SCN3230-ABED_1: Module 6 Lesson 8 - 2

Lesson 8 — Crossing Over Frequencies and Gene Mapping

Recombinant Offspring

Read pages 599 - 601

In the previous lesson on sex linkage, you learned that Thomas Hunt Morgan provided experimental evidence that genes occur on chromosomes. This discovery immediately produced a new idea from Mendel's laws: traits could move together if they are on the same chromosome.

Mendel did not find this with any of his seven traits. For Mendel's work, each trait had no relation to another. Each assorted independently. Now, we know that each of his traits was found on separate chromosomes, and we expect them to move independently.

Many traits have now been found to be coded on the same chromosome. These genes tend to move together and are, therefore, called linked genes. You might think these genes always move together; however, that is not the case. Although they are on the same chromosome, an exchange of pieces of homologous chromosomes during meiosis can occur through a process called crossing over.

The farther apart two genes are on a chromosome, the greater the number of cross over events that will occur between them. That the two traits will move together is also less likely. The closer together two genes are on a chromosome, the lower the number of crossing over occurs between them. This means these two genes are more likely to move together.

The number of crossing overs directly relates to distance on a chromosome. When genes are located side by side on one chromosome, they are said to be completely linked because the chance that cross over will occur between them thereby separating the genes is extremely small.

Read about linked genes, crossing over, and chromosome mapping on pages 599 - 601 of your text.

In this example, note that the two initial, parental chromosomes, located on the far left of the diagram, are carrying genes abc and ABC. The white chromosome is carrying genes abc and the blue chromosome is carrying genes ABC. The white and blue chromosomes are homologous. They have the same genes but different alleles. This is prior to interphase when the chromosomes are replicated to produce sister chromatids.

The original arrangements, abc and ABC, are called the parental genotypes because they show the order of alleles on the chromosome prior to meiosis. The genes a and b are linked on the white homologous chromosome and the genes A and B are linked on the blue homologous chromosome. Linked means that they are physically located close to each other on the chromosome. Therefore, genes a and b and A and B are more likely to move together during cross overs. Gene c and C are located farther away on the chromosome and are more likely to separate during cross overs. When crossing over occurs, two new and different types of arrangements occur: abC and ABc. These new, non-parental genotypes are called the recombinants because they are not the same as the parental genotypes.

Going Further

Linked Gene Notation

If genes A, B and C were on different chromosomes (unlinked) the genotype would be written AaBbCc.

Since these genes are linked (on the same chromosome) this should be indicated in the genotype by separating the chromosomes using a slash symbol (/). The alleles on one chromosome would be written in order, separated by a slash and the alleles of other chromosome would be written in same order. Using this nomenclature, you would write the genotype for the above chromosomes as ABC/abc.

Determining Recombinant Offspring

Direct relationships occur among

the number of recombinants
the cross over rate (recombinant frequency)
the distance apart the genes are on a chromosome

map units apart = recombinant frequency (%)

The number of recombinants relates directly to the cross over rate, which relates directly to the map units apart. One genetic map unit is defined as the distance between genes where one gamete from meiosis out of 100 is a recombinant. In other words, if the frequency of obtaining a recombinant in meiosis is 1% (or 0.01), it is equal to 1 map unit (m.u.).

If 30% of the offspring are recombinants, then the recombinant frequency is 30% and the genes are 30 map units apart. Conversely, a distance of 30 map units means a 30% recombinant frequency, and 30% of the offspring likely are recombinants. It is an extension of the formula you read about in the text and is shown below.

$R e c o m b i n a n t F r e q u e n c y = \frac{N u m b e r o f R e c o m b i n a n t T y p e s}{T o t a l N u m b e r o f O f f s p r i n g} \times 100 %$

Recombinant means a new combination (recombined set) of alleles as a result of crossing over. Recombinant offspring are not carrying the expected set of parental alleles.

Example

A chromosome has the following genes located on it. Their distances apart are provided in map units.

If this chromosome were test-crossed and 400 offspring resulted, how many would be recombinants for genes C and E?

Solution

There would be 40 recombinants.

The map units between C and E are 4 + 6 = 10 m.u.
The relationship is map units apart = recombinant frequency = % of recombinants.
10 map units = 10% chance of crossing over = 10% of offspring will be recombinants.

Thus, 10% of the 400 offspring are recombinants, or 40.

10 % = 0.1 0.1 \times 400 = 40

Watch and Listen

Watch the following lecture video on Linkage and Recombination, Genetic Maps by Professor Eric Lander at Massachusetts Institute of Technology (MIT) and answer the following questions for your own understanding.
1. How is the phenotypic ratio of the cross GgRr × ggrr different for independent assortment than for linked genes (chromosomal theory)?
2. What is meant by "parental types" of chromosomes?
3. What is meant by "non-parental types" of chromosomes?
4. How does recombination frequency relate to map distance?

The following three videos provide the same information in various formats at Youtube.