• Meiosis division occurs during gametogenesis in higher organism (eukaryotic cells).  The events of meiosis of a cell includes duplication of chromosomes which is followed by one reductional division and other equational division resulting into four haploid cells, each containing half the number of chromosomes.
• Meiosis consists of two rounds of chromosome segregation following a single replication.
• The phases of meiosis is divided into two; meiosis-I and meiosis-II.
• Before the onset of meiotic division, the chromosomes of the pre-gametic cell are replicated to yield two pairs of sister chromatids after which they enter a long meiotic prophase interval.
• This prophase-I is divided into five distinct stages which are termed as (a) leptotene, (b) zygotene, (c) pachytene, (d) diplotene, and (e) diakinesis
• In leptotene stage, the chromosomes condense along their axial elements.
• During the zygotene stage, the parental homologous chromosomes pair with the help of a structure known as the Synaptonemal complex (SC). These Synaptonemal complex have a tripartite structure, comprising a central element bounded by two lateral elements.
• The pachytene is the longest stage of the meiotic prophase, during which crossing over and genetic recombination takes place between the paired homologous chromosomes.
• In diplotene and diakinesis stages, nuclear membrane disappears and spindle fibres formations begins.
• Prophase I: chromosomes condense, undergo duplication and crossing over, and centrioles move to the poles
• Metaphase I: microtubules connect from the centrioles to the centromeres, and the paired chromosomes line up along the equator of the cell
• Anaphase I: paired chromosomes separate and are pulled to alternate poles
• Telophase I: chromosomes decondense, nuclear membrane and nucleolus forms, centrioles move back together
•  Prophase II: chromosomes condense and centrioles move to the poles
• Metaphase II: microtubules connect from the centrioles to the centromeres, and the paired chromatids line up along the equator of the cell
• Anaphase II: paired chromatids separate and are pulled to alternate poles
• Telophase II: chromosomes decondense, nuclear membrane and nucleolus forms, centrioles move back together.
• With the separation of the parental chromosome pairs in the subsequent stages, the first meiotic division results in the reduction of chromosomes by half, which is followed by another division, analogous to mitosis to produce haploid gametes.
• During meiosis, replicated homologous chromosomes pair and engage in recombination and then segregate to opposite poles. Sister chromatids are separated at meiosis II, and four recombined haploid gametes are formed.

Cytological basis of segregation of alleles:

• The cytological basis of segregation of alleles can be explained if we take the example of an individual which possesses two allelomorphs ‘A’ and ‘a’ on two different homologous chromosomes i.e., A on one chromosome and a on the second chromosome.
• In the prophase stage the two chromosomes will pair and duplicate
• In meiosis-I, allele Aand allele a from homologouschromosome will segregate or separate from each other.
• In the meiosis-II, the two chromatids of A chromosomes and those of a chromosome separate. Thus it becomes clear that both alleles of heterozygote separate during meiosis (gametogenesis) to produce gametes with only one chromosome with either dominant allele or recessive allele.
• No gamete will have both the genes and chromosomes of an allelic pair.

Historically, there have been three general approaches for the study of meiosis,

1. Morphological and cytological approaches
2. Genetic approaches:
3. Biochemical approaches: Characterization of the biochemical processes of meiosis.

1. Morphological and cytological approach:

• In this approach, morphological and cytological investigation of dividing cell was carried out and from these studies it was clear that chromosome behaves in certain ways during meiosis.
• From the behavior study of chromosome during prophase stage, it was found that chromosome comes together forming homologous pair which is brought together by a protein called synaptonemal complex.
• So, it is now very clear that these protein structures are an integral part of meiosis all living organisms.

2. Genetic approach:

• In genetic approach, from the study of several mutants such as yeast, Neurospora etc, it was found that several genes are responsible for meiosis process.

