Category: Biology

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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.

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

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:

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 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

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.

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.

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.

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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.

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.

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.

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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.

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, I^A and I^B 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.

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

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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Monohybrid, Dihybrid, Cross, Backcross And Testcross

Monohybrid inheritance is the inheritance of a single character i.e. plant height. It involves the inheritance of two alleles of a single gene. When the F₁ generation was selfed Mendel noticed that 787 of 1064 F₂ plants were tall, while 277 of 1064 were dwarf. The dwarf trait disappeared in the F₁ generation only to reappear in the F₂ generation.

The term genotype is the genetic constitution of an individual. The term phenotype refers to the observable characteristic of an organism. In a genetic cross the genotypes and phenotypes of offspring, resulting from combining gametes during fertilization can be easily understood with the help of a diagram called Punnett’s Square named after a British Geneticist Reginald C.Punnett.

It is a graphical representation to calculate the probability of all possible genotypes of offsprings in a genetic cross. The Law of Dominance and the Law of Segregation give suitable explanation to Mendel’s monohybrid cross.

Reciprocal cross:

In one experiment, the tall pea plants were pollinated with the pollens from a true-breeding dwarf plants, the result was all tall plants. When the parental types were reversed, the pollen from a tall plant was used to pollinate a dwarf pea plant which gave only tall plants.

The result was the same – All tall plants. Tall (img 1) x Dwarf (img 2) and Tall (img 3) x Dwarf (img 4) matings are done in both ways which are called reciprocal crosses. The results of the reciprocal crosses are the same. So it was concluded that the trait is not sex dependent. The results of Mendel’s monohybrid crosses were not sex dependent.

The gene for plant height has two alleles:
Tall (T) x Dwarf (t). The phenotypic and genotypic analysis of the crosses has been shown by Checker board method or by Forkline method.

Mendel’s analytical and empirical approach

Mendel chose two contrasting traits for each character. So it seemed logical that two distinct factors exist. In F₁ the recessive trait and its factors do not disappear and they are hidden or masked only to reappear in ¼ of the F₂ generation. He concluded that tall and dwarf alleles of F₁ heterozygote segregate randomly into gametes.

Mendel got 3:1 ratio in F₂ between the dominant and recessive trait. He was the fist scientist to use this type of quantitative analysis in a biological experiment. Mendel’s data is concerned with the proportions of offspring.

Mendel’s analytical approach is truly an outstanding scientifi achievement. His meticulous work and precisely executed breeding experiments proposed that discrete particulate units of heredity are present and they are transmitted from one generation to the other.

Now they are called as genes. Mendel’s experiments were well planned to determine the relationships which govern hereditary traits. This rationale is called an empirical approach. Laws that were arrived from an empirical approach is known as empirical laws.

Test cross

Test cross is crossing an individual of unknown genotype with a homozygous recessive. In Mendel’s monohybrid cross all the plants are tall in F₁ generation. In F₂ tall and dwarf plants in F₃ and F₄ generations.
So he concluded that the genotype of dwarf was homozygous (tt). The genotypes of tall plants TT or Tt from F₁ and F₂ cannot be predicted.

But how we can tell if a tall plant is homozygous or heterozygous? To determine the genotype of a tall plant Mendel crossed the plants from F₂ with the homozygous recessive dwarf plant. This he called a test cross. The progenies of the test cross can be easily analysed to predict the genotype of the plant or the test organism.

Thus in a typical test cross an organism (pea plants) showing dominant phenotype (whose genotype is to be determined) is crossed with the recessive parent instead of self crossing. Test cross is used to identify whether an individual is homozygous or heterozygous for dominant character.

Back cross

• Back cross is a cross of F₁ hybrid with any one of the parental genotypes. The back cross is of two types; they are dominant back cross and recessive back cross.
• It involves the cross between the F₁ offspring with either of the two parents.
• When the F₁ offsprings are crossed with the dominant parents all the F₂ develop dominant character and no recessive individuals are obtained in the progeny.
• If the F₁ hybrid is crossed with the recessive parent individuals of both the phenotypes appear in equal proportion and this cross is specifid as test cross.
• The recessive back cross helps to identify the heterozygosity of the hybrid.

