1. Module 6

1.43. Page 4

Lesson 9

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

Lesson Summary


During this lesson you were to examine the following focusing questions:

  • How does crossing-over in chromosomes relate to finding the locations of genes on chromosomes?
  • What is the importance of knowing where genes are located on a chromosome?

Genes on the same chromosome tend to be inherited together. When they are not, it can be the result of a crossover event during meiosis. The farther apart the genes are on a chromosome, the more likely a crossover event will occur, and the higher the recombinant frequencies are in the offspring.

 

By understanding where genes are located on chromosomes, geneticists can better predict the inheritance of those genes. Scientists can also design tests that detect the presence of those genes, or genes known to be nearby, just like the old-fashioned stud finders could find wooden studs in walls by detecting the nails in them.

 

Lesson Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

crossover percentage: same as recombination frequency

 

linked genes: genes that exist on the same chromosome (e.g., if the acne gene and belly fat gene are on the same chromosome, then they are linked; with A and B linked together and a and b linked together, or vice versa)

 

map distance: how far apart (in centimorgans) two genes are on a chromosome; the recombination frequency from a linkage cross is used as the map distance between two genes (e.g., 44% of the offspring of the linkage cross were recombinants, therefore the two genes are 44 map units or 44 cM apart)

 

map unit: the unit used is the centimorgan (cM); because the recombination frequency is a percentage, the largest map distance is 100 cM

 

non-parental type: in a linkage cross, any offspring of the above cross that don’t look like the parents (Aabb or aaBb) are non-parental types, are the results of crossing over, and are recombinants

 

parental type: in a linkage cross, if the offspring look like either parent, then they are parental types (e.g., if we assume A and B are linked together and a and b are linked together in the linkage cross AaBb X aabb, any offspring that are AaBb or aabb are parental types)

 

recombination: during crossing over in prophase I of meiosis, genes from non-sister chromatids trade places, making new allele combinations in the gametes (e.g., if A is linked to B and a to b, then any gametes that are Ab or aB are the results of recombination or crossingover)

 

recombination frequency: the number of recombinant offspring divided by the total number of offspring X 100; expressed as a percentage; used as a map distance between the two genes (e.g., the recombination frequency was 32%, therefore, the two genes are 32 map units apart on the chromosome)