GENE LINKAGE AND MAPPING
REVIEWING MENDEL’s PRINCIPLES
Segregation  alleles for a given trait located at the same locus on homologous chromosomes will
separate from each other during gamete formation ie. meiosis (e.g. AaBb make AB, Ab, aB and ab)
Independent Assortment  each pair of homologous chromosomes will separate independently of
other homologous chromosomes
LINKED GENES
- Genes that are located on the same chromosomes tend to be inherited together
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Linked genes can sometimes become unlinked to produce new combinations of genes through
the process of CROSSING OVER
o Chromosomes that undergo crossing over are referred to as recombinants
Genes located farther apart on the same chromosome experience higher rates of crossing over in
meiosis, therefore are less likely to be linked
The law of independent assortment does not always apply with linked genes
o This reduced the # of different possible gametes
o E.g. A-B and a-b, if you inherit A, then you also automatically inherit B
o Crossing over can SOMETIMES still separate the alleles, but this happens less often as the
genes are linked
o As a result, ratios from F1 crosses are not 9:3:3:1 when the genes in a cross are linked
Purple flower colour is
dominant over red flower
colour.
Long pollen is dominant over
round pollen.
The offspring that result from crossing over can tell us a lot of information  WHY?
- Recall: crossing over results in new combinations of gametes  also called “recombinants”
- The smaller the number of recombinant, the closer the genes are on the chromosome
o If the genes are really close together, then crossing over is not going to be able to
separate or segregate those genes very often  results in a low number of recombinants
- If the number of recombinants is higher, we say that those genes are further apart
o Crossing over can separate or segregate the genes often  results in a higher number of
recombinants
TEST CROSSES & PROBLEMS
- A test cross for independently assorting alleles, should exhibit an expected 1:1:1:1 phenotypic
ratio in the progeny  cross of AaBb x aabb  lethal alleles can also skew ratios
o HINT!!  when doing problems and you do not get this expected ratio, then the genes
are either linked or lethal
o E.g. expect 1:1:1:1, but get 5:5:1:1, then the large numbers are most likely due to linked
genes and the smaller number are a result of crossing over (see pg 173)
GENE MAPPING
- Identifies the location of genes on chromosomes and the relative distance between genes based
on recombination frequencies (ie. the occurrence of crossing over)
- If two genes always showed up together, we can assume these genes are linked and very close
together
- REMEMBER: if gene recombination frequencies are high = increased chance of crossing over and
the genes are further apart from each other
USING GENE LINKAGE TO DEVELOP CHROMOSOME MAPS
- RULES
o always start with gene at extreme end
o use bugger # first
o always connect to the end first and then build connections