Principles of Genetics

Introduction to Heredity and Genetics

Why do families look alike, yet each individual is unique?

Why is it that families resemble each other? Why might you look similar to a brother, sister or parent yet still appear uniquely you? This is due to heredity, or the inheritance of traits from generation to generation.

What is a trait? A trait is a feature that an organism displays that is inherited; traits can vary from individual to individual. Traits can be physical (human height), behavioral (a cat’s purr), or they may indicate a predisposition to disease (diabetes, heart disease).


An organism’s environment may influence traits. For instance, perhaps you aren’t born with the gene to roll your tongue naturally, but with practice, you may learn how. Another example of environmental influence would be related to nutrition. A plant (or even a human) that has the genes to be tall will not grow to its expected height if it does not get the required nutrients from its environment. The environment shapes the expression of the gene, but does not change the DNA for that trait.

Where do our traits come from? We inherited them from our parents when we were conceived. Recall from an earlier module that meiosis allows each parent to contribute half of their DNA in either sperm or egg to make an offspring. Look at the picture below that simplifies this. Two haploid cells combine their 23 chromosomes to give the baby a diploid set of chromosomes, or two copies of each type of chromosome. This means that you have two genes for each trait in your genome.


Every offspring is a unique combination of traits from the mother and the father because parents contributed their chromosomes randomly to their own sperm/egg in meiosis. This genetic diversity that results is responsible for the diversity of life on the planet. It also explains why there are so many different and unique individuals within a species.

Genetics is the branch of biology that studies heredity, or how traits are inherited through genes. It also involves examining the variation of traits within organisms.

Gregor Mendel: The Father of Genetics

Humans have long used a basic understanding of inheritance in agriculture to increase crop or livestock yield. They did thisDogs through selective breeding,or artificial selection. Prior the mid-19th century, before any knowledge of DNA, there were a two main ideas that attempted to explain how offspring inherited traits from their parents:

  1. Blending Model: Both parents traits are blended to produce a new trait in the offspring. This does not allow for variation over time.
  2. Particulate Model: Some sort of particles from each parent are passed on to the offspring. The particles combine to produce an offspring’s traits. This does allow for variation over time.

Which model was more accurate?

The answers began to surface in the 1800s when a monk named Gregor Mendel asked some scientific questions about peas. Watch the video.

Let’s Review!

Click through the interactive called ‘Some Genes Are Dominant‘ from the Dolan DNA Learning Center. Answer the questions below the interactive as you work through it.

  1. What color of pea seed is dominant?  (answer: yellow)
  2. Which color is recessive?  (answer: green)
  3. What is the genotype of a purebred green pea?  (answer: yy)
  4. What is the genotype of a purebred yellow pea? (answer: YY)
  5. What is the term for each “form” of the gene for pea color?  (answer: allele)
  6. What is the homozygous recessive genotype for pea color?  (answer: yy)
  7. What is the homozygous dominant genotype for pea color?  (answer: YY)
  8. When the YY and yy peas were crossbred, what was the phenotype of all the offspring?  (answer: yellow )
  9. When the YY and yy peas were crossbred, what was the genotype of all the offspring?  (answer: Yy)
  10. Were the offspring in the YY x yy cross homozygous dominant, heterozygous or homozygous recessive?  (answer: Heterozygous (dominant))