When babies are on the way, it is fun to imagine what they will look like. Will they have the father’s brown eyes or the mother’s blue ones? Will they be right-handed or left-handed? Thanks to a monk named Gregor Mendel, we know that some of these traits are more likely to occur than others. Eye color, dominant hand, and attached or detached ear lobes are all examples of genetically determined traits given to a child by the parents. One reason we know this is because Gregor was interested in pea plants in the mid-1800s.
At about the same time that the United States was embroiled in the Civil War, Gregor studied pea plants in his Austrian monastery, St. Thomas’ Abbey in current day Brno, Czechia. A gardener from an early age, he also excelled in physics, biology, and mathematics. Gregor noticed that the pea plants growing in the abbey grew quickly and had easily identifiable traits, such as bloom color, plant height, and the texture of the seed, all qualities that made them easy to study.
He began a seven-year experiment to see which traits could be manipulated by cross-pollenating one plant with another. He discovered that crossing a green plant with a yellow one would not result in a blended greenish-yellow colored bloom; rather, the combination resulted in more yellow flowers. Yellow blooms were dominant over green. It was Gregor who figured out that each plant had two copies of a trait, now called an allele (ah-LEEL) and that he could chart the probability of these blending results. After Gregor’s death, he became known as the father of genetics.
In the years since Gregor tended his pea plants, the field of knowledge surrounding genetics has exploded. We know about that most famous of spiral ladders, deoxyribonucleic acid, or DNA, and how a map of our entire makeup exists in the tiny spaces of its rungs. To add further complexity, environmental triggers are often the deciding factor on whether certain genes become manifest in a person or not. When sperm meets egg, each contributes 23 chromosomes, including the X and Y chromosomes, to determine gender.
Human traits can be located on either autosomal — or non-sex — chromosomes, or they can be linked to the sex chromosomes. Autosomal and X-chromosome traits can be dominant or recessive. With codominant traits, each allele has equal weight. Codominant traits produce a combined physical characteristic. In addition to codominant traits are traits with incomplete dominance when neither trait dominates the other entirely.
One example of an autosomal trait is the widow’s peak. When the hairline comes to a point near the middle of the forehead, it is known as a widow’s peak. This dominant trait is particularly annoying when center-part hairstyles are in vogue; only those who have inherited the recessive straight hairline can achieve the look. Freckles are also autosomal dominant.
We also know that while some traits, like eye color, are benign, genes sometimes carry damage that can be passed to a child, resulting in disease. Huntington’s disease is a progressive brain disorder caused by a defective, autosomal dominant gene. If passed from one parent to a child, the child will typically develop the disease in adulthood, which manifests itself in abnormal, involuntary movements, problems with reasoning, and mood disorders.
In autosomal recessive diseases such as cystic fibrosis, both parents must pass on a copy of the abnormal gene for the disease to develop; however, environmental triggers may determine severity. When a child is born to two people who both carry the gene for an autosomal recessive disease, the child has a 25 percent chance of being born with two normal genes; a 50 percent chance of being born with one of the genes, meaning they are a carrier; and a 25 percent chance of being born with two abnormal genes and thus are at risk of developing the disease.
While both the X chromosome and the Y chromosome carry traits, the X chromosome carries far more than the Y chromosome, some 2,000 as opposed to fewer than 80. One such X-chromosome trait is tetrachromatism. Whereas most people have three cone cells that allow them to see about a million colors, those with tetrachromatism have four cone cells, possibly increasing the ability to see up to 100 million different colors. A tetrachromat logically excels in art. With training, they are better able to identify and mix paint colors. They paint more quickly and use less paint than their three-cone-cell counterparts. They also spend less time editing their work. X-chromosome-related disorders include Duchenne muscular dystrophy, hemophilia, and red-green color blindness.
Y-chromosome traits occur only in males. Female offspring do not develop any Y-chromosome-related diseases, nor can they pass them to their offspring. Y-chromosome disorders include hypertrichosis (hairiness of the ears) and XYY syndrome, manifesting as increased height, severe cystic acne in adolescence, behavioral problems, and learning disabilities. Fortunately, Y-related disorders are rare because there are so few genes on the Y chromosome and they do not pass to females, even as carriers.
Incomplete and codominant genes are similar but not the same. An incomplete pairing of genes dilutes the dominant gene, allowing the recessive gene to play a part, too. Height is one example of incomplete allele pairing. When one parent is tall and the other is short, resulting children are usually anywhere in the range between the two parents’ heights. Other incomplete-gene traits include eye color, hand size, skin color, and voice pitch.
When codominance occurs, two alleles are expressed together, rather than there being a gene-driven decision of one or the other. One obvious example of codominance is blood type. The blood type AB is the result when one parent has type A and the other has type B. Hair texture is another example of codominant genes. Even though curly hair is considered dominant, when a curly haired gene matches with a straight one, the result is a pleasant wavy coexistence of both.
The explosion of information about genetics has given rise to a needed specialty: genetic counseling. One might wish to see a genetic counselor for many reasons. Couples considering having children may consult a genetic counselor to learn what awaits when one set of chromosomes meets the other. A genetic counselor can also use a patient’s medical and family history to suggest tests, help make diagnoses, or answer questions about their own risk of developing an inherited disorder.
Being informed can allow a person to take steps to lessen the effects of a disease or at least better plan for the eventuality of developing it. Thanks to giant steps in the field of genetics, a genetic counselor can answer questions, educate, and assess an individual’s risk, as well as provide guidance and support regarding the conditions individuals have in their DNA. For example, many women with a family history of genetic breast cancer consult a genetic counselor if considering a preventative mastectomy and/or hysterectomy.
“Most people do not know what exactly goes on at a genetics clinic until they are in need of services,” says Lori Bassett, MS, CGC, director of communication at the Greenwood Genetic Center and a board-certified genetic counselor. “At the Greenwood Genetic Center’s office in Columbia, patients might be referred because of a family history of a genetic disorder or if they have a child who has health or developmental challenges that could be genetic, such as developmental delay, autism, or a birth defect. Clinical geneticists (MDs) and genetic counselors work with patients and their families to identify the cause of a disorder, order genetic testing to confirm the diagnosis, and educate them about the disorder and the chances for it to happen again. They also manage treatment for genetic conditions and support families though all sorts of decision making about genetic testing, treatments, and planning for future children.”
It is astounding to think how far the field of genetics has come since the days when Gregor was quietly nursing his pea plants. He studied only seven traits. Today, we know that genes number between 20,000 to 25,000, with more being discovered all the time. Science teaches so much about the many characteristics that make humans unique — that determine whether we will write with our left hand or our right, if we are short or tall, athletically gifted or not so much, or whether we are brunette or blonde or red-headed or somewhere in between … or whether we have hair at all. It is our genes that determine the shapes we can roll with our tongues, whether we can lift one eyebrow, and they decide if we like cilantro. Genetics, without question, make an enormous impact on our lives.