Solutions For An Introduction to Genetic Analysis Chapter 4 Problem 38P

1. Fungi are generally haploid in nature with only one copy of chromosomes.

2. There are eight ascospores in the ascus of Neurospora present as 4 pairs. No, the answer does not match since only one member of each pair is presented in the data.

3. A mating type in fungi is the same as gender in humans. Two fungi of different mating types can mate to produce progeny. Mating type is determined experimentally by seeing the progeny results from specific crosses.

4. The symbols A and a is not related to dominant or recessive nature. They are used to differentiate between the different mating types.

5. The symbol arg-1 indicates that the organism requires the amino acid arginine in the media for its growth. We can test for this genotype by isolating nutritional mutants and later adding arginine to see if growth occurs.

6. The symbol arg-1⁺ indicates that the organism is wild type and does not require arginine in the media for growth.

7. The expression wild type refers to the common expression of alleles of an organism seen in the natural population.

8. The word mutant means that the organism has an allele which differs from the wild type allele.

9. The biological function of the alleles does not matter when solving this problem.

10. The expression “linear octad analysis” refers to the arrangement of ascospores in the ascus. This arrangement reflects the order of the two meiotic divisions and also the subsequent mitotic division. By tracking the position of the ascospores, we can detect crossing-over events which occurred at the tetrad stage.

11. In linear tetrad analysis, mapping of centromeres in relation to genes can be done. This cannot be done in the case of unordered tetrad analysis.

12. In a fungus like Neurospora, a cross is made by placing the two organisms in the same test tube or Petri plate and allowing them to grow. Gametes develop in both organisms. When the gametes meet, they fuse and fertilize. The processes of meiosis, mitosis, and ascus formation occur. These asci are isolated from the plate and dissected to remove ascospores.

An ascus has eight spores present [octad] as four pairs [tetrad]. In this way, the term tetrad is associated with the terms ascus and octad.

13. Meiosis occurs immediately after fertilization in Neurospora. The life cycle of Neurospora is as follows:

14. In this problem, the ascospores produced by the process of meiosis were analyzed.

15. The genotypes of the parental strains are as follows:

A arg-1 x a arg-1⁺

16. Since the eight ascospores occur as four pairs, each pair represents one chromatid of the original pair of chromosomes. In the fungal convention, both members of a pair are represented as a single genotype. Hence, only four genotypes are shown in each class.

17. The seven classes represent the seven different types of outcome.

In the above table, PD stands for parental ditype, NPD stands for non-parental ditype, T for tetratype, I for first division segregation and II for second division segregation.

Another way of classifying can be:

The above asci are equal with same genotypes of ascospores.

The first type of classification can be used both for linear and unordered tetrad analysis. Yes, the classification can be applied to the tetrads in this problem.

Since about 400 progeny were isolated and examined, there may not be any more possibilities in this cross.

18. Yes, there are several different spore orders within each class. These different spore orders will not change the class since the genotypes remain the same. A change in arrangement will not change the class.

19. The following class is not included in the list given:

a +

a +

A arg

A arg

The above class is not listed since it is identical with class I, only inverted.

20. The expression “linkage arrangement” refers to the relative positions of two genes with respect to the centromere along the length of the chromosome.

21. A genetic interval is the region between two gene loci. It is measured in map units.

22. Since the problem does not specify whether the genes are on the same chromosomes or on different chromosomes, the problem indicates it as centromeres.

The general method for mapping centromeres is to calculate the distance of a locus to its centromere. This can be measured by the percentage of tetrads that show second division segregation patterns for that specific locus and divide by 2.

This helps us to determine whether both the genes share a common centromere or have different centromeres.

23. The total frequency of A + ascospores can be calculated by inspection in the following way:

Yes, this is a recombinant genotype. The other recombinant genotype is a arg-1.

24. The first two classes are the most common and approximately equal in frequency. The first class is parental, while the second class is non-parental ditype. Since they occur in equal frequencies, they are not linked.

a) The cross is A arg-1 x a arg-1⁺.

To find out if the two genes are linked, we need to calculate the recombination frequency.

Since the recombination frequency is not below 50%, it indicates that the genes are not linked.

The distance between each gene and its centromere can be calculated using the formula:

For A – centromere:

For arg ⁺ – centromere:

The map distance from the centromere can be shown as:

b) The meiotic divisions that led to class 6 are diagrammed below. This class can be obtained with a single crossover between chromatids 2 and 3 between each gene and its centromere.