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Review
• Mendelian: genes are NOT linked and follow
rules of simple dominance
• Linked genes
• Meiosis: tetrads and chromatid segregation
• Crossing over/recombination
• Clarify “recombination frequency”
– 20%, 50%
Crossing-Over
Perspective: Historically, the conclusions of
genetic experiments were based on the results of
selected matings; In other words, we didn’t know
what was happening inside the cell, but we could
make conclusions based on the phenotypic results
(e.g. ratios) of the offspring.
It was only recently that scientists were able to
physically illustrate what happens during many of
these complicated events, such as “crossovers”
Perspective: These lectures are based on the
relationship between genes; within a cell,
within an individual, and within a population.
When we throw terms out there like
“Mendelian” or “recombinant” or
“nonparental” or “independent (segregation)”,
we are not making statements about all of the
genes, but rather about one gene relative to
another; or one gene relative to a few others…
…If we were to pick 1 gene at random, it
might have 2 alleles that act in a manner of
simple dominance; if we were to then pick
another gene at random, it might be on a
separate chromosome, which means it
segregates independently of Gene 1; and if the
trait associated with Gene 2 also exhibited
simple dominance, then we would have
picked two genes that fall under the
description of Mendelian genetics…
…If instead, our Gene 2 was located on the
same chromosome, we would now refer to the
relationship between the 2 genes as “linked”,
which is non-Mendelian. If we couldn’t
actually “see” the 2 genes to know they were
linked, we could find out by conducting
several matings and counting the number of
offspring with each possible phenotype.