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Modern Genetics

Defines and discusses genotype and phenotype.

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Modern Genetics

Did Mendel know about DNA?

No, people did not understand that DNA is our hereditary material until long after Mendel's time. Our modern understanding of DNA and chromosomes helped to explain how Mendel's rules worked.

Modern Genetics

Mendel laid the foundation for modern genetics, but there were still a lot of questions he left unanswered. What exactly are the dominant and recessive factors that determine how all organisms look? And how do these factors work?

Since Mendel’s time, scientists have discovered the answers to these questions. Genetic material is made out of DNA. It is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge of DNA and chromosomes, we can explain Mendel’s findings and build on them. In this concept, we will explore the connections between Mendel’s work and modern genetics.

Traits, Genes, and Alleles

Traits are characteristics that can be inherited or acquired.  Some examples are blood type, eye color, riding a bike, and hitting a baseball. 

Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not thousands, of genes. Genes are units of heredity that occupy a specific location on a chromosome and code for a product.  Each of these genes can have slightly different versions from individual to individual. These variants of genes are called alleles.

For example, remember that for the height gene in pea plants there are two possible factors. These factors are alleles. There is a dominant allele for tallness (T) and a recessive allele for shortness (t).

Genotype and Phenotype

Genotype is a way to describe the combination of alleles that an individual has for a certain gene (Table below). For each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes (think of it as one allele from Mom, one allele from Dad). The genotype is represented by letter combinations, such as TT, Tt, and tt.

When an organism has two of the same alleles for a specific gene, it is homozygous (homo- means "same") for that gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles (Tt) for a certain gene, it is known as heterozygous (hetero- means different).

Genotype Definition Example
homozygous two of the same allele TT or tt
heterozygous one dominant allele and one recessive allele Tt
homozygous dominant two dominant alleles TT
homozygous recessive two recessive alleles tt

Phenotype is a way to describe the traits you can see. The genotype is like a recipe for a cake, while the phenotype is like the cake made from the recipe. The genotype expresses the phenotype. For example, the phenotypes of Mendel’s pea plants were either tall or short, or they were purple-flowered or white-flowered.

Can organisms with different genotypes have the same phenotypes? Let’s see.

What is the phenotype of a pea plant that is homozygous dominant (TT) for the tall trait? Tall. What is the phenotype of a pea plant that is heterozygous (Tt)? It is also tall. The answer is yes, two different genotypes can result in the same phenotype. Remember, the recessive phenotype will be expressed only when the dominant allele is absent, or when an individual is homozygous recessive (tt) (Figure below).

Different genotypes (AA, Aa, aa or TT, Tt, tt) will lead to different phenotypes, or different appearances of the organism.


  • allele: Variant of genes.
  • chromosome: Structure composed of DNA wrapped around proteins.
  • DNA (deoxyribonucleic acid): Hereditary material of a cell.
  • gene: units of heredity on a chromosome
  • genotype: Describes the combination of alleles that an individual has for a certain gene.
  • homozygous: Having two of the same alleles for a specific gene.
  • heterozygous: Having two different alleles for a specific gene.
  • phenotype: Describes observable traits.
  • traits: inherited or acquired characteristics


  • Mendel's hereditary "factors" are variants of genes called alleles.
  • Genotype describes the combination of alleles that an individual has for a certain gene, while phenotype describes the traits that you can see.


Use the resources below to answer the questions that follow.

  1. When geneticists look at genotype, what are they really studying?
  2. Why do geneticists like to turn genes off? Explain your answer as fully as you can.
  3. If a geneticist turns off a gene and describes "what" happened, is his/her work done? Explain your answer as fully as you can.
  1. Do most of the phenotypes we observe come from a single gene? Given this situation, how valuable to you feel Mendel's work was? Be specific in your reasoning and explain your answer fully.
  2. What has led to the rapid analysis of DNA? Where do scientists now hope to apply these tools?
  3. What are some of the phenotypic traits that scientists are investigating? Why do you think these traits were chosen?


  1. Name the two types of traits. 
  2. What is the type of allele that only affects the phenotype in the homozygous condition?
  3. If two individuals have a certain phenotype, does that mean they must have the same genotype?

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