Linked Genes
1) Linked Genes – genes whose loci are located on the same chromosome, therefore the two
traits tend to be inherited together
2) When following linked genes in breeding experiments, the results deviate from those
expected from Mendel’s law of independent assortment ( not 9:3:3:1). Instead, the results
are skewed to most of the offspring resembling one of the parents.
3) The reason that genes are linked is because of the distance between them on the
chromosome
a. The closer 2 alleles are located to each other no the same chromosome, the more
likely they will be inherited together; or, the less likely they will be split up by
crossing-over
4) Morgan’s Fruit Fly Experiment
a. Characteristics being followed:
i. Body Color
1. Wild type: Grey body (b+) - dominant
2. Mutant type: Black body (b) - recessive
ii. Wing size
1. Wild type: Normal-sized wing (vg+) - dominant
2. Mutant type: Vestigial wing (vg) - Recessive
iii. Neither gene is located on a sex chromosome (a.k.a. not sex-linked)
b. Morgan had double wild-type flies (b+b+ vg+vg+) and double mutant type
flies(bbvgvg).
c. P generation: b+ b+ vg+ vg+ x bb vgvg
i. Grey body, normal wing x black body, vestigial wing
ii. All of the offspring are hybrid grey body, vestigial wing (b+b vg+vg)
d. F1 generation: b+b vg+vg x bb vgvg
i. Hybrid offspring x double mutant
ii. Expected phenotypic ratio = 1:1:1:1
iii. However, when the genes are located on the same chromosome and
parental alleles are always inherited together, the ratio is skewed and
becomes = 1:1:0:0. Majority of offspring look like either parent.
1. 1 – wild type
2. 1 – double mutant
b+
b
3. 0 – gray-vestigial
vg+
vg
4. 0 – black- normal
iv. Actual Results:
1. 965 – Wild type
2. 944 – double mutant
Wild type
Mutant
3. 206 – gray-vestigial
chromosome
Chromosome
4. 185 – black – normal
5) Genetic Recombination
a. Production of offspring with combinations of traits that differ from these found in
either parent (Ex. Gray-vesigial or black – normal)
b. Morgan suggested that some process was responsible for the occasional break in
the physical connection between the genes
c. Crossing over – accounts for the recombination of linked genes
i. Set of proteins orchestrates an exchange of corresponding segments of one
maternal and one paternal chromatid
ii. May bring alleles together in new combinations
6) Recombination Frequency
a. Percentage of recombinant offspring (offspring with different phenotypes from
the parents) RF = # recombinant offspring/total # offspring
b. Related to the distance between linked genes
i. The greater the distance between genes, the greater the frequency/chance
of recombination(crossing-over) occurring.
7) Genetic maps
a. Ordered list of genetic loci along a particular chromosome
b. Assumptions:
i. 1. crossing over is random,
ii. 2. The chance of crossing over is approximately equal at all points along
the chromosome
c. The further apart two genes are, the higher the probability that a crossover will
occur between them and therefore the higher the recombination frequency.
d. Map based on recombination frequencies are called a linkage map
8) Linkage map for body-color, wing-size, and cinnabar eye color
a. Cinnabar (cn) = affects eye color, mutant phenotype
b. Recombination frequencies
i. Cn – b = 9%
ii. Cn – vg = 9.5%
iii. B – vg = 17%
c. What does this mean?
i. These percentages mean that DNA between these genes is broken by
crossing over X% of the time.
ii. Crossovers between cn and b or cn and vg occur half as often as
crossovers between b and vg.
d. The distances between genes calculated in this fashion are called map units.
i. 1 map unit = 1% recombination
e. Genes on the same chromosome further than 50 map units apart do not appear to
be linked because crossing over is almost certain.