lecture 2 Flashcards
(22 cards)
blending hypothesis
One possible explanation for heredity is a “blending” hypothesis.
o This hypothesis proposes that genetic material contributed by each parent mixes in a manner
analogous to the way blue and yellow paints blend to make green.
o With blending inheritance, a freely mating population would eventually give rise to a uniform
population of individuals.
o Everyday observations and the results of breeding experiments tell us that heritable traits do
not blend to become uniform.
particulate inheritance
An alternative hypothesis, “particulate” inheritance, proposes that parents pass on discrete
heritable units, genes, that retain their separate identities in offspring.
o Genes can be sorted and passed on, generation after generation, in undiluted form.
wher did modern genetics begin
Modern genetics began in an abbey garden, where a monk named Gregor Mendel documented a
particulate mechanism of inheritance.
Mendel used the scientific approach to identify two laws of inheritance.
- Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned
experiments, carried out several decades before chromosomes were observed under the
microscope
Pea plants have several advantages for genetic study
o Pea plants are available in many varieties that have distinct heritable features, or characters,
with different variant traits.
o Peas have a short generation time; each mating produces many offspring.
o Mendel was able to strictly control the matings of his pea plants.
o Each pea plant has male (stamens) and female (carpal) sexual organs.
o In nature, pea plants typically self-fertilize, fertilizing ova with the sperm nuclei from their
own pollen.
o Mendel could also use pollen from another plant for cross-pollination.
Mendel tracked only those characters that varied in an “either-or” manner, rather than a “more-
or-less” manner.
o For example, he worked with flowers that were either purple or white.
o He avoided traits such as seed weight, which varied on a continuum.
Mendel started his experiments with varieties that were true-breeding.
A true breeding is a kind of breeding wherein the parents would produce offspring that would carry the same phenotype. This means that the parents are homozygous for every trait. An example of true breeding is that of the Aberdeen Angus cattle.
o When true-breeding plants self-pollinate, all their offspring have the same traits as their
parents.
In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting,
true-breeding pea varieties.
o The true-breeding parents are the P (parental) generation, and their hybrid offspring are
the F1 (first filial) generation.
Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 (second filial)
generation.
* It was mainly Mendel’s quantitative analysis of F2 plants that revealed two fundamental principles
of heredity: the law of segregation and the law of independent assortment.
The Law of Segregation– blending inheritance proved
- If the blending hypothesis were correct, the F1 hybrids from a cross between purple-flowered
and white-flowered pea plants would have pale purple flowers. - Instead, the F1 hybrids all have purple flowers, just as purple as their purple-flowered parents.
- When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-
flowered and white-flowered plants.
o The white trait, absent in the F1 generation, reappeared in the F2. - Mendel used very large sample sizes and kept accurate records of his results.
o Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants.
o This cross produced a ratio of three purple flowers to one white flower in the F2 offspring. - Mendel reasoned that the heritable factor for white flowers was present in the F1 plants but did
not affect flower color.
o Purple flower color is a dominant trait, and white flower color is a recessive trait.
○ The reappearance of white-flowered plants in the F2 generation indicated that the heritable
factor for the white trait was not diluted or lost by coexisting with the purple-flower factor in
F1 hybrids. - Mendel found similar 3:1 ratios of two traits in F2 offspring when he conducted crosses for six
other characters, each represented by two different traits.
○ For example, when Mendel crossed two true-breeding varieties, one producing round seeds
and the other producing wrinkled seeds, all the F1 offspring had round seeds.
o In the F2 plants, 75% of the seeds were round and 25% were wrinkled.
Mendel developed a hypothesis to explain these results that consisted of four related ideas. We
will explain each idea with the modern understanding of genes and chromosomes.
- Alternative versions of genes account for variations in inherited characters.
- For each character, an organism inherits two alleles, one from each parent.
- If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s
- Mendel’s law of segregation states that the two alleles for a heritable character segregate
Alternative versions of genes account for variations in inherited characters.
o The gene for flower color in pea plants exists in two versions, one for purple flowers
and one for white flowers.
o These alternative versions of a gene are called alleles.
o Each gene resides at a specific locus on a specific chromosome.
o The DNA at that locus can vary in its sequence of nucleotides.
o The purple-flower and white-flower alleles are two DNA variations at the flower-color
locus.
