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Biology 22 Problems in Genetic Mapping
1. In corn, a strain homozygous for the recessive alleles a (green), d (dwarf) and rg (normal leaves) was crossed to a strain homozygous for the dominant alleles of each of these genes, namely A (red), D (tall) and Rg (ragged leaves). Offspring of this cross were then crossed to plants that were green, dwarf and had normal leaves. The following phenotypic classes were observed.
Phenotype red, tall, ragged green, dwarf, normal red, tall, normal
Number
265
275
24 green, dwarf, ragged red, dwarf, normal green, tall, ragged red, dwarf, ragged green, tall, normal
16
90
70
120
140 a. Propose a linkage map with distances between the three genes. b. What is the coefficient of coincidence for these genes? c. What degree of interference occurred in this experiment?
2. In a Neurospora cross of ab x a + b + , the following classes and numbers of tetrads were produced. Neurospora produces ordered tetrads that undergo a single mitosis after formation. Pairs of spores are listed below for simplicity. ab ab a + b + a + b +
71 ab + ab + a + b a + b
1 ab ab + a + b + a + b
18 ab a + b a + b + ab +
1 a ab a +
+ b + b ab
8
+ ab + a + b a + b ab +
0 ab + a + b a + b + ab
1 a. Determine the gene-centromere distance for each gene. b. Determine the distance between genes a and b. c. Draw a genetic map that integrates the information from parts a and b.

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3. In yeast, the ura1 locus produces an enzyme that acts in the pathway for synthesis of the nucleotide uracil and the lys5 locus produces an enzyme that acts in the pathway for the synthesis of the amino acid lysine. Two haploid strains are crossed, ura1 + lys5 x ura1 lys5 + , and the resulting diploid is allowed to undergo meiosis. Unordered asci are scored with the following results: ura1 + lys5 ura1 + lys 5 ura1 lys5 + ura1 lys5 + ura1 + lys5 + ura1 + lys 5 + ura1 lys5 ura1 lys5 ura1 + lys5 ura1 + lys 5 + ura1 lys5 + ura1 lys5
330 30 140
Draw a genetic map to illustrate the linkage arrangement for these loci.
4. Hybrid cells containing human and mouse chromosomes were analyzed. The following grid shows the presence or absence of each of four human chromosomes in hybrid cell lines A through D.
Human Chromosome
5 7 11 18
A -- + + --
B
C
+
--
--
+
+
+
+
+
D +
The following grid shows the presence or absence of human enzyme activity in each of the cell lines.
Human Enzyme
ADH
+
PEP
--
HexA
--
GAPDH
A
B
+
+
--
+
+
--
--
+
C + -- + +
D -- + + --
Assign the gene for each enzyme to the chromosome that carries the gene.