Yes, two organisms can have the same phenotype but different genotypes, especially in cases involving dominant and recessive alleles. For instance, consider a trait governed by two alleles: A (dominant) and a (recessive). An organism with the genotype AA (homozygous dominant) and an organism with the genotype Aa (heterozygous) will both exhibit the dominant phenotype. This is because the presence of the dominant allele A masks the expression of the recessive allele a. This phenomenon is a cornerstone in Mendelian genetics and underlies the importance of understanding that phenotype does not always reveal the exact genetic makeup (genotype) of an organism.

A test cross is used to determine the genotype of an organism exhibiting a dominant phenotype but with an unknown genotype. This is achieved by crossing the organism with a homozygous recessive individual. If the organism in question is homozygous dominant, all offspring will display the dominant phenotype. Conversely, if the organism is heterozygous, the offspring will exhibit a 1:1 ratio of dominant to recessive phenotypes. This is because the heterozygous organism can pass on either the dominant or the recessive allele, whereas the homozygous recessive organism can only pass on the recessive allele. Test crosses are fundamental in genetics for revealing the underlying genetic makeup and can be applied to any organism where the genotypes of the parents can be controlled or predicted.

Environmental factors can significantly influence the expression of phenotypes, even in organisms with identical genotypes. This is due to the concept of gene-environment interaction, where the environment interacts with the genetic makeup to influence the final phenotype. For example, in plants, the colour of the flowers can be influenced by soil pH, temperature, and light exposure. Similarly, in animals, factors like nutrition, stress, and exposure to chemicals can affect various traits, from fur colour to behaviour. This shows that genetics is not the sole determinant of phenotype; environmental factors can modulate or even completely change the expression of certain traits. Understanding this interaction is crucial in fields like evolutionary biology and ecology, where the adaptability and variation of organisms in different environments are studied.

In codominance, both alleles in a heterozygote are fully expressed, leading to a phenotype that distinctly exhibits traits from both alleles. This differs from complete dominance where only the dominant allele's trait is expressed. In a genetic cross involving codominant alleles, the resulting phenotypes clearly show the influence of both alleles. A classic example is the ABO blood group system. Individuals with IAIB genotype express both A and B antigens on their red blood cells, leading to the AB blood type. In a genetic cross, this results in more varied and complex phenotypic ratios than simple dominance, as neither allele can mask the other. Codominance also provides a unique opportunity for studying the interaction of different alleles and can be crucial in understanding certain types of inheritance patterns and genetic diseases.

The law of independent assortment states that alleles for different traits segregate independently during gamete formation. However, this principle applies strictly to genes on different chromosomes. For genes located on the same chromosome, known as linked genes, they tend to be inherited together unless crossing over occurs during meiosis. Crossing over can exchange segments between homologous chromosomes, creating new combinations of alleles. The closer the genes are on the chromosome, the less likely they are to be separated by crossing over, thus they are more likely to be inherited together. This linkage reduces the genetic variation that would be expected from independent assortment. However, recombination frequency can be used to map the distance between genes on a chromosome, with higher recombination frequencies indicating greater physical separation.