Biology Review of Genetic Behavior

Overview:  What Is Genetic Behavior?
Morgan’s discovery of gender-linked traits was further evidence that genes were located on chromosomes.  However, other research indicated that the behavior of genes during meiosis was sometimes more complex than with gender-linked traits.  With the development of more powerful techniques of observation, specific genes could sometimes be located.

What Is Gene Linkage?
Morgan studied other traits in fruit flies besides eye color.  One of his experiments involved crossing flies that had the genotype homozygous for gray bodies (G) and long wings (L), all showing the dominant pattern GGLL,  with flies that were homozygous for black bodies (g) and short wings (l), all showing the recessive pattern ggll.  As was expected, the F1 generation all had gray bodies and long wings.  If body color and wing size were independent of each other, there should have been a mixture of genotypes in the 9:3:3:1 ratio in the F2 generation, similar to the experiments with round melons and striped melons discussed with Mendelian inheritance.  However, three-fourths of the flies in the F2 generation had gray bodies and long wings, and one-fourth had black bodies and short wings, about what would be expected if the two traits were linked.  When genes for a trait occur on the same chromosome, they are linked, and there are as many linkage groups as there are pairs of chromosomes.

What Is Crossing Over?
Some flies had a mixture of dominant and recessive traits, such as gray bodies with short wings or black bodies with long wings.  During meiosis, alleles for body color exchanged places with alleles for wing length so that both dominant traits or both recessive traits were not inherited together.  When the cells completed division, the alleles were in different positions on the chromosomes of each daughter cell.  Later experiments with other organisms showed that crossing over is a common phenomenon that accounts for genetic diversity within species.

What Is Chromosome Mapping?
The phenomenon of crossing over led scientists to believe that genes were located in fixed positions on chromosomes, similar to an arrangement of beads on a string.  If their position was fixed, then it could be mapped, as more was discovered about genes and chromosomes.  In addition, since crossing over results  when chromosomes break apart during meiosis, it is more likely to occur when chromosomes are further apart than when they occur close together. Because a mixture of dominant and recessive traits are found in the offspring of a testcross, the percentage of times the mixture occurs might be related to how far apart the genes are on the chromosome that contains them.

What Are Giant Chromosomes?
In most organisms, chromosomes are so small that only a few details about them can be seen with a microscope.  However, the larval form of the fruit fly has giant chromosomes in the salivary glands, more than a thousand times larger than those same chromosomes in the mature adult.  They are visible under a regular light microscope.  When the cell is stained to show the chromosomes, dark and light bands correspond to the locations of specific genes.

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