Why did Mendel used two traits like yy or tt why didn't he use one trait like y or t

To find the chance of having offspring with specific traits. To find which allele/trait is dominant, recessive, or co-dominant.

Explanation: Diploid organisms have two copies of each chromosome, meaning they have two of each gene. This means that each trait has two alleles coding for different traits. E.G. Brown eyes (B) could be dominant to Blue eyes (b) meaning that an organism has the coding for both on their chromosomes but one is dominant and is expressed. Mendel studied the offspring for statistics so he could figure out which alleles were dominant, or recessive and the statistics for the chances of having an offspring with a specific trait.. Most animals and cells are diploid organisms that have haploid sex cells (sperm, egg) ( One chromosome, Only one allele for a specific trait) .  If a single Haploid cell were to produce an offspring on its own it would only have the specific traits passed down by the single parent cell meaning there would be nothing to estimate/ collect statistics on. So using diploid organisms with YY or TT allowed Mendel to actually find the chances for specific traits of offspring, and dominant/recessive traits.

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