AGR 3303 - Genetics 2 Oct 2000

University of Florida - Fort Lauderdale

Exam #1: PARTICULATE GENETICS:  Explanations

Multiple choice (75 pts.)

Please read these carefully. One and only one response (a, b, c, d, or e) completely and correctly answers the question, or completes the statement. Circle the appropriate response and turn in this exam. Make sure your circle is unambiguous. Take time to relax. (Suggestion: put the problem into gene symbols, if that helps you to visualize the problem and its solution.)  (Comments attempt to explain why the true answer is true, and the other answers are false.  Page numbers refer to pages in Klug and Cummings, 6th ed.)

1. Compare these two of Mendel's postulates: segregation vs. independent assortment.
   Comment:  Mendel (pages 46-54) based his work on the segregation of phenotypes in peas, from which his postulates of segregation and independent assortment were described in the context of unit factors in cells.  Mendel was not aware of chromosomes which were not discovered until 1879.  The chromosomal theory of inheritance was proposed in 1902 that the chromosomal theory of inheritance was proposed by Sutton and Boveri.  Both segregation and independent assortment involve the reductional division of chromosomes.  Segregation does not depend on two or more genes for expression, as one gene will demonstrate segregation, and a 9:3:3:1 ratio will not result unless their are two or more genes with independent assortment.
   
1. Segregation involves longitudinal division while independent assortment involves reductional division.
2. Mendel observed the chromosomal basis for segregation, but could not see the basis for independent assortment.
3. Both segregation and independent assortment involve unit factors in the cells
4. Both segregation and independent assortment require two or more genes.
5. Either segregation or independent assortment result in a 9:3:3:1 phenotypic ratio.
2. A single gene has three alleles. In a diploid organism, how many possible genotypes are there involving this gene?
   Comment:  This can be solved by just listing the combinations, or one can make a 3 x 3 table in which the rows and columns each represent the three possible alleles for each position.  There will be three unique homozygous combinations running from the upper-left to the lower-right corner of the table.  Among the remaining six heterozygous combinations, the upper-right and lower-left halves of the table are mirror-images, so only three unique genotypes are present, plus the three homozygous combinations, or six altogether.  (Not Punnett's square, but the table is still a nice recordkeeping device.)
   
1. 3
2. 4
3. 6
4. 8
5. 9
3. Two genes each have two alleles. In a diploid organism, how many possible genotypes are there?
   Comment:  Mendel's dihybrid cross on page #53 is an example.  F or each gene there are three unique combinations [e.g., (AA, Aa, and aa) and (BB, Bb, bb)] so in combination there are nine combinations, AABB, AABb, AAbb, AaBB, AaBb, Aabb, aaBB, aaBb, and aabb.  This can be solved using a table similar to #2, except for #3 the rows and columns represent the genotypic condition for each of the two genes.  (Not Punnett's square, but the table is still a nice recordkeeping device.)
1. 3
2. 4
3. 6
4. 8
5. 9
4. Two genes each have two alleles, one dominant and the other recessive. In a diploid organism, what is the maximum possible number of phenotypes?
   Comment:  See the table at the bottom of page 53.  For each gene there are two unique combinations [e.g., (A_and aa) and (B_and bb)] so in combination there are four combinations, A_B_, A_bb, aaB_, and aabb.  Similar to questions #2 and #3.
   
1. 3
2. 4
3. 6
4. 8
5. 9
5. Green colorblindness in humans is a sex-linked recessive trait. In a family, the daughter has normal vision, whereas the son is colorblind. One can realistically surmise:
   Comment:  Sex-linkage is explained on pages 93-96.  Because color blindness is an X-linked trait, and male humans always receive their X from their moms, the mom is either heterozygous or homozygous.  Because color blindness is not common, the likelihood of a homozygous mom would be rare, and it is reasonable to surmise that she was heterozygous.  The father has nothing to do with his son's X. 
   
