AP Syllabus focus:
‘Monohybrid crosses use Punnett squares to predict genotypic and phenotypic ratios for a single gene.’
Monohybrid crosses connect meiosis, fertilization, and allele segregation to predictable inheritance patterns. By organizing possible gametes in a Punnett square, you can forecast offspring genotypes and phenotypes for one gene.
Core idea: one gene, two alleles
A monohybrid cross tracks inheritance for a single gene with two alternative alleles (often represented by letters such as A and a). Predictions depend on the dominance relationship and the parental genotypes.
Practice Questions
FAQ
During meiosis, allele segregation means each gamete receives one allele.
Equal proportions assume:
normal segregation
no meiotic drive
equal gamete viability
Use letters matching the dominant phenotype name (e.g., $P$ for purple), or use gene-based notation.
Avoid value-laden labels; dominance does not mean “stronger” or “more common”.
It may indicate violated assumptions, such as:
non-random fertilisation
reduced viability of a genotype
misidentified parental genotype
Large sample sizes help distinguish chance from bias.
Yes, but you may generate multiple predicted phenotype patterns under different dominance models.
You then compare which model best matches the observed categories and proportions.
State both:
the raw counts (e.g., 47:18)
the simplified ratio (approximate, e.g., $\sim$3:1)
Using “approximately” clarifies that ratios are expectations, not exact outcomes.
