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CH 5 Genetics
| Question | Answer |
|---|---|
| What is genetic linkage? | the observation that certain alleles do not assort independently during meiosis and tend to travel together for vertical generations |
| where are linked genes found? | on the same chromosome, often together |
| what is recombination? | alleles on the same chromosome but appear to assort independently if they are far enough apart from one another |
| What is crossing over? | the exchange of DNA between paired homologous chromosomes that occurs during meiosis |
| What does it mean that 2 genes are linked? | they are located close together on the same chromosome and tend to be inherited together during meiosis. |
| when genes are linked, how do they violate Mendel's law of independent assortment | Because of their proximity, crossing over between them is less likely, so they do not assort independently |
| syntenic | genes that are located on the same chromosome and have alleles that determine different things |
| What is an example of syntenic? | the alleles for a fruit fly's eye color and body are both found on X-chromosome. |
| How do linked genes affect the production of different kinds of gametes? | Linked genes reduce genetic variety in the gametes because fewer recombinant types are formed compared to unlinked genes, where all combinations are equally likely |
| What is the effect on recombination if genes are closer together? | less crossing over & fewer recombinants |
| What is the effect on recombination if genes are farther apart? | more crossing over & more recombinants |
| Recombination frequency is roughly proportional to | distance between genes (basis of genetic maps) |
| How is the 9:3:3:1 Mendelian ratio for two genes affected if both genes are linked? | linked genes travel together = more parental type offspring, fewer recombinants so the ratio deviates from 9:3:3:1. |
| How is the 9:3:3:1 Mendelian ratio for two genes affected if genes are unlinked? | unlinked genes independent assortment = 9:3:3:1 phenotypic ratio |
| What is a testcross? | unknown x homozygous recessive |
| In a testcross, the parental types =? | more frequent offspring (higher #s) |
| in a testcross, the recombinant types = ? | less frequent offspring due to crossing over (smaller #s) |
| What is recombination frequency | measures how often crossing over occurs between two linked genes during meiosis. It reflects how far apart two genes are on a chromosome |
| How is RF measured? | RF= (#recombinants/total offspring)x100 --> gives map distance in centimorgans |
| In RF, the greater the distance, the more | likely a crossover will occur between them |
| 1% recombination = ? | 1 map unit (or 1 centimorgan, cM) |
| What is a map unit? a CentiMorgan? | A map unit (m.u.) also called a centimorgan (cM) is a unit of genetic distance on a chromosome |
| Why do recombination frequencies never exceed 50%. ? | RF ≤ 50% because independent assortment produces a 50:50 ratio of parental and recombinant gametes. Genes with RF < 50% are linked; genes with RF = 50% are unlinked or on different chromosomes |
| Crossing over beyond 50% makes the genes appear to | assort independently (just like unlinked genes) |
| Single crossover? | one exchange → two recombinant, two parental. |
| Double crossover? | two exchanges → can restore parental arrangement |
| frequency of crossovers? | single > double crossovers |
| What is the purpose of crossovers? | used in genetic mapping to determine gene distances and order |
| How are genes mapped using recombination frequency? | Genes are mapped by measuring how often crossing over occurs between them — each 1% recombination equals 1 map unit, and testcross data reveal distances and order of genes along a chromosome |
| What is a two point cross? | a genetic cross used to map the distance between two genes by determining the frequency of recombination between them |
| How is the two point cross used to map gene distances? | The RF is calculated by # of offspring recombinant phenotypes/by the total # of offspring, and this percentage is equivalent to the distance in cM between the genes |
| What are the limitations of a two point cross? | this method is less accurate for genes that are far apart because it underestimates the distance due to a higher chance of double crossovers occurring between them, which are counted as a single crossover event |
| What is a three point cross? | a genetic experiment that tests for linkage and determines the order and distance of three genes on a chromosome by crossing a heterozygous individual with a homozygous recessive one |
| WHy is the three-point cross more accurate? | because including a third gene in the middle allows for the detection of double crossovers (an underestimation of the distance between the outer genes) |
| How do you use a threepoint cross to map genes? | 1.testcross, 2. grouped by phenotype. 3.most frequent (parental) vs least frequent (double crossover) phenotypes = the gene in the middle. 4.calc distances between each pair of genes based on the number of single and double crossovers |
| Genetic maps differ from reality because: | They are estimates based on recombination, so unequal crossover rates make them differ from physical maps. |
| Recombination hotspots | high recombination zones that make nearby genes appear farther apart than they really are |
| Chromosomal interference | one crossover can block another close by |
| linkage group | a set of genes located on the same chromosome form a linkage group and are inherited together. |
| Chi Square test | Tests whether your data fits Mendelian expectations, it is a test comparing observed data vs expected |
| Degree of freedom | df=# of phenotype classes -1 |
| Null Hypothesis | The null says your data matches the predicted ratio |
| p value is | the probability that the observed deviation happened by chance if the null hypothesis is true |
| If p< 0.05 then | it's unlikely due to chance and reject the null because the results are statistically significant |
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