Page 28 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

20 Chapter 2 Mendel’s Principles of Heredity

essential concepts Monohybrid Crosses Reveal
the Law of Segregation
• People practiced artificial selection of crop plants and
domestic animals for thousands of years without Once Mendel had isolated pure-breeding lines for several
understanding how heredity works. sets of characteristics, he carried out a series of matings
• Mendel established pure-breeding lines of peas in which between individuals that differed in only one trait, such as
a specific characteristic would remain constant from one seed color or stem length. In each cross, one parent carries
generation to the next. one form of the trait, and the other parent carries an alterna-
• When Mendel crossed pure-breeding lines with tive form of the same trait. Figure 2.9 illustrates one such
alternative traits, the hybrid progeny always had the mating. Early in the spring of 1854, for example, Mendel
characteristics of one parent. planted pure-breeding green peas and pure-breeding yellow
• In Mendel’s experiments, the hybrid progeny produced by peas and allowed them to grow into the parental (P)
reciprocal cross-fertilizations had the same characteristics; it generation. Later that spring when the plants had flowered,
did not matter which parent was male and which was female. he dusted the female stigma of green-pea plant flowers with
pollen from yellow-pea plants. He also performed the recip-
rocal cross, dusting yellow-pea plant stigmas with green-
pea pollen. In the fall, when he collected and separately
2.2 Genetic Analysis According analyzed the progeny peas of these reciprocal crosses, he
to Mendel found that in both cases, the peas were all yellow.
These yellow peas, progeny of the P generation, were
the beginning of what we now call the first filial (F 1 ) gen-
learning objectives eration. To learn whether the green trait had disappeared
entirely or remained intact but hidden in these F 1 yellow
1. Explain Mendel’s law of segregation and how it predicts peas, Mendel planted them to obtain mature F 1 plants that
the 3:1 dominant-to-recessive phenotypic ratio among he allowed to self-fertilize. Such experiments involving hy-
the F 2 generation of a monohybrid cross. brids for a single trait are often called monohybrid crosses.
2. Distinguish between a monohybrid cross and a testcross. He then harvested and counted the peas of the resulting
3. Explain Mendel’s law of independent assortment and second filial (F 2 ) generation, progeny of the F 1 generation.
how the 9:3:3:1 phenotypic ratio among the F 2 of a Among the progeny of one series of F 1 self-fertilizations,
dihybrid cross provides evidence for this law. there were 6022 yellow and 2001 green F 2 peas, an almost
4. Interpret phenotypic ratios of progeny to infer how
particular traits are inherited.
5. Predict the genotypic and phenotypic ratios among Figure 2.9 Analyzing a monohybrid cross. Cross-pollination
progeny of complex multihybrid crosses using simple of pure-breeding parental plants produces F 1 hybrids, all of which
rules of probability. resemble one of the parents. Self-pollination of F 1 plants gives rise
6. Cite the most common molecular explanations for to an F 2 generation with a 3:1 ratio of individuals resembling the
dominant and recessive alleles. two original parental types. For simplicity, we do not show the
plants that produce the peas or that grow from the planted peas.
Generation
In early 1865 at the age of 43, Gregor Mendel presented a Parental (P)
paper entitled Experiments on Plant Hybrids before the (pure-breeding) Yellow peas Green peas
Natural Science Society of Brünn. Despite its modest head- ( : pollen) ( : eggs)
ing, this was a scientific paper of uncommon clarity and
simplicity that summarized a decade of original observa- First filial (F 1 )
tions and experiments. In it Mendel describes in detail the All yellow
transmission of visible characteristics in pea plants, defines
unseen but logically deduced units (genes) that determine Self-fertilization
when and how often these visible traits appear, and ana-
lyzes the behavior of genes in simple mathematical terms
to reveal previously unsuspected principles of heredity.
Published the following year, the paper would eventu- Second filial (F )
ally become the cornerstone of modern genetics. Its stated 2
purpose was to see whether there is a “generally applicable
law governing the formation and development of hybrids.” 6022 yellow : 2001 green
Let us examine its insights. 3 : 1