Genetics
Genetics of Drosophila
Why study
Drosophila?
Genetic terminology
Dominant vs. recessive
Homozygous vs.
heterozygous
Wildtype vs. mutant
Test cross
Monohybrid vs.
dihybrid cross
Chi-squared analysis
Fly lab data
F1
RED/WHITE FEMALE MALE F2 FEMALE MALE
red eye w+ 18 0 78 69
white eye w 0 29 62 73
PURPLE/
VESTIGIAL F1 F2
purple/ normal p/v+ 154
red/ normal p+/v+ 98 1339
purple/vestigial p/v 100 1195
red/vestigial p+/v 151
Incomplete dominance
Incomplete dominance:
-1 allele is not completely dominant over the
other
-Result? A heterozygous genotype produces a
phenotype in-between the dominant and
recessive phenotypes
Problem
4 o’ clock flowers can either be red (RR), white
(WW), or pink (RW). A red flowered parent is
bred with a pink flowered parent. What will be
the probable genotype and phenotype
frequencies of the offspring? Use a punnett
square to show this.
Multiple alleles and Codominance
Codominance
-2 alleles contribute to the phenotype
-Result: Having 2 dominant alleles will result in a phenotype expressing both
alleles (not blending)
Multiple alleles
-More than 2 alleles for a gene
Result: More combinations of
genotypes and phenotypes
(blood types)
Epistasis
(“stopping”
Gene at one
location ALTERS
the phenotypic
expression of a
gene at a second
location
Often by masking
Pigmentation
Polygenic inheritance
A trait controlled
by more than 1
gene
Result: A
distribution of
phenotypes
(height, skin color,
MANY other traits)
Nature and nuture revisited
Norm of reaction
Multifactorial
Phenotype depends on the level of
organization – polypeptide, protein,
observable appearance, behavior
What EXACTLY is a gene?
A DNA segment has information for
making the protein hemoglobin, which
carries oxygen in your red blood cells
One allele will give information for
producing normal hemoglobin
-Another allele (ONLY 1 base different)
produces hemoglobin with 1 different
amino acid
This difference makes the hemoglobin
less soluble
When Oxygen levels are low, the
hemoglobin molecules start sticking
together, resulting in the red blood cell’s
“sickle-shape”
Having both defective alleles will lead to
multiple effects shown to the right
What EXACTLY is a gene?
Having only 1 defective allele (heterozygous)
will not be fatal and actually beneficial!
Malaria is a disease spread by mosquitois that
infects red blood cells
Being heterozygous results in your body
destroying the red blood cells as well as the
Malaria, leaving enough of the normal blood
cells.
Pedigree charts
1. Establish relationships based on information
2. Fill it appropriate circles or squares to represent phenotypes
3. With this information decide if the disorder (shaded shapes) were inherited through a
dominant or a recessive allele (best hint: if 2 shaded individuals produce an unshaded
individual, it must be a carried by a dominant allele)
4. Infer the genotypes of each individual
Linked genes
Genes located on the same
chromosome that tend to be inherited
together
Linked genes vs. Mendel’s independent
assortment
Recombinant offspring
Linkage mapping
Smaller the recombination frequency the
CLOSER together 2 genes are
If frequency = 50%, genes are considered NOT
linked. They DO assort independently. Why?
Can the frequency be greater than 50%?
Linkage mapping
Lab 7: Genetics (Fly Lab)
Probability Degrees of Freedom (df)
(p) 1 2 3 4 5
.05 3.84 5.99 7.82 9.49 11.1
The formula for Chi-squared is:
(observed – expected)2
2 = expected
Chromosomal basis
of sex
SRY development of
testes
SRY development of
ovaries
SRY regulates other
genes involved in
anatomical,
physiological,
biochemical
development of sex
Sex linked genes
Genetic disorders
Sex linked vs. sex influenced genes
X inactivation and Barr Bodies
You might notice that normal females have 2 X chromosomes
while males have 1. Does this mean females have twice as
many alleles on the X chromosome?
No: To compensate, 1 random X chromosomes becomes
inactive early in development and turns in a bar body
This is what causes the variety in fur color in calico cats
Nondisjunction
Abnormal chromosome number
Aneuploidy is caused by nondisjunction of 1 chromosome
When cell has 2n + 1 When cell has 2n - 1
chromosomes is called chromosomes is called
Trisomic Monosomic
Polyploidy is caused
by nondisjunction of
ALL chromosomes
Triploidy vs. tetraploidy
Alterations of chromosome structure
Some genetic disorders
Down syndrome cystic fibrosis,
Turner syndrome Tay-Sachs,
chronic myelogenous sickle-cell anemia
leukemia phenylketonuria
Klinefelter syndrome Achondroplasia
cri du chat syndrome Huntington’s
Mitochondrial disease.
myopathy