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Chapter 4: Problem 36

From the fungal cross arg- \(6 \cdot a l-2 \times\) arg\(-6^{+} \cdot\) al- \(2^{+},\) what will the spore genotypes be in unordered tetrads that are (a) parental ditypes? (b) tetratypes? (c) nonparental ditypes?

Short Answer

Expert verified
(a) arg-6.al-2 and arg-6+.al-2+; (b) arg-6.al-2, arg-6.al-2+, arg-6+.al-2, arg-6+.al-2+; (c) arg-6.al-2+ and arg-6+.al-2.

Step by step solution

01

Understand the Fungal Cross

In this fungal cross, we have two different loci: arg-6 and al-2. Each of these loci has two alleles. For arg-6, we have the alleles arg-6 and arg-6+; for al-2, we have the alleles al-2 and al-2+. The fungal cross is between the genotype arg-6.al-2 and the genotype arg-6+.al-2+.
02

Recognize Spore Types for Parental Ditypes

Parental ditypes (PD) are tetrads where all spores have the same genotype as the parents. From this cross, the parental genotypes are arg-6.al-2 and arg-6+.al-2+. Therefore, the spores in a PD tetrad will be two of arg-6.al-2 and two of arg-6+.al-2+.
03

Identify Spore Types for Tetratypes

Tetratypes (T) contain all possible combination of alleles, resulting in all spores having different genotypes. From this cross, the possible spore genotypes include arg-6.al-2, arg-6.al-2+, arg-6+.al-2, and arg-6+.al-2+.
04

Determine Spore Types for Nonparental Ditypes

Nonparental ditypes (NPD) are tetrads where all spores have genotypes that are recombinant, meaning they are not seen in the original parental types. The recombinant genotypes from this cross would be arg-6.al-2+ and arg-6+.al-2, each appearing twice in the NPD.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Fungal Cross
A fungal cross involves mating two different strains or types of fungi, much like in other organisms such as plants or animals. In genetics, a fungal cross can be an essential method for studying gene segregation and recombination. Fungi, particularly certain kinds of yeasts like * Saccharomyces cerevisiae offer an outstanding model for genetic studies due to their well-defined genetic platform. In this specific example, the fungal cross involves two loci. Loci are places on a chromosome where genes reside. The loci are labeled as arg-6 and al-2, each containing two alleles: arg-6/arg-6+ and al-2/al-2+. When these cross, they combine in various ways to give rise to different spore genotypes. Understanding this concept is crucial for predicting offspring genotypes, observing inheritance patterns, and studying mutations.
Tetrad Analysis
Tetrad analysis is a powerful genetic tool used predominantly in fungi to analyze meiotic events. Tetrads are structures comprised of four spores resulting from a single meiotic event, helpful for determining genetic linkage and gene mapping. When conducting tetrad analysis, it helps to know the various types of tetrads:
  • **Parental Ditype (PD):** All four spores contain only the parental type genotypes. This occurs without any recombination.
  • **Nonparental Ditype (NPD):** All spores are recombinant, meaning the genes have crossed over, leading to new allele combinations.
  • **Tetratype (T):** Contains all possible combinations of parental and recombinant genotypes.
Analyzing these types gives insight into the linkage between genes and hypothetical crossover events during meiosis. This is fundamental for genetic investigations aimed at decoding hereditary information.
Recombinant Genotypes
Recombinant genotypes are the result of genetic recombination during meiosis, a process where chromosomes exchange segments. This leads to new combinations of alleles in the offspring compared to the parental lines. Recombinants are a focal point in genetics because they introduce diversity within a population. In the context of a fungal cross, a recombinant genotype occurs when the genotypes of the spores differ from those of the parent strains. For instance, in the case given,
  • **Recombinant genotypes** include these combinations: arg-6.al-2+ and arg-6+.al-2, not present in either parental strain.
Studying recombinant genotypes can help illuminate the processes of evolution and natural adaptation by highlighting genetic variability.
Allelism
Allelism is a concept in genetics that describes the existence of different versions (alleles) of the same gene. Alleles contribute to genetic diversity and are responsible for the variability in phenotype among individuals. In the provided scenario, alleles appear at multiple gene loci. For example,
  • **arg-6 and arg-6+:** These are alleles for a gene located on the arg-6 locus.
  • **al-2 and al-2+:** These represent alleles for the gene positioned on the al-2 locus.
The presence of alleles contributes to the complexity of genetic crosses and explains how different traits can be exhibited based on dominant or recessive alleles within a population. Allelism is essential in understanding inheritance as it illustrates why offspring differ in genotype and phenotype, allowing scientists to map hereditary patterns.

