Diversity in Chromosome Numbers Across Species
- A species is partly defined by its characteristic chromosome number, which remains stable in normal individuals.
- Chromosome number is conserved across all diploid cells within a species, ensuring accurate inheritance during reproduction.
- Occasional mutations or chromosomal rearrangements may alter this in individuals, but the species norm remains stable.
- Across species, chromosome number varies dramatically—from just two chromosomes in some nematodes to hundreds in ferns.
Chromosome Numbers Are Shared Traits Within a Species
Definition
Chromosomes
Chromosomes are the carriers of genetic information in all living organisms. They consist of two essential components: deoxyribonucleic acid (DNA) and proteins.
- One of the defining characteristics of a species is that its members typically share the same number of chromosomes.
- This uniformity is essential for successful reproduction.
Diploid Cells and Even Chromosome Numbers
- Most plants and animals are diploid, meaning their body cells contain two sets of chromosomes, one from each parent.
- Humans: Somatic cells have 46 chromosomes (23 pairs).
- Each pair consists of homologous chromosomes that carry the same sequence of genes.
- Haploid Gametes: During fertilization, sperm and egg cells (haploid) combine to restore the diploid number.
Remember, the diploid chromosome number is always even because it represents two complete sets of chromosomes, one from each parent.
Chromosome Numbers Are Usually Even
- Diploid organisms inherit two sets of chromosomes: one paternal, one maternal.
- During gamete formation, chromosomes must pair during meiosis; this is only possible if the diploid number is even.
- After fertilization, the zygote is restored to diploid status.
- In IB exams, always state whether you are referring to the haploid (n) or diploid (2n) number.
- Saying “humans have 23 chromosomes” is incorrect.
- It should be “23 pairs in diploid cells” or “23 chromosomes in haploid gametes.”
How Chromosome Numbers Change Over Time
Changes in chromosome numbers are rare events, but when they do occur, they can be caused by:
1. Chromosome Fusion
- Two chromosomes join together end-to-end.
- This reduces the chromosome number.
- Human chromosome 2 is the result of a fusion of two ancestral ape chromosomes.
- This explains why humans have 46 chromosomes while chimpanzees, gorillas, and orangutans have 48.
2. Chromosome Fission
- A single chromosome splits into two smaller chromosomes.
- This increases the chromosome number without increasing the DNA content.
3. Polyploidy
- Entire sets of chromosomes duplicate.
- Common in plants (e.g., wheat is hexaploid, 6n).
- Polyploidy can lead to instant speciation because polyploid individuals cannot breed with diploid members of the same species.
Wheat is hexaploid (six sets of chromosomes).
Example- A horse has 64 chromosomes, while a donkey has 62.
- Their offspring, a mule, inherits 63 chromosomes.
- This odd number prevents proper chromosome pairing during meiosis, leading to infertility.
Diversity in Chromosome Numbers Across Species
- Chromosome numbers vary dramatically among species, ranging from as few as 2 to over 100.
- Humans share close genetic similarity with chimpanzees, yet differ in chromosome number (46 vs 48) due to a fusion event in human ancestry.
Humans vs. Chimpanzees
- Humans: 46 chromosomes.
- Chimpanzees: 48 chromosomes.
- Cause of Difference: Research suggests that human chromosome 2 resulted from the fusion of two ancestral chromosomes after humans and chimpanzees diverged.
Polyploidy in Plants
- Unlike animals, plants frequently exhibit polyploidy (more than two sets of chromosomes).
- Polyploidy increases genetic diversity and often leads to new species.
Polyploidy is rare in animals but common in plants, as plants can reproduce asexually, bypassing the challenges of uneven chromosome numbers during meiosis.
Chromosome Size vs. Number
- The same total DNA content can be arranged in a few large chromosomes or many small chromosomes.
- Laboratory experiments in yeast show that reducing chromosomes from 16 to just 2-4 still allowed survival, proving that chromosome number itself is not the limiting factor.
- Instead, it is the presence of all essential genes.
Some plants have very high chromosome numbers but are not genetically more complex than humans.
Theory of KnowledgeHow does the study of chromosome numbers challenge traditional definitions of species? Consider hybrids like ligers or polyploid plants that blur the boundaries.
Self review- Define chromosome number, diploid, and haploid, using humans as an example.
- Why are diploid chromosome numbers nearly always even?
- Describe two mechanisms that can increase and one mechanism that can decrease chromosome numbers.
- State the diploid chromosome numbers of humans and chimpanzees, and explain the difference.
