal pigmentation have an albino child. What is the probability that their next child will be an albino girl?
1/8
Dentinogenesis imperfecta is a rare, autosomal, dominantly inherited disease of the teeth that occurs in about one in 8000 people (Witkop, 1957). The teeth are somewhat brown in color, and the crowns wear down rapidly. Assume that a male with dentinogenesis imperfecta and no family history of the disease marries a woman with normal teeth. What is the probability that: their first child will be a girl with dentinogenesis imperfecta?
25%
The following F2 results occur from a typical dihybrid cross: purple: A_B_ 9/16 white: aaB_ 3/16 white: A_bb 3/16 white: aabb 1/16 If a double heterozygote (AaBb) is crossed with a fully recessive organism (aabb), what phenotypic ratio is expected in the offspring? Enter the phenotype and number in ratio format (ie, 1pink:2yellow:1white)
3 (white):1 (purple)
Assuming a typical monohybrid cross in which one allele is completely dominant to the other, what phenotypic ratio is expected if the F1's are crossed?
3:1
Klinefelter syndrome in humans, which leads to underdeveloped testes and sterility, is caused by which chromosomal condition?
47, XXY
Assume that there are 12 map units between two loci in the mouse and that you are able to microscopically observe meiotic chromosomes in this organism. If you examined 200 primary oocytes, in how many would you expect to see a chiasma between the two loci mentioned above?
48
In a mating between individuals with the genotypes IAIB X IOIO, what percentage of the offspring are expected to have the O blood type?
50%
Assume that two genes are 80 map units apart on chromosome II of Drosophila and that a cross is made between a doubly heterozygous female and a homozygous recessive male. What maximum percent recombination would be expected in the offspring of this type of cross?
50% recombination going to independently assort
An organism has a haploid number of 6. How many possible assortments of chromosomes are possible based on the independent assortment of the chromosomes?
64
A recessive allele in tigers causes the white tiger. If two normally pigmented tigers are mated and produce a white offspring, what percentage of their remaining offspring would be expected to have normal pigmentation?
75%
Assume that a dihybrid cross is made in which the genes' loci are autosomal, independently assorting, and incompletely dominant. How many different phenotypes are expected in the offspring?
90
Typical ratios resulting from epistatic interactions in dihybrid crosses would be ________.
9:3:4, 9:7
Describe the difference between a "character" and a "contrasting trait" with respect to Mendelian inheritance. Include an example of each in your answer.
A character is that characteristic of an orgainsim that is being examined, eg. seeds are a character of a plant. A contrasting trait is two different "versions" or phenotypes of that character, eg. round or wrinkled are contrasting traits of the seed character. NOTE: other examples of characters and contrasting traits are acceptable.
What is a dominant epistatic gene?
A dominant allele that, if present, determines the phenotype of a given trait regardless of which other alleles for the trait are present
Assume that the somatic cells of a male contain one pair of homologous chromosomes (e.g., A1 and A2), and an additional chromosome without a homolog (e.g., W). What chromosomal combinations would be expected in the meiotic products (spermatids) of a single primary spermatocyte? (There may be more than one answer.)
A1, WA1, WA2 A2 AND A1 A1 A2, WA2, W
Given that loci A and B in Drosophila are sex-linked and 20 map units apart, what phenotypic frequencies would you expect in 100 offspring resulting from the following cross? (Assume A and B are dominant to a and b, respectively.) AaBb (cis) female X ab/Y male AB = ab = Ab = aB =
AB = 40; ab = 40; Ab = 10; aB = 10 (sexes have the same phenotypes)
Which of the following human genotypes is associated with Klinefelter syndrome?
All of the above
A diploid somatic cell from a rat has a total of 42 chromosomes (2n = 42). As in humans, sex chromosomes determine sex: XX in females and XY in males.
An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.
An allele is
An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.
Why is it possible to "map" the centromere in Neurospora and not in most other organisms?
Ascospores are held in order as meiotic products. This order is dependent on centromere migration, and a comparison of ascospore patterns allows one to determine whether crossovers have occurred. Quantifying such crossovers allows one to estimate map distance between the gene in question and the centromere.
What is a primary function of the centromeric DNA region found on chromosomes?
Centromeric DNA, specific DNA (tandem repeated sequences) that allows binding by centromeric proteins, which are in turn bound by Kinetochore proteins, which allows attachment to spindle fibers (made of alpha, beta, gamma tubulin), which facilitates chromosome transport to poles
Homologous chromosomes are those that can be matched by virtue of their similar structure and function within a nucleus. What chromosomes making up the human genome do not follow the same characteristics of homology?
