General Agriculture & Cell Biology

In Mendelian genetics, a standard monohybrid cross begins by mating two true-breeding homozygous parental strains possessing contrasting traits (e.g., $AA \times aa$). All resulting offspring in the first filial ($F_1$) generation receive one dominant allele ($A$) and one recessive allele ($a$), rendering 100% of the $F_1$ individuals uniformly and completely heterozygous ($Aa$).

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• test cross involves mating an individual showing the dominant phenotype ($A\_$) with a homozygous recessive tester ($aa$).
• If the dominant parent is homozygous dominant ($AA$):
  $$AA \times aa \implies 100\% \text{ Aa (All dominant phenotype, 0\% recessive)}$$
• If the dominant parent is heterozygous dominant ($Aa$):
  $$Aa \times aa \implies 50\% \text{ Aa (dominant)} : 50\% \text{ aa (recessive)}$$
  Here, exactly half ($1/2$) of the progenies display the recessive trait.
• Thus, obtaining half recessive progenies confirms that the dominant parent is heterozygous dominant (Option 2).