Linkage – Eye Colour In Drosophila And Seed Colour In Maize

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Linkage – Eye Colour In Drosophila And Seed Colour In Maize

The genes which determine the character of an individual are carried by the chromosomes. The genes for diffrent characters may be present either in the same chromosome or in different chromosomes. When the genes are present in diffrent chromosomes, they assort independently according to Mendel’s Law of Independent Assortment. Biologists came across certain genetic characteristics that did not assort out independently in other organisms after Mendel’s work.

One such case was reported in Sweet pea (Lathyrus odoratus) by William Bateson and Reginald C. Punnet in 1906. They crossed one homozygous strain of sweet peas having purple flowers and long pollen grains with another homozygous strain having red flowers and round pollen grains.

All the F1 progenies had purple flower and long pollen grains indicating purple flower long pollen (PL/PL) was dominant over red flower round pollen (pl/pl). When they crossed the F1 with double recessive parent (test cross) in results, F2progenies did not exhibit in 1:1:1:1 ratio as expected with independent assortment.

A greater number of F2 plants had purple flowers and long pollen or red flowers and round pollen. So they concluded that genes for purple colour and long pollen grain and the genes for red colour and round pollen grain were found close together in the same homologous pair of chromosomes. These genes do not allow themselves to be separated. So they do not assort independently. This type of tendency of genes to stay together during separation of chromosomes is called Linkage.
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Genes located close together on the same chromosome and inherited together are called linked genes. But the two genes that are suffiently far apart on the same chromosome are called unlinked genes or syntenic genes (Figure 3.3). Such condition is known as synteny.

It is to be diffrentiated by the value of recombination frequency. If the recombination frequency value is more than 50 % the two genes show unlinked when the recombination frequency value is less than 50 %, they show linked. Closely located genes show strong linkage, while genes widely located show weak linkages.

Coupling and Repulsion theory

The two dominant alleles or recessive alleles occur in the same homologous chromosomes, tend to inherit together into same gamete are called coupling or cis confiuration (Figure: 3.5). If dominant or recessive alleles are present on two different, but homologous chromosomes they inherit apart into diffrent gamete are called
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Kinds of Linkage

T.H. Morgan found two types of linkage. They are complete linkage and incomplete linkage depending upon the absence or presence of new combination of linked genes.

Complete Linkage
If the chances of separation of two linked genes are not possible those genes always remain together as a result, only parental combinations are observed. The linked genes are located very close together on the same chromosome such genes do not exhibit crossing over. This phenomenon is called complete linkage. It is rare but has been reported in male Drosophila.

Incomplete Linkage
If two linked genes are sufficiently apart, the chances of their separation are possible. As a result, parental and non-parental combinations are observed. The linked genes exhibit some crossing over. This phenomenon is called incomplete linkage. This was observed in maize. It was reported by Hutchinson.

Linkage Groups

The groups of linearly arranged linked genes on a chromosome are called Linkage groups. In any species the number of linkage groups corresponds to the number haploid set of chromosomes. Example:
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Linkage and crossing over are two processes that have opposite effects. Linkage keeps particular genes together but crossing over mixes them. The differences are given below.
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Chromosomal Theory 0f Inheritance

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Chromosomal Theory Of Inheritance

G. J. Mendel (1865) studied the inheritance of well-defined characters of pea plant but for several reasons it was unrecognized till 1900. Three scientists (de Vries, Correns and Tschermak) independently rediscovered Mendel’s results on the inheritance of characters. Various cytologists also observed cell division due to advancements in microscopy. This led to the discovery of structures inside nucleus.

In eukaryotic cells, worm-shaped structures formed during cell division are called chromosomes (colored bodies, visualized by staining). An organism which possesses two complete basic sets of chromosomes are known as diploid. A chromosome consists of long, continuous coiled piece of DNA in which genes are arranged in linear order.

Each gene has a definite position (locus) on a chromosome. These genes are hereditary units. Chromosomal theory of inheritance states that Mendelian factors (genes) have specific locus (position) on chromosomes and they carry information from one generation to the next generation.

Historical development of chromosome theory

The important cytological fidings related to the chromosome theory of inheritance are given below.

Wilhelm Roux (1883):
postulated that the chromosomes of a cell are responsible for transferring heredity.

