As we know, one of the hallmarks of the
production of gametes by meiosis is the formation (a process called "synapsis")
of bivalents (paired homologous chromosomes consisting of a "tetrad"
of four chromatids) during prophase I. These bivalents align on the metaphase
plate in metaphase I, and then separate as anaphase I begins. The random nature
of the alignment accounts for the Mendelian "independent assortment"
of genes on different chromosomes.
1. What is meant by the term "crossing-over"?
Figure 4.9 (page 146) shows a photograph
of a bivalent, which in this case shows two sites at which the chromosomes are
attached to each other. Other such photographs would show various numbers and many possible locations of such contact points. What could happen at these contact points? A possibility is that some sort of chromatid
exchange might sometimes occur. This is, in fact, the case, and we call such an exchange "crossing-over".
The details of how crossing-over occurs, in terms of the proteins involved in
the process, and how it happens accurately and only at the right time (during prophase I), are still
being studied.
2. What is meant by the quantitative term "recombination frequency"?
Before we start considering the occurrence and genetic outcomes of crossing-over, let's look again at something we
already know, "Simple Mendelian Assortment for Two Genes on Different Chromosomes".
In the standard Mendelian dihybrid cross involving two genes "alpha" and "beta" that undergo independent assortment (i.e., the two genes are
on different chromosomes), we know that homozygous parents (AABB and aabb) produce heterozygotic offspring (AaBb), and then these F1s
produce gametes that are 1/4 AB, 1/4 Ab, 1/4 aB, and 1/4
ab. Half of these gametes (AB and ab) are the same genotype as those produced by the original homozygous parental (P) organisms, and the other half (Ab and aB) are different. So, we say that the gametes produced by the F1 organisms are 50% "parental-type" and 50% "non-parental-type".
If, however, the two genes alpha and beta are on the same chromosome,
we would expect to get only two types of gametes, AB and ab,
produced by the F1s. These gametes are genetically the same as those produced
by the original parents P; that is, 100% of the F1 gametes are parental-type. Figure 5.3(a) depicts this situation, using "+" (meaning wildtype) in place of A and B allele designations, and using the word "nonrecombinant" to mean the same thing as parental-type.
Now we are ready to start considering the genetic effects of crossing-over. Figure 5.3(b)
shows a breakage and cross-reunion of chromatids at the meiotic prophase I stage, at the site of a chiasma in a
pair of homologous chromosomes. This produces two "recombinant" chromatids,
while leaving the other two chromatids unaltered. Thus, the four haploid gametes
(also called a "tetrad") have genotypes of four types, two of which
are recombinant and two of which are nonrecombinant. Note ( aha ! ) that these are
the same four types that would have resulted from independent assortment of
genes on separate chromosomes.
So, for the case of independent assortment OR for the case of two genes on the same
chromosome but with a crossing-over event occurring between them during meiotic
prophase I, we say we have a "recombination frequency" of 50% ( because half
the gametes are non-parental type and half are parental type).
3. What does a measurement of recombination frequency tell us about the locations of the two genes involved?
Two genes that are close together, or a moderate distance apart, on the same
chromosome (i.e., close enough such that it will not always be that a crossing
over event occurs between them) are said to be "linked". That is, they will
segregate together in meiosis more often than if they were on different chromosomes
("unlinked").
So, for any two genes in any organism, there is a numerical value for the recombination frequency
between them, and this number must be between 0% and 50%. Genes on separate (non-homologous)
chromosomes have a recombination frequency of 50% and are "unlinked". Genes
that are very close together on the same chromosome have a recombination frequency
very close to 0% and are "tightly linked".