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Most popular questions from this chapter

A Neurospora cross was made between a strain that carried the mating-type allele \(A\) and the mutant allele arg- 1 and another strain that carried the mating-type allele \(a\) and the wild-type allele for arg- \(1(+)\). Four hundred linear octads were isolated, and they fell into the seven classes given in the table below. (For simplicity, they are shown as tetrads.) a. Deduce the linkage arrangement of the mating-type locus and the arg-1 locus. Include the centromere or centromeres on any map that you draw. Label all intervals in map units. b. Diagram the meiotic divisions that led to class 6\. Label clearly. $$\begin{array}{ccccccc}\mathbf{1} & \mathbf{2} & \mathbf{3} & \mathbf{4} & \mathbf{5} & \mathbf{6} & \mathbf{7} \\\\\hline A \cdot \text {arg} & A \cdot+ & A \cdot \text {arg} & A \cdot \text {arg} & A \cdot \text {arg} & A \cdot+ & A \cdot+ \\\A \cdot \text {arg} & A \cdot+ & A \cdot+ & a \cdot \text {arg} & a \cdot+ & a \cdot \text {arg} & a \cdot \text {arg} \\\A \cdot+ & a \cdot \text {arg} & a \cdot \text {arg} & A \cdot+ & A \cdot \text {arg} & A \cdot+ & A \cdot \arg \\\A \cdot+ & a \cdot \text {arg} & a \cdot+ & a \cdot+ & a \cdot+ & a \cdot \text {arg} & a \cdot+ \\\\\hline 127 & 125 & 100 & 36 & 2 & 4 & 6 \end{array}$$ 1\. Are fungi generally haploid or diploid? 2\. How many ascospores are in the ascus of Neurospora? Does your answer match the number presented in this problem? Explain any discrepancy. 3\. What is mating type in fungi? How do you think it is determined experimentally? 4\. Do the symbols \(A\) and \(a\) have anything to do with dominance and recessiveness? 5\. What does the symbol arg-1 mean? How would you test for this genotype? 6\. How does the arg-1 symbol relate to the symbol +? 7\. What does the expression wild type mean? 8\. What does the word mutant mean? 9\. Does the biological function of the alleles shown have anything to do with the solution of this problem? 10\. What does the expression linear octad analysis mean? 11\. In general, what more can be learned from linear tetrad analysis that cannot be learned from unordered tetrad analysis? 12\. How is a cross made in a fungus such as Neurospora? Explain how to isolate asci and individual ascospores. How does the term tetrad relate to the terms ascus and octad? 13\. Where does meiosis take place in the Neurospora life cycle? (Show it on a diagram of the life cycle.) 14\. What does Problem 38 have to do with meiosis? 15\. Can you write out the genotypes of the two parental strains? 16\. Why are only four genotypes shown in each class? 17\. Why are there only seven classes? How many ways have you learned for classifying tetrads generally? Which of these classifications can be applied to both linear and unordered tetrads? Can you apply these classifications to the tetrads in this problem? (Classify each class in as many ways as possible.) Can you think of more possibilities in this cross? If so, why are they not shown? 18\. Do you think there are several different spore orders within each class? Why would these different spore orders not change the class? 19\. Why is the following class not listed? $$\begin{array}{ll} a \cdot+ & A \cdot \arg \\\a \cdot+ & A \cdot \arg\end{array}$$ 20\. What does the expression linkage arrangement mean? 21\. What is a genetic interval? 22\. Why does the problem state "centromere or centromeres" and not just "centromere"? What is the general method for mapping centromeres in tetrad analysis? 23\. What is the total frequency of \(A\). \(+\) ascospores? (Did you calculate this frequency by using a formula or by inspection? Is this a recombinant genotype? If so, is it the only recombinant genotype?) 24\. The first two classes are the most common and are approximately equal in frequency. What does this information tell you? What is their content of parental and recombinant genotypes?

For a certain chromosomal region, the mean number of crossovers at meiosis is calculated to be two per meiosis. In that region, what proportion of meioses are predicted to have (a) no crossovers? (b) one crossover? (c) two crossovers?

In the model plant Arabidopsis thaliana, the following alleles were used in a cross: \(\begin{array}{ll}T=\text { presence of trichomes } & t=\text { absence of trichomes } \\ D=\text { tall plants } & d=\text { dwarf plants } \\ W=\text { waxy cuticle } & w=\text { nonwaxy } \\ A=\text { presence of purple } & a=\text { absence (white) } \\ & \text { anthocyanin pigment }\end{array}\) The \(T / t\) and \(D / d\) loci are linked 26 m.u. apart on chromosome 1 , whereas the \(W / w\) and \(A / a\) loci are linked 8 m.u. apart on chromosome 2 A pure-breeding double-homozygous recessive trichomeless nonwaxy plant is crossed with another pure-breeding double-homozygous recessive dwarf white plant. a. What will be the appearance of the \(\mathrm{F}_{1}\) ? b. Sketch the chromosomes 1 and 2 of the parents and the \(F_{1},\) showing the arrangement of the alleles. c. If the \(\mathrm{F}_{1}\) is testcrossed, what proportion of the progeny will have all four recessive phenotypes?

You have a Drosophila line that is homozygous for autosomal recessive alleles \(a, b,\) and \(c,\) linked in that order. You cross females of this line with males homozygous for the corresponding wild-type alleles. You then cross the \(\mathrm{F}_{1}\) heterozygous males with their heterozygous sisters. You obtain the following \(F_{2}\) phenotypes (where letters denote recessive phenotypes and pluses denote wild-type phenotypes): \(1364+++365\) a \(b c, 87 a b+, 84++c\) \(47 a++, 44+b c, 5 a+c,\) and \(4+b+\) a. What is the recombinant frequency between \(a\) and \(b\) ? Between \(b\) and \(c ?\) (Remember, there is no crossing over in Drosophila males. b. What is the coefficient of coincidence?

In corn, the cross \(W W\) ee \(F F \times w w E E f f\) is made. The three loci are linked as follows Assume no interference. a. If the \(F_{1}\) is testcrossed, what proportion of progeny will be ww ee ff? b. If the \(F_{1}\) is selfed, what proportion of progeny will be ww ee ff?
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