In the 1850s, Austrian monk Gregor Mendel experimented with pea plants to unravel the basic principles of inheritance. His findings were ignored at the time, but later, when scientists learned about chromosomes and the process of meiosis, Mendel's theory gained prominence as the physical basis of inheritability became clearer. One of the principles Mendel described, now called Mendel's second law, was that alleles of different genes assort independently during gamete formation. Since Mendel's time, we have discovered that the law of independent assortment applies to many genes, but is not universal. When geneticist T. H. Morgan began studying the genetics of fruit flies, he discovered that some genes sort together when they are located on the same chromosome.
Gregor Mendel devised the law of independent assortment based on his data with pea plants. He created pea plants heterozygous for two characters, seed shape and seed color. For both characters, the dominant alleles—spherical and yellow—came from the mother and the recessive alleles—wrinkled and green—came from the father.
Mendel found that when he crossed these heterozygous plants with themselves, the offspring could be either spherical and yellow, wrinkled and yellow, spherical and green, or wrinkled and green. Therefore, he concluded, the allele for seed shape was passed down independently of the allele for seed color. Does this principle of independent assortment apply to all pairs of genes?
T. H. Morgan pioneered the study of the genetics of Drosophila melanogaster, the fruit fly, which remains an important organism in genetics research today. Morgan crossed Drosophila of two known genotypes for two different traits: body color and wing shape. Wild-type gray body is dominant over black body. Wild-type normal wing is dominant over "vestigial," a very small wing.
In each cross, the mother was heterozygous for both traits, and the father was homozygous for the recessive trait.
Assuming that the alleles for body color and wing shape assort independently, as Mendel predicted, what would Morgan have observed when he crossed these flies?
Mendel's law predicts equal numbers of each of the four possible types of offspring. The father always contributes a recessive allele, and the mother contributes the dominant allele 50 percent of the time. The dominant allele for one gene should be equally likely to pair with either the dominant or recessive allele for the other gene.
However, Morgan observed something different in his fruit flies. The alleles for body color and wing shape seemed to be inherited together most of the time. These results made sense when Morgan realized that the two genes are on the same chromosome; that is, they are linked.
During meiosis, chromosome pairs line up at the midplane of the cell. When genes are located on different chromosomes, random chance determines which alleles line up and are transmitted together to the germ cell. This is why the traits observed by Mendel appeared to be inherited independently of each other.
When genes are located on the same chromosome, they do not line up randomly during meiosis. In Morgan's experiment, the recessive allele encoding black body, b, lines up with the recessive allele for vestigial wings, vg because both are on the same chromosome. Similarly, the two wild-type alleles, B and Vg, also stay together. The two genes are thus transmitted to the offspring as a set, rather than independently.
If the genes for body color and vestigial wings are located on the same chromosome and always segregate together, what phenotypes should Morgan have observed in the F1 generation?
If linkage were complete, Morgan's cross could only have produced gray flies with normal wings, or black flies with vestigial wings.
Yet this is not what he observed. Some flies were born with gray bodies and vestigial wings and some had black bodies and normal wings. How could the "gray body" and "vestigial wings" alleles, or the "black body" with "normal wings" alleles, end up in the same cell?
It is very rare that two genes are inseparably linked, thanks to a complex process called recombination. Chromosomes are not unbreakable, so genes at different loci on the same chromosome do sometimes separate from one another during meiosis. After the DNA replicates during S phase, each chromosome consists of two chromatids. In prophase I, homologous chromosome pairs come together to form tetrads.
At this point, the homologous chromosomes can physically exchange corresponding segments. The exchange can occur at any point along the chromosome. Both chromatids then end up with genes from both of the organism's parents. The result is two recombinant gametes from each event of crossing over.
When crossing over takes place, not all offspring will have the parental phenotypes. Instead, as in Morgan's cross, recombinant offspring will appear. How often two genes recombine depends upon how close together they are on the chromosome. A chromatid exchange is more likely to occur between genes that are far apart than between genes that are close together.
Geneticist T. H. Morgan, in his work with fruit flies, showed that traits encoded by genes that are located on the same chromosome may not obey Mendel's second law—the law of independent assortment. Because they are physically linked, alleles of these genes are less likely to separate from one another during gamete formation than are alleles of genes located on different chromosomes.
However, alleles of linked genes can be shuffled by crossing over, in which two homologous chromatids exchange corresponding chromosomal segments. A portion of the chromatid physically breaks off and is reattached to the homologous chromatid. In this way, the maternal allele of one gene can be passed down together with the paternal allele of another gene on that same chromosome. Crossing over is an essential process for maintaining a wealth of genetic diversity, even though a species may have a small number of chromosomes.