Recall that homologous chromosomes pair during Prophase 1 of meiosis, and later, each member of the pair will be packaged into separate gametes. Therefore alleles on homologous chromosomes will segregate during Meiosis I (The Law of Segregation), and if the genes for different characters reside on different chromosomes, they will also independently assort during Metaphase I. (The Law of Independent Assortment).
As the connection between genes and chromosomes started to become more clear in the years after Mendel's death, scientists noted what seemed to be a paradox in Mendel's Law of Independent Assortment and the behavior of chromosomes during gamete formation. Complex organisms must have huge numbers of genes to govern their biochemical processes, and yet, the number of chromosomes in these organisms is often small. For example, humans have 23 pairs of chromosomes, whereas pea plants have 7 pairs. Therefore, each chromosome must have many genes.
But if there are many genes on each chromosome, how can two genes on the same chromosome be separated from one another? The chromosome doesn't break apart during meiosis; therefore, there must be another explanation. The answer is that some genes are physically linked to one because they are located on the same chromosome, but they appear to independently assort because the chromosomes recombine (exchange genetic material) with one another. The frequency with which they do this enables scientists to "map" gene locations on chromosomes. We will examine recombination in greater detail in upcoming exercises.
Phenotypes and Genotypes Punnett Squares VoiceThread Transcript