Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 13.2 Fertilization and Meiosis Alternate in Sexual Life Cycles
In sexually reproducing organisms, meiosis and fertilization alternate to maintain a constant chromosome number across generations. This cycle allows genetic information to be transmitted from parents to offspring while generating genetic diversity.
Chromosome Sets in Humans
- Humans have 46 chromosomes in somatic (body) cells, arranged as 23 pairs
- Each pair consists of homologous chromosomes—one inherited from each parent
- The 23rd pair determines sex: XX for females, XY for males
- Gametes (sperm and egg) are haploid (n = 23); somatic cells are diploid (2n = 46)
- Karyotype: displays a complete set of chromosomes, often used to screen for genetic disorders
Human Life Cycle
- Begins with the union of two haploid gametes during fertilization
- Fertilization produces a diploid zygote (2n), which undergoes mitosis to form a multicellular organism
- Gametes develop from germ cells via meiosis, reducing chromosome number by half
- Meiosis prevents chromosome number from doubling in each generation
- Mitosis enables growth and development from the zygote and replaces somatic cells
Types of Sexual Life Cycles
- All sexual life cycles include meiosis and fertilization, but timing and dominance of haploid/diploid stages vary
1. Animal Life Cycle
- Diploid stage dominates
- Gametes are the only haploid cells
- No multicellular haploid stage
2. Plants and Some Algae (Alternation of Generations)
- Both haploid and diploid stages are multicellular
- Diploid sporophyte → meiosis → haploid spores
- Spores → mitosis → haploid gametophyte → gametes
- Fertilization produces new diploid sporophyte
3. Fungi and Some Protists
- Haploid stage dominates
- Zygote is the only diploid stage, which immediately undergoes meiosis
- Produces haploid cells that divide by mitosis
- Haploid cells become gametes through further mitosis
Key Definitions and Concepts
- Diploid (2n): two sets of chromosomes, one from each parent
- Haploid (n): one set of chromosomes
- Homologous chromosomes: pairs of chromosomes with genes for the same traits at the same loci
- Alleles: different versions of a gene at the same locus
- Meiosis: reduces chromosome number from diploid to haploid
- Fertilization: restores diploid number by uniting two haploid gametes
In a Nutshell
Sexual life cycles alternate between meiosis and fertilization to preserve chromosome number and promote genetic diversity. Though the timing differs among animals, plants, fungi, and protists, this alternation is fundamental to inheritance and evolution.