3. Biochemical approach:

• In this approach of characterizing biochemical processes in cell during meiosis, it was found that several enzymes and protein are responsible for chromosome pairing as well as DNA replication and other events.
• This approaches helps to characterized the role of enzymes and protein in DNA synthesis, chromosome pairing and in crossing over (recombination).

Physical behaviour of chromosomes during meiosis

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• In 1902 and 1903, Sutton and Boveri published independent papers proposing the chromosome theory of inheritance.
• According to Chromosomal theory of inheritance, chromosome is the genetic material responsible for Mendelian inheritance. At that time, Mendel knew nothing of the chromosomes and meiosis.
• In order to explain the law of independent assortment Sutton and Boveri suggested that different genes were in different chromosomes completely independent of one another.
• This theory states that individual genes are found at specific locations on particular chromosomes, and that the behavior of chromosomes during meiosis can explain why genes are inherited according to Mendel’s laws.
• Thus, similarity between the Mendelian factors and chromosomes became apparent.

Chromosomal Theory of Inheritance supports Mendel’s law

• In somatic cell, the chromosome or gene occurs in pairs because one chromosome is from father and other from mother to form homologous pair.
• During meiosis or gametogenesis, homologous chromosome pairs segregates independent of other chromosome pairs. Thus, a gamete contains only one particular type of chromosome ie. a gamete contains only one of two allele of particular trait.
• The sorting of chromosomes from each homologous pair into pre-gametes appears to be random and it is similar to Mendel’s law of independent assortment
• Similarly, Chromosome are transferred from one generation to another. And the number of chromosome is fixed in each organism.
• Even though male and female gametes differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.
• The gametes combine during fertilization to produce offspring with the same number of chromosome number as their parents

Morgan’s experiments on Dorsophila melanogaster to explain chromosomal theory of inheritance

• In 1910, Thomas Hunt Morgan performed experiment on Drosophila melanogaster
• Dorsophila melanogaster is commonly known as a fruit fly. It has only four pair of chromosomes (three pair of autosomes and a pair of sex chromosome). At the time of experiment, it was knew that X and Y chromosomes are associated with gender.

I. Cross between Red eyed female (Xw+ Xw+) and white eyed male (Xw Y)

• In Drosophila, normal flies have red eyes and mutated flies have white eyes.
• In the experiment, normal flies with red eyes and mutated flies with white eyes are crossed and offspring are observed.
• Dominant allele: Red eye color.
• Recessive allele (mutated): white eye  
• When a red eyed female (Xw+ Xw+) and white eyed male (Xw Y) were mated, all the progeny in F1 generation had red eyes
• According to Dominant and recessive inheritance pattern, this result makes perfect sense.

II. Reciprocal cross- red eyed male (Xw+ Y) and white eyed female (Xw Xw)

• A surprising result was obtained when the reciprocal cross was performed by mating white eyed females to red eyed males. 
•  In F1 generation, instead of obtaining all the progeny with red eye, the result showed that all the progeny females had red eyes and all the progeny males had white eyes. 
• This result shows traits for gender and eye color are linked, which is not accordance with Mendel’s law of independent assortment.
• This result can be explained only if gene for color of eye is present on X chromosome and are linked.
• Thus, this experiment on eye color of Drosophila supports the chromosome theory of inheritance. Now, all the patterns of inheritance observed by Mendel, including the principle of segregation and the principle of independent assortment can be explained by the behavior of chromosomes during meiosis and chromosome theory of inheritance.

Note: Males are known as Hemizygous because they have only one allele for any X-linked traits. Therefore, a male with the recessive allele will always show the recessive trait, because he only has one copy of the allele. 

• We know, most genes are located on autosomes. This experiment of Morgan establish that gene for eye color in Drosophila is on X-chromosome and it is the first X-linked trait to be identified.