Dihybrid cross

It is a genetic cross which involves individuals differing in two characters. Dihybrid inheritance is the inheritance of two separate genes each with two alleles.

Law of Independent Assortment:

When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent to the other pair of characters. Genes that are located in different chromosomes assort independently during meiosis. Many possible combinations of factors can occur in the gametes.

Independent assortment leads to genetic diversity. If an individual produces genetically dissimilar gametes it is the consequence of independent assortment. Though independent assortment, the maternal and paternal members of all pairs were distributed to gametes, so all possible chromosomal combinations were produced leading to genetic variation.

In sexually reproducing plants/organisms, due to independent assortment, genetic variation takes place which is important in the process of evolution. The Law of Segregation is concerned with alleles of one gene but the Law of Independent Assortment deals with the relationship between genes.

The crossing of two plants diffring in two pairs of contrasting traits is called dihybrid cross. In dihybrid cross, two characters (colour and shape) are considered at a time. Mendel considered the seed shape (round and wrinkled) and cotyledon colour (yellow & green) as the two characters. In seed shape round (R) is dominant over wrinkled (r); in cotyledon colour yellow (Y) is dominant over green (γ).

Hence the pure breeding round yellow parent is represented by the genotype RRYY and the pure breeding green wrinkled parent is represented by the genotype rryy. During gamete formation the paired genes of a character assort out independently of the other pair.

During the F₁ × F₁ fertilization each zygote with an equal probability receives one of the four combinations from each parent. The resultant gametes thus will be genetically different and they are of the following four types:

1. Yellow round (YR) – 9/16
2. Yellow wrinkled (Yr) – 3/16
3. Green round (yR) – 3/16
4. Green wrinkled (yr) – 1/16

These four types of gametes of F₁ dihybrids unite randomly in the process of fertilization and produce sixteen types of individuals in F₂ in the ratio of 9:3:3:1 as shown in the fiure. Mendel’s 9:3:3:1 dihybrid ratio is an ideal ratio based on the probability including segregation, independent assortment and random fertilization.

In sexually reproducing organism/plants from the garden peas to human beings, Mendel’s fidings laid the
foundation for understanding inheritance and revolutionized the field of biology. The dihybrid cross and its result led Mendel to propose a second set of generalisations that we called Mendel’s Law of independent assortment.

The Dihybrid test cross

Extensions of Mendelian Genetics

Apart from monohybrid, dihybrid and trihybrid crosses, there are exceptions to Mendelian principles, i.e. the occurrence of different phenotypic ratios. The more complex patterns of inheritance are the extensions of Mendelian Genetics. There are examples where phenotype of the organism is the result of the interactions among genes.

Gene interaction:
A single phenotype is controlled by more than one set of genes, each of which has two or more alleles. This phenomenon is called Gene Interaction. Many characteristics of the organism including structural and chemical which constitute the phenotype are the result of interaction between two or more genes.

Mendelian experiments prove that a single gene controls one character. But in the post Mendelian fidings, various exception have been noticed, in which different types of interactions are possible between the genes. This gene interaction concept was introduced and explained by W. Bateson. This concept is otherwise known as Factor hypothesis or Bateson’s factor hypothesis. According to Bateson’s factor hypothesis, the gene interactions can be classifid as

• Intragenic gene interactions or Intra allelic or allelic interactions
• Intergenic gene interactions or inter allelic or non-allelic interactions

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Laws Of Mendelian Inheritance

Mendelian inheritance – Mendel’s Laws of Heredity

Mendel proposed two rules based on his observations on monohybrid cross, today these rules are called laws of inheritance The first law is The Law of Dominance and the second law is The Law of Segregation. These scientific laws play an important role in the history of evolution.

