Question

What is the difference between the law of segregation and the law of independent assortment?

Answer 1

Both laws are referring to how genes are organized during the process of meiosis (i.e. the cell divisions that produce haploid gametes). As you know, humans have a total of 46 chromosomes, each of which contains hundreds if not thousands of genes, but really there are 22 pairs of similar or homologous chromosomes plus 2 sex chromosomes (also paired). One chromosome of each pair is from your father, and the other is from your mother.

The law of segregation states that each pair of homologous chromosomes is separated such that each gamete only contains one chromosome from each homologous pair. This way, the gamete will have 23 DIFFERENT chromosomes.

The law of independent assortment states that different genes are segregated independent of one another (provided they aren't on the same chromosome), so for each gamete that is produced, the likelihood it receives one gene does not depend on whether it has received another gene (again, provided they are on different chromosomes).

As an example for independent assortment, say you're looking at two genes located on different chromosomes, one for height (T=tall, t=short), and one for eye color (C=brown, c=blue). In a person with the genotype TtCc, the gametes that form can have genotype TC or Tc or tC or tc. Whether a gamete receives T or t does not influence whether it receives C or c, meaning all combinations of the two genes are possible. This is what is meant by independent assortment.

However, if the two genes were on the same chromosome, and one chromosome had the TC alleles while the other had tc alleles, the gametes can only be either TC or tc, but never Tc or tC (ignoring crossing over). This is because the genes are linked - if a gamete happened to receive the T allele, it MUST have also received the C allele, because it's on the same chromosome. Similarly, if the gamete instead received the t allele, it MUST have also received the c allele for the same reason. Now the different genes are NOT assorting independently, because the presence of one gene DEPENDS on the presence of the other!