Download Dihybrid Crosses and Linked Genes

Dihybrid Crosses and
Linked Genes
Chapter 10
Pages 312 - 316
Grasshopper Phenotype
•
Found on chromosome 14
•
Autosomal recessive
•
Alleles:
 G – normal gait
 g – grasshopper gait
•
To have this trait the mouse would have to be homozygous
recessive (gg) for the allele.
Prune Phenotype
•
Found on chromosome 19
•
Autosomal recessive
•
Alleles
 P – normal skin
 p – prune skin
•
To have this trait the mouse would have to be homozygous
recessive (pp) for the allele.
Dihybrid test cross
Take a mouse that is heterozygous for both traits PpGg and cross it with a
mouse that is homozygous recessive ppgg for both gene loci.
PG
Pg
pG
pg
PpGg
pg PpGg Ppgg ppGg ppgg
ppgg
What’s the phenotypic ratio?
Same as the parent
combinations so it is
referred to as
parental phenotypes
Different to the parent
combinations so are
referred to as
recombinant
phenotypes
Lets say that a mouse breeder actually performed the test cross we just did and got the
following results.
Normal
Normal skin,
skin, normal grasshopper
gait
gait
47
Prune Skin,
normal gait
Prune skin,
grasshopper
gait
3
44
6
1) What do the results indicate about the loci of the gene for skin type and the gene for
gait?
The genes are linked
2) What was the genotype of the heterozygous normal skin, normal gait mouse?
PG/pg
Why does that happen?
•
We have already looked at inheritance of unlinked genes which are usually located on
non-homologous chromosomes. Their alleles sort independently into gametes.
•
Linked genes are located close together on a chromosome.
•
The combinations of their alleles on homologous chromosomes tend to stay together,
but they can, on occasions, be separated by crossing over during meiosis.
•
Genes that are closer together on a chromosome are less likely to be separated during
crossing over.
•
Genes that are further apart on a chromosome are more likely to be separated during
crossing over.
Yes, crossing over
No crossing over
Detecting linkage.
•
If two gene loci are linked, the outcome of the test cross can
reveal that linkage.
•
There will be four classes of offspring but the proportions of these
will not be equal. Instead, there will be an excess of offspring
from parental gametes and a deficiency of offspring from
recombinant gametes.
•
This is all because the location of the gene loci are close together.
Normal
Normal skin,
skin, normal grasshopper
gait
gait
47
Prune Skin,
normal gait
Prune skin,
grasshopper
gait
3
44
6
You can also estimate the distance of the linked gene loci from one another. This estimate is
based on the percentage of recombinant offspring.
In this case:
No. of recombinant phenotype offspring = 6 + 3 = 9
Total no. of offspring = 100
9/100 = 9%
This 9% of recombinant offspring corresponds to the number of map units separating the two genes.
So in this case the loci of the two genes are about 9 map units apart (which is roughly 9 million base
pairs apart)
Predicting outcomes of crosses for
linked genes
When a test cross is carried out with two genes that are
known to be linked and are separated by a known number
of map units (but fewer than 40), the outcome of the cross
can be predicted.
e.g. If two linked genes are separated by 8 map units, the
a test cross involving these genes would produce about 8%
of the recombinant type offspring and about 92% of the
parental type offspring.
Identify whether each of the
following statements is true or false.
•
An organism that is heterozygous at two gene loci will produce two kinds
of gametes.
•
An offspring from a dihybrid cross of two heterozygotes has a 1 in 16
chance of showing both recessive traits.
•
The expected outcome from a dihybrid test cross is four phenotypes in
the ratio of 1 : 1 : 1 : 1
What is the difference between a parental and a recombinant gamete?