Mapping the genes in a correct order helps to know the location of the genes on the chromosomes. Various techniques achieved success in mapping the genes. Mapping techniques achieved success with eukaryotic organisms such as Drosophila and plants before humans. Gene mapping not only helps us in knowing the exact location of the gene but also helps in conducting various other experiments based on the gene location. Thomas Morgan worked with Drosophila strains and found out recombination mechanisms. An experiment sometimes gives an idea of other hidden strategies. For example, gene mapping studies revealed the mechanism of recombination. The studies revealed that the progeny obtained by crossing some strains of the eukaryotic organisms also showed phenotype differing from the parental phenotype.
Morgan’s experiment:
Thomas Morgan and his colleagues worked with Drosophila strains. They cultured Drosophila with a particular X-linked phenotype. The experimenters selected certain strains of fruit flies. The female flies had two X chromosomes with linked genes. The males had one X and one Y chromosome. The female flies had a phenotype of white eyes and miniature wings. The male flies were wild-type flies. The cross between these two types of flies gave rise to an F1 generation having wild-type females, and white-eyed, miniature winged male flies. Interbreeding of the F1 progeny gave different kinds of flies. There were total 2241 flies in the F2 generation. Out of these, total 900 flies had a non-parental phenotypic combination of white eyes and normal wings. Other types of non-parental strains included red-eyed, miniature winged flies. The non-parental ones are known as recombinants. The recombinants arise due to the crossing over between the homologous chromosomes. The theory involves two key concepts. The first one is the site of physical exchange. It is known as the chiasma. The second one involves the genetic recombination between the linked genes. It is known as crossing over. It also involves a reciprocal exchange of chromosome segments.
Image 2: Morgan's experiment
Stern’s Experiment:
Stern worked with X-linked gene loci in Drosophila. The experimenters conducted a cross between the wild-type bar eyed females and carnation type round-eyed males. The female flies had two X chromosomes. One of them additionally had a detached piece of X chromosome. The other X chromosome had an additional attachment of a piece of the Y chromosome. The chromosomes in the males flies had no extra pieces attached. The interbreeding of the F1 progeny gave rise to different types of flies. Four main types of progeny observed included carnation bar, red round, carnation round, and red bar eyed males and females respectively. The results of the experiment revealed the genetic recombination and exchange of identifiable segments.
The corn species selected for the experiment consisted of heterozygotes for the two genes on the 9th chromosome. One of the genes gave a phenotype of colored versus colorless. The other type of genes resulted in the phenotypes such as standard type starch with amylose and amylopectin versus waxy plants having the only amylopectin. The chromosomes had genes cWx giving normal phenotype. The homologs of the chromosomes having genes cWx had the genotype of Cwx. These homologs had a large double stained knob and a piece of 8th chromosome attached near the wx gene. It was a translocated segment. These features are known as the cytological markers. Hence, the corn experiments revealed the process of genetic recombination associated with the physical exchange between the parts of the homologous chromosomes.
Linkage studies using testcross:
A cross involving a normal individual with an individual who is homozygous recessive for all the genes is known as a testcross.
· Two point test cross
Consider the autosomal recessive individuals. Suppose there involves a cross between the double heterozygotes with a genotype of a+b+/ a+b+ and double homozygous recessives with a genotype of ab/ab. The F1 generation revealed progeny with a wild-type a+b+/ab genotype. Upon conducting a testcross with double homozygous recessives, the progeny had 50% parental non-recombinants and 50% recombinant progeny. The formula for the recombination frequency involves (Number of recombinants/ Number of testcross progeny) x 100. The recombination frequency cannot exceed 50%.
Image 2: Two-point test cross
· Three-point test cross:
Consider a cross between the triple heterozygotes with a genotype of a+b+c+/abc and triple homozygous recessives with a genotype of abc/abc. These crosses reveal the genetic recombination. Consider another example of flowering plants having three linked genes controlling the fruit phenotype. The recessive p allele gives a purple phenotype versus the wild-type yellow phenotype. The recessive r allele gives a round shape versus the wild-type elongated one. The recessive j allele gives juicy phenotype versus the wild-type dry fruit. The order of genes gets determined through a three-point test cross. Two parentals and six recombinants arise due to crossing over. The frequency of the double crossovers was found less than the frequency of the single crossovers.
Gene-centromere distance studies in Neurospora crassa:
The products of meiosis get a specialized arrangement depicting the four chromatids of each of the homologous pair of chromosomes. It usually reflects during the metaphase I. Neurospora consists of ordered tetrads. Meiotic and the mitotic divisions in the tetrads help in studying the process of recombination. It becomes easy to map the distance between the gene and the centromere using the ordered tetrads. The first division segregation tetrad consists of a parental type occupying half the ordered tetrad and another parental type in the other half of the tetrad. It occurs when there is no crossover. A single crossover between the gene and the centromere gives different types of tetrad segregation patterns (the second division segregation). The percentage of the second division tetrads divided by 2. It is known as the gene-centromere map distance. Tetrad analysis also helps in mapping two linked genes.
Mitotic recombination:
Crossing over is also known as genetic recombination between the linked genes or the reciprocal exchange of chromosome segments. It occurs during the mitosis as well as meiosis. The mitotic crossing over is also known as mitotic recombination. It leads to the production of the progeny cells having a combination of genes differing from the diploid parental cell entering the mitotic cycle. A classic example of the mitotic recombination includes fungus Aspergillus nidulans. It has a parasexual cycle of genetic systems. The genetic recombination in Aspergillus occurs through the processes other than regular alteration of meiosis and fertilization. The heterokaryon forms due to the mycelial fusion and the fusion of the two haploid nuclei. It gives rise to a diploid nucleus. The parasexual cycle also consists of mitotic crossing over within the diploid nucleus or haploidization of the diploid nuclei without meiosis. It becomes easy to calculate the gene order and the map distances.
Human gene mapping:
Physical mapping techniques help in mapping human genes. This technique mainly involves large genomes. It is not possible to set up a testcross for human genes since the human genome is vast. We obtain the recombination data from the pedigree analysis in humans. Gene mapping involves the use of gene markers and DNA markers.
References:
[1] Genetics: Analysis of Genes and Genomes, Daniel L. Hartl, Elizabeth W. Jones
[2] Biology, Raven
[3] Biology, Pages 172-180, Neil A. Campbell, Jane B Reece
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