Final answer:
The parental genotypes producing a 1:1 ratio of phenotypes in the progeny are Aa (heterozygous) crossed with aa (homozygous recessive). This genetic cross creates a 50% chance for each progeny to be heterozygous (Aa) or homozygous recessive (aa), resulting in a 1:1 phenotypic ratio.
Step-by-step explanation:
The desired phenotypic ratio of 1:1 in the progeny is achieved through the second option, Aa × aa. When the heterozygous genotype (Aa) is crossed with the homozygous recessive (aa), it results in a 1:1 ratio of phenotypes (Aa:aa).
The mechanics of this genetic cross can be outlined using a simple Punnett square that demonstrates the segregation of alleles. Since Aa produces both A and a alleles in equal proportion, and aa can only produce the a allele, the possible genotypes of their offspring can only be Aa or aa. Since A is dominant over a, the Aa progeny will exhibit the dominant phenotype, while aa progeny will express the recessive phenotype, leading to a 1:1 phenotype ratio among the progeny.