LECTURE
CONNECTIONS
7 | Linkage, Recombination, and Eukaryotic
Gene Mapping
© 2009 W. H. Freeman and Company
7.1 Linked Genes Do Not Assort Independently
•
Mendel’s work:
- Segregation: each individual diploid organism possess
two alleles at a locus that separate in meiosis.
- Independent: the alleles at a locus act independently of at
other loci.
•
Genes located close together on the same
chromosome are called linked gens and belong to the
same linkage group.
•
Linked genes travel together during meiosis and are
not expected to assort independently.
7.2 Linked Genes Segregate Together and Crossing
Over Produces Recombination between Them
Occasionally, genes switch from one homologous
chromosome to the other through the process of
crossing-over.
7.2 Linked Genes Segregate Together and Crossing
Over Produces Recombination between Them
• Notation for Crosses with Linkage
• Complete Linkage Leads to Nonrecombinant Gametes
and Nonrecombinant Progeny
• Crossing Over with Linked Genes Lead to Recombiant
Gametes and Recombinant Progeny
Non-recombinant gametes:
gametes that contain only
original combinations of
alleles present in the parents.
Recombinant gametes:
gametes with new
combinations of alleles.
No crossing-over = Non-recombinant gametes
Single Crossover = ½ of gametes are recombinants +
½ of gametes are non-recombinants
Concept Check 1
For single crossovers, the frequency of recombinant
gametes is half the frequency of crossing over
because:
a. a test cross between a homozygote and heterozygote
produces ½ heterozygous and ½ homozygous progeny.
b. the frequency of recombination is always 50%.
c. each crossover takes place between only two of the four
chromatids of a homologous pair.
d. crossovers occur in about 50% of meiosis.
Application about
linkage and
recombination
Calculating Recombination Frequency
• Recombination frequency =
Number of recombinant progeny
Total number of progeny
X 100%
Coupling and Repulsion Configuration of
Linked Genes
• Coupling (cis configuration): Wild type alleles are
found on one chromosome; mutant alleles are found on
the other chromosome.
p+
p
b+
b
• Repulsion (trans configuration): Wild-type allele and
mutant allele are found on the same chromosome.
p+
p
b
b+
Predicting the Outcome of crosses with
Linked genes
Determining the proportions of the types of offspring
requires an additional piece of information: the
recombination frequency.
Example:
Cucumbers smooth fruit (t) is recessive to warty fruit (T)
and glossy fruit (d) is recessive to dull fruit (D)
Page 170: Suppose we cross a plant HETEROZYGOUS
for warty and dull fruit with a plant homozygous for
smooth and glossy fruit…