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 dataF1
FEMALE MALE F2 FEMALE MALE RED/WHITE
red eye w + 18 0 78 69
white eye w 0 29 62 73
F1 F2
PURPLE/
VESTIGIAL
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 phenotypesProblem4 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 geneResult: 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 charts1. Establish relationships based on information2. Fill it appropriate circles or squares to represent phenotypes3. 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)
The formula for Chi-squared is:
Probability (p)
Degrees of Freedom (df)
1 2 3 4 5
.05 3.84 5.99 7.82 9.49 11.1
2 = (observed – expected)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 numberAneuploidy is caused by nondisjunction of 1 chromosome
When cell has 2n + 1 chromosomes is called
When cell has 2n - 1 chromosomes is called
Trisomic Monosomic
Polyploidy is caused by nondisjunction of ALL chromosomesTriploidy vs. tetraploidy
Alterations of chromosome structure
Some genetic disorders
Down syndrome Turner syndrome chronic myelogenous
leukemia Klinefelter syndrome cri du chat syndrome Mitochondrial
myopathy
cystic fibrosis, Tay-Sachs, sickle-cell anemia phenylketonuria Achondroplasia Huntington’s
disease.