This much was well known that the purpose of a gene is to store the recipe for making proteins and that we inherit from our parents a gigantic list of recipes for making proteins and protein-making machines. What was not clear as late as the early 20th century until Gregor Mendel’s experiments were published and understood was that when crossed, the expression of these genes skipped a generation.
Mendel discovered that when white flower and purple flower plants are crossed, the result is not a blend. Rather than being a mix of the two, the offspring was purple flowered. He then conceived the idea of heredity units, which he called "factors", one of which is a recessive characteristic and the other dominant. Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. Mendel crossed over 29,000 plants including inflated seed pods with wrinkled seed pods, green unripe pods with yellow unripe pods, and axial flowers with terminal flowers. In every case the resulting hybrids were just like one parent with the essence of other parent missing. However, as these hybrids were allowed to self-fertilize, the essence of the recessed parent re-appeared in exactly one third of the cases. This re-appearance is called the law of thirds. This law is not just obeyed in plants but also in every living species as demonstrated by Nageli’s experiment on crossing angora cats with another breed. Nageli observed that the angora coat disappeared in the next generation but re-appeared in the kittens in the third.
Mendel’s law has been put to a variety of uses such as selecting the right plants to hybridize to get the desired mix of dominant factors. A surprising use of the law has been to explain Alkaptonuria in which the patients suffered from arthritis and their urine and the ear wax turned reddish black upon exposure to air. Rare in the general population but frequent in children of first-cousin marriages, the incidence of Alkaptonuria is attributed to expression of recessive factors.
Mendel discovered that when white flower and purple flower plants are crossed, the result is not a blend. Rather than being a mix of the two, the offspring was purple flowered. He then conceived the idea of heredity units, which he called "factors", one of which is a recessive characteristic and the other dominant. Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. Mendel crossed over 29,000 plants including inflated seed pods with wrinkled seed pods, green unripe pods with yellow unripe pods, and axial flowers with terminal flowers. In every case the resulting hybrids were just like one parent with the essence of other parent missing. However, as these hybrids were allowed to self-fertilize, the essence of the recessed parent re-appeared in exactly one third of the cases. This re-appearance is called the law of thirds. This law is not just obeyed in plants but also in every living species as demonstrated by Nageli’s experiment on crossing angora cats with another breed. Nageli observed that the angora coat disappeared in the next generation but re-appeared in the kittens in the third.
Mendel’s law has been put to a variety of uses such as selecting the right plants to hybridize to get the desired mix of dominant factors. A surprising use of the law has been to explain Alkaptonuria in which the patients suffered from arthritis and their urine and the ear wax turned reddish black upon exposure to air. Rare in the general population but frequent in children of first-cousin marriages, the incidence of Alkaptonuria is attributed to expression of recessive factors.
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