Ever wonder why children resemble their parents, grand parents, or siblings? These questions can be answered by inheritance, which is the reception of genetic qualities by transmission from parent to offspring. Inheritance is why organisms look the way they look, and why their genetic make-up is the way it is. While learning about inheritance, we learned about the studies of Mendel, genes and dominance, segregation, and probability and Punnett Squares.

Mendel's Genetics

o Gregor Mendal was an Austrian monk who laid the foundations of genetic science. A native of Austria, born in 1822, Mendal used the peas in his garden to make one of the most important discoveries in the history of genetics. His experiment:

  • Mendal noticed the peas in his garden were only true breeding, meaning if they were allowed to self- pollinate, they would produce offspring identical to themselves.
  • He cut away the pollen- bearing male parts of the flower and dusting pollen from another plant onto the flower. This process is known as cross-pollination. This made it possible for Mendel to cross-breed plants with different characteristics and study the results.
  • Mendel studies seven different pea plant traits, a trait is a specific characteristic. He crossed seven contrasting plants and studied their offspring. These offspring of crosses between parents with different traits were called hybrids.
  • Mendel drew two conclusions; one was that biological inheritance is determined by factors that are passed from one generation to the next. The other was the principle of dominance, which states some alleles are dominant and some are recessive.

Left: picture of pea plants Mendel used in his experiments.

 Photo Courtesy of Google Images

Genes and Dominance
  • -  In his studies, Mendel studied seven different pea plant traits. A trait is a specific characteristic that varies from one individual to another.
  • - Mendel crossed the traits of seven different pea plants. The offspring of these plants were called hybrids, the offspring of crosses between parents with different traits.
  • - Scientists call the chemical factors that determine traits genes, and each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms. The forms of these genes are called alleles.
  • - Through Mendel’s principle of dominance, he states that some alleles are dominant, meaning they hide other traits, and others are recessive, meaning they are hidden by other traits.

- Mendel classified the parts of his experiments into three generations: the P, F1, and F2 generations.
o The P generation was the original parent plants.
o The F1 generation was the offspring plants that showed one of the parental traits.
o The F2 generation was the offspring plants that showed both of the parental traits.

- Mendel asked how the segregation, or separation, of alleles occurred. Mendel used the traits for tallness and shortness as an example, and recommended that the alleles separated during the formation of the gametes, or sex cells.
- Mendel established two laws, referred to as Mendel’s Laws:
o The law of segregation states that members of each pair of alleles of a gene separate when gametes are produced in meiosis.
o The law of independent assortment states that pairs of alleles separate independently of each other during gamete formation.

For more on Mendel's Laws:
 Law of Segregation:
 Law of Independent Assortment:

Probability and Punnett Squares
- Probability- the likelihood that a particular event will occur.
- The principles of probability can be used to predict the outcomes of genetic crosses.
- Calculating the probability of offspring’s traits can be accomplished by utilizing the Punnett Square, named after scientist Reginald Punnett.
- The Punnett Square works by using genotypes, an organism’s genetic make-up, represented by two letters.
o An uppercase letter shows a dominant trait and a lowercase letter shows a recessive trait.
- Organisms that show two identical alleles are said to be homozygous.
o Ex. TT or tt (trait could be tallness)

- Organisms that show two different alleles are said to be heterozygous.
o Ex. Tt (trait could be tallness)
- Organisms can have the same phenotype, physical appearance, but have different genotypes because an organism can be heterozygous dominant (Tt) or homozygous dominant (TT).
- Punnett Squares can be monohybrids, a cross of one trait, or dihybrids, a cross of two traits.

For more practice on Punnett Squares:

Below: sample Punnett Square


Photo Courtesy of Google Images