Genetics Problems from Book
Assume the trihybrid cross AA BB rr x aa bb RR is made in a plant species in which A and B are dominant but there is no dominance between R and r. Consider the F2 progeny from this cross, and assume independent assortment. 1. How many phenotypic classes are expected? 2. What is the probability of the parental aa bb RR genotype? 3. What proportion would be expected to be homozygous for all genes?
1. 2 are expected for AA, tow for Bb, and 3 for RR. = 2x2x3 = 12 2. Probability of getting aa bb RR offspring is 1/4 (aa) x 1/4 (bb) x 1/4 (RR) = 1/64 3. Because the probability of homozygosity for eithher allele is 1/2 for each gene, the proportion expected to be homozygous for all three genes is 1/2 x 1/2 x 1/2 = 1/8.
Consider the accompanying diagram of a region of duplex DNA, in which the Bs represent bases In Watson-Crick pairs, specify as precisely as possible the identity of: 1. B5, assuming that B1 = A 2. B6, assuming that B2 = C 3. B7, assuming that B3 = Purine 4. B8, assuming that B4 = A or T
1. T 2. G 3. Pyrimidine 4. T or A
Which of the following sequences are palindromes, and which are not? 1. 5'-AATT-3' 2. 5'-AAAA-3' 3. 5'-AANTT-3' 4. 5'-AA(A/T)AA-3' 5. 5'-AA(G/C)TT-3'
1. Yes, partner strand reads 3'-TTAA-5' 2. no, partner strand reads 3'-TTTT-5' 3. Yes, partner strand reads 3'-TTNAA-5' Where the two n's must be watson-crick base paired 4. No, partner strands read 3'-TT(T/A)TT-5' 5. Yes, partner strand reads 3'-TT(C/G)AA-5'
Assuming equal numbers of boys and girls, if a mating has already produced a girl, what is the probability that the next child will be a boy? On what type of probability argument do you base your answer?
1/2; 1/2. The probability argument is that eac birth is independent of all the previous ones, so the number of boys and girls has no influence on the sexes of future children.
How many different gametes can be formed by an organism with genotype AA Bb Cc Dd Ee and in general, by an organism that is heterozygous for m genes and homozygous for n genes?
1x2x2x2x2 = 16 possible gametes In general, 1^n x 2^m which equals 2^m.
What important observation about S and R strains of streptococus pneumoniae prompted Avery, Macleod, and McCarty to study this organism?
A mixture of heat-killed S-cells and living R cells causes pneumonia in mice, but neither heat killed S-cells nor living R cells do so alone.
A homozygous recessive results from the mating of a heterozygote and a parent with the dominant phenotype. What does this tell you about the genotype of the parents with the dominant phenotype?
A. The parent with the dominant phenotype must carry one copy of the recessive allele and hence must be heterozygous.
A double stranded DNA molecule is separated into its constituent strands, and the strands are separated in an ultracentrifuge. In one of the strands the base composition is 24 percent A, 28 percent T, and 22 percent G, and 26 percent C. What is the base composition of the other strand?
Because A pairs with T and G with C, the composition of the other strand must be 24 percent T, 28 percent A, 22 percent C and 26 percent G.
The pattern of coat coloration in dogs is determined by the alleles of a single gene, with S (solid) being dominant over s (spotted). Black coat color is determined by the dominant allele A of a second gene, tan by homozygosity for the recessive allele a. A female having a solid tan coat is mated with a male having a solid black coat and produces a litter of six pups. The phenotypes of the pups are 2 solid tan, 2 solid black, 1 spotted tan, and 1 spotted black. What are the genotypes of the parents? (assume independent assortment)
Because both parents have solid coats but produce some spotted offspring, they must be Ss. With respect to the A, a pair of alleles, the female parent (tan) is aa, and because there are some ta offspring, the genotype of the black male parent must be Aa. Parental genotypes are thus Ss aa, and Ss Aa.
The DNA extracted from a bacteriophage contains 28 percent A, 28 percent T, 22 percent G, and 22 percent C. What can you conclude about the structure of this DNA molecule?
Because the amount of T equals that of A, and the amount of G equals that of C, it can be inferred that the DNA in this bacteriophage is double stranded. 56 percent are AT pairs, 44 percent are CG pairs.
Like DNA, molecules of RNA contain large amounts of phosphorus. When Hershey and Chase grew their T2 phage in bacterial cells that had grown in the presence of radioactive phosphorus, the RNA must also have incorporated the labeled phosphorus and yet the experimental result was not compromised. Why not?
Because the mature T2 phage contains only DNA and protein; the labeled RNA was left behind in material released by the burst cells.
Two parents with the dominant phenotype produce nine offspring. Two have the recessive phenotype. What does this tell you about the genotype of the parents?
Becuase some of the progeny are homoyzgous recessive, both parents must be heterozygous.
One strand of a DNA duplex as the base sequence 5'-ATCGTATGCACTTTACCCGG-3'. What is the base sequence of the complimentary strand?
Complimentary strand has the sequence: 3'-TAGCATACGTGAAATGGGCC-5'
Phenylketonuria is a recessive inborn error of metabolism of the amino acid phenylalanine that results in severe mental retardation of affected children. The female II-3 (red circle) in the pedigree shown on the next page is affected, whereas II-2 and II-4 are not affected, If persons III-1 and III-2 (first cousins) mate, what is the probability that their offspring will be affected?
