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)
A 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.