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This article explores the contributions of Gregor Mendel, a Czech monk, to Darwin's theory of natural selection. Mendel's experiments on pea plants provided answers to Darwin's dilemma of how new traits survive through generations. The article discusses Mendel's background, his experiments, and the concept of particulate inheritance.
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Darwin’s Dilemma: Remember that Darwin’s theory of natural selection claimed that “there is a struggle for existence and that in that struggle, the fittest survive” Natural selection therefore implies that an organism that is best-equipped to survive the rigors of its environment is most likely to bear offspring and that these offspring will pass on advantageous traits to their offspring Implications: “wonderous monsters” with new, advantageous traits will eventually replace “less well-adapted” individuals and give rise to new species
Problem: How do new characters survive through several generations ? Remember that at this time, people believed that traits were passed on through the blood (so why don’t new traits just get diluted out ?) Darwin never figured out this problem !
The ultimate irony Among several books on Darwin’s bookshelf were two books, one by H. Hoffmann, and another by W.O. Focke that referred to the work of Gregor Mendel, It was Mendel’s experiments on pea plants that provided the answers to Darwin’s dilemma Apparently, Darwin never had a chance to read Mendel’s work before his death (D’oh !)
Gregor Mendel (1822-1884) Born to peasant parents in Heizendorf, Austria (now Hyncice in Czechoslovakia) Entered monastery Brunn in Moravis, (now Brno in Czechoslovakia). Attended University of Vienna to get a teaching diploma University examiner failed him with the comments, " he lacks insight and the requisite clarity of knowledge". For many years took care of a monastry garden (apparently had lots of time on his hands)
From 1858 to 1866, Mendel bred garden peas in his monastery garden and analyzed the offspring of these matings. • The garden pea was good choice of experimental organism because: • many varieties were available that bred true for clear-cut, traits like: • seed texture (round vs wrinkled) • seed color (green vs yellow) • flower color (white vs purple) • tall vs dwarf growth habit
Anatomy of a flowering plant • Pea plants have both male and female reproductive organs. • As a result, they can either self-pollinate themselves or cross-pollinate with another plant. • In his experiments, Mendel was able to selectively cross-pollinate “purebred” plants with particular traits and observe the outcome over many generations.
One of his simple experiments: crossing purebred round and wrinkled-seeded plants The first cross: Mendel crossed a pure-breeding round-seeded variety with a pure-breeding wrinkled-seeded one. Result: All the peas produced in the second or hybrid generation were round. So…round + wrinkled = round
Second cross: Mendel then allowed round hybrid peas to self pollinate Result: The wrinkled trait (which had disappeared in his hybrid generation) reappeared in 25% of the new crop of peas. So…round + round = 75 % round + 25 % wrinkled
Third cross: Mendel then allowed some of each offspring from the second-cross generation to self-pollinate Results: 1. Crossing of wrinkled seeds from second cross produced only wrinkled seeds 2. Round seeds produced 2/3 round seeds and 1/3 wrinkled seeds
Explanation • In the organism there is a pair of factors that controls the appearance of a given characteristic. (We call them genes.) • The organism inherits these factors from its parents, one from each. • Each is transmitted from generation to generation as a discrete, unchanging unit. • When the gametes are formed, the factors separate and are distributed as units to each gamete (sex cell). This statement is often called Mendel's rule of segregation. • If an organism has two unlike factors in a factor pair (we call members of such pair alleles) for a certain characteristic, one may be expressed to the total exclusion of the other (dominant vs. recessive).
First Cross R = round allele (dominant – always expressed) R= wrinkled allele (recessive – hidden when combined with dominant) So we have a genotype (organism’s characteristics in genetic code) and a phenotype (physical expression of genotype)
Second Cross Recessive allele is hidden when combined with dominant allele Recessive allele only shows up when combined with another recessive allele
Third Cross Genotype 16 RR 32 Rr 16 rr Phenotype 48 round 16 wrinkled So crossing all types, get ratio of 1:2:1 for RR:Rr:rr But ratio of 1:3 for round: wrinkled
Mendel’s “Particulate Inheritence”: Significance to Evolution Indicated that in sexual organisms, each parent has two factors for each trait (these split when sex cells are made, then recombine in offspring) Also indicated that traits are passed on more like “particles” than as a “fluid” (i.e. factors are discrete entities that are not diluted). Natural selection can, however, act on populations on organisms according to the “fitness” of the phenotypes
But… Some characteristics such as hair and eye colour may not significantly affect the survival of organisms such as humans (at least at the present time- but who knows for the future) What might the future hold for the apparently “neutral” characteristics humans hold ?
