genetics a short history

7/29/2019 Genetics a Short History

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At the dawn of the 20th century, when the writings of an Austrian monk

named Gregor Mendel were rediscovered, genetics became a science.

Notions of heredity and inheritance were already prevalent. Having

domesticated plants and animals, people realized that in many cases, "like

begat like." Herdsmen and farmers selectively bred plants such as wheat and

maize without understanding the underlying mechanisms. People seemed to

grasp intuitively that attributes could be passed from one generation to the

next.

The 1800s saw several revolutions in the biological sciences. Charles Darwin

transformed the way humans look at the natural world by introducing the

theory of natural selection. Darwin got a lot right, but not when it came to

heredity. About the only thing he understood was that both parents

contribute to their offspring. Mendel was rumored to have sent his writings to

Darwin, but Darwin either disagreed with the papers or simply did not

understand their significance.

Mendel, besides being fond of growing thingspeas in particularhad an

astounding talent for observation. In 1857, Mendel began to study the

mechanisms of inheritance while working with two varieties of garden peas

from the genus Pisum, one with a yellow seed and one with a green seed.

First he crossed a parent purebred for yellow seeds with a parent purebred for

green seeds. All of the offspring gave yellow seeds. (This first cross is called

the F1, or filial cross, and the offspring are called F1s.) This observation was

nothing new. Many traits in plants and animals seem to be swamped by other

traits. But Mendel somehow reasoned that the green trait did not get altered

it was just hidden.

So he took the F1s and crossed them, probably reasoning that the green trait

would come back. He counted the peas in all the offspring (called F2s) and

observed that 6,022 F2s were yellow and 2,001 F2s were green. Do the math,

and you, like Mendel, will recognize a 3:1 ratio.

This observation by itself would not have meant much, but Mendel's scientific

thinking led him to do several other crosses, with these results:


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Table 1: The Results of Mendels Crossing Experiments

Trait Number of Plants Having Trait in F2 Trait Number of Plants Having

Trait in F2

Round Seed 5,474 Wrinkled Seed 1,850

Gray Seed Coat 705 White Seed Coat 224

Green Pods 428 Yellow Pods 152

Inflated Pods882 Constricted Pods 299

Long Stems 787 Short Stems 277

Axial Flowers 651 Terminal Flowers 207

Do the math again and you recognize a trendall the ratios are 3:1.

Controllers of these traits (the genes) were coming from the two parents, and

when the F1s were mated, the controllers were "segregating" in a random

fashion. Based on these observations, Mendel described his first law, the Law

of Segregation. The two versions of a gene segregate and are distributed to

different sex cells.

Even more remarkably, he constructed all possible combinations of two of the

seven traits together (the six listed above, plus the green or yellow seeds).

The real kicker came when he crossed the yellow and smooth F1s with each

other and counted the number of offspring. There are four possible

combinations of these two traits of seed color and textureyellow round

seeds, yellow wrinkled seeds, green round seeds, and green wrinkled seeds.

The ratio of the numbers of F2 with these traits was: 9 yellow round seeds to

3 yellow wrinkled seeds to 3 green round seeds to 1 green wrinkled seed.

Mendel reasoned that mating the F1 plants with these two traitsseed color

and texturewas like throwing the controllers of each of the traits into

separate hats, and mating the F2 plants was like randomly drawing thecontrollers out of the hats. While the choices are random, the outcome is

remarkably regular. This second observation is known as the Law of

Independent Assortment. Each member of a pair of chromosomes segregates

independently of the members of other pairs, so that alleles carried on

different chromosomes are distributed randomly to the sex cells.


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Some feel that Mendel anticipated his results. Others have gone so far as to

suggest that Mendel even cheated by massaging the numbers in his crosses.

Two things are sure: Mendel was extremely observant, and he was RIGHT.

Unfortunately, Mendel's observations disappeared as readily as those traits in

the F1. Luckily for the world, Carl Correns, Hugo de Vries, and Erich von

Tschermak-Seysenegg rediscovered his writings, including his 1865 paper

"Experiments in Plant Hybridization," at the turn of the 20th century, and the

science of genetics was born.

Mendel's laws explain a lot, but more had to be clarified. What is a gene?

What is it made from? How are traits stored? How do different versions of a

gene arise? How can exceptions to Mendel's laws be explained? Throughout

the 20th century, exceptional scientists such as Thomas Hunt Morgan,

George Beadle, Edward Tatum, Sydney Brenner, Barbara McClintock, and a

host of other geneticists looked for systems that could help them understand

more about heredity. They looked for organisms that grew rapidly and that

could produce large populations whose traits could be counted the way

Mendel had done with his peas. Initially called "genetic pets," these plants

and animals are now called "model systems" and they have become the

focus of present-day genome-sequencing projects. Bacteria, yeast, nematode

worms, Drosophila (fruit flies), mice, Arabidopsis weed (mustard plant), rice,

and corn are all model systems.

Humans have also been the subjects of genetic studies. Of course, scientists

can't grow humans in petri dishes or in fly bottles. Even if humans could be

bred like flies or worms, ethical considerations would prohibit it. So

geneticists have devised other methods for examining the genetics of

humans. They include pedigree analyses and twin studies, both of which help

demonstrate the genetic component to a trait.

Concepts such as the gene and one gene/one protein were developed, and

genetics dovetailed with molecular biology and biochemistry. Traits are

inherited through confined units called genes. Genes code for proteins, or as

was once taught, one gene codes for one protein. Nowadays we know that

one gene can code for a variety of similar proteins. Over the last few

decades, the discipline of genetics has changed from a science that described

and investigated the inheritance patterns of certain traits to a science that


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primarily investigates inheritance at the molecular level.

The central discovery might be considered the elucidation in 1953 of the

structure of the genetic materialdeoxyribonucleic acid (DNA). The ultimate

data set could most certainly be considered the entire genome sequence of

an organism. The first complete genomes to be sequenced in the 1970s were

from viruses, and contained about 5,000 base pairs. Next, in the mid-1990s,

came bacterial genomes with about 2 million base pairs. In comparison,the

human genome, with more than 3 billion base pairs, has now been

completely sequenced in almost two-dozen humans. In addition, clever

shortcuts to generating human-genome-level information have contributed to

an unprecedented understanding of our genomes and our biology.

While it's tempting to think that genome sequences represent the ultimate

explanation of genetics, to grasp the importance of any discovery we must

always return to the laws that Mendel discovered.

genetics a short history

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