Montgomery (1901):
Was first to suggest occurrence of distinct pairs of chromosomes and he also concluded that maternal chromosomes pair with paternal chromosomes only during meiosis.

T. Boveri (1902):
supported the idea that the chromosomes contain genetic determiners, and he was largely responsible for developing the chromosomal theory of inheritance.

W.S. Sutton (1902):
A young American student independently recognized a parallelism (similarity) between the behaviour of chromosomes and Mendelian factors during gamete formation. Sutton and Boveri (1903) independently proposed the chromosome theory of inheritance. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance.

Salient features of the Chromosomal

Somatic cells of organisms are derived from the zygote by repeated cell division (mitosis). These consist of two identical sets of chromosomes. One set is received from female parent (maternal) and the other from male parent (paternal). These two chromosomes constitute the homologous pair.

Chromosomes retain their structural uniqueness and individuality throughout the life cycle of an organism. Each chromosome carries specific determiners or Mendelian factors which are now termed as genes.

The behaviour of chromosomes during the gamete formation (meiosis) provides evidence to the fact that genes or factors are located on chromosomes.

Comparison between gene and chromosome behaviour

Around twentieth century cytologists established that, generally the total number of chromosomes is constant in all cells of a species. A diploid eukaryotic cell has two haploid sets of chromosomes, one set from each parent. All somatic cells of an organism carry the same genetic complement. The behaviour of chromosomes during meiosis not only explains Mendel’s principles but leads to new and different approaches to study about heredity.
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Extra Chromosomal Inheritance – Cytoplasmic Inheritance In Chloroplast

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Extra Chromosomal Inheritance – Cytoplasmic Inheritance In Chloroplast

DNA is the universal genetic material. Genes located in nuclear chromosomes follow Mendelian inheritance. But certain traits are governed either by the chloroplast or mitochondrial genes. This phenomenon is known as extra nuclear inheritance.

It is a kind of Non-Mendelian inheritance. Since it involves cytoplasmic organelles such as chloroplast and mitochondrion that act as inheritance vectors, it is also called Cytoplasmic inheritance. It is based on independent, self-replicating extra chromosomal unit called plasmogene located in the cytoplasmic organelles, chloroplast and mitochondrion.

Chloroplast Inheritance

It is found in 4 o’ Clock plant (Mirabilis jalapa). In this, there are two types of variegated leaves namely dark green leaved plants and pale green leaved plants.

When the pollen of dark green leaved plant (male) is transferred to the stigma of pale green leaved plant (female) and pollen of pale green leaved plant is transferred to the stigma of dark green leaved plant, the F1 generation of both the crosses must be identical as per Mendelian inheritance. But in the reciprocal cross the F1 plant differs from each other. In each cross, the F1 plant reveals the character of the plant which is used as female plant.
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This inheritance is not through nuclear gene. It is due to the chloroplast gene found in the ovum of the female plant which contributes the cytoplasm during fertilization since the male gamete contribute only the nucleus but not cytoplasm.
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Recently it has been discovered that cytoplasmic genetic male sterility is common in many plant species. This sterility is maintained by the inflence of both nuclear and cytoplasmic genes. There are commonly two types of cytoplasm N (normal) and S (sterile).

The genes for these are found in mitochondrion. There are also restores of fertility (Rf) genes. Even though these genes are nuclear genes, they are distinct from genetic male sterility genes of other plants. Because the Rf genes do not have any expression of their own, unless the sterile cytoplasm is present. Rf genes are required to restore fertility in S cytoplasm which is responsible for sterility.

So the combination of N cytoplasm with rfrf and S cytoplasm with RfRf produces plants with fertile pollens, while S cytoplasm with rfrf produces only male sterile plants.

Atavism

Atavism is a modifiation of a biological structure whereby an ancestral trait reappears after having been lost through reemergence of sexual reproduction in the flowering plant Hieracium pilosella is the best example for Atavism in plants.

Polygenic Inheritance In Wheat Kernel Colour, Pleiotropy – Pisum Sativum

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Polygenic Inheritance In Wheat Kernel Colour, Pleiotropy – Pisum Sativum

Polygenic inheritance – Several genes combine to affct a single trait.

A group of genes that together determine (contribute) a characteristic of an organism is called polygenic inheritance. It gives explanations to the inheritance of continuous traits which are compatible with Mendel’s Law.

