BIOL 4003, Week 4 Notes
BIOL 4003, Week 4 Notes 4003
U of M
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This 4 page Class Notes was uploaded by Rachel Heuer on Wednesday February 10, 2016. The Class Notes belongs to 4003 at University of Minnesota taught by Robert Brooker in Spring 2016. Since its upload, it has received 18 views. For similar materials see Principles of Genetics in Biology at University of Minnesota.
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Date Created: 02/10/16
Chapter 6 (Linkage and Mapping): Part 1 - Species have hundreds to thousands of genes o Most species have a few dozen or less chromosome o Therefore, each chromosome has many many genes - If genes are close on same chromosome they don’t follow the laws of independent assortment - Each linear chromosome has a long piece of DNA with hundreds of genes - Synteny: Two or more genes are located on the same chromosome and are physically linked - Genetic Linkage: Phenomenon that genes close together on a chromosome tend to be transmitted as a unit, influencing inheritance patterns - Dihybrid should yield 9:3:3:1 ratio (some traits did not yield that) o Bateson and Punnett followed flower color and pollen shape and noticed surprising results (6.1) o Genes linked on chromosome will have a higher ratio of phenotype appearance (purple and long/red and round) o Parental combinations found to be more common than expected/predicted o They suggested that the transmission of genes is somewhat coupled § Not completely (because there was purple rounds and red longs) § Due to chromosome - Linkage can be altered during meiosis o Crossing over (Figure 6.2) § Occurs during prophase I of meiosis I § Non-sister chromatids of homologous chromosomes exchange DNA segments (associated as bivalents) o Non-recombinant cells: When haploid cells contain the same combination of alleles as the original chromosomes § Produces non-recombinant offspring o Recombinant cells: when haploid cells contain a combination of alleles NOT found in the original chromosomes; result of crossing over § Produces recombinant offspring o Thomas Hunt Morgan provided more evidence for the linkage of several X- linked genes (Fig 6.3) § Used fruit flies and studied three traits § Morgan observed a much higher proportion of the combinations of traits found in the parental generation • Said that all three genes are located on the X chromosome • Therefore, they tend to be transmitted together as a unit § Why did F2 still have some recombinant offspring (not completely linked)? • Less common than expected by independent assortment but still there § Why was there a quantitative difference between the various numbers of recombinant offspring (not constant rate of linkage between the 3 genes)? • A lot more recombinance between eye color and wing size than with body color and eye color (meaning that they are farther apart on chromosome à not linked) o Morgan’s 3 hypotheses § Genes for body color, eye color and wing length are all located on the X-chromosome • Alleles on same chromosome tend to be inherited together § Due to crossing over, the homologous X chromosomes (in the female) can exchange pieces of the chromosomes • new combinations of alleles § Likelihood of crossing over depends on distance between the two genes • Closer genes do not cross over as much as far away genes • Double crossover are extremely unlikely Chapter 6: Part 2 - Chi Square analysis: Used to determine if outcome of a dihybrid cross is consistent with linkage or independent assortment o Step 1: propose a hypothesis § Hypothesis is always that genes are sorting independently (can calculate expected values for all genes regardless of whether location is known, even if data appears inconsistent with this hypothesis) o Step 2: Based on the hypothesis, calculate the expected values of each of the four phenotypes § If linked, 2 outcomes more likely § If independent assortment, all four phenotypes are equally likely (1/4) o Step 3: ap2ly the c2i square formula § X = (O-E) /E + … o Step 4: Interpret the calculated chi square value § N=4, degrees of freedom (df)= n-1 = 3 § If we assume that the law of segregation is occurring, there are just two categories, recombinant and non-recombinant, so n=2 and df=1 o If linked, the deviation between observed/expected is ginormous § Therefore, can reject the null hypothesis (independent assortment) and accept the alternative hypothesis (linkage) § Deviation between observed and expected values Is very large; such a large deviation is expected to occur by chance alone less than 1% of the time § Accepted hypothesis doesn’t always mean linkage (could be viability or other stuff) - Genetic Mapping of plants and animals o Purpose is to determine linear order of genes along the same chromosome o Each gene has its own unique locus at a particular site within chromosome o Many uses § Allows us to understand the complexity and genetic organization of a species § Allows molecular geneticists to clone genes § Improve understanding of evolutionary relationships between species § Diagnosing and treating diseases and genetic counseling § Selective breeding for agriculture o Genetic maps allow us to estimate the relative distances between linked genes on chromosomes, based on the likelihood that a crossover will occur between them o Percentage of recombinant offspring is correlated with the distance between the two genes § Far apart = many recombinant offspring § Close = few recombinant offspring o Map distance= # of recombinant offspring / total # of offspring x 100% = map units (mu) or centiMorgans (cM) o One map unit is equivalent to 1% recombination frequency o Genetic mapping experiments are typically accomplished by carrying out a testcross § Mating of a heterozygous for two or more genes and one that is homozygous recessive (for autosomes) § If for sex-linked, the homozygote would be hemizygous (only 1 allele) o Non-recombinant: Alleles not due to crossing over o Crossover occurs in the heterozygote parent (can’t occur in the homozygote because they are the same genes/alleles) - Recombinant offspring is usually present in the lower number than non-recombinant offspring - MU = 12.3 = 12.3% of offspring are recombinant - If genes are over 50 map units apart, they normally appear to independently assort (especially if chromosome is relatively large) o Only genes that are relatively close together will defy Mendel’s law of independent assortment o Genes that are too far apart will not show linkage in a test cross
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