Welcome to the Genome. Michael Yudell

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Welcome to the Genome - Michael Yudell


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was “blended inheritance.” (17) This theory held that the characteristics of parents blended in their offspring. Experimentation in this area failed because, as Mendel was able to eventually determine, heredity was not a lump sum but rather a series of individual traits.

Portrait of Gregor Mendel.

       Credit: American Museum of Natural History

      Mendel began an experiment with purebred peas. One breed had yellow seeds, the other green seeds. When purebred yellow‐seeded peas were bred with each other, their offspring through the generations would have yellow seeds. Under the same circumstances, the green‐seeded peas would always have green‐seeded progeny. However, when he bred the purebred pea with yellow seeds to a purebred pea with green seeds, the offspring, or the first generation of this breeding cross, always had yellow seeds. The green seed trait seemed to be gone. Mendel called traits like the yellow‐seed trait dominating (now called dominant) because in first‐generation crosses they would always appear. (19) Traits like the green‐seed trait were called recessive—although they disappeared completely in the first generation, they reappeared in the second. Thus, when Mendel took the yellow seeds from the first generation and either self‐pollinated them or pollinated them with pollen from other yellow peas from the same first‐generation breed, he discovered that some of these offspring, the second generation, again had the green seed trait. The plants, Mendel concluded, retained the ability to produce green seeds—of the second‐generation seeds, 6022 were yellow and 2001 were green. Likewise, when he used six other traits, he found the same pattern in the second generation—traits that had disappeared in the first generation reappeared in the second. (20) The chart below shows the relationship between dominant and recessive traits in second‐generation pea plants in the seven traits Mendel experimented with.

Dominant trait Recessive trait
Round seeds 5474 Wrinkled seeds 1850
Yellow seeds 6022 Green seeds 2001
Gray seed coats 705 White seed coats 224
Green pods 428 Yellow pods 152
Inflated pods 882 Constricted pods 299
Long stems 787 Short stems 277
Axial flowers 651 Terminal flowers 207

      (21)

Diagram illustrating Mendel's law of segregation from figures labeled Yellow, round and Green, wrinkled leading to figures labeled Yellow, round; Green, round; Yellow, wrinkled; and Green, wrinkled from left to right.

      Figure 1.4 Mendel’s first law of segregation says that alleles will segregate randomly between generations. Mendel’s second law, the law of independent assortment, represented in the figure above, says that pairs of alleles will segregate independently between generations. (P = parents, F1 = first generation, F2 = second generation).

       Credit: Wiley

      These observations are now known as Mendel’s second law, the law of independent assortment—if two traits (genes) are being controlled with different controllers (alleles), offspring will be produced by random combinations of the controllers (alleles). (24) In other words, a trait is independently and randomly distributed among offspring.

      Mendel was either very lucky or very perceptive: it turns out that seven is the number of chromosomes of Pisum. For all seven of the traits he examined to show true independent assortment with respect to one another, none of them can be linked—that is, none of them can be on the same chromosome (or in the case of one of the traits he examined, they have to be very far apart on the same chromosome). (25) Mendel must have watched his peas very closely. Perhaps he recognized the pattern of segregation as he was weeding his garden and thus performed his experiment with an expectation based on his knowledge as a pea


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