Download Genetics Notes #3

Genetics Notes #3
Other Patterns of Inheritance
The traits Mendel studied showed little variation
Each only had two alleles where one trait was clearly
dominant and one was clearly recessive
Many traits are more complicated than this
Pleiotropic Genes
Pleiotropic genes affect many different characteristics
Example: sickle cell anemia
- Normal hemoglobin is produced by allele HbA
- Sickle cell has two mutated copies of the allele - HbS
- The mutation leads to abnormally shaped hemoglobin
that interlock leading to a blood cell with a sickle shape
that have a hard time passing through capillaries
Multiple Alleles
When traits are determined by more than two alleles, the
most commonly seen trait is called wild type (instead of
dominant)
The allele that determines wild-type is called the wild-type
allele
Non-wild type traits are said to be mutant
Multiple Alleles
Members of Drosophilia melanogaster (fruit fly) have more
than two alleles for eye colour
Because there are multiple alleles, there is a dominance
hierarchy.
Red eyes are considered to be the wild-type allele colour
Multiple Alleles
Practice
What will be the phenotypic ratio of the offspring from the
mating of the following Drosophilia individuals:
E1E4 (wild-type eye colour) x E2E3 (apricot eye colour)
Practice
Create a Punnett square of
the cross. Use the hierarchy
table in your notes to
determine the phenotype
ratio
2 red: 1 apricot: 1 honey
Practice
What will be the phenotypic ratio of the offspring from the
mating of the following Drosophilia individuals:
E1E2 (wild-type eye colour) x E2E4 (apricot eye colour)
Practice
E1
E2
E2
E1E2
E2E2
E4
E1E4
E2E4
E1E2: Red
E1E4: Red
E2E2: Apricot
E2E4: Apricot
Phenotypic Ratio: 2 Red : 2 Apricot
Incomplete Dominance
When two alleles are equally dominant, they interact to
produce a new phenotype
When an individual is heterozygous for two alleles that
show incomplete dominance, both alleles are expressed
equally but at lower levels
Incomplete Dominance
The phenotypic ratio
for this cross would
be
1 red:2 pink:1 white
Codominance
This is where two alleles are fully expressed in a
heterozygous individual but not in the same cells
Coat colour in cattle show a good example of this
Red coats have all red hairs and white coats have all white
hairs but heterozygous individuals have both red and white
hairs
Environment?
The environment can have a profound effect on phenotype
Look at humans:
Caucasian individuals (white skin) tend to be more pale in
areas with less sun exposure (like Canada).
Case Study
Look at the Case Study on page 611
Read through and complete questions 1-3