The first experiment on polygenic inheritance was demonstrated by Swedish Geneticist H. Nilsson – Ehle (1909) in wheat kernels. Kernel colour is controlled by two genes each with two alleles, one with red kernel colour was dominant to white. He crossed the two pure breeding wheat varieties dark red and a white.

Dark red genotypes R1R1R2R2 and white genotypes are r1r1r2r2. In the F1 generation medium red were obtained with the genotype R1r1R2r2. F1 wheat plant produces four types of gametes R1R2, R1r2, r1R2, r1r2.
The intensity of the red colour is determined by the number of R genes in the F2 generation.

Four R genes:

A dark red kernel colour is obtained. Three R genes: Medium – dark red kernel colour is obtained. Two R genes: Medium-red kernel colour is obtained. One R gene: Light red kernel colour is obtained. Absence of R gene: Results in White kernel colour.

The R gene in an additive manner produces the red kernel colour. The number of each phenotype is plotted against the intensity of red kernel colour which produces a bell shaped curve. This represents the distribution of phenotype. Other example: Height and skin colour in humans are controlled by three pairs of genes.
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Conclusion:

Finally the loci that was studied by Nilsson – Ehle were not linked and the genes assorted independently. Later, researchers discovered the third gene that also affect the kernel colour of wheat. The three independent pairs of alleles were involved in wheat kernel colour. Nilsson – Ehle found the ratio of 63 red : 1 white in F2 generation – 1 : 6 : 15 : 20 : 15 : 6 : 1 in F2 generation.
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From the above results Nilsson – Ehle showed that the blending inheritance was not taking place in the kernel of wheat. In F2 generation plants have kernels with wide range of colour variation. This is due to the fact that the genes are segregating and recombination takes place.

Another evidence for the absence of blending inheritance is that the parental phenotypes dark red and white appear again in F2. There is no blending of genes, only the phenotype. The cumulative effect of several pairs of gene interaction gives rise to many shades of kernel colour. He hypothesized that the two loci must contribute additively to the kernel colour of wheat. The contribution of each red allele to the kernel colour of wheat is additive.

Interaction Of Genes – Intragenic And Intergenic Incomplete Dominance, Lethal Genes, Epistasis

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Interaction Of Genes – Intragenic And Intergenic Incomplete Dominance, Lethal Genes, Epistasis

Interactions take place between the alleles of the same gene i.e., alleles at the same locus is called intragenic or intralocus gene interaction. It includes the following:

  1. Incomplete dominance
  2. Codominance
  3. Multiple alleles
  4. Pleiotropic genes are common examples for intragenic interaction.

Incomplete dominance – No blending of genes

The German Botanist Carl Correns’s (1905) Experiment – In 4 O’ clock plant, Mirabilis jalapa when the pure breeding homozygous red (R1R1) parent is crossed with homozygous white (R2R2), the phenotype of the F1 hybrid is heterozygous pink (R1R2). The F1 heterozygous phenotype differs from both the parental
homozygous phenotype.

This cross did not exhibit the character of the dominant parent but an intermediate colour pink. When one allele is not completely dominant to another allele it shows incomplete dominance. Such allelic interaction is known as incomplete dominance.

Such allelic interaction is known as incomlete dominance. F1 generation produces intermediate phenotype pink coloured flower. When pink coloured plants of F1 generation were interbred in F2 both phenotypic and genotypic ratios were found to be identical as 1 : 2 : 1(1 red : 2 pink : 1 white). Genotypic ratio is 1 R1R1 : 2 R1R2 : 1 R1R2. From this we conclude that the alleles themselves remain discrete and unaltered proving the Mendel’s Law of Segregation.

The phenotypic and genotypic ratios are the same. There is no blending of genes. In the F2 generation R1 and R2 genes segregate and recombine to produce red, pink and white in the ratio of 1 : 2 : 1. R1 allele codes for an enzyme responsible for the formation of red pigment. R2 allele codes for defective enzyme. R1 and R2 genotypes produce only enough red pigments to make the flower pink.

