Biology

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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.