Mendelian inheritance is a basic idea in genetics that was introduced by Gregor Mendel in the 1800s. It explains how parents pass traits to their kids through alleles. Alleles are the different forms of a gene.

In genetics, there are two main types of alleles: dominant and recessive. Knowing the difference between these two is important to help us understand how traits can change from one generation to the next.

Dominant Alleles

• A dominant allele shows up in a person's traits, even if there is just one copy of it.
• We use a capital letter to represent it, like “A”.
• If someone has at least one dominant allele (like AA or Aa), we will see the dominant trait.
• For example, if the allele for brown eyes (B) is dominant over the allele for blue eyes (b), people with BB or Bb will have brown eyes.

Recessive Alleles

• A recessive allele only shows up in a person's traits if there are two copies of it, making the person homozygous recessive (bb).
• It is shown with a lowercase letter, like “b”.
• If a person has just one recessive allele (like AB or Aa), the dominant trait will cover it up.
• Traits that are controlled by recessive alleles are often less common. For example, if blue eyes (b) are rare, only about 25% of the kids in a group might have blue eyes (bb), depending on what alleles the parents have.

Punnett Squares
Punnett squares are handy tools used to predict the genetic results when two individuals have offspring. They show how alleles combine and help make sense of Mendelian inheritance visually.

• Let’s look at a cross between two parents who both have a heterozygous genotype (Bb x Bb):

  1. Write the alleles of one parent on the top and the other parent on the side.

     | B | b | |-------|-------| | B | BB | Bb | | b | Bb | bb |

  2. Fill in the squares to find the possible genotypes:

     • BB: 1 (25%)
     • Bb: 2 (50%)
     • bb: 1 (25%)

This table shows that there is a 25% chance to produce a homozygous dominant (BB), a 50% chance for heterozygous (Bb), and a 25% chance for homozygous recessive (bb) kids.

In summary, understanding dominant and recessive alleles helps show how traits are passed down. These ideas not only help us predict genetic outcomes using Punnett squares but also help us understand genetic differences in groups of living things. This knowledge is important for grasping how inheritance and evolution work.