Since prehistoric times, people have noticed how children tend to resemble their parents and grandparents. People also recognized that some human diseases tended to run in families as well. Learn why this happens and how it was discovered.
What is the substance in cells that carries the heritable trait information? Further, what is the structure of this substance?
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Since prehistoric times, people have noticed how children tend to resemble their parents and grandparents. People also recognized that some human diseases tended to run in families as well.
People have always had an idea about the inheritance of traits from parents to children to grandchildren. Nearly all people throughout history in cultures around the world had kept detailed genealogies of their families going back many generations. The Hebrew and later Christian scriptures contain detailed records of a great number of generations. Most other cultures around the world kept their ancestral records in the form of oral tradition, handed down from generation to generation. Clearly, family lines have been very important to nearly all human cultures. Lines of inheritance, sometimes called bloodlines, have always had a great significance to people, affecting wealth, status, and societal privilege. However, physical traits were also seen to run in families.
What appeared to be inherited traits seemed to be true for animals and plants, as well. People learned to breed animals and plants that had desirable traits to produce offspring that also had the same desirable traits. For instance, over thousands of years dogs had been selectively bred for a variety of specialized traits such as having skills helpful for hunters or being good companion animals.
Over many generations, selective breeding, also known as artificial selection, turned a poor yielding grass into wheat with much larger kernels. Selective breeding was applied to many plants important to agriculture and animals important to duties such as hunting, animal herding, and companionship.
Although over the millennia these people did not understand the molecular basis for genetics, they did have a practical understanding of it. One of the questions that remained about selective breeding was why did some traits disappear in one generation and reappear in the next?
In 1837, sheep breeder Abbot Cyril Napp begin asking specific questions about the nature of heredity. What traits are inherited? How are they inherited? What is the role of chance in heredity?
These questions intrigued Gregor Mendel. As both an Augustinian friar and a scientist, he decided to begin formal experiments to try to answer these questions. Mendel decided to use pea plants for his experiments. They grew fast, so many generations could easily be studied within a reasonable time. And the flowers were shaped so he could also control their fertilization easily. In this way he could control in-breeding and cross-breeding of his plants.
When he performed statistical analyses on the results of some of his breeding experiments, he showed that when a plant that only produced yellow peas was cross-bred with a plant that only produced green peas, the offspring always produced yellow peas. But in the next generation, the ratio of green to yellow peas was 1:3. He called the green pea trait “recessive” and the yellow pea trait “dominant.”
Mendel had shown that there was a systematic process of discrete traits whose inheritance ratios could be predicted mathematically. He also provided insight into the original question of why some traits disappear in one generation and reappear in the next.
Although Mendel did not know of the modern concepts of genes, he did establish a new scientific discipline known as genetics. He published his work in 1866, but it wasn’t until about 1900 that his work was re-discovered and built upon by others.
Now new questions were raised. What was the substance in cells that carried the heritable trait information? Further, what was the structure of this substance?
Mysteries remained to be solved. It was decades later that deoxyribonucleic acid, or DNA, was shown to be the molecule responsible for inherited traits. We take knowledge of DNA for granted now, but for most of the 20th century, no one knew much about DNA or what it did, let alone its structure and how it functions as the molecule of heredity. That will be the topic of the next two episodes.