Fundamentals of Genetics: DNA, Hereditary Traits, and Chromosomes

Understanding Genetics

DNA is the genetic material that all living things have in their cells. It is transmitted from one generation to the next. Characteristics like blue eyes, hair color, etc., are examples of the diversity and variability that exists within a species.

Hereditary vs. Acquired Traits

Characteristics of an organism that are transmitted from parents to children are called hereditary traits. In contrast, acquired characteristics are those developed during an organism's life, such as a tattoo.

Genetics studies the specific characteristics encoded in DNA, how they are expressed in organisms, and explains how this information is passed to the next generation—that is, their inheritance—and why traits vary from generation to generation.

Mendel's Factors

Mendel concluded that each characteristic is determined by factors that are inherited from the parents. He called these factors dominant and recessive.

Mendel also concluded that hereditary factors did not mix. That is, the resulting combination of two factors is held independently and is transmitted as such to the next generation.

Modern Genetics

Walter Sutton observed structures, which he called chromosomes, that behaved in the same way as Mendel's "factors."

Sutton's Chromosome Theory of Inheritance

Sutton proposed the following:

• The chromosomes were the carriers of Mendel's factors, now known as genes.
• This became known as the Chromosome Theory of Inheritance, which holds that genes are physical units located on chromosomes. Each gene is a DNA fragment that contains information for a particular feature.
• Sutton also proposed that since chromosomes are the carriers of genes, when chromosomes separate during meiosis, each gamete receives only one gene from each allele pair.

The gene fragments that determine a characteristic are known today as alleles. Each gene is composed of two alleles, and each allele is a variant of a gene.

Chromosomes and Inheritance

Each particular species has a specific number of chromosomes; in humans, there are 46. These 46 chromosomes are present in somatic cells as 23 pairs. The chromosomes of each pair are called homologous chromosomes (this is visualized in a human karyotype).

The DNA that makes up each pair of homologous chromosomes includes genetic information for the same characteristics. This means that for each feature, an individual has double information: one set from the maternal side and one from the paternal side.

Each gamete (sperm or egg), originated during meiosis, provides one chromosome from each homologous pair.

The Punnett Square

A Punnett square is a tool that helps us visualize the possible combinations of gametes.