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Linked Genes & Probability (CHAPTER 2- Brooker Text) Nov 13 & 18, 2008 BIO 184 Dr. Tom Peavy • Scientific basis for chromosome theory of inheritance – 1. Analysis of the transmission of traits from parent to offspring – 2. Inquiry into the material basis of heredity – 3. Microscopic examination of the processes of mitosis, meiosis and fertilization Principles of Chromosome Theory of Inheritance – 1. Chromosomes contain the genetic material – 2. Chromosomes are replicated and passed along from parent to offspring – 3. The nuclei of most eukaryotic cells contain chromosomes that are found in homologous pairs – 4. During the formation of gametes, different types of (nonhomologous) chromosomes segregate independently – 5. Each parent contributes one set of chromosomes to its offspring PROBABILITY • Probability calculations are used in genetic problems to predict the outcome of crosses • To compute probability, we can use three mathematical operations – Product rule – Sum rule – Binomial expansion equation The Chi Square Test • A statistical method used to determine goodness of fit – Goodness of fit refers to how close the observed data are to those predicted from a hypothesis • Note: – The chi square test does not prove that a hypothesis is correct • It evaluates whether or not the data and the hypothesis have a good fit The Chi Square Test • The general formula is c2 = S (O – E)2 E • where O = observed data in each category E = observed data in each category based on the experimenter’s hypothesis S = Sum of the calculations for each category 2-75 Sex-Linked Transmission (CHAPTER 3 & 5- Brooker Text) BIO 184 Dr. Tom Peavy Sex Determination • In many animal species, chromosomes play a role in sex determination • Homogametic vs Heterogametic • The Y chromosome determines maleness • The sex chromosomes are designated Z and W to distinguish them from the X and Y chromosomes of mammals • W chromosome determines femaleness • In some insects, – Males are XO and females are XX • In other insects (fruit fly, for example) – Males are XY and females are XX • The Y chromosome does not determines maleness • Rather, it is the ratio between the X chromosomes and the number of sets of autosomes (X/A) – If X/A = 0.5, the fly becomes a male – If X/A = 1.0, the fly becomes a female • Males are known as the drones – They are haploid – Produced from unfertilized eggs • Females include the worker bees and queen bees – They are diploid – Produced from fertilized eggs Transmission of Genes Located on Human Sex Chromosomes • Genes that are found on one of the two types of sex chromosomes but not on both are termed sex-linked – Indeed, sex-linked and X-linked tend to be used synonymously, but there are Y-linked genes • Males have only one copy of the X chromosome – They are said to be hemizygous for their Xlinked genes Reciprocal crosses • Crosses between different strains in which the sexes are reversed • These crosses reveal whether a trait is carried on a sex chromosome or an autosome – X-linked traits do not behave identically in reciprocal crosses In poultry, sex is determined by the ZW rather than the XY system. A dominant sex-linked gene (B) produces barred feathers, and the Recessive allele (b), when homozygous produces nonbarred feathers. Suppose a nonbarred male is crossed with a barred female. What will be the appearance of the F1 birds? • The X and Y chromosomes also contain short regions of homology at one end – These promote the necessary pairing of the two chromosomes in meiosis I of spermatogenesis • The few genes found in this homologous region follow a pseudoautosomal pattern of inheritance – Their inheritance pattern is the same as that of a gene found on an autosome Contains many X-linked genes Involved in antibody production Y-linked gene Follows a pseudoautosomal pattern of inheritance Necessary for proper male development