Two R1R1 are needed for producing red flowers. Two R2R2 genes are needed for white flowers. If blending had taken place, the original pure traits would not have appeared and all F2 plants would have pink flowers. It is very clear that Mendel’s particulate inheritance takes place in this cross which is confirmed by the reappearance of original phenotype in F2.
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Codominance (1 : 2 : 1)

This pattern occurs due to simultaneous (joint) expression of both alleles in the heterozygote – The phenomenon in which two alleles are both expressed in the heterozygous individual is known as codominance. Example: Red and white flowers of Camellia, inheritance of sickle cell haemoglobin, ABO blood group system in humanbeings. In humanbeings, IA and IB alleles of I gene are codominant which follows Mendels law of segregation.

The codominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flvonoid substance. Example: Gossypium hirsutum and Gossypium sturtianum, their F1 hybrid (amphiploid) was tested for seed proteins by electrophoresis. Both the parents have different banding patterns for their seed proteins. In hybrids, additive banding pattern was noticed. Their hybrid shows the presence of both the types of proteins similar to their parents.

The heterozygote genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes. The F1 heterozygotes produce a F2 progeny in a phenotypic and genotypic ratios of 1 : 2 : 1.

Lethal genes

An allele which has the potential to cause the death of an organism is called a “Lethal Allele”. In 1907, E. Baur reported a lethal gene in snapdragon (Antirrhinum sp.). It is an example for recessive lethality. In snapdragon there are three kinds of plants.

  1. Green plants with chlorophyll. (CC)
  2. Yellowish green plants with carotenoids are referred to as pale green, golden or aurea plants (Cc)
  3. White plants without any chlorophyll. (cc)

The genotype of the homozygous green plants is CC. The genotype of the homozygous white plant is cc.

The aurea plants have the genotype Cc because they are heterozygous of green and white plants. When two such aurea plants are crossed the F1 progeny has identical phenotypic and genotypic ratio of 1 : 2 : 1 (viz. 1 Green (CC) : 2 Aurea (Cc) : 1 White (cc)) Since the white plants lack chlorophyll pigment, they will not survive. So the F2 ratio is modifid into 1 : 2. In this case the homozygous recessive genotype (cc) is lethal.
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The term “lethal” is applied to those changes in the genome of an organism which produces effects severe enough to cause death. Lethality is a condition in which the death of certain genotype occurs prematurely. The fully dominant or fully recessive lethal allele kills the carrier individual only in its homozygous condition. So the F2 genotypic ratio will be 2 : 1 or 1 : 2 respectively.

Pleiotropy – A single gene affects multiple traits

In Pleiotropy, the single gene affcts multiple traits and alter the phenotype of the organism. The Pleiotropic gene inflences a number of characters simultaneously and such genes are called pleiotropic gene were crossed with a variety of peas having white flowers, light coloured seeds and no spot on the axils of the leaves, the three traits for flwer colour, seed colour and a leaf axil spot all were inherited together as a single unit. Another example is: sickle cell anemia.

Intergenic gene interactions

Interlocus interactions take place between the alleles at different loci i.e between alleles of diffrent genes. It includes the following:

Dominant Epistasis

It is a gene interaction in which two alleles of a gene at one locus interfere and suppress or mask the phenotypic expression of a different pair of alleles of another gene at another locus. Th gene that suppresses or masks the phenotypic expression of a gene at another locus is known as epistatic.

The gene whose expression is interfered by non-allelic genes and prevents from exhibiting its character is known as hypostatic. When both the genes are present together, the phenotype is determined by the epistatic gene and not by the hypostatic gene.

In the summer squash the fruit colour locus has a dominant allele ‘W’ for white colour and a recessive allele ‘w’ for coloured fruit. ‘W’ allele is dominant that masks the expression of any colour. In another locus hypostatic allele ‘G’ is for yellow fruit and its recessive allele ‘g’ for green fruit. In the first locus the white is dominant to colour where as in the second locus yellow is dominant to green.

When the white fruit with genotype WWgg is crossed with yellow fruit with genotype wwGG, the F1 plants have white fruit and are heterozygous (WwGg). When F1 heterozygous plants are crossed they give rise to F2 with the phenotypic ratio of 12 white : 3 yellow : 1 green
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Since W is epistatic to the alleles ‘G’ and ‘g’, the white which is dominant, masks the effect of yellow or green. Homozygous recessive ww genotypes only can give the coloured fruits (4/16). Double recessive ‘wwgg’ will give green fruit (1/16). The Plants having only ‘G’ in its genotype (wwGg or wwGG) will give the yellow fruit(3/16).

Intra – genic or allelic interaction

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