Understanding Mendelian Genetics

Mendelian genetics was started by a scientist named Gregor Mendel in the 1800s. His research helped us learn how traits are passed from parents to their children. Mendel's work became the foundation of genetics today. He identified important ideas like dominant and recessive traits, using Punnett squares, and the difference between genotype and phenotype.

Key Principles of Mendelian Genetics

1. Law of Segregation:

   • Every person has two alleles for each gene, getting one from each parent.
   • When cells make gametes (which can become sperm or eggs), these alleles separate. This means each gamete carries just one allele for each gene.
   • This separation creates genetic variety in the offspring.

2. Law of Independent Assortment:

   • Genes for different traits mix freely when gametes are formed, as long as the genes are on different chromosomes.
   • This explains why offspring can have different combinations of traits, which adds to genetic diversity.

3. Dominant and Recessive Traits:

   • Dominant traits show up in the phenotype if at least one dominant allele is present (like $A$).
   • Recessive traits only show up if there are two recessive alleles (like $a$).
   • For example, in pea plants, the allele for purple flowers ($P$) is dominant over the allele for white flowers ($p$). A plant with the genotype $PP$ or $Pp$ will have purple flowers, but only $pp$ will have white flowers.

Punnett Squares

Punnett squares are helpful tools that show the possible genetic outcomes of a cross between two organisms. They help us see what allele combinations offspring might have based on the parents' genotypes.

• For a simple cross between two plants that are both heterozygous ($Pp \times Pp$), we can make a Punnett square:

$$\begin{array}{c|c|c} & P & p \\ \hline P & PP & Pp \\ \hline p & Pp & pp \\ \end{array}$$

• From this square, we can expect the genotypes to show up in this ratio:
  • 1 $PP$: 2 $Pp$: 1 $pp$ (with a phenotype ratio of 3 purple: 1 white).

Genotype vs. Phenotype

• Genotype is the genetic makeup of an individual (like $PP$, $Pp$, or $pp$).
• Phenotype is about what traits we can see (like purple or white flowers).

Mendel's discoveries gave us important insights into how traits are inherited. He showed that there are predictable patterns. These ideas are important in fields like genetics, farming, and medicine. Studies show that around 75% of offspring will have dominant traits in a typical cross, which highlights the role of dominant and recessive alleles in inheritance.