Module 6 Mendelian Genetics

3.6.9 page 3

Reflect and Connect

By analyzing linked-gene crosses for recombinant phenotypes, and by calculating recombinant frequencies, you can create accurate gene maps for traits. This allows for a much more clear understanding of how or why certain traits tend to move together while others may not, even if found on the same chromosome.

 

Discuss

The Human Genome Project was one of the largest genetic projects ever completed. It involved the participation of public universities and private companies from around the world, and took over 13 years to complete. Now that it is complete, scientists have a relative map of all human genes found on all chromosomes.

 

However, the project was not without controversy. In order to encourage private companies to take part, these companies were allowed to patent certain tests that can locate specific genes in people; genes that may predict if a person is going to develop cancer or not. As a result, if a person is at risk for developing cancer due to family histories, hospitals need to pay these companies in order to be granted the right to conduct these tests. In addition, tests for the presence of certain genes may also provide information about other genes that were not originally considered. This kind of information may be very dangerous if the information became available to insurance companies, or to a person’s place of employment.

 

Diploma Connection

Answer the following questions from a previous Biology 30 Diploma Exam.

 

 

1. During meiosis, which of the following pairs of genes has the greatest chance of being separated by crossing over?
   
   
   1. (m) and (d)
   2. (ne) and (p)
   3. (m) and (lc)
   4. (p) and (o)

Use the following additional information to answer the next question.

Genes	Cross-Over Frequency
normal leaf (M) and tall plant (D)	12%
normal leaf (M) and normal tomato (O)	33%
normal leaf (M) and simple inflorescence (S)	64%
tall plant (D) and normal tomato (O)	21%
tall plant (D) and simple inflorescence (S)	52%

2. The cross-over frequency between genes O and S is
   
   
   1. 6%
   2. 29%
   3. 31%
   4. 97%

Use the following information to answer the next question.

Cross-over Frequencies of Some Genes on Human Chromosome 6
Genes	Approximate Cross-over Frequencies
Diabetes mellitus (1) and ovarian cancer (2) Diabetes	21%
mellitus (1) and Rhesus blood group (3) Ragweed	12%
sensitivity (4) and ovarian cancer (2) Ragweed	10.5%
blood group (3) and ovarian cancer (2) Ragweed	9%
sensitivity (4) and ovarian cancer (2)	19.5%

 

Numerical Response

3. On human chromosome 6, the order of the genes numbered above is ___, ___, ___ and ___.
4. What is the approximate cross-over frequency between the diabetes mellitus gene and the ragweed sensitivity gene?
   
   
   1. 1.5%
   2. 10.5%
   3. 15.0%
   4. 22.5%

Use the following information to answer the next question.

Gregor Mendel examined the inheritance of two traits in pea plants: seed coat texture and colour. Seed coat texture can be represented as S–smooth and s–wrinkled, and seed coat colour can be represented as Y–yellow and y–green. SSYY plants were crossed with ssyy plants to yield F1 pea seeds that were all smooth and all yellow. By crossing plants grown from these F1 seeds, Mendel obtained four different phenotypes of F2 seeds:

• smooth, green seeds

• wrinkled, green seeds

• smooth, yellow seeds

• wrinkled, yellow seeds

5. If the traits for seed coat texture and seed coat colour had been located close together on the same chromosome, Mendel might not have conceptualized
   
   
   1. gene pairs
   2. dominance
   3. the Law of Segregation
   4. the Law of Independent Assortment

Going Beyond

For a challenge, go back to the Arizona biology project website and work through the second set of sex-linked inheritance problems. In this set, each question builds on the next, and the last question requires an understanding of linked genes.

 

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. If they are not, it is a 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 stud finders could find wooden studs in walls by detecting the nails in them.