Genetics: How do we inherit traits from our ancestors?

DNA, which is short for "deoxyribonucleic (dy-AHK''-see-RY'-boh-noo-KLAY-ik) acid," is the molecule that makes up chromosomes. It holds the entire instruction manual for an organism. DNA molecules look like zany ladders, with two long strands joined by "rungs" at the center and twisted into a 3D shape called a double helix. The sides of the ladder are sugars and phosphates. The ladder's rungs are paired combinations of four molecules called nucleotides: adenine, cytosine, guanine and thymine (A, C, G and T). These are the "letters" that make up DNA's code.

There are approximately 3 billion of these letters in the human genome, and they are arranged in different pairs and in different sequences. These sequences translate into instructions for making proteins, which, in turn, affect specific traits. These sequences are known as genes.

Between genes are other DNA sections that do not include protein-making instructions. These bits are called noncoding DNA, which helps cells function in other ways; for instance, the noncoding DNA may turn other genes "up" or "down." About 98% to 99% of our DNA is noncoding.

Assisting DNA in its work is RNA, or ribonucleic acid, which is also found in every cell. RNA are copies of DNA sequences that read and carry DNA's instructions out of the nucleus and into the cell. RNA is required to make proteins, and it also assists in sparking chemical reactions and controlling genes.

Most DNA is nuclear DNA, meaning it's located in the cells' nuclei in chromosomes. But some DNA is in the fluid surrounding the nucleus, found in structures called mitochondria. This mitochondrial DNA, or mtDNA, helps mitochondria produce energy for cells; that's why the mitochondria are known as the "powerhouses" of the cell. But unlike nuclear DNA, which comes from both parents, mtDNA is usually inherited only from the mother.

All living things have DNA, and all DNA molecules contain sequences of nucleotides — A, C, G and T — that code for proteins. Differences between species come from the order and length of those DNA sequences. Scientists study these differences by comparing the percentage of DNA sequences that species have in common.

From one human to the next, our genomes are about 99.9% alike. As a result of evolution, the arrangement of genetic instructions in our DNA is also very similar to that of other animals.

Earth's first life is thought to have appeared at least 3.77 billion years ago. Over billions of years, increasingly complex forms of life evolved and passed down their DNA. Animal species that are closely related to each other — and thereby closer to the same shared ancestor — carry similar genetic instructions. Even if they look very different, outwardly, the genomes of closely related species are more similar than those of distantly related species.

For example, manatees, also known as sea cows, have bodies that are streamlined for living in water. But even though manatees may look somewhat like seals and walruses, they are actually more closely related to elephants. And the closest land-animal relatives of seals and walruses are bears!

Except for modern humans (Homo sapiens), all species on the human family tree are long extinct. Our closest living relatives are primates — namely, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Until recently, scientists had proposed that chimps and humans shared 98.8% of their DNA. However, researchers recently suggested that this percentage omits key parts of the two genomes that are harder to compare. After considering these other bits of the human and chimp genomes, their overall similarity may be closer to 90%.

Animals that aren't primates still have many genes in common with humans. Mice and humans are both mammals and vertebrates (animals with backbones). On average, the protein-coding parts of the mouse and human genomes are about 85% similar. By comparison, we share just 70% of our genome with zebrafish, another animal commonly studied in the lab.

As researchers continue to map the genomes of even more animal species, they will learn more about evolution, heredity and genetics throughout the tree of life.

The field of modern genetics is rooted in plant experiments performed by pioneering scientist Gregor Johann Mendel (1822-1884). In 1865, Mendel began studying heredity by experimenting with pea plants (Pisum sativum). Physical traits in pea plants are easy to see. For example, the peas can be smooth or wrinkled, the flowers can be purple or white, and the plants can be tall or short.

Mendel selected seven characteristics to observe in the plants. To study how these traits were inherited, he created hybrids by passing pollen between pea plants with different traits. He then observed which traits in hybrids were passed down to the next generation of plants and which were not inherited or would skip generations.