1. the mother was heterozygous for colorblindness
2. the father was heterozygous for colorblindness
3. the father was homozygous for color blindness
4. the mother most probably was colorblind
5. the father most probably was colorblind
6. The idea of unit factors of inheritances was not obvious because:
   Comment:  DNA was not mentioned on pages 45-54.  Mendel did not base his idea in any way on the presence of DNA or any other chemical of inheritance.  Genetics flourished for decades before anyone knew that DNA was responsible, in the mid 1900s.  Mendel postulated unit factors in pairs in 1865, before DNA was first described in 1868.  DNA was determined to be the genetic material in 1944, at least in prokaryotes, and this was idea was solidified with the Watson-Crick model of double-stranded DNA in 1953.  Early plant breeders had made tens of thousands more crosses than Mendel, but they weren't structured in design of experiments or recordkeeping the way Mendel was.
   
1. the religious view of fixity of species would not allow it
2. early plant breeders were not systematic in their recordkeeping
3. "The Origin of Species" by Darwin had not yet been published
4. DNA had not been discovered
5. early plant breeders did not make as many hybrids as Mendel
7. Which human blood phenotypes are always associated with phenotypically homogeneous (all the same) offspring?
   Comment:  O is the only phenotype always associated with a homozygous genotype.  In contrast, heterozygous genotypes always have the potential to not breed true. 
   
1. Mother A mates with father O
2. Mother A mates with father B
3. Mother AB mates with father O
4. Mother O mates with father O
5. Mother A mates with father A
8. A dominant lethal allele can exist in a population:
   Comment:  Dominant alleles are always expressed, therefore if the allele is lethal, the only way the organism can reproduce is if the gene represented by the lethal allele is expressed after sexual maturity, e.g., "late onset."  In the movie "Alice's Restaurant," Arlo, the son of folk singer Woody Guthrie (who died of Huntington disease, page 85) worries if he might have the lethal allele.  Generational test:  At Alice's Restaurant you can get anything except _______________.
   
1. only if the gene is expressed after sexual maturity
2. only if it is recessive
3. only if it is heterozygous
4. only if it is on a sex chromosome
5. only if it expresses an enzyme
9. The Russian Lysenko proposed the cold treatment of potatoes to make the next crop of potatoes, and those that followed, more resistant to winter. This is an example of:
   Comment:  This is pretty straightforward.  Lysenko took his ignorance of genetics to the lowest possible level, including the execution of his colleagues. Well, he didn't actually pull the trigger.  The consequence was the shambles of Soviet agriculture and the inability to compete in grain production with the West.  Unfortunately the text book has dropped this section, which is why I brought a book on the subject to class.
   
