Yeasts are unicellular organisms from the kingdom Fungi. Saccharomyces cerevisiae is one
species of yeast that can carry out either asexual reproduction by mitosis or sexual reproduction
by meiosis.
Budding in S. cerevisiae is a process where a small daughter cell forms as a bud on the parent
cell. The bud contains a copy of the parent cell nucleus and it eventually separates from the
parent cell to form a new cell.
S. cerevisiae can exist in two forms: haploid cells or diploid cells.
• Haploid cells can be one of two different mating types: a and α.
• Haploid cells can only mate with other haploid cells of the opposite mating type.
Fig. 2.1 shows the life cycle of S. cerevisiae with its asexual and sexual reproductive stages.
Key: a mating type a a α
α
a budding budding
α mating type α
α
a
In pea plants, tall (T) is dominant over dwarf (t), and yellow seeds (Y) is dominant over green (y). Two heterozygous tall plants with yellow seeds are crossed.
(a) Write the genotypes of the parent plants.
(b) Construct a Punnett square to show the possible offspring genotypes.
(c) State the phenotypic ratio expected in the offspring.
(d) Explain what is meant by the term independent assortment.