A plantlike organism on the planet Pandora can have three recessive genetic traits: bluish leaves, due to an allele (a) of gene A; a feathered stem, due to an allele (b) of gene B; and hollow roots due to an allele (c) of gene C. The three genes are linked and recombine as follows:
A geneticist did a testcross with an organism that had been found to be heterozygous for the three recessive traits and she was able to identify progeny of the following phenotypic distribution (+ = wild type):
Phenotypes Leaves Stems Roots Number
1 a + + 14
2 a + c 0
3 a b + 32
4 a b c 440
5 + b + 0
6 + b c 16
7 + + c 28
8 + + + 470
Total 1,000
1) Which of the following are the phenotypes of the parents in this cross?
A) 2 and 5
B) 1 and 6
C) 4 and 8
D) 3 and 7
E) 1 and 2
Answer: C
2) In which progeny phenotypes has there been recombination between genes A and B?
A) 1, 2, 5, and 6
B) 1, 3, 6, and 7
C) 2, 4, 5, and 8
D) 2, 3, 5, and 7
E) in all 8 of them
Answer: A
3) If recombination is equal to distance in centimorgans (cM), what is the approximate distance between genes A and B?
A) 1.5 cM
B) 3 cM
C) 6 cM
D) 15 cM
E) 30 cM
Answer: B
4) What is the greatest benefit of having used a testcross for this experiment?
A) The homozygous recessive parents are obvious to the naked eye.
B) The homozygous parents are the only ones whose crossovers make a difference.
C) Progeny can be scored by their phenotypes alone.
D) All of the progeny will be heterozygous.
E) The homozygous recessive parents will be unable to cross over.
Answer: C
5) The greatest distance among the three genes is between a and c. What does this mean?
A) Gene a is closest to b.
B) Genes are in the order: abc.
C) Gene a is not recombining with c.
D) Gene a is between b and c.
E) Distance ab is equal to distance ac.
Answer: D
The Chromosomal Basis of Inheritance
- A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's father was 6 feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive.
- The pedigree in Figure 15.3 shows the transmission of a trait in a particular family. Based on this pattern of transmission, the trait is most likely
- In a series of mapping experiments, the recombination frequencies for four different linked genes of Drosophila were determined as shown in Figure 15.2. What is the order of these genes on a chromosome map?
- Between which two genes would you expect the highest frequency of recombination?
- Suppose that a gene on human chromosome 18 can be imprinted in a given pattern in a female parent but not in a male parent. A couple in whom each maternal meiosis is followed by imprinting of this gene have children. What can we expect as a likely outcome?
- Which of the following produces a Mendelian pattern of inheritance?
- A certain kind of snail can have a right-handed direction of shell coiling (D) or left-handed coiling (d). If direction of coiling is due to a protein deposited by the mother in the egg cytoplasm, then a Dd egg-producing snail and a dd sperm-producing snail will have offspring of which genotype(s) and phenotype(s)?
- Mitochondrial DNA is primarily involved in coding for proteins needed for electron transport. Therefore, in which body systems would you expect most mitochondrial gene mutations to be exhibited?
- Correns described that the inheritance of variegated color on the leaves of certain plants was determined by the maternal parent only. What phenomenon does this describe?
- Genomic imprinting is generally due to the addition of methyl (CH3) groups to C nucleotides in order to silence a given gene. If this depends on the sex of the parent who transmits the gene, which of the following must be true?
- A gene is considered to be non-Mendelian in its inheritance pattern if it seems to "violate" Mendel's laws. Which of the following would be considered Mendelian?
- Which of the following is true of aneuploidies in general?
- At what point in cell division is a chromosome lost so that, after fertilization with a normal gamete, the result is an embryo with 45, X?
- Which of the following is known as a Philadelphia chromosome?
- What is a syndrome?
- At which phase(s) is it preferable to obtain chromosomes to prepare a karyotype?
- In 1956 Tijo and Levan first successfully counted human chromosomes. What is the reason it took so many years to do so?
- A couple has a child with Down syndrome. The mother is 39 years old at the time of delivery. Which of the following is the most probable cause of the child's condition?
- Down syndrome has a frequency in the U.S. population of ~1/700 live births. In which of the following groups would you expect this frequency to be significantly higher?
- What is the source of the extra chromosome 21 in an individual with Down syndrome?
- An inversion in a human chromosome often results in no demonstrable phenotypic effect in the individual. What else may occur?
- Abnormal chromosomes are frequently found in malignant tumors. Errors such as translocations may place a gene in close proximity to different control regions. Which of the following might then occur to make the cancer worse?
- A phenotypically normal prospective couple seeks genetic counseling because the man knows that he has a translocation of a portion of his chromosome 4 that has been exchanged with a portion of his chromosome 12. Although he is normal because his translocation is balanced, he and his wife want to know the probability that his sperm will be abnormal. What is your prognosis regarding his sperm?
- Of the following human aneuploidies, which is the one that generally has the most severe impact on the health of the individual?