1. the idea that inherited traits can be acquired by environmental influences
2. evolution by the survival of species occurs
3. that Mendel's postulates can be extended to new situations
4. a mutation
5. something that happens readily in polyploids
10. A family of four genetically related siblings has phenotypes A, B, O, and AB, therefore:
    Comment:  To get four possible phenotypes containing all four different alleles, each parent must contribute two uniquely different alleles, thus the only way this could happen is if the parental genotypes were AO and BO. 
1. the parents could be A and B phenotypes
2. the parents could be A and O phenotypes
3. they may be related on their mom's side, but there must be at least two daddies
4. the parents could be AB and AB phenotypes
5. not sure about their dads, but the siblings must have different mamas
11. An alien geneticist escaping from Planet X brought to earth two pure-breeding breeds of frogs. One characteristically croaked "rib-it rib-it" and the other croaked "knee-deep knee-deep." Continuing his research on Planet Earth, the geneticist discovered that when the Rib-it breed was crossed with the Knee-deep breed, all the tadpoles grew up saying "bud-weis-er." Realizing the market potential, a large brewery company intermated the Bud-weis-er frogs and discovered that they were not true-breeding, as they segregated in the ratio of 1:1:2 for Rib-it, Knee-deep, and Bud-weis-er. The results are consistent with the hypothesis:
    Comment:  This is the same question as #21 thus it has the same answer for the same reason. 
1. the gene for croak sound has three alleles, with dominance for Bud-weis-er over Rib-it and Knee-deep
2. there are two alleles of the croak sound gene and they are related by codominance
3. the croak sound gene is linked to the X-chromosome
4. there must be complete dominance of Rib-it over Knee-deep
5. croaking sound is controlled by two tightly linked genes that never cross over.
6. there is not such thing as a croak sound genes, the whole thing is based on ventriloquy
12. An inheritance study evaluates deviations from the null hypothesis of Mendel's 3:1 ratio, comparing the observed to the expected distribution of progeny. The result is P = 0.50, therefore:
    Comment:  When we are talking about the P value in most common hypothesis tests, such as the ones in the book, we are always, we are always looking for an arbitrary small value, such as P < 5% or P < 0.05 in order to reject the null hypothesis.  See also #19 . 
1. accept the null hypothesis
2. reject the null hypothesis
3. there must be independent assortment
4. there must be epistasis
5. there must be two genes involved
13. Thalassemia is an inherited anemia controlled by a single allele. Homozygous persons have Thalassemia major, which is almost always fatal in childhood. Heterozygous persons have Thalassemia minor, and are mildly affected. An infant has Thalassemia major. What is the likelihood that a subsequent child of the same parents will have either Thalassemia major or Thalassemia minor?
    Comment:  This is a perfect problem for using Punnett's square, using the rows and columns as the gametic genotypes from the two parents, both of whom must be heterozygous or else they would be dead.  Use whatever symbols work for you.  For example, if T1T1 is normal, T1T2 is Thalassemia minor, then T2T2 is Thalassemia major.  The male gametic genotypes for the columns would be T1 and T2, and the female gametic genotypes would be T1 and T2, which would be the rows.  Three of the four resulting cells have either T1T2 or T2T2.  Unfortunately, the book discusses the disease only in the context of genetic screening.
1. 0%
2. 25%
3. 50%
4. 75%
5. 100%
14. An autosomal gene is one located on a chromosome other than a sex chromosome, therefore:
    Comment:  Certainly it's not a sex-chromosome, e.g., an X-chromosome, if it's an autosome, and an autosomal gene is typical of the kind that were studied by Mendel (Fortunately he didn't work with sex-linked traits.)
1. it is sex-linked
2. it is X-linked
3. it shows Mendelian patterns of segregation
4. it does not show Mendelian patterns
5. It only expresses in the male
   
15. Mendel's fourth postulate was independent assortment, which is observed:
    Comment:  Independent assortment was observed in dihybrid crosses, and the two genes have to be far apart, or on separate chromosomes, otherwise linkage will make them segregate together.  For further ideas, please read the "What if" on page 165. 
1. when two genes are tightly linked on the same chromosome
2. with monohybrids
3. when two genes are distantly linked or on different chromosomes
4. only in peas
5. during mitosis
16. One of the features of polyploids in plants is that:
    Comment:  This is on page 259 and was explained in class.  Polyploids, a form of euploids, are by definition organisms with extra whole sets of chromosomes (assuming a base for most organisms of 2n, where n is the of unique chromosomes.  Deleterious dominant alleles are always expressed, regardless of ploidy, because this is the nature of dominance.  This problem was written around plants, because for animals polyploidy is often lethal.  Animals are much less tolerant of chromosome number variations than are plants. 
1. they have higher fertility and seed set than regular diploids
2. their chromosomes pair and disjoin neatly
3. deleterious dominant alleles are exposed
4. they do not survive
5. they have extra whole sets of chromosomes
17. What is true about the two divisions of meiosis?
    Comment:  Meiotic divisions are described on pages 29-34.  Reductional division occurs only in the first division of meiosis so a, b, and e are false.  Answer c is false because the amount of DNA is reduced by half during the reductional division, thus the daughter cells do not have the same amount of DNA as the mother cell, or sporocyte.  The "sporocyte" is the cell in plant tissue which undergoes meiosis, either the megasporocyte (female) or microsporocyte (male).  It was explained briefly on the blackboard.  However, the book prefers to use an animal example, thus it refers on page 34 to the spermatocyte and the oocyte.
    