DNA content essentially doubles
These two terms generally refer to meiosis I or II. Enter the correct stage of meiosis in the box below each term. Equational? (I or II) Reductional? (I or II)
Equational = prophase II Reductional = prophase I
Suppose you repeated Thomas Morgan's experiments by crossing Drosophila females of wild type red eye appearance and normal wing length with males showing two autosomal recessive traits: purple eyes and vestigial wings. If 1000 progeny of this cross are distributed in the following phenotypic classes, is this ratio diagnostic of two linked genes and why?Wild type 441Purple eyes, vestigial wings 484 Red eyes, vestigial wings 30 Purple eyes, normal wings 45
From inspection, this ratio easily deviates from the 1:1:1:1 Mendelian ratio expected for a testcross. We see that the wild type and double recessive offspring are the largest phenotypic classes, while the other two classes are fewer, a characteristic of recombination resulting in recombinant offspring. It points to close linkage between the two genes.
Describe the difference between meiosis I and meiosis II.
Homologs pair and segregate in meiosis I. Sister chromatids are paired and segregate in meiosis II. Crossing over occurs in meiosis I, but not in meiosis II.
What is the maximum recombination frequency observable from a cross? Why is it less than the true genetic distance in some cases?
If a double heterozygote is testcrossed and gametes are produced in a ratio of 1:1:1:1, this can be interpreted as unlinked genes or as genes that are 50 map units apart. Because gametes this far apart or more cannot be distinguished from gametes that are unlinked, it's not possible to ever get a ratio of gametes produced that indicates a distance of greater than 50 map units.Some genes that are farther apart but are still on the same chromosome may appear to be closer because the observed proportion of recombinant gametes neglects to count those double crossover events that occur between the genes. When a double crossover occurs, the parental gametes are formed again and it looks as if no crossovers have occurred at all.
"The occurrence of partial expressivity and incomplete penetrance demonstrates that gene expression can be influenced by biological or environmental context." Explain this statement and give examples to support it.
Individuals with identical genotypes can exhibit different degrees of given phenotypes based on their sex, stress levels, lifestyle, nutritional status, and so forth. An individual's genotype defines genetic potential; the expression of genetic potential can vary greatly depending on other factors that determine the overall context of gene expression. For example, men and women with the same genotype for pattern baldness exhibit very different phenotypes due to the biological context (i.e., male or female) the genotype exists in. The high relative level of testosterone produced in men provides a biological context whereby the genotype for baldness can be more fully expressed. The variation in the degree of pattern baldness in men and women with identical "baldness" genotypes is an example of partial expressivity. Note also that stress, nutritional deficiency, and so forth, can also result in loss of hair even in individuals not having a "baldness" genotype.
What is the purpose of synteny testing?
It tests to see whether genes reside on the same chromosome.
Most calico cats are female, but you can find male calico cats. How would this be possible?
Male calicos can be XXY individuals, in other words, Kleinfelters cats having the Y chromosome, which would determine maleness, and the 2 X chromosomes producing the color pattern. GRADE NOTE: an alternative answer would be that the male cat SRY region could be translocated to one of the X chromosome, which may also produce this phenotype
a.Meiosis I anaphase b.Meiosis I prophase c.Meiosis II anaphase d.Meiosis II prophase e.Mitosis telophase f.Mitosis prophase Reference: Ref 2-2 Chromosomes are in unseparated, sister-chromatid form, at the end of which phase(s)? Separate multiple answers with commas, and enter only the letter of your choice(s).
Meiosis I prophase
The stage at which "sister chromatids go to opposite poles" immediately follows which stage listed below?
Mitotic metaphase
. Explain why large recombination frequency values are less accurate measures of map distance than smaller ones.
Multiple crossovers often lead to the parental arrangement of alleles and so are not counted when measuring recombination frequency. Multiple crossovers are expected more over longer regions and so the problem is worse for larger recombination frequencies.
Provide a brief definition for positive interference
Positive interference occurs when a crossover in one region of a chromosome reduces crossovers in nearby regions.
What is the mechanism that ensures Mendel's First Law of segregation?
Segregation of homologous chromosomes during meiosis I
Two pea plants with purple flowers are crossed. Among the offspring, 63 have purple flowers, and 17 have white flowers. With a chi-square test, compare the observed numbers with a 3:1 ratio and determine if the the difference between observed and expected is statistically significant. Note: Chi-square critical value with 1 degree of freedom is 3.84 with alpha equal to 0.05.
Sum (O-E)^2/E = .592; .592 < 3.84, therefore the observed are not statistically significantly different from the expected and differences are due to random chance
A chromosome with a centromere at the very end is called
Telocentric
What is the chi-square test used for, and what does it tell you?
The chi-square test is a statistical tool for analyzing data generated from genetic experiments to determine if observed results are consistent with a hypothesis proposed to explain them. The calculated chi-square statistic may be used to determine whether or not to accept or reject the proposed hypothesis within pre-determined confidence limits.
The expected ratio of phenotypes among the progeny of a test cross is 1:1:1:1. Out of 200 total resulting progeny, 48 occur in one of the four phenotypic classes. Given this information, which of the following must also be true?
The value of observed - expected for this cell = -2.
