A3.1.6 Diversity in chromosome numbers of plant and animal species Notes

Diversity in Chromosome Numbers Across Species

1. A species is partly defined by its characteristic chromosome number, which remains stable in normal individuals.
2. Chromosome number is conserved across all diploid cells within a species, ensuring accurate inheritance during reproduction.
3. Occasional mutations or chromosomal rearrangements may alter this in individuals, but the species norm remains stable.
4. 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

1. One of the defining characteristics of a species is that its members typically share the same number of chromosomes.
2. This uniformity is essential for successful reproduction.

Diploid Cells and Even Chromosome Numbers

1. Most plants and animals are diploid, meaning their body cells contain two sets of chromosomes, one from each parent.
2. Humans: Somatic cells have 46 chromosomes (23 pairs).
   1. Each pair consists of homologous chromosomes that carry the same sequence of genes.
   2. Haploid Gametes: During fertilization, sperm and egg cells (haploid) combine to restore the diploid number.

Chromosome Numbers Are Usually Even

1. Diploid organisms inherit two sets of chromosomes: one paternal, one maternal.
2. During gamete formation, chromosomes must pair during meiosis; this is only possible if the diploid number is even.
3. After fertilization, the zygote is restored to diploid status.

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

1. Two chromosomes join together end-to-end.
2. This reduces the chromosome number.