1. both involve reductional division
2. both involve equational or longitudinal division
3. the amount of DNA is the same in the daughter cells as in the sporocyte
4. the first division results in two daughter cells
5. the second division results in a reduction of the chromosome number
18. Bread wheat arose under human cultivation through a process of
    Comment:  Answers b, c, d, and e are discredited ideas of inheritance, e.g., pages 3-4.  In class the interspecific hybridization of wheat wild ancestors was explained as occurring in the foothills of the Zagros Mountains in modern day Iraq, and it also involved polyploidy.  The book uses the term "doctrine of use and disuse" to refer to the same concept described in class as "acquired inheritance."
1. interspecific hybridization
2. acquired inheritance
3. spontaneous generation
4. pangenesis
5. epigenesis
19. In the chi-square test the P refers to:
    Comment:  If the null hypothesis is true, results will still deviate based on sampling problems as well sometimes as other forms of variation.  If a very low P value is observed, however, so low that it would occur very rarely with the true null hypothesis, the null hypothesis is rejected.  It street language we would say, "That could never be."  Which is statistical language translates, "There is only a small P, less than 5%, that these results would be expected.  So, in statistical terms, what is the P value and the frequency of occurrence for "The hogs ate my little brother."
1. the chance that results could have been caused by a true null hypothesis
2. the chance that the results could have been caused by a false null hypothesis
3. the chance that the results could have been caused by a true alternative hypothesis
4. the null hypothesis should be accepted
5. the chance that the alternative hypothesis is correct.
20. Calico ("tortoise-shell") cats are always female because:
    Comment:  Heterochromatization and inactivation can affect either X chromosome, but it has to happen early enough that the kitty gets patches.  The Lyon hypothesis is explained on pages 240-241.
1. the maternal X chromosome is always heterochromatized into a Barr body
2. she always receives her X chromosomes from her mother, which may also be calico
3. either the maternal or paternal X chromosome is inactivated so all cells throughout the calico cat have the same inactivated chromosome
4. either the maternal or paternal X chromosome is randomly inactivated at an early stage of development
5. codominance
21. Coat color of horses can be cremello (almost white), palomino (golden coat with lighter mane and tail), or chestnut (brown). Among these phenotypes, palomino never breed true. Cremello and chestnut always breed true when they are mated with other horses of the same coat color. What is the simplest explanation for these relationships?
    Comment:  There are many explanations for this phenomena, but codominance is simplest because it involves only one gene with two alleles.  If the palomino is not true-breeding it must be the heterozygote.  This was illustrated on the chalk board and is book problem #14 on page 107. 
1. multiple alleles
2. epistasis
3. codominance
4. inactivation of the X-chromosome
5. Lyon hypothesis
22. Coat color of mice is determined by interacting loci: bb gives albino, A_B_ gives agouti (an alternating light and dark combination), and aaB_ gives black. What are the phenotypic frequencies of the offspring from the dihybrid (a cross between two double heterozygotes, that is, a cross between two AaBb)?
    Comment:  See page 89.  Only safe way to solve this is to do a dihybrid cross, and then intercross the F1s and then find the F2 progeny based on a Punnett's Square, using as rows the gametic genotypes AB, Ab, aB, and aa for the females, and the same for the columns representing the gametic genotypes for the males. 
1. 15 agouti :1 albino
2. 9 agouti : 3 black : 4 albino
3. 9 agouti : 4 black : 3 albino
4. 12 agouti : 3 black : 1 albino
5. 3 black : 1 albino
23. Down syndrome is a human genetic condition involving:
    Comment:  Down syndrome (pages 255-257) is a trisomy, involving a single extra chromosome, and the class of chromosome aberrations that include trisomy are called aneuploids. 
1. a point mutation
2. polyploidy
3. polygenic inheritance
4. an extra autosome
5. XYY
24. Of the four main genetic capabilities, the one explained by the Central Dogma is:
    Comment:  Genes are transcribed then mRNA is translated into polypeptides which are essentially proteins, even though they need a little finishing.  The whole process is the expression of the gene, as explained on page 284 and during the first day of class. 
1. mutation
2. storage
3. pangenesis
4. replication
5. expression
25. In the Punnett's square, the way it is used in Klug and Cummings, the rows and columns represent:
    Comment:  As explained on page 50, rows are the female gametes, and columns are the male gametes (or vice-versa) and the cells represent the genotypic combinations of male and female gametes.  This table is used to find the phenotypes, but the rows and columns are still the gametic genotypes.  
1. the distribution of the phenotypes
2. heterozygotes
3. homologous sets of chromosomes
4. possible gametic genotypes
5. sex chromosomes

