Download Further Clarification of GENE LINKAGE When you did Gamete

Further Clarification of GENE LINKAGE
When you did Gamete Formation and Dihybrid Cross, all the different genes you considered (represented
by different letters of the alphabet) were assumed to be on different chromosomes. (Example#1)
This is why the inheritance of the Tall trait did not mean that a gamete (or offspring) was any more likely
to inherit the Green or Yellow trait in particular. Inheritance of color was independent of inheritance of
height.
Example #1
Gamete formation following two traits with
genes on different chromosomes
Example #2
Gamete formation following two traits with
genes on the same chromosome
Diploid parent: GgNn
Diploid parent: GgNn
G
N
G
N
g
g
n
n
G
G
g
g
N
N
n
n
MEIOSIS
MEIOSIS
G
G
g
N
n
N
GN
Gn
gN
g
n
gn
There are four possible combinations in the
gametes formed during meiosis. They are all
equally likely to form.
G
G
g
g
N
N
n
n
GN
GN
gn
gn
There are only two possible combinations in the
gametes formed during meiosis. These two
possibilities are equally likely to form.
What if two genes were located on the same chromosome, however? (see Example#2) We call genes
on the same chromosome linked genes, and they are inherited together when sorted into gametes. Only
the allele combinations that exist in the parent body cells will be found in their gametes (if we only take
independent assortment of chromosomes into account, not crossover).
Since we only see these two allele pairings (GN & gn) in the gametes, we can tell these genes are linked!
We can figure out which alleles are paired with each other in the parent if we cross the parent with a
recessive homozygote (like in a testcross).
GgNn x ggnn normally produces what offspring ratios in a dihybrid cross?
Well, if these genes were not linked, we’d expect a 1:1:1:1 ratio of G,N : G,n : g,N : g,n phenotypes.
If these genes are linked however, this cross will produce a 1:1 ratio of G,N : g,n phenotypes (without
taking crossover into account). These phenotypes are what we call Parentals.
Now, in reality, crossover happens. And when it happens between linked gene loci G and N, it can
produce different allele combinations, such as Gn and gN (what we call Recombinants). BUT, the largest
number of offspring will still have the parental phenotypes. Usually, a much smaller percentage of
offspring will be recombinants. The likelihood of producing recombinants is directly related to the
likelihood that crossover will happen between two gene loci. The farther apart two gene loci are on a
chromosome, the greater the chance there is of crossover occurring between them.