The Law of Dominance:

The characters are controlled by discrete units called factors which occur in pairs. In a dissimilar pair of factors one member of the pair is dominant and the other is recessive. This law gives an explanation to the monohybrid cross (a) the expression of only one of the parental characters in F₁ generation and (b)
the expression of both in the F₂ generation. It also explains the proportion of 3:1 obtained at the F₂.

The Law of Segregation (Law of Purity of gametes):

Alleles do not show any blending, both characters are seen as such in the F₂ generation although one of the characters is not seen in the F₂ generation.

During the formation of gametes, the factors or alleles of a pair separate and segregate from each other such that each gamete receives only one of the two factors. A homozygous parent produces similar gametes and a heterozygous parent produces two kinds of gametes each having one allele with equal proportion. Gametes are never hybrid.

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An Overview of Mendelism

The contribution of Mendel to Genetics is called Mendelism. It includes all concepts brought out by Mendel through his original research on plant hybridization. Mendelian genetic concepts are basic to modern genetics. Therefore, Mendel is called as Father of Genetics.

Father of Genetics – Gregor Johann Mendel (1822 – 1884)

The first Geneticist, Gregor Johann Mendel unraveled the mystery of heredity. He was born on 22^nd July 1822 in Heinzendorf Silesia (now Hyncice, Czechoslovakia), Austria. After school education, later he studied botany, physics and mathematics at the University of Vienna. He then entered a monastery of St.Thmas at Brunn in Austria and continued his interest in plant hybridization.

In 1849 Mendel got a temporary position in a school as a teacher and he performed a series of elegant experiments with pea plants in his garden. In 1856, he started his historic studies on pea plants. 1856 to 1863 was the period of Mendel’s hybridization experiments on pea plants.

Mendel discovered the principles of heredity by studying the inheritance of seven pairs of contrasting traits of pea plant in his garden. Mendel crossed and catalogued 24, 034 plants through many generations. His paper entitled “Experiments on Plant Hybrids” was presented and published in The Proceedings of the Brunn Society of Natural History in 1866. Mendel was the fist systematic researcher in the field of genetics.

Mendel was successful because:

• He applied mathematics and statistical methods to biology and laws of probability to his breeding experiments.
• He followed scientifi methods and kept accurate and detailed records that include quantitative data of the outcome of his crosses.
• His experiments were carefully planned and he used large samples.
• The pairs of contrasting characters which were controlled by factor (genes) were present on separate chromosomes. (Figure 2.4)
• The parents selected by Mendel were pure breed lines and the purity was tested by self crossing the progeny for many generations.
  

Mendel’s Experimental System – The Garden pea.

He chose pea plant because,

• It is an annual plant and has clear contrasting characters that are controlled by a single gene separately.
• Self-fertilization occurred under normal conditions in garden pea plants. Mendel used both self-fertilization and crossfertilization.
• The flowers are large hence emasculation and pollination are very easy for hybridization.

Mendel’s experiments on pea plant

Mendel’s theory of inheritance, known as the Particulate theory, establishes the existence of minute particles or hereditary units or factors, which are now called as genes. He performed artificial pollination or cross pollination experiments with several true-breeding lines of pea plants. A true breeding lines (Pure-breeding strains) means it has undergone continuous self pollination having stable trait inheritance from parent to offspring.

Matings within pure breeding lines produce offprings having specific parental traits that are constant in inheritance and expression for many generations. Pure line breed refers to homozygosity only. Fusion of male and female gametes produced by the same individual i.e pollen and egg are derived from the same plant is known as selffertilization.

Self pollination takes place in Mendel’s peas. The experimenter can remove the anthers (Emasculation) before fertilization and transfer the pollen from another variety of pea to the stigma of flowers where the anthers are removed.

This results in cross-fertilization, which leads to the creation of hybrid varieties with different traits. Mendel’s work on the study of the pattern of inheritance and the principles or laws formulated, now constitute the Mendelian Genetics.

Mendel worked at the rules of inheritance and arrived at the correct mechanism before any knowledge of cellular mechanism, DNA, genes, chromosomes became available. Mendel insights and meticulous work into the mechanism of inheritance played an important role which led to the development of improved crop varieties and a revolution in crop hybridization.