Please write your answers in the white region below and number each.

36. What are some ways that a sex-linked gene differs in inheritance from an autosomal gene?
    Comment:  Maternal carrier, skips generations, male is hemizygous thus he is more likely to express the trait than the female. In the book there is an example of a human pedigree with explanations, on page 96. 
    
37. Tifway bermudagrass (T-419) has 27 chromosomes because it is an artificial interspecific hybrid between an 18-chromosome female parent and a 36-chromosome male parent. What kind of plant does that make Tifway, and what kind of fertility characteristic would you expect?
    Comment:  It's a polyploid (a triploid to be exact) because it has three sets of the basic complement of 9 chromosomes.  As such, the chromosomes pair badly and disjoin or separate badly at meiosis, so the plant is sterile, and doesn't produce seed.  Explained also on page 259.  Triploid watermelons are the same way, but as I explained in class, some people have uses for watermelon seeds.. 
38. What is an enzyme and what does it do? Assuming that a gene has the genetic code for a particular enzyme, explain how alleles of that gene often show complete dominance and complete recessiveness.
    Comment:  An enzyme is a catalyst that enables chemical reactions by reducing the energy of activation.  Consequently, a little goes a long way.  Enzymes can function in small dosages.  A heterozygote with a defective copy of the enzyme gene can still produce enzyme based on the DNA in the normal copy of the gene.  Consequently, such an heterozygote will look "normal" phenotypically, in most cases, thus the trait of "normal" will dominate.  The idea of "inborn errors of metabolism," observed by Garrod and Bateson, is described on pages 390-391, in the context of the one-gene-one-enzyme hypothesis.
39. Describe the alternation of generations starting with haploid gametes. What are the main events? What happens to the chromosome number?
    Comment:  haploid gametes (1n) fertilize to produce a zygote (2n) which grows up, produces germ cells that undergo meiosis, and the products are haploid pollen and eggs (plants) or sperm and eggs (animals) and the world goes round.  Alternation of generations is illustrated on page 36.
40. The book used an example of pink snapdragons to illustrate incomplete or partial dominance. Make up gene symbols and whatever else you need (e.g., arrows, tables) to show the results of a monohybrid cross, and the resulting phenotypic ratio.
    Comment:  The beautiful example of the snapdragons is illustrated on page 79 and in the book example the alleles used are R1 and R2, but anything that you want to use will work.   If R1R1 is red and R2R2 is white, the heterozygous R1R2 is pink and intermediate between the parents. 
41. How can continuous or quantitative gene effects be obtained from unit factors of inheritance?
    Comment:  Additive alleles (page 118) explain continuous variation because even though they represent unit factors, there can be quite many genes involved, called polygenes. 
42. Illustrate and name the major components of the cell, making sure to include those that are important in genetics.
    Comment:  See page 18.  The parts that are really, really important in genetics are chromosomes, nucleus, ribosomes, and mitochondria.  It would also be nice to see a cell wall, some chloroplasts if it's a plant, and maybe some endoplasmic reticulum.