Question

When Sutton and Boveri published the chromosome theory of inheritance, research on meiosis had not yet established that paternal and maternal homologs of different chromosomes assort independently. Then, in 1913 , Elinor Carothers published a paper about a grasshopper species with an unusual karyotype: One chromosome had no homolog (meaning no pairing partner at meiosis \(\mathrm{I}\); another chromosome had homologs that could be distinguished under the light microscope. If chromosomes assort independently, how often should Carothers have observed each of the four products of meiosis shown in the following figure? a. Only the gametes with one of each type of chromosome would occur. b. The four types of gametes should be observed to occur at equal frequencies. c. The chromosome with no pairing partner would disintegrate, so only gametes with one copy of the other chromosome would be observed. d. Gametes with one of each type of chromosome would occur twice as often as gametes with just one chromosome.

Short answer

The correct scenario that aligns with the concept of independent assortment is option b: The four types of gametes should be observed to occur at equal frequencies.

Step-by-step solution

Understand Independent Assortment

Independent assortment is a fundamental principle of genetics that states that genes for different traits can segregate and recombine independently of one another during meiosis. It is one of Mendel's laws of inheritance.

Analyze the Possible Outcomes

Since chromosomes are supposed to assort independently, we should expect the gametes with one of each type of chromosome to occur. In addition, we should also expect the presence or absence of other chromosomes to not affect the frequency of a particular gamete. Therefore, we can now try to figure out which of the given options best fits this expectation.

Evaluate Each Scenario

a. Only the gametes with one of each type of chromosome would occur: This statement implies that only one specific combination of chromosomes is possible, which doesn't match the expectation of independent assortment. b. The four types of gametes should be observed to occur at equal frequencies: If chromosomes assort independently, it means that the products of meiosis will have different combinations of paternal and maternal chromosomes. This option fits the expectation of independent assortment. c. The chromosome with no pairing partner would disintegrate, so only gametes with one copy of the other chromosome would be observed: This statement contradicts the idea of independent assortment, as it suggests that the presence or absence of a single chromosome would affect the other chromosomes. d. Gametes with one of each type of chromosome would occur twice as often as gametes with just one chromosome: This statement suggests an unequal distribution of gametes, which does not align with the concept of independent assortment.

Identify the Correct Scenario

Based on our evaluation, the correct scenario that aligns with the concept of independent assortment is option b. The four types of gametes should be observed to occur at equal frequencies.

Key concepts

Independent Assortment

Independent assortment refers to the way genes and chromosomes are randomly distributed into gametes during meiosis. This happens because homologous chromosome pairs line up independently of each other along the metaphase plate in meiosis I. Each pair of chromosomes separates without being influenced by the behavior of other pairs. As a result, the genetic combinations in gametes can vary considerably.

Mendel discovered this principle through his work with pea plants, noticing that different traits could be inherited independently of each other. He identified it as one of the essential principles of inheritance. This principle is crucial because it enhances genetic diversity among offspring by combining alleles in various ways. Thus, independent assortment helps ensure that offspring have a mix of genetic traits from both parents.

• Different gene pairs separate independently.
• Promotes genetic diversity by mixing alleles.
• Key part of Mendel’s laws of inheritance.

Meiosis

Meiosis is the process by which reproductive cells, or gametes, are formed in organisms. It involves two rounds of cell division, meiosis I and meiosis II, and results in four non-identical daughter cells, each containing half the number of chromosomes of the original cell. This halving is crucial for maintaining the species' chromosome number from generation to generation.

During meiosis I, homologous chromosomes pair up and then separate. It is during this phase that independent assortment occurs, leading to varied combinations of genes. Meiosis II resembles a normal cell division, where sister chromatids separate, resulting in gametes, ready for fertilization.

• Two-part division process: meiosis I and II.
• Creates genetic diversity through independent assortment.
• Produces gametes with half the original chromosome number.

Mendel's Laws of Inheritance

Mendel's laws are fundamental to understanding how traits are inherited in organisms. They were identified by Gregor Mendel during his experiments with pea plants in the mid-1800s and comprise three main principles: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance.

The Law of Segregation states that allele pairs separate during gamete formation, so each gamete carries only one allele for each gene. The Law of Independent Assortment, as discussed earlier, explains how different gene pairs separate independently. The Law of Dominance states that some alleles are dominant while others are recessive; a dominant allele will mask the expression of a recessive allele in heterozygotes. These laws laid the groundwork for the field of genetics, influencing research and genetic understanding for over a century.

• Segregation: allele pairs separate in gametes.
• Independent assortment: combinations vary due to random separation.
• Dominance: dominant alleles mask recessive in heterozygotes.

Gamete Formation

Gamete formation is a vital process of sexual reproduction, involving the creation of reproductive cells like sperm and eggs. These cells are haploid, meaning they contain half the usual number of chromosomes, which is achieved following meiosis. Each gamete carries a mix of alleles derived from both parental chromosomes, thanks to independent assortment and sometimes crossing over.

When gametes from two individuals merge during fertilization, they form a new diploid organism, with a complete set of chromosomes. This fusion ensures genetic variation among offspring, contributing to evolutionary processes in populations. Gamete formation also includes the phase where cells undergo specialized division to ensure that only one copy of each chromosome passes into each gamete, making each reproductive cell unique.

• Involves meiotic division to create haploid cells.
• Enables genetic variation and diversity.
• Essential step in sexual reproduction and formation of new organisms.