BSC 314 Lecture Notes - Lecture 36: Vegetative Reproduction, Quantitative Trait Locus, Epistasis
27 Jun 2018
School
Department
Course
Professor
Intricacies of Inheritance
Continued breeding experiments, better microscopes, and more scientists working in
the field have advanced the knowledge of inheritance in organisms and, at the same
time, complicated the simple patterns discovered by Mendel. This article covers some of
the intricacies.
Shortly after the genetic community accepted Mendel's Law of Independent Assortment,
several exceptions to its operation were found. Most of these exceptions were the result
of linkage of the genes being studied on one chromosome. When the usual crosses
were made (P 1: parents pure line dominant for two traits × pure line recessive for two ‐ ‐
traits), the F 1 individuals of the cross were all dominant and presumably heterozygous.
Selfing (transferring pollen from the anthers to the stigma of the same flower) of the
F 1 resulted in no predictable ratios, and never the expected 9:3:3:1. Two phenotypes,
those of the original P 1 parents, were in high frequency in the F 2 and two other
phenotypes, in low frequency, combined the phenotypes of the two original parents. In
searching for explanations for the phenomena, the scientists followed the principle of
parsimony—that is, they looked first for the simplest explanation that fits all the facts. In
this instance, the simplest interpretation—and the correct one—is that the genes for the
traits lie close together on the same chromosome.
Linkage and crossing over
Linkage might properly explain the high frequencies of two phenotypes, but what of the
low frequency, other combinations? The most logical explanation is that during
Prophase I of meiosis when the four chromatids of two homologous chromosomes lie
close together, crossing over occurs; that is, there is a physical exchange of material
between non sister chromatids and a‐ genetic recombination. Thus, unexpected gene
frequencies occur because genes no longer travel in their previous sequences. The X‐
shaped location of the crossover is called the chiasma (plural, chiasmata), and there
may be several in each pair.
If genes lie close together on a chromosome, there is less chance of crossing over
taking place than if they lie farther apart. After tabulating the frequency of crossing over
for known genes, it is possible to construct linkage maps of the chromosomes and
determine approximate locations of genes.
Incomplete dominance
Incomplete dominance occurs when both alleles in a heterozygous individual are
expressed, producing a phenotype different from either single allele. For example, red
snapdragons crossed with white snapdragons produce pink snapdragons. A cross of
two pinks restores the red and white in a 1 red:2 pink:1 white ratio. The dominant allele
does not completely mask the recessive in this case. Although the phenotype is
changed, the alleles themselves are unaltered, as can be shown by a backcross in
which they segregate and express their original trait in the homozygous condition.
Mutations
A mutation is defined as any change in the DNA of an organism—a sufficiently broad
definition to include all manner of changes: deletions (a piece of the chromosome
breaks off and is lost), translocations (pieces of material are exchanged between two
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Continued breeding experiments, better microscopes, and more scientists working in the field have advanced the knowledge of inheritance in organisms and, at the same time, complicated the simple patterns discovered by mendel. Shortly after the genetic community accepted mendel"s law of independent assortment, several exceptions to its operation were found. Most of these exceptions were the result of linkage of the genes being studied on one chromosome. When the usual crosses were made (p 1: parents pure line dominant for two traits pure line recessive for two traits), the f 1 individuals of the cross were all dominant and presumably heterozygous. Selfing (transferring pollen from the anthers to the stigma of the same flower) of the. F 1 resulted in no predictable ratios, and never the expected 9:3:3:1. Two phenotypes, those of the original p 1 parents, were in high frequency in the f 2 and two other phenotypes, in low frequency, combined the phenotypes of the two original parents.
