Ch. 14 Reminder: Unlinked Genes & Independent Assortment. 1. Cross: F1 dihybrid test cross: DO the Punnett Square

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1 Ch. 14 Reminder: Unlinked Genes & Independent Assortment 1. Cross: F1 dihybrid test cross: DO the Punnett Square b + b vg + vg (gray body, normal wings) with bb vgvg (black body vestigial wings) 2. Results Independent Assortment occurred 1

2 15.3 Linked Genes A. How it Affects Inheritance 1. Genes located near each other on the same chromosome 2. Tend to be inherited together in genetic crosses 3. Do not undergo independent assortment 4. Only options for offspring are those of the parents a. example: b + (grey) is dominant to b(black) vg + (normal wings) is dominant to vg(vestigial wings) b. Cross b + b vg + vg with bb vgvg phenotypes of the parents c. If the genes are completely linked, offspring will have phenotypes of both parents i. 1:1 ratio 2

3 B. Morgan's Experiments with Linked Genes 1. Wanted to know whether the genes for body color & wing size were genetically linked a. and if so, how this affected their inheritance. b. Did the same experiment as on previous page c. Received different results than what was predicted Ind.Assort.would occur No Ind. Assort. Genes are inherited together Unlinked genes Linked genes Incompletely linked genes parental types recombinants Recombination of linked gene due to crossing over 2. Conclusions: a. Most offspring had a parental (P generation) phenotype = i. genes for body color & wing size are genetically linked on the same chromosome. b. However, the fact that there were a small number of recombinants = i. some mechanism (crossing over) occasionally breaks the linkage between specific alleles of genes on the same chromosome = incompletely linked genes 3

4 C. Chromosomal basis for recombination of linked genes 1. Recreating the testcross we just saw, but tracking chromosomes as well as genes this time crossing over between b & vg loci produces new allele combinations in some but not all eggs no new allele combinations produced separation of chromatids produces recombinant gametes with the new allele combinations = # of map units apart the genes are 4

5 D. Constructing a Chromosome Linkage Map 1. Linkage map shows the relative locations of gene along a chromosome a. based on the probability of crossing over between 2 genes is proportional to the distance separating the genes b. % crossing over/recombination frequency = # of map units 5

6 2. Linkage Groups on Chromosome 2 a. # of map units between each gene locus b. more than 1 gene can determine a trait (polygenic trait) 6

7 E. Calculating Recombination of Linked Genes 1. Example: What is the crossing over frequency between gray body & normal wings? = 27 map units = 27% rate of crossing over 2. Tells us that: 27% of gametes would have this combination of phenotypes, known as recombinant gametes. 3. The farther apart 2 genes are, the higher the probability that a crossover will occur between them & therefore the higher the recombination frequency 4. Note: If the frequency of recombination is 50% then the two genes are likely to be located on different chromosomes. 7

8 F. Creating Chromosome Maps: How it's Done 1. Need to determine the total % of recombinant phenotypes(phenotypes in offspring that are different from the parents) produced during a cross. 2. This % of recombinants will give us the map units in between genes. 3. Example: Heterozygous grey bodied & red eyed female x mutant black bodied & purple eyed male. GgRr x ggrr a. What are the predicted offspring genotypes & phenotypes if Independent Assortment occurs? 8

9 4. What if though, body color and eye color are on the same chromosome? a. Linked genes (on the same chromosome), alleles will NOT separate = no recombinant phenotpyes (offspring can only be the same as parents) b. Female produces gametes: GR & gr (no independent assortment so NO foiling) c. Male only produces: gr d. Offspring either: GgRr or ggrr. e. Produce a 1:1 ratio (50% grey body/red eyes & 50% black body/ purple eyes) 9

10 5. Actual results: Black/purple = 470 Grey/red = 470 Black/red = 30 Gray/purple = Conclusions: a. Crossing over lead to these recombinant phenotypes b. Use these # to determine the frequency of crossing over = map units on chromosome map Total % recombinants = 6% = 6 map units in between black body & purple eyes c. These genes are incompletely linked - genes do NOT get passed on together due to crossing over 10

11 Complete Linkage Incomplete Linkage due to Crossing Over 11

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