The white-eye gene in Drosophila is recessive and sex-linked. Assume that a white-eyed female is mated to a wild-type male. What would be the phenotypes of the offspring? Explain your answer.
The white-eyed female has the recessive allele on each X chromosome. The red-eyed (wildtype) male has the wildtype allele on his single X chromosome. So for the offspring, the X of the female combined with the X of the male will give wildtype (heterozyous) females (XX). Also, the X of the female combined with the Y of the male (hemizygous) will give white-eyed males (XY).
Which of the following species is considered a genetic model organism?
The worm, Caenorhabditis elegans
Which of the following statements about homologous chromosomes are true?
They always contain the same alleles. They occur together in diploid cells.
Under what conditions does one expect a 1:1:1:1 phenotypic ratio among the offspring of a cross? What must be true about the parents? What must be true about the genes in question?
This occurs in a cross involving doubly heterozygous individuals crossed to fully recessive individuals. The genes involved assort independently of each other.
n humans, the presence of the SRY gene, normally on Y, determines maleness. In Drosophila, an X: A (autosome) ratio of 0.5 determines maleness.
XXY human is male. The SRY on the Y chromosome determines maleness, in spite of the two X chromosomes. XXY Drosophila is female. Drosophila is diploid, so there are two of each autosome. If there are two X chromosomes, the X:A ratio is 1.0, which is a female.
Is it possible that the characteristic in the pedigree below could be autosomal dominant? If so, what are the genotypes of each individual (use alleles D and d)? If not, explain why not, giving specific genotypes.
Yes, it is possible. If I-1 is Dd, I-2 is dd, II-1 is dd, and II-2 is Dd
The rule of independent assortment states that alleles of one gene can segregate independently of alleles of other genes. In other words, the allele you pass on for one trait (for example, eye color) does not have any effect on the allele you pass on for some other trait (for example, height). Imagine you have conducted the following cross using a type of pea plant similar to Mendel. P1 Round/Yellow (RR/YY) P2 Wrinkled/Green (rr/yy) a) What is (are) the phenotype(s) and genotype(s) of the F1 generation?b) You cross the F1's. What are the resulting genotype/phenotype ratios of the offspring?
a) They are all RrYy: round and yellowb) RrYy x RrYy will result in a 9:3:3:1 ratio of round yellow:round green:wrinkled yellow: wrinkled green. Genotype ratios can be determined from the Punnett square. It would probably be useful to have the student draw out the Punnett square for this cross.
a. Meiosis I anaphase b. Meiosis I prophase c. Meiosis II anaphase d. Meiosis II prophase e. Mitosis telophase f. Mitosis prophase Reference: Ref 2-2 Chromosomes are in unseparated, sister-chromatid form, at the end of which phase(s)?
a, c, e
Drosophila melanogaster, the fruit fly, has a 2n chromosome number of 8. Assuming that a somatic G2 nucleus contains about 8.0 picograms of DNA, how many picograms of nuclear DNA would you expect in: a) Mitotic metaphase b) a primary oocyte (metaphase) c) a secondary oocyte (metaphae) d) a first polar body (metaphase)
a-d: 8, 8, 4, 4
In a three-point mapping experiment, what three general classes of offspring are expected (assuming crossovers occur)? Select all those that apply.
a. noncrossover, single crossover, and double crossover
Assume that a black guinea pig crossed with an albino guinea pig produced 5 black offspring. When the albino was crossed with a second black one, 4 black and 3 albino offspring were produced. What genetic explanation would apply to these data?
albino=recessive; black=dominant
What inheritance pattern is suggested by the following observations of a particular pedigree? Extensive pedigree analysis on a characteristic shows all of the following: • the characteristic affects males and females equally. • two unaffected parents can have an affected child. • in families in which the parents are unaffected but the children are affected, 1/4 of the children are affected.
autosomal recessive
Polygenic traits ______________________
both a and b
In anaphase
chromosomes move to opposite poles
Interactions among the human ABO blood group alleles involve _______ and ________. a.expressivity b.continuous variation c.complete dominance d.incomplete dominance e.co-dominance
co-dominance; complete dominance
Red-green color blindness is X-linked recessive. A woman with normal color vision has a father who is color blind. The woman has a child with a man with normal color vision. Which phenotype is NOT expected?
color-blind female
A condition in which one gene pair masks the expression of a nonallelic gene pair is called ________.
epistasis
Because of the mechanism of sex determination, males of many species can be neither homozygous nor heterozygous. Such males are said to be:
hemizygous
Ability to roll the tongue is caused by a dominant allele. A woman is a "roller," but one of her parents is not.
heterozygous
In Drosophila, an individual female fly was observed to be of the XXY chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's mother and father had red eyes. The mother, however, was heterozygous for the gene for white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, white eyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.
meiosis II in the mother
A rare dominant trait, when exhibited in men, is transmitted to half their sons and to half their daughters. The gene for this trait is carried:
on an autosome.
