Question

What is mitotic recombination? A heterozygous individual \((B b)\) with brown eyes has one eye with a small patch of blue. Provide two or more explanations for how the blue patch may have occurred.

Short answer

Mitotic recombination or a mutation during early eye development could explain the blue patch in the brown eye of the heterozygous individual. In the first case, an early recombination event in a precursor cell lead to a clone of cell with blue pigmentation, hence the blue patch. In the second case, a mutation in the dominant allele \(B\) to become recessive allele \(b\) would create a blue patch as the mutated cell reproduces.

Step-by-step solution

Understand the Concept of Mitotic Recombination

Mitotic recombination is a rare event that can occur in the somatic cells of an organism during mitosis. It contributes to genetic variability in somatic cells, and resultant mosaicism if it occurs early in development. In a heterozygous individual, recombination can result in a homozygous cell. For instance, a heterozygous individual with a \(Bb\) genotype where \(B\) (brown) is dominant to \(b\) (blue) could undergo mitotic recombination leading to two daughter cells, one of which is \(BB\) (brown) and another one is \(bb\) (blue).

Generate the First Explanation Based on Mitotic Recombination

One possible explanation for the blue patch in the brown eye could be mitotic recombination. At an early stage in eye development, recombination may have occurred in a precursor cell (initially with \(Bb\) genotype) producing a cell with \(bb\), which resulted in a clonal patch of cells with blue pigmentation. This early switch from brown to blue in a subset of cells meant that as these cells reproduced, the patch of blue continued to grow.

Generate Other Possible Explanation

Another explanation could be a mutation in the gene. A mutation in the \(B\) allele in a certain cell during early eye development could turn it into a recessive \(b\) allele. The cell is then \(bb\) and will give rise to a blue patch of cells as it divides. This mutation would have to occur early in the development of the eye so that the blue patch has time to expand as the eye grows.

Key concepts

genetic variability

Genetic variability refers to the diversity in gene frequencies within a population. It arises from different processes, including mutations, natural selection, genetic drift, and recombination. In the context of mitosis, where typically variability is limited as cells should ideally create exact copies of themselves, any genetic changes can lead to new traits.
In somatic cells, genetic variability is rarer but can happen through events like mitotic recombination. This is particularly significant because it involves exchanges of genetic material between homologous chromosomes which can make two genetically different cells from what was initially one heterozygous cell.
Mitotic recombination can therefore contribute to variability in traits among cells in a body. This is especially true if the recombination happens early during organism development when cells are rapidly dividing and differentiating. By creating cells with different genetic makeups, genetic variability as a result of mitotic recombination can lead to mosaic patterns, like patches of different-colored skin, hair, or eyes, as seen in the blue and brown eye patch example.

mosaicism

Mosaicism describes a condition where different cells within the same organism have different genetic compositions. This results from mutations or recombination events occurring after fertilization, during cell division.
Due to these events, some cells will have a genetic makeup different from the rest of the organism. This can lead to different patches or regions of distinct characteristics such as coloration or texture in an organism's phenotype. A classic example of mosaicism is seen in the case of calico cats that possess patches of different colors due to differences in active X chromosomes in their cells.

• This can arise from:
  • Mitotic recombination - as cells divide during development, varying genotypes and phenotypes can appear.
  • Mutations that create new traits in some but not all cells.

Understanding mosaicism is crucial for explaining phenomena like different eye color patches, as it highlights the resulting differences in local patches of tissue that have diverged genetically from the surrounding cells.

somatic cells

Somatic cells are all the non-reproductive cells in an organism's body, distinct from germ cells which contribute to an organism's reproduction. These cells form the complex tissues and systems of bodies, like skin, organs, and bones.
In humans, the bulk of our cells are somatic, undergoing mitosis for growth and repair. Normally, mitosis produces identical cells, ensuring consistent characteristics are maintained throughout tissues. However, rare events like mitotic recombination can induce genetic changes within somatic cells.
Genetic changes in somatic cells, like seen in the formation of a blue eye patch, don't typically pass on to offspring but can cause visible or functional changes in the individual, leading to differences such as color patches in the eyes, skin, or hair.
Thus, the genetic changes within somatic cells have direct immediate impacts on the individual but do not influence the genetic makeup of future generations.

mutation

A mutation is a change in the DNA sequence of a cell. These changes can occur spontaneously and can be driven by environmental factors or errors during cell division.
Mutations can have varied effects depending on where they occur within the genome and can lead to new traits, diseases, or sometimes have no noticeable effect at all. In the context of mitotic recombination and somatic cells, a mutation can change the genotype of a cell, creating new phenotypes like color patches.

• Mutations can lead to:
  • Creation of homozygous recessive cell patches if a dominant allele mutates into a recessive one.
  • Unexpected phenotypic traits appearing in certain tissues due to localized genetic changes.
• These mutations, when occurring early in development, have more pronounced effects since they can contribute to mosaicism.

For example, a mutation in the gene responsible for eye color can lead to a blue patch in an otherwise brown-eyed individual if the mutation affects melanin production. This illustrates how genetic mutations can contribute to visible differences in phenotype.