For the child to be affected, both III-1 and III-2 must be heterozygous Aa. For III-1 to be Aa, individual II-1 must be Aa and must transmit the recessive allele to III-1. The probability that II-2 is Aa equals 2/3 and then the probability of transmitting the a allele to III-1 is 1/2. Altogether the probability is 2/3 x 1/2 = 1/3.
Apart from the base sequence, what is different about the ends of restriction enzymes? (* represents line of cleavage) 1. HaeIII (5'-GG*CC-3') 2. MaeI (5'-C*TAG-3') 3. CfoI (5'GCG*C-3')
MaeIII produces blunt ends MaeI produces a 5' overhang CfoI produces a 3' overhang
A chemical called phenol destroys proteins but not nucleic acids, and a strong alkali such as sodium hydroxide destroys both proteins and nucleic acids. In the transformation experiments with streptococcus pn. what result would be expected if the S-strain extract had been treated with phenol? What would be expected if it had been treated with a strong alkali?
Phenol would not destroy the transforming activity; a strong alkali would.
What is the ed result of replication of a duplex DNA molecule?
Replication results in two daughter DNA duplexes, each identical in base sequence to the parental molecule (except for possible mutations in the sequence). Each of the daughter molecules contains one of the original intact parental strands.
The linear DNA fragment shown here has cleavage sites for BAMHI (B) and EcoRI (E.) In the accompanying diagram of an electrophoresis gel, indicate the positions at which bands would be found after digestion with: 1. BamHI Alone 2. EcoRI Alone 3. BamHI and EcoRI Together
See text book page 796
You have sequenced both strands of a double stranded DNA molecule. To inspect the potential amino acid coding content of this molecule, you conceptually transcribe it into RNA and then conceptually translate the RNA into a polypeptide chain. How many reading frames will you have to examine?
Six. Either DNA strand could be transcribed, and each transcript could be translated in any one of the three reading frames.
One parent has a dominant phenotype and the other has a recessive phenotype. Two offspring result, and both have the dominant phenotype. What genotypes are possible for the parent with the dominant phenotype?
The parent with the dominant phenotype could be either homozygous dominant or heterozygous; the occurrence of no homozygous recessive offspring encourages the suspicion that the parent may be homozygous dominant, but because there are only two offspring, heterozygosity cannot be ruled out.
A duplex DNA molecule contains a random sequence of the four nucleotides with equal proportions of each. What is the average spacing between consecutive occurrences of the sequence 5'-GGCC-3'? Between consecutive occurrences of the sequence 5'-GAATC-3'?
The probability that four particular bases ave te sequence 5'-GGCC-3' is (1/4)^4=1/256. So 256 base pairs is te average spacing between consecutive occurrences; similarly, the average spacing for the sequence 5'-GAATTC-3' is 4096 base pairs.
A synthetic mRNA molecule consists of the repeating base sequence 5'-AUCAUCAUCAUCAUCAUC...-3'. When this molecule is translated in vitro, the result is a mixture of three different polypeptide chains. One consists of repreating isoleucines (Ile, ile...) another of repeating serines (ser-ser-ser) and the third of repeating histidines (His His His). What does this imply about the manner in which an mRNA is translated?
The result means that an mRNA is translated in nonoverlapping groups of three nucleotides: The genetic code is a triplet code.
The duplex nucleic acid molecule shown here consists of a strand of DNA paired with a complementary strand of RNA. Is the RNA the top or the bottom strand? One of the base pairs is mismatched. Which pair is it? 5'-AUCGGUUACAUUCCGACUGA-3' 3'-TAGCCAATGTAAGGGTGACT-5'
The top strand is the RNA because it contains U instead of T; the mismatched base pair is the sixth from the right.
From their examination of the structure of DNA, what were Watson and Crick able to infer about the probable mechanisms of DNA replication, coding capability, and mutation?
They noted that the strands were used as a template for the formation of a new daughter strand having complementary sequence of bases. They also noted that genetic information could be coded by the sequence of bases along the DNA molecule, analogous to letters of the alphabet printed on a strip of paper. Finally, they noted that changes in genetic information could result from errors in replication and the altered nucleotide sequence could then be perpetuated.
If human DNA were essentially a random sequence of 3x10^9 bp with equal proportions of all four nucleotides, approximately how many restriction fragments would be expected from cleavage with: 1. A "4-Cutter" restriction enzyme? 2. A "6-Cutter" restriction enzyme? 3. A "8-Cutter" restriction enzyme?
a. 3x10^9/256 = 12 million b.3x10^9/4096 = 730,000 c.3x10^9/65536 = 46,000
An RNA molecule folds back upon itself to form a hairpin structure held together by a region of base pairing. One segment of the molecule in the paired region has the base sequence 5'-AUACGAUA-3'. What is the base sequence with which this segment is paired?
sequence is: 3'-UAUGCUAU-5'
In a random sequence consisting of equal portions of all four nucleotides, what is the probability that a particular short sequence of nucleotides matches a restriction site for: 1. A restriction enzyme with a 4-base cleavage site? 2. A restriction enzyme with a 6-base cleavage site? 3. A restriction enzyme with a 8-base cleavage site?
the probability of a restriction site is 1/4^n, where n is the number of nucleotides in the site. 1. 1/4^4 = 0.0039 2. 1/4^6 = 0.00024 3. 1/4^8 = 0.000015