A yeast strain of mating type alpha that cannot grow in the absence of the amino

Question

A yeast strain of mating type alpha that cannot grow in the absence of the amino acid histidine (his2) is mated with a yeast strain of mating type a that cannot grow in the absence of the amino acid lysine (lys4). Among the 500 unordered tetrads resulting from this mating, 180 were PD, 20 were NPD, and 300 were T. a. What is the genotype of each haploid parent and the diploid they form when mated. Include the mating type. b. Indicate the genotypes of the spores that are in the PD, NPD, and T tetrads below. Do not include mating type in the genotype of the spores: c. Are the genes linked? State your reasoning. What is the distance in map units between the his and lys genes? If they are not linked just state that they are more than 50 m.u. apart

Explanation / Answer

Mutant yeast cannot grow on a minimal medium that consists of trace elements, salts, some sort of nitrogen and sugar.

Vital growth factors like specific amino acids are added to this minimal medium to see the growth of the mutant yeast. If the yeast can grow if histidine is added to the minimal medium, but cannot grow if histidine is absent; this mutant strain of yeast cannot make histidine for itself; it is a his mutant strain.

a. Haploid yeast strains mutant in different genes are mated together, a process of fusion of two cells to form a new diploid cell.

his- lys+ * his+ lys-       = his- his+ lys+ lys- = 180 PD, 20 NPD, 300 T

()        *          (a)

The genotype of this diploid could be HIS/his and LYS/lys, where the HIS and LYS genes are on the same piece of DNA on one of the pairs of homologous chromosomes, and the his and lys genes are on the same piece of DNA but on the complementary homologous chromosome.

b. The four haploid cells or the spores from the meiosis are kept together as a tetrad. Three kinds of patterns of growth can be observed on the testing media depending on the meiosis of the original diploid

A Parental Ditype (PD) pattern will have two different genotypes both of which are exactly like the parents i,e. two colonies of cells will have the same genes as one of the haploid parents and the other two colonies of cells that have exactly the same genes as the other original haploid parent

A Non-parental Ditype (NPD) pattern will have four colonies of haploid cells none of which are carrying the same combination of genes found in the original haploid parents

A Tetratype (T) pattern will have one colony that carries the same gene combination found in one of the parents, one colony that carries the same gene combination found in the other parent, and two colonies that have combinations of the two genes found in neither of the original haploid parents

c. The distance (in map units called centimorgans) between the two genes can then be calculated using the formula:

distance im m.u. between his and lys genes = 100( T+ 6 NPD) / 2 ( PD + NPD + T)

= 100(300 + 6*20) / 2(180 + 20 + 300)

=48000/1000

= 48 m. u.

Therefore the his and lys genes are linked.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.