Mendel died in 1884. In 1900 the work of Mendel’s experiments were rediscovered by three biologists, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tschermak of Austria.

Terminology related to Mendelism

Mendel noticed two different expressions of a trait – Example: Tall and dwarf. Traits are expressed in different ways due to the fact that a gene can exist in alternate forms (versions) for the same trait is called alleles.

If an individual has two identical alleles of a gene, it is called as homozygous (TT). An individual with two different alleles is called heterozygous (Tt). Mendels non-true breeding plants are heterozygous, called as hybrids. When the gene has two alleles the dominant allele is symbolized with capital letter and the recessive with small letter.

When both alleles are recessive the individual is called homozygous recessive (tt) dwarf pea plants. An individual with two dominant alleles is called homozygous dominant (TT) tall pea plants. One dominant allele and one recessive allele (Tt) denotes nontrue breeding tall pea plants heterozygous tall.

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Heredity And Variation

Genetics is oftn described as a science which deals with heredity and variation.

Heredity:
Heredity is the transmission of characters from parents to off springs.

Variation:
The organisms belonging to the same natural population or species that shows a diffrence in the characteristics is called variation. Variation is of two types

1. Discontinuous variation and
2. Continuous variation

1. Discontinuous Variation:

Within a population there are some characteristics which show a limited form of variation. Example: Style length in Primula, plant height of garden pea. In discontinuous variation, the characteristics are controlled by one or two major genes which may have two or more allelic forms. These variations are genetically determined by inheritance factors.

Individuals produced by this variation show diffrences without any intermediate form between them and there is no overlapping between the two phenotypes. The phenotypic expression is unaffcted by environmental conditions. This is also called as qualitative inheritance.

2. Continuous Variation:

This variation may be due to the combining effects of environmental and genetic factors. In a population most of the characteristics exhibit a complete gradation, from one extreme to the other without any break. Inheritance of phenotype is determined by the combined effects of many genes, (polygenes) and environmental factors. This is also known as quantitative inheritance. Example: Human height and skin color.

Importance of variations

• Variations make some individuals better fited in the struggle for existence.
• They help the individuals to adapt themselves to the changing environment.
• It provides the genetic material for natural selection.
• Variations allow breeders to improve better yield, quicker growth, increased resistance and lesser input.
• They constitute the raw materials for evolution.

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Asexual and Sexual Reproduction of Parthenocarpy

As mentioned earlier, the ovary becomes the fruit and the ovule becomes the seed after fertilization. However in a number of cases, fruit like structures may develop from the ovary without the act of fertilization. Such fruits are called parthenocarpic fruits. Invariably they will not have true seeds. Many commercial fruits are made seedless. Examples: Banana, Grapes and Papaya.

Signifiance

• The seedless fruits have great signifiance in horticulture.
• The seedless fruits have great commercial importance.
• Seedless fruits are useful for the preparation of jams, jellies, sauces, fruit drinks etc.
• High proportion of edible part is available in parthenocarpic fruits due to the absence of seeds.

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Polyembryony Types and its Occurence

Occurrence of more than one embryo in a seed is called polyembryony (Figure 1.24). The first case of polyembryony was reported in certain oranges by Anton von Leeuwenhoek in the year 1719. Polyembryony is divided into four categories based on its origin.

1. Cleavage polyembryony (Example: Orchids)

2. Formation of embryo by cells of the Embryo sac other than egg (Synergids – Aristolochia; antipodals – Ulmus and endosperm – Balanophora)

3. Development of more than one Embryo sac within the same ovule.
(Derivatives of same MMC, derivatives of two or more MMC – Casuarina)

4. Activation of some sporophytic cells of the ovule (Nucellus / integuments-Citrus and Syzygium).

Practical applications

The seedlings formed from the nucellar tissue in Citrus are found better clones for Orchards. Embryos derived through polyembryony are found virus free.