AP Bio: Thursday 3/17/11Genetic Patterns of Inheritance • Happy St. Patrick’s Day / Evacuation Day! • Homework: PS 16 #1-7 (be ready to discuss these on Monday) • Do Now: Get some breakfast, get out your notebooks, get ready to learn. • Today’s Goals: • Explain how Mendel’s Laws of Inheritance are based on the events of meiosis • Solve genetics problems involving one gene, two genes, multiple alleles, incomplete dominance, sex-linked traits, and epistasis • Agenda: Lecture mixed w/ practice problems
Genetic Patterns of Inheritance AP Biology Chapters 14-15
Karyotypes Autosome = Chromosomes 1-22 Sex Chromosome = X or Y Trisomy 21 (Down Syndrome)
A A Mendel’s 1st Law:Law of Segregation • Alleles “segregate” into different gametes during meiosis A A a a a a
Monohybrid Cross A a Phenotypes: Ratios: A a Involves one gene Crossing two heterozygotesyields:
Mendel’s 2nd Law:Law of Independent Assortment • Alleles on one set of chromosomes segregate independently from alleles on other sets of chromosomes. • Results in different allele combinations in different gametes, and in different combinations of traits
Dihybrid Cross Ry rY ry RY RY Ry rY ry • Involves two genes on different chromosomes • First step is always to arrange the gametes • Crossing two heterozygotesyields:
Beyond the Dominant/Recessive Paradigm… • Codominance • Both alleles fully expressed • 1:2:1 phenotypic ratio • Ex: red, white, red/white spots • Incomplete Dominance • Dominant allele partially expressed • 1:2:1 phenotypic ratio • Ex: red, white, pink • Multiple Alleles • More than two versions (alleles) of the gene
ABO Blood Types –Show Multiple Alleles AND Co-dominance Gene I controls ABO Blood Type. Three possible alleles: IA – makes A antigens IB – makes B antigens i – makes no antigens Possible genotypes (sets of two alleles):
Beyond the Dominant/Recessive Paradigm… • Polygenic traits • Controlled by many genes • Often show continuous variation in phenotypes
Beyond the Dominant/Recessive Paradigm… • Epistasis • The effects of one gene hide or alter the effects of another gene • 9:3:4 phenotypic ratio
Sex-Linked Traits • Controlled by genes on sex chromosomes • X-linked traits – on X chromosome • Hemophilia • Red-green colorblindness XR – normal vision Xr – red-green colorblind (recessive) XRXr = carrier female (normal vision) XrY = colorblind male (only needs one recessive allele to be colorblind)
More on X-Linked Traits • Try these sample problems… • Carrier female x normal male • Normal female x colorblind male
Hairy Ears Why do only males have them? Why do only males produce the SRY (testis-determining) protein? They are Y-linked!
Hairy Ears If a man with hairy ears marries a woman with non-hairy ears, what is the chance that • their daughters will have hairy ears? • their sons will have hairy ears?
Pedigree Analysis • Get a worksheet…
Evaluating the Validity of Genetics Experiments… Using Chi-Squared • Statistics! Wootwoot! • Get a worksheet… enough of this powerpoint business.
More Chi-Squared Practice Two Drosophila genes • Body color: gray (G) or black (g) • Wing shape: normal (N) or vestigial (n) • Cross a double heterozygote with a double recessive • GgNn x ggnn
Linked Genes • Chi-Squared Test REJECTS our null hypothesis. • Independent Assortment of the two genes doesn’t seem to apply • Why? • Conclusion: Two genes on same chromosome (linked) • Recombinant genotypes created by crossing over between the two genes • Frequency of recombinant genotypes based on the distance between genes on the chromosome
Finding Distances Between Linked Genes • Calculate the recombination frequency – how frequent are the genotype combinations that AREN’T the same as the parents? • Percentage of recombinants = distance in map units • Practice this with the fruit fly example…
Linkage Maps • Show the relative location of genes on a chromosome • First, determine recombination frequencies • Body color & wing shape = 17% recombinants • Body color & eye color = 7% recombinants • Wing shape & eye color = 23% recombinants • 1 % recombination frequency = 1 map unit (Can’t be over 50%... why not?) • Create a linkage map (aka genetic map)