Gene mapping in eukaryotes

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. 

Barbara McClintock’s corn experiment:

The corn species selected for the experiment consisted of heterozygotes for the two genes on the 9^th 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 8^th 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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Non-Mendelian inheritance

Non-Mendelian inheritance helps in knowing more about the extranuclear genes and their pattern of inheritance. A typical feature of non-Mendelian inheritance includes the absence of meiosis based segregation pattern or Mendelian inheritance pattern. Hence, this type of inheritance does not include the ratios depicting the Mendelian type of inheritance. The results site the differences between the crosses involving the non-mendelian genes in comparison with the crosses involving the nuclear genes. The inheritance pattern of the extranuclear genes differs a lot. The Non-Mendelian pattern of segregation mainly affects the results of the reciprocal crosses. The maternal, paternal or uniparental pattern of inheritance usually gets involved in the Non-Mendelian inheritance. Most of the extranuclear genes such as mitochondria and chloroplast genes follow the maternal pattern of inheritance or the cytoplasmic pattern of inheritance. From generations to generations, these organelles follow this type of inheritance since they belong to the cytoplasmic content. The concept of cytoplasmic inheritance is very simple. Upon the fusion of the male and the female gametes, the zygote receives the cytoplasm from the oocyte or the female gamete.

Examples of non-Mendelian inheritance:

Image 1: Mirabilis jalapa

·        Shoot variegation in four o'clock plant:

The common name of Mirabilis Jalapa plant is also known as four o'clock plant. It possesses a variegated phenotype of the shoot belonging to the albomaculata strain. It consists of the shoots with variegated leaves. These leaves exhibit yellowish-white colored patches. The shoots also show the presence of complete green leaves or yellowish-white leaves. These leaves have no patches. Various crosses between the shoot bearing male and the female parents revealed the striking phenotypes. The crosses between the white female plants with any of the male plants (green, white or variegated) gave rise to the progeny plants consisting of the white phenotype. The crosses between the green female plant with white, green or variegated male plants gave rise to the progeny having a green phenotype. The crosses between the variegated female plants and white male plants resulted in three types of progeny such as variegated, white or green. Similarly, the crosses with the other two combinations of parents gave rise to the progeny with variegated, green or white phenotypes, respectively. The white coloration indicates the lack of chlorophyll pigment.

Such plants do not carry out photosynthesis and die early. Hence, the phenotype of progeny depended only on the type of the maternal phenotype. It led to the neglection of the paternal phenotype. The abnormal coloration or white coloration due to leukoplasts ( those lacking chlorophyll pigment) occurs as a result of cpDNA mutations. Two types of organelles exist in these plants such as the chloroplast and leukoplasts. Chloroplasts get involved in the process of photosynthesis. Colorless chloroplasts are known as leukoplasts. During organelle segregation, some zygotes receive chloroplasts. The others receive only leukoplasts. The remaining zygotes receive a combination of chloroplasts and leukoplasts. Hence, some of the plants show variegated phenotypes.

·        Poky mutants of Neurospora:

The fungi known as Neurospora require aerobic respiration. However, the mutant strains carry out the defective type of aerobic respiration. The Neurospora mutant strains are also known as poky mutants. The poky mutants show the presence of mutations in the mitochondrial DNA. Poky female and the wild-type male parents give rise to the poky mutants. Wild-type female and the poky male parents give rise to the progeny with wild-type phenotype. Hence, the progeny had maternal phenotypes. The tetrads analysis involving a cross between the poky female and the wild-type male gave rise to all poky spores. The tetrads analysis involving a cross between the normal female and poky male gave rise to all normal spores. The cytochrome deficiency in the Neurospora leads to the poky strains. It involves a four base pair deletion mutation in the promoter region of the 19s rRNA of the small mitochondrial ribosome.

·        Yeast petite mutants:

The small colonies are known as petite mutants. The large wild-type colonies are known as grandes. There are two mating types in yeast such as "a" and "alpha" respectively. Crosses involving the petite and the wild-type mutants produced wild-type colonies. Nuclear petite occur less than the extranuclear petites. The cross between the neutral petite and wild-type petite produce wild-type colonies. It follows uniparental inheritance since all the progeny have the phenotype of one parent. The cross between the suppressive Petites with wild-type strains give rise to diploid with respiratory properties between the petite and the normal properties. After meiosis, the tetrads produces four petite colonies.

·        Non-Mendelian inheritance in Chlamydomonas:

It has two mating strains such as mt+ and mt- strains. A chloroplast trait known as erythromycin resistance gets inherited through a non-Mendelian pattern of inheritance. A cross between the erythromycin resistant mt + and erythromycin sensitive mt - strain gives rise to all erythromycin-resistant progeny. A cross between the mt - erythromycin-resistant strain and mt + erythromycin sensitive strain gives rise to all the progeny showing erythromycin sensitivity. Cross between both the mating types results in syngamous mating. The diploid zygote after meiosis gives rise to tetrads segregating in different strains.

Image 2: Non-Mendelian inheritance in Chlamydomonas

Many human genetic diseases arise due to mtDNA mutations. Leber's hereditary optic neuropathy, Kearns Sayre syndrome and Myoclonic epilepsy arise due to mutations in the mitochondrial inheritance. The corn plants show cytoplasmic male sterility and hybrid seed production.

·        Maternal effects:

The maternal nuclear genome also affects the phenotype of the progeny. It is known as a maternal effect. Example of maternal effect includes the inheritance of the coiling direction in a snail known as Limnaea peregra. A cross between the dextral coiling female and the sinistral coiling male gives rise to dextral coiling progeny in the generation. Selfing the F1 progeny leads to a phenotypic ratio of 1:2:1. Most of them are dextral. A cross between the sinistral coiling females and dextral coiling males gave rise to F1 snails with sinistral phenotype. Selfing of the F1 gave rise to all dextral phenotype. Hence, the coiling phenotype follows the nuclear genotype of the mother. Thus, it is an example of the maternal effect.

References:

[1] Essentials of Genetics, Pragya Khanna
[2] Genetics, 9th Edition (Multicolour Edition), Verma P.S. & Agarwal V.K.
[3] Cytology genetics and molecular genetics, Pandey

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