For each character, an organism inherits two alleles, one from each parent.
o A diploid organism inherits one set of chromosomes from each parent.
o Each diploid organism has a pair of homologous chromosomes and, therefore, two
copies of each gene.
o These homologous loci may be identical, as in the true-breeding plants of the P
generation.
o Alternatively, the two alleles may differ, as in the F1 hybrids
If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s
appearance. The other, the recessive allele, has no noticeable effect on the organism’s
appearance.
o In the flower-color example, the F1 plants inherited a purple-flower allele from one
parent and a white-flower allele from the other.
o The plants had purple flowers because the allele for that trait is dominant.
Mendel’s law of segregation states that the two alleles for a heritable character segregate
(separate) during gamete production and end up in different gametes.
o This segregation of alleles corresponds to the distribution of homologous chromosomes
to different gametes in meiosis.
o If an organism has two identical alleles for a particular character, then that allele is
present as a single copy in all gametes.
o If different alleles are present, then 50% of the gametes will receive one allele and 50%
will receive the other
Useful Genetic Vocabulary
- An organism with two identical alleles for a character is homozygous for the gene controlling
that character. - An organism with two different alleles for a gene is heterozygous for that gene.
- An organism’s observable traits are called its phenotype.
○ “Phenotype” refers to physiological traits as well as traits directly related to appearance. - An organism’s genetic makeup is called its genotype.
- Two organisms can have the same phenotype but different genotypes if one is homozygous
dominant and the other is heterozygous.
o PP and Pp plants have the same phenotype (purple flowers) but different genotypes
(homozygous dominant and heterozygous). - For flower color in peas, the only individuals with white flowers are those that are homozygous
recessive (pp) for the flower-color gene.
The Testcross
- How can we determine the genotype of an individual that has the dominant phenotype?
o The organism must have one dominant allele but could be homozygous dominant or
heterozygous. - The answer is to carry out a testcross.
o The mystery individual is bred with a homozygous recessive individual.
o If any of the offspring display the recessive phenotype, the mystery parent must be
heterozygous.
mendel showed
Mendel showed that white-flowered plants could be
produced by crossing two purple-flowered plants, but only if the purple-flowered plants themselves
had at least one white-flowered parent (Fig. 1.3.11). This was evidence that the genetic factor that
produced white-flowers had not blended irreversibly with the factor for purple-flowers
Mendel’s
observations disprove blending inheritance and favor an alternative concept, called
particulate
inheritance. in which heredity is the product of discrete factors that control independent traits.
Inheritance of flower color in peas.
Mendel observed that a cross between pure
breeding, white and purple peas (generation P)
produced only progeny (generation F1) with
purple flowers. However, white flowered plant
reappeared among the F2 generation progeny
of a mating between two F plants. The symbols
P, F1 and F2 are abbreviations for parental, first
filial, and second filial generations, respectively.
Because of this work, blended inheritance was discredited as a theory of inheritance.
Through careful study of patterns of inheritance, Mendel recognized that a single trait could exist in different
versions, or alleles, even within an individual plant or animal. For example, he found two allelic forms of a gene
for seed color: one allele gave green seeds, and the other gave yellow seeds. Mendel also observed that although
different alleles could influence a single trait, they remained indivisible and could be inherited separately. This is
the basis of Mendel’s First Law, also called The Law of Equal Segregation, which states: during gamete formation,
the two alleles at a gene locus segregate from each other; each gamete has an equal probability of containing either
allele.
overview of mendel
Mendel first made his discoveries of inheritance in
the 1850’s. In his 1866 publication he didn’t use the word
“gene” as the fundamental unit of heredity because it
wasn’t coined until 1909 by Danish botanist Wilhelm
Johannsen. Thomas Hunt Morgan proposed that genes
resided on chromosomes in 1910, and occupied distinct
regions on those chromosomes. DNA as a substance
was discovered in the 1860’s, but it took until the 1940s
to realize that DNA was the molecule that contained
the genetic information. Then in the 1950’s Watson and
Crick discovered the structure of DNA
__________________ is the science of solving biological questions using
controlled matings of model organisms. It began with
Mendel in 1865 but did not attain wide spread usage
until Mendel’s work was rediscovered in 1903 by four
researchers (E. von Tschermak, H. de Vries, C. Correns,
and W. J. Spillman).
Classical genet-
ics is the science of solving biological questions using
controlled matings of model organisms. It began with
Mendel in 1865 but did not attain wide spread usage
until Mendel’s work was rediscovered in 1903 by four
researchers (E. von Tschermak, H. de Vries, C. Correns,
and W. J. Spillman).
