The Sordaria Cross

Summary

In this video, Mr. Andersen explains the Sodaria cross using the fungi *Sodaria fimicola*, commonly found in herbivore feces, which is used in AP Biology to study meiosis. He demonstrates how a cross between wild-type and tan mutant fungi reveals the processes of meiosis, mitosis, and genetic recombination. The focus is on observing how crossing over occurs, with emphasis on the 4:4 and recombinant ratios of spores. He also explains how to calculate the distance between a gene and the centromere using crossover frequency. The lab procedure involves counting asci to determine genetic variation and recombination rates.

Takeaways

• 😀 *Sodaria fimicola* is a fungus found in herbivore feces and is commonly used in AP Biology classrooms to study genetics.
• 😀 The cross involves two types of fungi: a wild-type (dark) and a tan mutant type, and the goal is to study meiosis and recombination.
• 😀 Meiosis in *Sodaria fimicola* starts with a diploid nucleus, undergoes DNA replication, and then divides through meiosis I and II, resulting in haploid cells.
• 😀 In fungi like *Sodaria fimicola*, the cells are typically haploid and fuse to form the next generation, with meiosis continuing in the life cycle.
• 😀 The crossing over event during meiosis can result in recombinant chromosomes, creating genetic variation between the parent and offspring.
• 😀 A 4:4 spore ratio indicates no crossing over occurred, while different ratios like 2:2:2:2 or 2:4:2 suggest recombination took place.
• 😀 Crossing over is crucial for genetic variation and recombination, and its frequency can provide insights into gene mapping.
• 😀 Thomas Hunt Morgan and Alfred Sturtevant helped establish the importance of crossing over for genetic recombination and gene mapping.
• 😀 To determine the frequency of crossing over, students count the parental and recombinant asci and calculate the crossover frequency.
• 😀 The gene distance from the centromere is calculated by dividing the recombinant count by the total asci number, then halving the result to get the map units.
• 😀 A small sample size can affect the accuracy of crossover frequency calculations, and a larger sample (at least 50 asci) is recommended for better data.

Q & A

What is Sodaria fimicola, and where can it be found?

Sodaria fimicola is a type of fungus found in the feces of herbivores, but it is also commonly found in AP Biology classrooms due to its ease of cultivation and use in studying meiosis.

What is the significance of the wild-type and tan-type fungi in the Sodaria cross experiment?

In the Sodaria cross experiment, the wild-type (dark) and tan-type fungi are crossed to observe meiosis. The cross allows for the observation of genetic recombination and the study of how meiosis occurs in fungi.

What are the key stages of meiosis as discussed in the video?

The key stages of meiosis outlined in the video include DNA replication, meiosis I, meiosis II, and the formation of haploid cells. In the case of Sodaria, the resulting haploid cells undergo fusion to form diploid cells that then proceed through meiosis again.

How does meiosis in Sodaria differ from meiosis in humans?

In humans, meiosis results in haploid sperm and egg cells, followed by fertilization. In Sodaria, most of the life cycle consists of haploid cells, and fertilization occurs when two haploid cells fuse to create a diploid cell that undergoes meiosis again.

What is the result when no crossing over occurs in the Sodaria cross?

When no crossing over occurs, the result is a 4:4 ratio of tan and dark spores, as no genetic recombination has taken place.

What does crossing over in meiosis lead to in terms of genetic diversity?

Crossing over results in recombinant chromosomes, which are mixtures of the parental chromosomes. This increases genetic diversity by creating new combinations of alleles.

How can you identify the occurrence of crossing over in the spores of Sodaria?

Crossing over can be identified by looking at the spore ratios. If the ratio is 4:4, no crossing over has occurred. If the ratio is 2:2:2:2 or 2:4:2, crossing over has occurred, resulting in a new combination of genetic material.

What role do Thomas Hunt Morgan and Alfred Sturtevant play in the context of the Sodaria experiment?

Thomas Hunt Morgan and his student Alfred Sturtevant were instrumental in understanding the concept of genetic recombination through crossing over, and their work showed how the frequency of crossing over can indicate the distance between genes on a chromosome.

How do you calculate the frequency of crossing over in the Sodaria cross?

To calculate the frequency of crossing over, you first count the number of asci (spore-containing structures) showing crossing over and divide that number by the total number of asci. This gives the crossover percentage, which is then divided by two to account for the mitotic division that follows meiosis in Sodaria.

Why is it necessary to divide the crossing over percentage by two in the Sodaria experiment?

The crossing over percentage is divided by two because, in Sodaria, after meiosis, a mitotic division occurs. This mitotic division doubles the number of spores, effectively halving the crossing over result to reflect the actual gene distance from the centromere.