Question

In diploid yeast strains, sporulation and subsequent meiosis can produce haploid ascospores, which may fuse to reestablish diploid cells. When ascospores from a segregational petite strain fuse with those of a normal wild-type strain, the diploid zygotes are all normal. Following meiosis, ascospores are petite and normal. Is the segregational petite phenotype inherited as a dominant or a recessive trait?

Short answer

Answer: The segregational petite phenotype in yeast is inherited as a recessive trait.

Step-by-step solution

Understand Dominant and Recessive Traits

A dominant trait is expressed when at least one copy of the responsible gene is present; if two recessive alleles are present, the recessive trait is expressed.

Observe the Fusion of Ascospores from Petite and Wild-Type Strains

When ascospores from a segregational petite strain fuse with those of a normal wild-type strain, the resulting diploid zygotes are all normal. This indicates that the normal (wild-type) phenotype is expressed in the presence of both petite and normal alleles.

Investigate the Resulting Ascospores after Meiosis

After meiosis, the resulting ascospores are petite and normal. Since both petite and normal phenotypes are present, this indicates that the two different alleles for the trait, one for petite and one for normal, have segregated and been passed on to the progeny.

Determine the Inheritance Pattern of the Petite Phenotype

From the observed results, we can conclude that the petite phenotype is inherited as a recessive trait because it's only expressed when two copies of the petite allele are present in the progeny, while the normal phenotype is expressed when at least one normal allele is present.

Key concepts

Dominant and Recessive Traits

In genetics, understanding dominant and recessive traits is crucial. These are the basic patterns that determine how a particular trait is inherited and expressed in an organism. A dominant trait requires only one copy of an allele for it to be exhibited in an organism's phenotype. To illustrate, if an individual inherits one allele for a dominant trait from one parent, that trait will be visible regardless of the second allele inherited from the other parent.

A recessive trait, on the other hand, will only appear in the phenotype if an individual inherits two copies of the allele, one from each parent. Therefore, if an individual has just one copy of a recessive allele and one dominant allele, the dominant trait will mask the recessive one, and it will not be expressed in the organism's phenotype. This is known as complete dominance. As the exercise indicates, the petite trait must be recessive since it is not expressed in the presence of the normal allele.

Gene Expression

The concept of gene expression is intimately related to how dominant and recessive traits manifest. Gene expression refers to the process by which information from a gene is used in the synthesis of a functional gene product, often a protein, which can affect the phenotype of the organism. The expression pattern (on or off) can depend on multiple factors, such as the presence of regulatory proteins and environmental conditions.

The exercise provided demonstrates that when the petite allele is by itself (recessive homozygous), the petite trait is expressed. However, when paired with a normal allele from the wild-type strain, the trait expressed is normal, indicating that the normal allele is likely contributing a functional protein that overrides the effect of the petite allele in the diploid zygotes.

Meiosis and Sporulation

During sexual reproduction, meiosis and sporulation are processes that ensure the continuation of genetic diversity. Meiosis is a type of cell division that reduces the chromosome number by half, forming four genetically diverse haploid cells from one diploid cell.

Sporulation

, particularly in yeast, is a form of reproduction that involves the formation of spores which can survive in harsh environmental conditions.

In the given exercise, sporulation leads to the creation of haploid ascospores. These spores then fuse to create diploid zygotes, which undergo meiosis to produce more ascospores. This cycle is essential for maintaining genetic diversity and demonstrates how traits can be passed on through generations.

Allelic Segregation

The principle of allelic segregation is a cornerstone of Mendelian genetics. It refers to the separation of different alleles for the same gene during gamete formation in meiosis. According to Mendel’s law of segregation, each individual carries pairs of alleles for any given trait, and these alleles are separated into different gametes.

In our exercise scenario, after meiosis occurs in the diploid zygotes, the alleles for the petite and normal phenotypes segregate into different ascospores. This is why some ascospores express the petite phenotype and others express the normal phenotype. The presence of both phenotypes among the ascospores confirms that the alleles segregated during meiosis, following Mendel’s law.