THE LIVING WORLD
Unit Three. The Continuity of Life
10. Foundations of Genetics
Mendel's First Law: Segregation
Mendel’s model brilliantly predicts the results of his crosses, accounting in a neat and satisfying way for the ratios he observed. Similar patterns of heredity have since been observed in countless other organisms. Traits exhibiting this pattern of heredity are called Mendelian traits. Because of its overwhelming importance, Mendel’s theory is often referred to as Mendel’s first law, or the law of segregation. In modern terms, Mendel’s first law states that the two alleles of a trait separate from each other during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy.
Mendel's Second Law: Independent Assortment
Mendel went on to ask if the inheritance of one factor, such as flower color, influences the inheritance of other factors, such as plant height. To investigate this question, he first established a series of true-breeding lines of peas that differed from one another with respect to two of the seven pairs of characteristics he had studied. He then crossed contrasting pairs of true-breeding lines. Figure 10.10 shows an experiment in which the P generation consists of homozygous individuals with round, yellow seeds (RRYY in the figure) that are crossed with individuals that are homozygous for wrinkled, green seeds (rryy). This cross produces offspring that have round, yellow seeds and are heterozygous for both of these traits (RrYy). Such F1 individuals are said to be dihybrid. The chromosomes are then allocated to the gametes during meiosis such that there are four types of gametes for these two traits.
Mendel then allowed the dihybrid individuals to self-fertilize. If the segregation of alleles affecting seed shape and alleles affecting seed color were independent, the probability that a particular pair of seed-shape alleles would occur together with a particular pair of seed-color alleles would simply be a product of the two individual probabilities that each pair would occur separately. For example, the probability of an individual with wrinkled, green seeds appearing in the F2 generation would be equal to the probability of an individual with wrinkled seeds (1 in 4) multiplied by the probability of an individual with green seeds (1 in 4), or 1 in 16.
In his dihybrid crosses, Mendel found that the frequency of phenotypes in the F2 offspring closely matched the 9:3:3:1 ratio predicted by the Punnett square analysis shown in figure 10.10. He concluded that for the pairs of traits he studied, the inheritance of one trait does not influence the inheritance of the other trait, a result often referred to as Mendel’s second law, or the law of independent assortment. We now know that this result is only valid for genes not located near one another on the same chromosome. Thus in modern terms, Mendel’s second law is often stated as follows: Genes located on different chromosomes are inherited independently of one another.
Figure 10.10. Analysis of a dihybrid cross.
This dihybrid cross shows round (R) versus wrinkled (r) seeds and yellow (F) versus green (y) seeds. The ratio of the four possible phenotypes in the F2 generation is predicted to be 9:3:3:1.
Mendel’s paper describing his results was published in the journal of his local scientific society in 1866. Unfortunately, his paper failed to arouse much interest, and his work was forgotten. Sixteen years after his death, in 1900, several investigators independently rediscovered Mendel’s pioneering paper while searching the literature in preparation for publishing their own findings, which were similar to those Mendel had quietly presented more than three decades earlier.
Key Learning Outcome 10.4. Mendel's theories of segregation and independent assortment are so well supported by experimental results that they are considered "laws."