5.5 Modern Genetics

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Learning Objectives

• Compare Mendel’s experiments with our modern understanding of genes.
• Explain how genes and alleles are related to an organism's genotype.
• Explain how an organism's genotype is related to its phenotype.

Introduction

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.

Guided Learning

Unraveling the Mystery

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

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. A gene codes for a specific characteristic in an organism. Each of these genes can have slightly different versions from individual to individual. These different forms of genes are called alleles.

In describing genetic crosses, letters are used to represent alleles. For example, remember that for the height gene in pea plants there are two possible factors. These factors are alleles. The dominant allele is represented by a capital letter (T for tall) while the recessive allele is represented by a lowercase letter (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 from Mom and one 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).

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.

Recall that, in his experiments, Mendel crossed plants with two contrasting traits. For example, he crossed a tall plant with a short plant. We now know that the tall plant had a genotype of TT and the short plant had a genotype of tt. The offspring receives one allele from the tall parent (T) and one allele from the short parent (t), resulting in a genotype of Tt for the F1 generation. Also recall that the F1 generation expressed only the dominant phenotype; all of the plants were tall. In this case, the genotypes of TT and Tt both result in the tall phenotype because the dominant allele masks the recessive allele when they are present together. So yes, different genotypes can result in the same phenotype. Remember, though, that the recessive phenotype will only be expressed when the dominant allele is absent, or when an individual is homozygous recessive (tt)

Environmental Influences

Sometimes, even though your genes code for a specific trait, your environment or lifestyle choices can alter a person's phenotype. For instance, your DNA may code for blonde hair. If you choose to dye your hair brown, you are altering the color of your hair through outside chemicals, not through your DNA. Another example would be if your DNA puts you at risk for heart disease. Through diet and exercise, you can minimize that risk.

Review

• 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.

Vocabulary

Alleles

Different forms of a gene.

Chromosome

Structure composed of DNA wrapped around proteins.

DNA (deoxyribonucleic acid)

Hereditary material of a cell.

Dominant Trait

Trait that is expressed if at least one copy of the allele is present. Represented by a capital letter.

Gene

A portion of DNA that codes for a specific trait.

Genotype

The combination of alleles an individual has for a certain gene. The alleles are inherited from both parents and code for a phenotype.

Homozygous

Having two of the same alleles for a specific gene.

Heterozygous

Having two different alleles for a specific gene.

Phenotype

An organism's appearance as coded for by the genotype.

Recessive Trait

Trait that is expressed only when two copies of the allele (one contributed by each parent) are present; the recessive trait is hidden when the dominant allele is present, such as in the F1 generation. Represented by a lowercase letter.