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.