At the time, most scientists thought offspring's traits resulted from a blending of their parents' traits. So by that logic, plants with bright purple flowers that were bred with plants with white flowers would produce hybrids with lilac flowers. In other words, the traits of each parent carried equal weight when contributing to the offspring.

Mendel's experiments proved otherwise. He showed that some traits were dominant and would override other genetic instructions for the trait. If just one parent had a dominant trait, such as purple flowers, their offspring would have it, too. By comparison, recessive traits require that both parents have a trait in order for it to be observable in their offspring.

(Although Mendel uncovered these key principles of genetics, it's important to note that scientists did not understand how traits were shaped by DNA and genes until the 1940s.)

Mendel spent nine years on his pea plant project. He bred and observed about 28,000 plants, creating hybrid strains that combined different traits. The pattern of dominant and recessive traits applied to all of the characteristics that Mendel tested.

While his scientific achievements were not recognized during his lifetime, Mendel is now known as "the father of genetics." His research offered a simple view of how traits work. Since then, the field of genetics has uncovered more complex mechanisms that control traits, including the influence of an organism's environment.

Genetic ancestry testing, also known as genetic genealogy, is a way to trace family history through DNA. Over time, genetic variations appear in groups of people. These variations are passed down through generations and are unique to those geographic regions. Having a certain type of variation in your DNA could mean that your ancestors originated from a place in the world where other people share that variation.

Commercially available kits can help to identify a person's genetic ancestry. Users submit their DNA in saliva samples, which are then sent to laboratories and compared with other DNA samples in a large database. Tests may look for genetic variations in the Y chromosome — the sex chromosome typically carried by males — to explore ancestry in the direct male lineage. They may seek matches in mitochondrial DNA, which reflects maternal lineage. Other tests investigate variations called single nucleotide polymorphisms (SNPs), which are differences in single "letters" within a given gene and thus appear in the entire genome.

DNA can even tell you if you have Neanderthals in your family tree. Neanderthals, H. sapien's closest extinct relatives, died out about 40,000 years ago. But for thousands of years before that, Neanderthals mated with H. sapiens in parts of Europe and Asia. Traces of Neanderthal DNA linger in the genomes of people whose ancestors came from those continents.

Still, ancestry represents only a partial picture of what traits you end up with. Genetics alone may shape certain traits, such as eye and hair color, or susceptibility to certain types of disease. However, a person's environment, lifestyle and other factors also affect their overall health and appearance, and distinguish one individual from another.

Certain health problems are caused by genetic abnormalities. These coding errors in DNA may be a mutation in a single gene, or missing or duplicated material in chromosomes.

Some genetic errors are inherited. The symptoms of hereditary disorders sometimes appear early in life, but this is not always the case. In Tay-Sachs disease, which affects nerves in the brain and spinal cord, symptoms typically emerge at 3 to 6 months old. By comparison, Huntington's disease, a neurodegenerative disorder, usually doesn't develop until adulthood.

Environmental factors and lifestyle choices may also combine with heredity to cause disease. These health conditions are called multifactorial disorders because they stem from a complex combination of factors.

Cancer, in which cells in different parts of the body multiply or grow uncontrollably, is one such disorder. There are more than 200 types of cancer, and genetic mutations can increase the risk of some cancers. Sometimes, these mutations are caused by external factors that damage DNA and cause errors in cell replication, like sun damage or exposure to toxins. But some genetic mutations that increase the risk of cancer are hereditary, inherited from a person's parents. For example, certain inherited mutations in genes called BRCA1 and BRCA2 are linked to an increased risk of breast cancer and ovarian cancer.

Inherited gene mutations have been linked to more than two dozen types of cancers, including cancers of the thyroid, stomach, prostate and pancreas. However, only about 5% to 10% of cancer cases are caused by hereditary mutations alone.