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Ch. 16 Part 2: Genetics, monohybrid vs. Dihybrid crosses, Chi Square • Genes – Section of DNA that codes for a specific protein • Alleles – Dominant: allele that has an affect in a heterozygous organisms – Recessive: allele that does not have an affect in heterozygous organism – Symbols: • • Use letters where capital letter looks different than lowercase letter Genotype – Homozygous (organism has TWO identical alleles for a gene…dominant or recessive) – Heterozygous • Phenotype – Observable characteristics of an organism; determined by genotype • F1 generation – Offspring resulting from cross between organism with a HOMOZYGOUS Dominant parent and a HOMOZYGOUS recessive parent • F2 generation – Offspring resulting from cross between two F1 generation (heterozygous organism) • Co-dominant – Both alleles express their characteristic at the same time • Multiple Alleles – When a gene has more than two versions (alleles) • Ex. Blood groups – Caused by genetic mutation: base substitution, deletion, insertion, etc. • Sex Inheritance – Alleles found on sex chromosomes (pair 23) – Sex-linked gene gene found on X chromosome and not matched on Y chromosome • Use X chromosome as gene symbol with superscript • Mother can be carrier • Male either has allele or does it have allele • Ex. FACTOR VIII – Gene that codes for production of protein needed for blood clotting » Dominant allele produces protein (H) » Recessive allele does NOT produce protein (h) Genetic Diagrams • Standard way of showing the genotypes of offspring that might be expected from two parents – Monohybrid Crosses • Diagrams showing inheritance of ONE gene • Test Cross – Cross organism showing dominant phenotype with organism that is homozygous recessive – Phenotypes of offspring will guide you to determine the phenotype of Dominant parent (homozygous or heterozygous) Dihybrid Crosses Diagrams that show inheritance of TWO genes at once – Independent assortment • Two ways in which 2 pairs of chromosomes can line up along equator during METAPHASE 1 – Many cells going through meiosis chromosomes in half will line up one way and the other half will line up the other way – Predict gametes formed from heterozygous cells appear in equal numbers): » AD » Ad » aD » ad Interaction between LOCI • Same Locus Interactions – Dominant alleles – Dominant & recessive alleles – Multiple Alleles • Different Loci Interactions – Genes at one loci affect inheritance of genes at another loci Autosomal Linkage • When two or more gene loci are on the same chromosome • Do not sort independently • Inherited together (usually) Crossing Over • Prophase 1 of meiosis 1 • A bivalent (pair of homologous chromosomes) joined by CHIASMATA – Chromatids of bivalent can break and reconnect to another NONSISTER CHROMATID • Leads to exchange of gene loci between maternal and paternal chromosome • Cross-over value – Percentages of offspring that belong to recombinant class – Measure of the distance apart of two gene loci on chromosome – Smaller cross-over value = closer gene loci are – Chance of crossing over DIRECTLY related to distance apart • Farther apart = higher chance of crossing over Chi-Squared Test • Statistical test that allows comparison of observed results with expected results to determine if there is a significant difference between the two • Ratios from genetic diagrams represent PROBALITLITIES of phenotypes – Do NOT always come out to that ratio • Expected phenotypic ratio vs. actual phenotypic ratio Stage 1 of Chi-squared Test • Complete dihybrid cross for expected results • Record expected results on table • Observe and record your actual observed results onto table • Calculate difference between each set of results – Squaring gets rid of negative signs (irrelevant) • Divide each square difference by expected value • Add up all answers for x2 value • • • • Finding expected value with ratio: (using 9:3:3:1) Take ratio (9/16) and multiply by total number of organisms actually produced Take ratio (3/16) and multiply by total number of organisms actually produced Take ratio (3/16) and multiply by total number of organisms actually produced Take ratio (1/16) and multiply by total number of organisms actually produced • Stage 2 of Chi-Squared Test Degrees of Freedom of Results – Takes into account the number of comparisons made – Calculation of degrees of freedom: • • Degrees of Freedom = # of classes of data – 1 • For dihybrid cross, we have 4 possible resulting phenotypes (classes of data) • Degrees of freedom = 4-1 = 3 Look at table of X2 probabilities – Probabilities in table are the probabilities that the differences between our respected and observed results are due to chance – Probability of 0.05 is CRITICAL in biological investigations • If X2 value represents probability of 0.05 or larger = differences b/t observed and expected results are due to chance • When x2 value is LESS than critical value for that specific degree of freedom, it is GOOD! – NOT SIGNIFICANT DIFFERENCE • If X2 value represents probability smaller than 0.05 = differences b/t observed and expected results are SIGNIFICANT, reconsider assumptions about cross Probability 0.1 means 1 in 10 chance Critical value tells us how often we expect to see difference between observed and expected result 0.05 means 5% Hypothesis vs. Null Hypothesis • Null hypothesis is always what is expected to be. – Looking at a dihybrid cross, our null hypothesis would be our expected results. – “There is NO significant difference between the observed and expected frequencies” • Hypothesis: The loss of my homework is due to an alien abduction. • Null Hypothesis: The loss of my homework has nothing to do with an alien abduction. • When X2 is LESS than critical value, null hypothesis is accepted (GOOD!) • When X2 is MORE than critical value, null hypothesis is NOT accepted (it is REJECTED) – Something other than chance causing observed results We want Chi-squared value to be LESS than critical value!