BSC 314 Lecture Notes - Lecture 37: Red Color, Backcrossing, Zygosity
27 Jun 2018
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Mendelian Genetics
The breeding experiments of the monk Gregor Mendel in the mid 1800s laid the ‐
groundwork for the science of genetics. He published only two papers in his lifetime and
died unheralded in 1884. The significance of his paper published in 1866 on inheritance
in peas (which he grew in the monastery garden) apparently went unnoticed for the next
34 years until three separate botanists, who also were theorizing about heredity in
plants, independently cited the work in 1900. During the next 30 years, the universality
of his findings was confirmed, and breeding programs for better livestock and crop
plants—and the science of genetics—were well under way.
At the time of Mendel's work, scientists widely believed that offspring blended the
characteristics of their parents, but Mendel's painstaking experimentation suggested this
was not so. Remember, no one had yet heard of genes, chromosomes, or meiosis, but
Mendel concluded from his breeding experiments that particles or “factors” that passed from
the parents to the offspring through the gametes were directly responsible for the physical
traits he saw first lost in the offspring's generation, then repeated in the next. Closer still to
the actual truth, Mendel even hypothesized that two factors, probably one from each parent,
interacted to produce the results. His “factors” were, of course, the genes, which do,
indeed, come in pairs or alleles for each trait.
Some say Mendel was lucky, others that his reported results are too good to be true, that
he (or someone else) must have fudged the data to make them “come out right.” His choice
of garden peas was fortuitous. Peas are self-pollinated, and the seven traits he chose to
measure are inherited as single factors, so Mendel could establish true-breeding lines for
each trait. Thus, he was able to select the parent traits, pollinate the flowers, and count the
results in the offspring with no complicating elements. He was mathematically trained, kept
accurate records, and applied mathematical analyses (and was among the first to do so with
biological materials).
Mendel's first law: Law of Segregation
Mendel did not formulate his conclusions as laws or principles of genetics, but later researchers have
done so. Restating and using modern, standardized terminology, this is the information that developed
and expanded from his early experiments.
Inherited traits are encoded in the DNA in segments called genes, which are located at particular
sites ( loci, singular locus) in the chromosomes. (Genes are Mendel's “factors.”)
Genes occur in pairs called alleles, which occupy the same physical positions on homologous
chromosomes; both homologous chromosomes and alleles segregate during meiosis, which results in
haploid gametes.
The chromosomes and their alleles for each trait segregate independently, so all possible
combinations are present in the gametes.
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The expression of the trait that results in the physical appearance of an organism is called
the phenotype in contrast to the genotype, which is the actual genetic constitution.
The alleles do not necessarily express themselves equally; one trait can mask the expression of
the other. The masking factor is the dominant trait, the masked the recessive.
If both alleles for a trait are the same in an individual, the individual ishomozygous for the trait,
and can be either homozygous dominant or homozygous recessive.
If the alleles are different—that is, one is dominant, the other recessive—the individual
is heterozygous for the trait. (Animal and plant breeders often use the term “true-breeding” for
homozygous individuals.)
Geneticists use a standard shorthand to express traits using letters of the alphabet, upper case for
dominant, lower case for recessive. Red color, for example, might be Ror r so a homozygous dominant
individual would be RR, a homozygous recessive individual, rr and a heterozygous individual Rr.
Crosses between parents that differ in a single gene pair (such as those that Mendel made) are
called monohybrid crosses (usually TT and tt). Crosses that involve two traits are called dihybrid
crosses. Symbols are used to depict the crosses and their offspring. The letter P is used for the parental
generation and the letter F for the filial or offspring generation. F1 is the first filial generation, F2 the
second, and so forth.
What kinds of crosses did Mendel make to conclude that factors/genes segregate? First of all, he made
certain that the plants that he planned to use in the experiment werepure line for the trait—that is, that
they bred true for the trait for two or more years. (Peas are self-pollinated so he simply grew the plants
and examined their offspring.) Other experimenters omitted this step, which confounded their results.
Mendel then made a series of monohybrid crosses for each of the seven traits he had identified using
parents of opposite traits—tall (TT) vs. dwarf (tt), yellow seed (YY) vs. green (yy) seed, round seed (RR)
vs. wrinkled (rr), and so forth. (He, of course, did not symbolize them with letters, but he did know that
seeds from his tall pure-line plants would always produce tall plants, seeds from the dwarfs would always
produce dwarf plants, and so on.)
Mendel then let the F1 plants self-pollinate: Tt × Tt and in the F2 generation counted the numbers of
individuals with each of the traits. For the tall × dwarf crosses he got 787 tall plants and 277 dwarf plants
(6,022 yellow seeds and 2,001 green seeds, and so forth).
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
The breeding experiments of the monk gregor mendel in the mid 1800s laid the groundwork for the science of genetics. He published only two papers in his lifetime and died unheralded in 1884. The significance of his paper published in 1866 on inheritance in peas (which he grew in the monastery garden) apparently went unnoticed for the next. 34 years until three separate botanists, who also were theorizing about heredity in plants, independently cited the work in 1900. During the next 30 years, the universality of his findings was confirmed, and breeding programs for better livestock and crop plants and the science of genetics were well under way. At the time of mendel"s work, scientists widely believed that offspring blended the characteristics of their parents, but mendel"s painstaking experimentation suggested this was not so. Remember, no one had yet heard of genes, chromosomes, or meiosis, but.