NON disjunction as proof of chromosomal theory of Inheritance:

• Morgan showed that a gene for eye color was on the X chromosome of Drosophila
• One of his students, C. B. Bridges, who secured proof of the chromosome theory by showing that exceptions to the rules of inheritance could also be explained by chromosome behavior.
• Bridges performed one of Morgan’s experiments on a larger scale.
• In his experiment, white-eyed female Drosophila (Xw Xw) crossed with red-eyed males (Xw+ Y) and examined many F1 progeny.
• In F1 generation, following results are obtained;
  • Almost all of the F1 progeny flies are either red eyed females or white eyed males which are normal
  • But few flies such as white-eyed females and red-eyed males are also obtained which are exceptional.
• When he tried to crossed red eyed F1 progeny males with normal white eyed females, it was found that all the F1 progeny red eyed males were sterile.
• However, the F1 progeny white eyed females were fertile. When these white eyed females were crossed with normal red eyed males, many F2 progeny were obtained as white eyed females and red eyed males.
• It seems, the exceptional F1 progeny females produced large number of exceptional progeny in successive generation.

Explanation of result:

• Bridges explained these results by proposing that the exceptional F1 flies were the result of abnormal X chromosome behavior during meiosis in the females of the P generation.
• Normally the X chromosome in females separates or disjoin during gametogenesis but Occasionally, they might fail to separate, producing an egg with two X chromosomes (diplo-X) or an egg with no X chromosome at all (nullo-X) known as NONDISJUNCTION
• Fertilization of such abnormal eggs by normal sperm would produce zygotes with an abnormal number of sex chromosomes.
• If an egg with two X chromosomes (XwXw) is fertilized by normal Y chromosome of sperm (Y)- the zygote will be XwXwY. Since each of the X chromosomes in this zygote carries a mutant Xw allele, the resulting fly will have white eyes.
• If an egg without an X chromosome (O) is fertilized by an X-bearing sperm (Xw+), the zygote will be Xw+ O. (O- denotes absence of a chromosome.) Because the single Xw+ in this zygote carries mutant Xw+ allele, the zygote will develop into a red-eyed fly
• It is inferred that XXY flies were female and that XO flies were male. The exceptional F1 white-eyed females that were observed were therefore XwXwY, and the exceptional F1 red-eyed males were Xw+O.
• Bridges confirmed the chromosome constitutions of these exceptional flies by direct cytological observation.
• Because the XO animals were male, Bridges concluded that in Drosophila the Y chromosome has nothing to do with the determination of the sexual phenotype. However, because the XO males were always sterile, he realized that this chromosome must be important for male sexual function.
• Bridges called the abnomaly nondisjunction because it involved a failure of the chromosomes to disjoin during meiotic divisions.

Causes of nondisjunction:

• faulty chromosome movement
  •  imprecise or incomplete homologous pairing,
  • centromere malfunction.
• From Bridges experiment, in F2 generation, exceptional XwXwY females produces high frequency of exceptional progeny, it means, the nondisjunction of sex chromosome goes on disjunction.
• XXY Sex chromosomes can disjoin in different ways:
• Either X chromosomes can disjoin from each other form to form: X and XY
• OR  X chromosome  can disjoin from the Y to form: XX and Y: In the latter case, a diplo- or nullo-X egg is produced because the X that does not disjoin from the Y is free to move to either pole during the first meiotic division. When fertilized by normal sperm, these abnormal eggs will produce exceptional zygotes.
• These early studies with Drosophila—primarily the work of Morgan and Bridges greatly strengthened the view that all genes were located on chromosomes. Thus chromosome are the genetic material for inheritance.

Chromosomal theory of inheritance

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1.Gene:

• Gene is the shortest segment of DNA responsible for the expression of its specific character.
• Gene is also called as the basic unit of heredity. It is small DNA segment that determines a biological character of an organism. Gene is situated on chromosomes.
• Mendel used gene as the term ‘elementi’ for the genes. The term ‘factor’ was actually given by Carl correns.

2. Genome:

• It is the total genetic composition of an organism.

3. Alleles (Allelomorph):

• An Allele is one of a particular form of a gene. Allele is short form allelomorph. The term was coined by Bateson and Saunders (1902) for characters which are contrasting or differing from one another.
• Allele is an alternate form of the same gene in which one is parental and other is maternal. In case of plant height, tallness and dwarfness are the two alleles of a gene.
• A pair of alternative corresponding genes that are occupying the same position or locus of the homologous chromosomes are called allele or allelomorph.

4. Locus:

• The point on a chromosomes where an allele is located is called locus.

5. Dominant allele:

• Out of two alleles, the one that is capable of expressing itself by hiding or suppressing its contrasting allele is known as dominant allele.
• Dominant allele is an allele that expresses itself in the same form whether present in the homozygous or in the heterozygous condition.
• It is represented by capital letter ‘T’ is the dominant allele for tallness.

6. Recessive allele:

• Out of two alleles, the one that is being suppressed by its alternative allele is called as recessive allele.
• Recessive is an allele that does not express itself when present with the dominant allele (in heterozygous form).
• It expresses itself only in the homozygous form. It is represented by small letter as small ‘t’ is the recessive allele for dwarfness.

7. Homozygous:

• A diploid individual carrying two identical alleles is known as homozygous.
• It is pure for a trait or character
• Capital T and T are the two identical alleles for tallness. Similarly, small t and t are the two identical alleles for dwarfness.
• The homozygous organisms when self-pollinated or inbred always gives rise to true breeding homozygous forms.

8. Heterozygous:

• A diploid individual carrying two different alleles is known as heterozygous or hybrid.
• It is impure for a trait (T t).
• This term was proposed by Bateson and Saunders in 1902 for a zygote or a diploid individual developed from it (zygote) which carries both factors of a pair of alleles.

9. Hemizygous:

• When a genotype is determined by a single gene/factor.

10. Genotype:

• A genotype is a genetic expression of an organism. The term was proposed by Johannason 1909 for hereditary or genetic constitution of an individual.
• Genotype is the genetic constitution of an organism.
• For plant height, TT, Tt and tt are the different genotypes.
• The genotype ratio of F2 in monohybrid cross is 1:2:3. It is observable phenomenon.

11. Phenotype (Trait):

• It is the physical or the observable expression of an organism.
• The term was coined by Johannason 1909 for the visible characters of external appearance of an organism with respect to particular character or a group of characters.
• It is the observable characteristic of an individual.
• Though, the phenotype is determined by the genotype, but certain genes do not express for number of reasons. So genotype and phenotype of an organism may not be the same.

12. Character:

• It refers to a general feature of an organism such as eye color, plant height etc.

13. Dominant character:

• The character which express itself in all the members of first filial generation from a cross between two pure breeding parents differing in respect of this character is called dominant character

14. Recessive character:

• The suppressed character which does not appear in first filial generation is called recessive.

15. Parental generation:

• The plants used as parents in a cross are said to represent parental generation, designed by ‘P’.

16. Hybrid:

• The product of a cross between two genetically different individual belonging to the same genus or species or between two different genera is called hybrid.
• It is product of cross in between two dissimilar parents having contrasting characters.

17. Hybridization:

• It is a phenomenon which speaks about the mixing of two unlike genetic constitutions.

18. Monohybrid cross:

• A cross between the two parents that are differing in one pair of alternating (contrasting) character is called monohybrid cross.
• In this type, single character is used.

19. Dihybrid cross:

• A cross between the two parents that are differing in two pairs of alternating character is called dihybrid cross.
• In this type, two contrasting characters are used. For example, seed shape and seed color (round yellow versus wrinkled green).

20. Polyhybrid cross:

• This is a cross between two parents differing in more than two sets of alternating characters.

21. Filial generation:

• The progeny obtained as a result of crossing between parents is a hybrid progeny and is called First filial generation, represented by F1 (filial=offsprings).
• The progeny obtained as a result of self-fertilization among F1 plants represents second filial generation represented as F2.

22. Pure line:

• It is the variety (line) which is homozygous in condition. It is called as true breeding varieties.

23. Reciprocal crosses:

• A set of two reciprocal crosses means that the same two parents are used in two experiments in such a way that in one cross one individual is used as female parent and other as male parent and in second cross of same genotype sexes are reversed. For example, cross between a tall male and dwarf female is reciprocal of a cross between a tall female and dwarf male.
• The purpose of the reciprocal crosses is to find out whether both parents are making equal contribution.

24. Back cross: 

• It is a cross between F1 hybrid with one of its parents (P1 or P2)
• Backcross is often used in genetics for the analysis of genetic constitution of the offsprings.
• It is quick way to make desirable homozygous individuals.

25. Test cross:

• In this cross, the individual of an unknown genotype is crossed with homozygous recessive parents.
• This cross is made in order to determine the genotype of the unknown individual.
• A test cross is conducted to know whether an individual is homozygous or heterozygous for a dominant character.
• The off springs will be 100% dominant, if the individual is homozygous dominant. But the ration will be 50% dominant and 50% recessive in case of hybrid or heterozygous individual.

Terminology used in Mendelian genetics

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• Mendel conducted hybridization experiments on garden pea. He studied the inheritance of seven different morphologically traits on pea plants. These traits are:
  • Height of plant: tall vs dwarf
  • Color 0f flower; purple vs white
  • Position of flower: axial vs terminal
  • Color of seed: yellow vs green
  • Shape of seed: round vs wrinkled
  • Color of pod: Yellow vs green
  • Shape of pod: smooth vs constricted

Steps of Mendel’s experiment

Step 1: Selection of true breeding varieties:

• Mendel selected the true breeding varieties for his experiments as parental generation (P generation). True breeding varieties are the varieties that give rise to same trait for number of successive generations.
• Mendel allowed several generations of pea plant to self-pollinate and verified that each variety was genetically pure (homozygous for each of the traits that he chose to study) by growing the plants for two generations and confirming that all offspring were same as their parents.

Step 2: Hybridization of true breeding varieties:

• Hybridization is a process when two individuals having contrasting character are crossed.
• Mendel conducted crosses between different true breeding plants to obtain hybrid plants. Theese hybrids were named as F1 (first filial) generation or F1 plants. When Mendel examined the F1 plants, he found that expressed only one of the phenotypes present in the parental generation.
• He also conducted reciprocal cross but result was same in F1 generation.

Step 3: Self-pollination of F1 plants:

• When two individuals having same genotype are crossed, are called selfing or self-pollination.
• Mendel planted F1 seeds and cultivated the plants and allowed them to self-pollinate producing a second generation or F2 plants in 3:1 ratio of parental triats.

Mendel’s result:

Mendel’s result appeared in the following pattern:

• For any character the F1 generation derived from crosses between different varieties showed only one of the trait and never the other. The form of trait expressed was also called as dominant and the one that remained hidden as recessive.
• It did not matter which parent variety provided the pollen and which the ova: but the results of both were always the same.
• The trait which was disappeared or been hidden in the F1 generation reappeared in the F2 generation but only in frequency one quarter that of the total number. It means that in F2 generation, both the parental forms are reappeared in the ratio of 3:1 of which 50% were recombinants or hybrids.
• The ratio of the dominant from to recessive form was always 3:1 that is the dominant form was three times the number of the recessive form.

Example: Mendel’s monohybrid between Tall pea plant and dwarf pea plant:

• In an artificial cross between two pure breeding plants (stock or parent), one tall plant and the other dwarf plant were cross pollinated.
• The seeds obtained from cross pollination  are cultivated to developed plants which represented the first filial generation (F1).

Result of Mendel’s monohybrid cross between Tall and dwarf pea plant:

• In F1 generation, all offspring were tall plant. They were not intermediate between the tall and dwarf as might be expected.
• Similar results were obtained regardless of whether pollen grains or ova taken from dwarf plants or tall plant.
• As Mendel noticed, one of the contrasting character is dominant and other is recessive. Tall character expressed among all F1 generation so, it is represented as dominant character whereas dwarf character remains unexpressed in F1 generation and is represented as recessive character.
• Mendel allowed the hybrid plants of first generation to self-pollination.
• The seeds obtained from F1 plants were planted which developed into plants of second filial generation (F2).
• In F2 generation both tall and dwarf characters appeared in the ratio of approximately 3 tall to one dwarf (3:1).
• Mendel look 1064 F2 hybrid and grew them into plants. Among F2 plants, 787 were tall and 277 short (dwarf), i.e. in the ratio of 2.84 tall to 1 dwarf or on allowing for experimental error due to chance, it comes to 3:1.
• Plants of F2 generation were allowed self-pollination and the F3 generation was raised.
• Result of F3 generation:
  • All F2 dwarf plants (1/4) on self-pollination produced seeds which developed into dwarf plants
  • Out of 3/4 Tall plant in F2, 1/3 produced only talls whereas the remaining 2/3 produced both tall and dwarf plants in 3:1 ratio as in F1 generation.

Mendel’s experiment on pea plant

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• Mendel started his work on pea in 1856 and derived law of inheritance in 1865.
• Mendel’s work was unfortunately paid no attention and ignored for 34 years. This is perhaps due to the controversies arisen from the publication of Darwin’s book on ‘origin of species’.
• In 1990, three biologists, who rediscovered the Mendel’s laws and found it correct. Three scientist were a Dutch biologist Hugo de Vries, a German botanist Carl Correns and an Austrian botanist Erich von Tschemark.
• After the rediscovery, Mendel’s laws were widely accepted and called Mendel as the ‘Father of Classical Genetics’.
• Before Mendel, other researches have been working on several varieties of plants differing from one another in many complex characters produced fertile hybrids by artificial pollination, but they failed to explain the mechanism of heredity.
• The following were the two reasons behind their failure:
  • They considered simultaneously many characters in which parents differed. This made them confused. Due to confusion they could neither trace the individual characters through successive generations nor they could maintain the complete numerical records of results
  • Secondly, they believed that the hereditary characters of two parents (mother and father) become thoroughly mixed in the offspring.
• Mendel did realize the above two causes of his predecessor’s failure. In order to overcome those difficulties he carefully planned experiments exactly on the same pattern as his predecessors had already followed.
•  He began his breeding experiments with different varieties of garden peas in 1856.

Reason for the selection of Pea plant

• Mendel made careful selection of garden pea, Pisum sativum as the plant material for his experiments, as it has the following advantages:
• Pea plants possess many varieties with well defined characters. Among 20-30 different characters he choose seven different ‘unit characters’ for his study.
• The flowers of pea plants are bisexual.
• Flower is closed typed such that pollination is limited to self.
• Cross pollination can be easily done by removing stamens before pistils of flower mature by the process called emasculation and cross with stigma of desired pea plant.
• Cultivation of pea plant is easy, economic and required small space.
• Life span of pea plant is short, thus many generation can be obtained in a single growing season.
• The hybrids of pea plant produced by cross pollination are perfectly fertile

Reasons for Mendel’s success:

• The Mendel’s success was dependent on his careful selection of pea plant. He was also lucky that the plant did not show incomplete dominance or epistasis.
• Mendel studied all the seven pairs of differing contrasting characters individually in both original stocks (parent pea plant) and then in the hybrids offspring in different generations. He classified the offspring according to their characters as dominant and recessive and also maintained the record of individuals having particular characters. This quantitative method of recording the number of individuals having particular characters is vital for Mendel’s success
• Mendel studied the inheritance of one character at a time, while his predecessors considered the organism as a whole.
• He carried out experiments to F2 and F3 generations only.

Mendel’s genetics: Selection of pea plant and Reasons for Mendel’s success

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