Short Answer 
Point mutations involve small changes in a single nucleotide, such as deletion, insertion, or substitution, affecting traits like fur color. Chromosome rearrangement entails structural changes in chromosomes, leading to deletions, duplications, or translocations, while nondisjunction refers to improper separation during cell division, resulting in abnormal chromosome numbers.
Step 1: Understand Point Mutation
A point mutation refers to a small-scale change that occurs in a single nucleotide of the DNA sequence. This mutation can manifest in various forms, including:
- Deletion – where a nucleotide is removed.
- Insertion – where a new nucleotide is added.
- Substitution – where one nucleotide is replaced by another.
For instance, in the example, an orange fur color allele transforming into black signifies a point mutation at the nucleotide level.
Step 2: Recognize Chromosome Rearrangement
Chromosome rearrangement involves significant alterations to chromosome structure. This can result in various changes such as:
- Deletion of sections of chromosomes.
- Duplication of chromosome segments.
- Translocation where parts of one chromosome detach and attach to another.
In the example illustrated, the presence of both orange and black fur suggests that a rearrangement has occurred in the chromosome structure.
Step 3: Identify Nondisjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This results in an abnormal number of chromosomes in the daughter cells. A key indication of nondisjunction, as discussed, is:
- Having two copies of each X chromosome within a single cell.
This abnormality is often observable in the third diagram, demonstrating how nondisjunction can affect genetic outcomes.