Three Main Criteria Are used To Classify Chromosomes
- Banding Patterns
- When stained with dyes like Giemsa, chromosomes show light and dark bands unique to each chromosome.
- These act like a barcode, helping identify specific chromosomes and detect deletions, duplications, or translocations.
- Length
- Chromosomes are arranged in a karyogram from longest to shortest.
- In humans, chromosome 1 is the longest and chromosome 21 is among the shortest.
- Centromere Position: The centromere divides the chromosome into two arms (p = short, q = long).
Karyotypes and Karyograms
- A karyotype describes the complete chromosome set of an organism (number, size, and structure).
- A karyogram is the visual arrangement of those chromosomes in homologous pairs, typically ordered by decreasing size.
- Humans have 46 chromosomes (23 pairs):
- 22 pairs of autosomes
- 1 pair of sex chromosomes (XX or XY)
Remember that sex chromosomes (pair 23) often break the size order (e.g., X is large, Y is small).
Evidence for the Fusion of Human Chromosome 2
- Humans have 46 chromosomes, while chimpanzees, gorillas, and bonobos each have 48.
- This led scientists to hypothesize that human chromosome 2 formed through the fusion of two ancestral ape chromosomes (chimpanzee chromosomes 12 and 13).
- The active centromere in human chromosome 2 corresponds to the centromere of chimpanzee chromosome 13,
- While remnants of the second centromere align with chimpanzee chromosome 12.
Nature of Science (NOS): Testable Hypotheses
- The origin of human chromosome 2 is a testable hypothesis because it can be supported or refuted by observable evidence, such as banding, centromere structure, and DNA sequence data.
- Testable hypotheses must:
- Make specific, evidence-based predictions.
- Allow for acceptance or rejection through data.
- Avoid vague or unverifiable claims.
- Testable: “Human chromosome 2 formed by fusion of two ancestral ape chromosomes.”
- Non-testable: “The ancestor of humans and chimpanzees preferred singing.” (no evidence possible)
- What three criteria are used to classify chromosomes?
- Why is the chromosome 2 fusion hypothesis considered testable?
- Give one example of a non-testable statement and explain why it fails the criteria.
- How does the fusion of chromosome 2 support the idea of shared ancestry among primates?
