Transcript Slide 1
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 red eye w+ white eye w MALE F2 FEMALE 18 0 78 69 0 29 62 73 PURPLE/ VESTIGIAL MALE F1 F2 154 purple/ normal p/v+ 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) Degrees of Freedom (df) Probability (p) 1 2 3 4 5 .05 3.84 5.99 7.82 9.49 11.1 The formula for Chi-squared is: 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 number Aneuploidy is caused by nondisjunction of 1 chromosome When cell has 2n + 1 chromosomes is called Trisomic Polyploidy is caused by nondisjunction of ALL chromosomes Triploidy vs. tetraploidy When cell has 2n - 1 chromosomes is called Monosomic 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.