Final answer:
Mendel's law of segregation explains that alleles for a trait separate during gamete formation, and a 1:1 test cross ratio implicates that the parent is heterozygous. This was determined through Mendel's pea plant experiments and the understanding of meiosis. The law of segregation allows the use of Punnett squares to predict genetic combinations.
Step-by-step explanation:
Mendel's law of segregation is a fundamental principle of genetics that explains how alleles for a specific trait are separated into gametes. Mendel observed that traits in pea plants followed specific patterns when they were passed on from generation to generation. Essentially, the law states that organisms have two alleles for each gene, which segregate during the formation of gametes, meaning that each gamete receives only one allele. The physical basis of this law is in the first division of meiosis, where the homologous chromosomes, each with different alleles, are separated.
When it comes to a test cross, it is a genetic cross between an organism exhibiting a dominant phenotype (but unknown genotype) and a homozygous recessive organism. The purpose of the test cross is to determine the genotype of the dominant-presenting individual. If the test cross results in a 1:1 ratio of offspring with dominant and recessive traits, it indicates that the unknown parent is heterozygous for the trait. Therefore, the separation of alleles as indicated by the law of segregation can be observed directly through test crosses.
The law of segregation is a crucial concept because it allows us to use tools like the Punnett square to predict genetic outcomes. For example, we can forecast the probability of an organism inheriting a particular allele from its parents. Mendel's careful breeding experiments with pea plants and subsequent observations of the resulting offspring's traits led him to establish the law of segregation, along with his other key genetic laws.