Overview
This lecture explains the life cycle and genetics of the sea fern, focusing on alternation of generations and how to track genetic inheritance using observable traits.
Chromosomes and Ploidy
- Humans have 46 chromosomes (23 pairs), with one chromosome of each pair from each parent.
- Diploid (2n) means having two sets of chromosomes; haploid (n) means having one set.
- Gametes (eggs and sperm) are haploid due to meiosis, while somatic cells are diploid.
Sea Fern Life Cycle
- The sea fern (Ceratopteris) alternates between diploid sporophyte and haploid gametophyte generations.
- Sporophyte (diploid) produces haploid spores via meiosis.
- Spores germinate into haploid gametophytes through mitosis.
- Gametophytes have antheridia (male, sperm-producing) and archegonia (female, egg-producing) structures.
- Fertilization restores diploidy, initiating a new sporophyte.
Genetics Experiment Procedure
- Two homozygous sea ferns (green and spotted) are crossed, producing all heterozygous (Gg) offspring.
- Meiosis produces haploid spores, each carrying either a dominant (G) or recessive (g) allele.
- Spores are hydrated and germinate for about three weeks to form gametophytes.
- Gametophyte gender is determined by microscopic observation of antheridia and archegonia.
- Count the number of green and spotted gametophytes to track inheritance.
- After adding water, fertilization occurs; new sporophytes observed after two more weeks.
- The experiment models a monohybrid cross and expected Mendelian ratios.
Data Collection and Analysis
- Count green and spotted forms among gametophytes and sporophytes under the microscope.
- Use observed ratios and simple statistics to confirm Mendelian inheritance.
Key Terms & Definitions
- Diploid (2n) — Cell with two sets of chromosomes.
- Haploid (n) — Cell with one set of chromosomes.
- Sporophyte — Diploid generation producing spores.
- Gametophyte — Haploid generation producing gametes.
- Meiosis — Cell division reducing chromosome number by half (for gametes/spores).
- Mitosis — Cell division resulting in two identical cells.
- Antheridia — Male gamete-producing structures.
- Archegonia — Female gamete-producing structures.
- Monohybrid cross — Genetic cross tracking inheritance of one trait.
Action Items / Next Steps
- Observe and count green and spotted gametophytes and later sporophytes.
- Identify and record presence of antheridia and archegonia under the microscope.
- Analyze ratios to compare with expected Mendelian outcomes.