Dihybrid Inheritance

What is Mendel’s Second Law?

Mendel’s Second Law – Law of Independent Assortment – states that when more than one pair of traits are involved in sexual reproduction for the formation of an offspring, each pair segregate independently from the other (just as a pair of homologous chromosomes separate) only one entering each gamete.

The segregation of one gene pair occurs independently of any other gene parent. In other words the alleles of two (or more) different genes get sorted into gametes independently of one another.

Dihybrid Cross Example in Pea Plants:

The parent genotypes in a dihybrid cross are both heterozygous for two genes. Let’s take a look at an example in pea plants;

Gene 1:

• The first Gene is for seed colour and has two alleles, a dominant allele that results in a yellow seed colour (Y) and a recessive allele that results in a green pea colour (y).
• The homozygous dominant (YY) and heterozygote genotypes (Yy) result in yellow seeds.
• The homozygous recessive genotype (yy) results in green seeds.

Gene 2:

• The second gene is for seed shape and has two alleles, a dominant allele that results in a round seed shape (R) and a recessive allele that results in a wrinkled seed shape (r).
• The homozygous dominant (RR) and heterozygote genotypes (Rr) result in a round shape.
• The homozygous recessive genotype (rr) results in a wrinkled shape.

This is illustrated below: