PRINCIPLES OF INHERITANCE AND VARIATION :

 

INTRODUCTION: The field of genetics was coined by W. Bateson and encompasses the collective study of heredity and variations in organisms. Heredity refers to the transmission of genetic traits from parents to their offspring. Variations are the differences between individuals of the same species. The history of genetics research includes notable figures such as G.J. Mendel, who is considered the father of genetics, W. Bateson, the father of modern genetics, and Morgan, who is considered the father of experimental genetics. Morgan conducted experiments on Drosophila and proposed various concepts such as linkage, sex linkage, crossing over, criss-cross inheritance, and linkage maps on Drosophila. A. Garrod is considered the father of human genetics and biochemical genetics. Garrod discovered the first human metabolic genetic disorder called alkaptonuria, also known as black urine disease.

SOME GENETICAL TERMS:

1. Genes: These are the units of heredity that are responsible for the inheritance and expression of traits. The term “gene” was coined by Johannsen in 1909, while Mendel used the terms “element” or “factor.” Morgan was the first to use symbols to represent genes. Dominant genes are represented by capital letters, while recessive genes are represented by small letters.
2. Alleles: These are alternative forms of a gene located at the same position (locus) on homologous chromosomes. The term “allele” was coined by Bateson.
3. Homozygous: An organism that has two identical alleles for a particular trait is said to be homozygous. For example, TT or rr.
4. Heterozygous: An organism that has two different alleles for a particular trait is said to be heterozygous. For example, Tt or Rr. The terms “homozygous” and “heterozygous” were coined by Bateson.
5. Hemizygous: An individual that carries only one copy of a gene pair is said to be hemizygous. Male individuals are always hemizygous for sex-linked genes.
6. Phenotype: The observable physical and morphological characteristics of an organism for a particular trait.
7. Genotype: The genetic makeup or genetic constitution of an organism for a particular trait. The terms “phenotype” and “genotype” were coined by Johannsen.
8. Phenocopy: When different genotypes are exposed to different environmental conditions and produce the same phenotype, they are referred to as phenocopies.
9. Hybrid Vigor/Heterosis: The superiority of offspring over their parents is called hybrid vigor or heterosis. This phenomenon arises due to heterozygosity. Hybrid vigor can be maintained for a long time in vegetatively propagated crops. However, it can be lost by inbreeding (selfing) as inbreeding induces homozygosity in offspring. The loss of hybrid vigor due to inbreeding is called inbreeding depression.

MENDELISM :

 

Mendelism refers to the experiments conducted by Gregor Johann Mendel on genetics, which involved the description of mechanisms of hereditary processes and the formulation of principles. Mendel postulated various experimental laws in relation to genetics.

Mendel was born on July 22, 1822, in Heinzendorf in Austria’s Silesia village and worked as a monk in the Augustinian Monastery at Brunn city, Austria.

In 1856-57, he started his historical experiments on the pea (Pisum sativum) plant, and his experimental work continued on the same plant until 1863 in the 19th century. The results of his experiments were published in 1865 in a paper named “Experiment in plant Hybridization.” However, Mendel was unable to gain any popularity, and no one understood his work. He died in 1884 without getting any credit for his work.

After Mendel’s death, his postulates were rediscovered 16 years later in 1900 by three scientists independently. Carl Correns (Germany) experimented on maize, Hugo de Vries (Holland) experimented on evening primrose, and Erich von Tschermak Seysenegg (Austria) experimented on different flowering plants.

The credit for rediscovery of Mendelism goes to these three scientists. Correns gave two laws of Mendelism, namely the Law of Heredity/Inheritance/Mendelism, which includes the Law of Segregation and the Law of Independent Assortment. Mendel’s experiments remained hidden for 34 years because communication was not easy at that time, and his work could not be widely publicized.

Additionally, Mendel’s concept of genes as stable and discrete units that controlled the expression of traits was not accepted by his contemporaries as an explanation for the continuous variation seen in nature. Finally, Mendel’s approach of using mathematics to explain biological phenomenon was new and unacceptable to many biologists of his time.

Despite the challenges, Mendel’s experiments succeeded for several reasons. Firstly, Mendel studied the inheritance of one or two characters at a time, unlike his predecessors who considered many characters at a time. Secondly, Mendel’s selection of garden pea plant was suitable for studies due to its many contrasting traits, natural self-pollination, and the possibility of artificial cross-pollination. Thirdly, Mendel quantitatively analyzed the inheritance of qualitative characters, maintained statistical records of all the experiments, and had a large sampling size.

 

MENDELS WORK

Gregor Mendel, an Austrian monk, is considered to be the father of genetics. He studied the inheritance of traits in pea plants and established the principles of heredity. Mendel studied seven pairs of contrasting traits, also known as characters, in garden peas. These traits included stem height, flower position, shape of pod, pod color, seed shape, seed color, and flower color.

Mendel observed these traits in the F2 progenies of garden pea plants and recorded the number of plants that exhibited the dominant and recessive traits. His data showed that the ratio of dominant to recessive traits was approximately 3:1 for all seven traits, with an average ratio of 2.98:1.

Mendel also discovered that the genes responsible for these traits were located on four different pairs of chromosomes – the first, fourth, fifth, and seventh chromosomes. Two of the genes were located on chromosome 1, and three were located on chromosome 4. The genes were found to be far apart on the chromosomes, except for the genes that controlled plant height and pod shape.

Interestingly, Mendel discovered that the gene that controlled seed coat color and flower color was the same gene. This gene is called a pleiotropic gene because it controls more than one character.

Mendel also observed that wrinkled seeds were caused by the absence of a starch branching enzyme (SBE). In these seeds, free sugar was present instead of starch.

Mendel’s work with garden pea plants helped establish the principles of inheritance and paved the way for modern genetics. His findings are still widely studied and applied in the field of genetics today.

Mendel’s Technique

Gregor Mendel used artificial pollination techniques to study the inheritance of traits in pea plants. He selected several true-breeding pea lines, which were plants that exhibited the same trait for several generations after undergoing continuous self-pollination.

Mendel chose 14 pairs of true-breeding pea plants that were similar in all aspects except for one character that had contrasting traits. To conduct the artificial pollination, Mendel developed a technique known as Emasculation and Bagging.

Pea plant flowers are bisexual, meaning that they contain both male and female reproductive structures. In Mendel’s technique, one flower is considered the male flower and the other the female flower. Emasculation involves removing the anther from the male flower at an immature stage to prevent self-pollination.

To prevent undesirable cross-pollination, the emasculated flower is then covered by a bag, which is called bagging. Mature pollen grains are collected from the male plants and spread over the emasculated female flower.

After pollination, the female flower produces seeds. The plants that grow from these seeds are known as the first filial generation or F1 generation. Mendel then self-pollinated the F1 plants to produce the second filial generation or F2 generation.

In conclusion, Mendel’s technique of artificial pollination, which involved emasculation and bagging, allowed him to selectively breed pea plants to study the inheritance of traits. His groundbreaking work with pea plants laid the foundation for the field of genetics.

Mendels Monohybrid Cross 

In Mendel’s monohybrid cross experiment, he studied the inheritance of seed color in pea plants. He selected two true-breeding pea plants: one with yellow seeds and the other with green seeds. A true-breeding plant is one that produces offspring with the same traits as the parent plant.

Mendel then crossed these two true-breeding plants by transferring pollen from the male flower of one plant to the female flower of the other plant. The resulting offspring, known as the first filial generation or F1 generation, all had yellow seeds.

Mendel then self-pollinated the F1 generation to produce the second filial generation or F2 generation. The F2 generation had a ratio of 3:1 for yellow seeds to green seeds. This result was unexpected, as many scientists of the time believed that the traits of the parent plants would blend in the offspring.

Mendel’s experiment showed that the traits of the parent plants did not blend in the offspring, but instead were passed down independently. He proposed that each trait was determined by a pair of factors, which we now know as genes. One of the factors was dominant and the other was recessive. In the case of seed color in pea plants, the dominant factor was for yellow seeds and the recessive factor was for green seeds.

Mendel’s monohybrid cross experiment was groundbreaking because it demonstrated the principles of segregation and independent assortment. Segregation refers to the separation of the two factors during the formation of gametes, or sex cells, so that each gamete carries only one factor. Independent assortment refers to the random distribution of the factors during the formation of gametes.

Mendel’s monohybrid cross experiment also showed that genes are responsible for the inheritance of traits and that they are passed down from generation to generation in predictable patterns. This laid the foundation for modern genetics and the study of heredity.

In conclusion, Mendel’s monohybrid cross experiment with pea plants was a significant breakthrough in the field of genetics. His experimental results showed that traits are determined by genes that are passed down independently from each parent and that these genes can be dominant or recessive. His work paved the way for modern genetics and our understanding of heredity.

Diagram : Cross

DIHYBRID CROSS 

A dihybrid cross is a breeding experiment in which the inheritance of two pairs of contrasting traits is studied. Gregor Mendel, the father of genetics, conducted a dihybrid cross with garden peas to study the relationship between two pairs of heterozygous traits.

Mendel selected two pairs of traits: seed shape (round or wrinkled) and seed color (yellow or green) in garden peas. He denoted the dominant trait by a capital letter and the recessive trait by a small letter. In this case, Round (R) was dominant over Wrinkled (r), while Yellow (Y) was dominant over Green (y).

Mendel crossed a homozygous round and yellow seeded plant (RRYY) with a homozygous wrinkled and green seeded plant (rryy). All the F1-generation plants produced had yellow and round seeds, indicating that these traits were dominant.

When the F1 plants were self-pollinated, four different kinds of plants were produced in the F2-generation. These were yellow-round, yellow-wrinkled, green-round, and green-wrinkled. The ratio of these plants was 9:3:3:1, which is known as the dihybrid ratio.

The phenotypic ratio of the F2 generation was 9:3:3:1, which means that out of 16 plants, 9 had round and yellow seeds, 3 had round and green seeds, 3 had wrinkled and yellow seeds, and 1 had wrinkled and green seeds.

The genotypic ratio of the F2 generation was 1:2:2:4:1:2:1:2:1, which means that out of 16 plants, 1 had the genotype RRYY, 2 had RrYY, 2 had RRYy, 4 had RrYy, 1 had rrYY, 2 had rrYy, 1 had RRyy, 2 had Rryy, and 1 had rryy.

Mendel also observed that the parental combination (Round yellow and Wrinkled green) was present in a ratio of 10:6. However, he also found new combinations in the F2 generation, due to independent assortment.

From these results, Mendel concluded the law of independent assortment. This law states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters. This conclusion was based on the F2-generation of the dihybrid cross.

However, there is an exception to this law, which is known as linkage. Linkage occurs when two genes are located close to each other on the same chromosome, and they tend to be inherited together.

Dia : Dihybrid cross

BACK CROSS

A back cross is a type of breeding experiment that involves crossing the first filial (F1) generation with one of its parents. The purpose of this cross is to determine the genotype of the F1 individuals by observing the phenotypic ratio of the offspring in the next generation.

In a back cross, there are two types: outcross and testcross. In an outcross, the F1 individual is crossed with a homozygous dominant parent. The resulting offspring will all have the dominant phenotype, making it impossible to determine the genotype of the F1 individual based on their phenotype alone. This type of cross is mainly used to obtain a large number of individuals with the dominant phenotype for further analysis.

For example, suppose a plant breeder wanted to determine if an F1 plant that produced purple flowers was heterozygous for flower color (Pp) or homozygous dominant (PP). The breeder would perform an outcross with a homozygous dominant parent (PP). The resulting offspring would all have purple flowers, but their genotype would be Pp. This would confirm that the F1 plant was indeed heterozygous for flower color.

In a testcross, the F1 individual is crossed with a homozygous recessive parent. The resulting offspring will have a 1:1 phenotypic ratio of the dominant and recessive phenotypes, allowing the genotype of the F1 individual to be determined based on the observed phenotypic ratio.

For example, suppose a plant breeder wanted to determine if an F1 plant that produced purple flowers was heterozygous for flower color (Pp) or homozygous dominant (PP). The breeder would perform a testcross with a homozygous recessive parent (pp). The resulting offspring would have a 1:1 phenotypic ratio of purple flowers (Pp) and white flowers (pp). If all the offspring had purple flowers, it would suggest that the F1 plant was homozygous dominant (PP), while a 1:1 ratio would suggest that it was heterozygous (Pp).

Diagram 

Reciprocal cross is a breeding experiment in which two parents are used in two separate experiments, but their roles as male and female are switched. In the first experiment, “A” is used as the female parent and “B” as the male parent, while in the second experiment, “A” is used as the male parent and “B” as the female parent. This type of cross is used to test whether there is any difference in the inheritance pattern of a trait depending on which parent is male and which parent is female.

Reciprocal cross is most effective for studying the inheritance of traits controlled by genes located on autosomes, i.e., non-sex chromosomes. In such cases, the results obtained from reciprocal crosses are expected to be the same. However, when the trait is controlled by genes located on sex chromosomes or by cytoplasmic factors, the results of reciprocal crosses may differ.

For example, let’s consider a hypothetical trait controlled by a gene located on an autosome. We perform a reciprocal cross between two pea plants, one homozygous dominant (TT) and the other homozygous recessive (tt). In the first experiment, the TT plant is used as the female parent, and the tt plant is used as the male parent. In the second experiment, the tt plant is used as the female parent, and the TT plant is used as the male parent. If the trait is controlled by autosomal genes, then the results of both experiments should be the same – all the offspring from both experiments should be heterozygous (Tt) and show the dominant phenotype.

However, if the trait is controlled by a gene located on the sex chromosome or by cytoplasmic factors, the results may differ. For example, if we perform a reciprocal cross between a male fruit fly with white eyes and a female with red eyes, we would get different results depending on which parent is male and which parent is female. If the male fruit fly is crossed with the female with red eyes, all the F1 offspring would have red eyes. However, if the female with white eyes is crossed with the male fruit fly, all the F1 offspring would have white eyes. This is because the gene for eye color in fruit flies is located on the X chromosome, and males only have one X chromosome. Therefore, the trait is sex-linked, and the results of the cross are affected by whether the male or female parent carries the dominant allele.

 

GENE INTERACTION

Gene interaction refers to the phenomenon where two or more genes work together to determine a particular trait. There are two types of gene interactions: allelic interaction or intragenic interaction, and non-allelic interaction or intergenic interaction.

(i) Allelic interaction/Intragenic interaction:

Allelic interaction takes place between alleles of the same gene that are present at the same locus. An example of allelic interaction is incomplete dominance.

Incomplete dominance: In incomplete dominance, both the dominant and recessive alleles are expressed, resulting in an intermediate phenotype. An example of incomplete dominance is the flower color in Mirabilis jalapa, also known as the “4 O’ clock plant” or “Gul-e-Bans”. When plants with red flowers are crossed with those having white flowers, the F1 generation has pink flowers. The F2 generation has a 1:2:1 phenotypic ratio of red, pink, and white flowers, and a 1:2:1 genotypic ratio of RR:Rr:rr.

Other examples of incomplete dominance are the flower color in Antirrhinum majus, also known as the “Snapdragon” or “Dog flower,” feather color in Andalusian fowls, and the size of starch grains in pea plants.

(ii) Non-allelic interaction/Intergenic interaction:

Non-allelic interaction takes place between genes that are not allelic, i.e., genes that are present on different loci. An example of non-allelic interaction is epistasis.

Epistasis: Epistasis is the interaction between genes where the expression of one gene is dependent on the presence of one or more other genes. An example of epistasis is the coat color in Labrador retrievers. The gene for coat color has two alleles, B (black) and b (brown). However, the expression of this gene is dependent on another gene, E, which determines the presence or absence of pigmentation. If the dog has the homozygous recessive ee genotype, it will have a yellow coat, regardless of its genotype at the B locus.

In conclusion, gene interaction plays an important role in determining traits, and its understanding is essential in fields such as genetics and breeding.

Co-dominance: In this phenomenon, both alleles of a gene express for a particular character equally in the F1 progeny, without any blending. Examples are: (i) Coat colour in cattle: Co-dominance is observed in the coat colour of cattle. When a black parent is crossed with a white parent, a roan colour F1 progeny is produced. Upon obtaining F2 generation from the F1 generation through sib-mating cross, the ratio of black: black-white (roan): white cattle is obtained in 1:2:1. BLACK × WHITE R1R1 R2R2 F1 generation R1R2 (Roan) Sib-mating cross R1 R2

R1 R1R1 R1R2 R2 R1R2 R2R2 R1R1 = Black -1 R1R2 = Roan – 2 R2R2 = White – 1 The ratio of phenotype and genotype in co-dominance is always 1:2:1. Other examples of Co-dominance: (ii) Inheritance of AB blood group (IAIB) (iii) Carrier of Sickle cell anaemia (HbAHbS)

 

Multiple Alleles: Multiple alleles refer to the presence of more than two alternative forms of the same gene at the same locus on homologous chromosomes. These multiple alleles arise due to mutations in the gene. Each diploid individual can contain only two alleles for a character, while each gamete can contain only one allele. The number of different possible genotypes for a gene with n alleles can be expressed as n(n+1)/2.

Example of Multiple Alleles: The ABO blood group system is determined by three alleles – IA, IB, and i. IA and IB are codominant and are both dominant over i, which is recessive. The possible phenotypes in the ABO blood group system are A, B, AB, and O.

 

Pleiotropic gene : A gene that controls multiple characters is called a pleiotropic gene. Such genes exhibit multiple phenotypic effects due to their influence on metabolic pathways that contribute towards different phenotypes. For example, the disease phenylketonuria in humans is caused by a mutation in a single gene that codes for the enzyme phenylalanine hydroxylase, resulting in mental retardation and reduced hair and skin pigmentation. While we have so far observed the effect of a gene on a single phenotype or trait, there are cases where a single gene can produce multiple phenotypic expressions. In the pea plant, for instance, seed shape and the size of starch grains are controlled by the same gene located on the 7th chromosome. The gene responsible for starch synthesis has two alleles (B and b), with BB homozygotes producing large starch grains and bb homozygotes producing small ones. The starch grains produced by heterozygotes (Bb) are of intermediate size, indicating incomplete dominance. Therefore, dominance depends on the gene product and the phenotype chosen for examination, rather than being an autonomous feature of the gene or its product.

 Linkage

Linkage refers to the phenomenon where certain genes remain together during inheritance because of their location on the same chromosome. This was first observed in Lathyrus odoratus and later in Drosophila, where the term “linkage” was coined. Linked genes are arranged linearly on the same chromosome and can be separated by crossing over. The strength of linkage is inversely proportional to the distance between the genes. Factors like temperature, X-rays, and distance affect the probability of crossing over.

Linked genes can be arranged in cis- or trans-arrangements, with cis-arrangement being an original form where two dominant or two recessive genes are located on one chromosome, and two types of gametes can be produced. In trans-arrangement, one dominant and one recessive gene are present on each chromosome, and crossing over can lead to two different types of gametes.

There are two types of linkage: complete linkage, where genes always show parental combinations and never form new combinations due to absence of crossing over, and incomplete linkage, where new combinations appear along with parental combinations due to crossing over. The percentage of new combinations is equal to the percentage of crossing over, and the distance can be identified by the percentage of recombination.

A linkage group is a group of linearly arranged linked genes that are inherited as a single unit due to their being present on a chromosome. All the genes located on one pair of homologous chromosomes form one linkage group, which is equal to the haploid number of homologous chromosomes.

SEX DETERMINATION :

Sex determination refers to the process by which an individual’s sex is established through differential development at an early stage of life. This process involves various methods such as environmental factors, non-allosomic genetic determination, allosomic sex determination, and haplodiploidy. In most plants and animals, sex determination is based on factors that determine whether an individual will develop as male, female, or hermaphrodite.

There are three types of sex determination based on fertilization: progamic, syngamic, and epigamic. Progamic sex determination occurs before fertilization, as seen in drones of honey bees. Syngamic sex determination occurs during fertilization, which is the case in most plants and animals. Epigamic sex determination occurs after fertilization, as seen in female honey bees.

The chromosomal theory for sex determination was proposed by Wilson and Stevens. Chromosomes can be divided into two types: autosomes and allosomes. Autosomes, also known as somatic chromosomes, regulate somatic characteristics. Allosomes, heterosomes, or sex chromosomes are associated with sex determination. Sex chromosomes were first discovered by McClung in grasshoppers. The X chromosome, also known as the X body, was discovered by Henking, who found that male bugs had one chromosome with no homologue in their testes.

There are different types of mechanisms for sex determination in organisms:

1. XX-XY type: In this type, females are homogametic and produce only one type of gamete, while males produce two types of gametes – X-chromosome containing (gynosperm) and Y-chromosome containing (androsperm). This type is found in humans, Drosophila, and dioecious plants like Coccinea and Melandrium.
2. ZW-ZZ type: In this type, females are heterogametic and produce two types of gametes (ZW), while males are homogametic and produce only one type of gamete (ZZ). This type is found in birds, fishes, reptiles, and some insects like butterflies and moths. It also occurs in the plant Fragaria elatior.
3. XX-XO type: In this type, males have a deficiency of one chromosome, and females are homogametic while males are heterogametic. It is found in grasshoppers, squash bugs, cockroaches, Ascaris, and in plants like Dioscorea sinuta and Vallisneria spiralis.
4. ZO-ZZ type: In this type, females have an odd sex chromosome while males have two homomorphic sex chromosomes (AA + ZZ). Females produce two types of eggs, one with the sex chromosome (A + Z) and the other without (A + O). Males produce similar types of sperm. This type is found in some butterflies and moths, and it is opposite to the condition found in cockroaches and grasshoppers.
5. Haplodiploidy: In this type, one sex is haploid (male) while the other is diploid (female). This type is found in honeybees, wasps, and ants. In honeybees, males (drones) develop from unfertilized eggs (haploid), while queens and worker bees develop from fertilized eggs (diploid).

SEX DETERMINATION IN HUMAN

In humans, sex determination is based on the presence of the Y chromosome. The Y chromosome carries a gene called the SRY (Sex-determining Region Y) gene, which is responsible for the development of male reproductive organs during embryonic development. The SRY gene is located on the Y chromosome, which means that only males can carry it.

During fertilization, the egg cell from the mother contains an X chromosome, while the sperm cell from the father can carry either an X or a Y chromosome. If the sperm cell carries an X chromosome, the resulting zygote will be XX and will develop into a female. However, if the sperm cell carries a Y chromosome, the resulting zygote will be XY and will develop into a male.

After fertilization, the presence of the SRY gene on the Y chromosome causes the undifferentiated gonads (the embryonic structures that will develop into testes or ovaries) to differentiate into testes. The testes then produce testosterone and other hormones that drive the development of male reproductive organs such as the penis, scrotum, and prostate gland.

In the absence of the SRY gene, the gonads will develop into ovaries, and the fetus will develop female reproductive organs such as the uterus, ovaries, and vagina. In addition to the SRY gene, other genes on the X and Y chromosomes also play a role in sex determination and development of reproductive organs.