Final answer:
There are four equally likely combinations of sperm and egg for one character of two traits following Mendelian genetics: YY, Yy, yy, or yy, with a genotypic ratio of 1:2:1.
Step-by-step explanation:
When dealing with one character of two traits in genetics, and considering a self-cross of Yy heterozygous offspring, the question of how many equally likely combinations of sperm and egg are there must consider the basic principles of Mendelian genetics. Using a simple 2 × 2 Punnett square, each parent can donate one of two different alleles, leading to four possible allele combinations: YY, Yy, yy, or yy. It is necessary to note that the Yy and yY combinations are equivalent, both resulting in the heterozygous genotype. Since fertilization is a random event, these combinations are equally likely, and the genotypic ratio expected is 1:2:1 for YY:Yy:yy. Mendel's experiments confirmed these expectations, observing this ratio in pea plants when dealing with individual traits.