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Essay on Genetics
Essay Contents:
- Essay on the Meaning of Genes
- Essay on Mendelian Genetics
- Essay on the Punnett Square
- Essay on the Mendelian Principles
- Essay on the Test Cross
- Essay on the Backcrossing
- Essay on the Limitations of Mendelian System
- Essay on the Polygenic or Quantitative Inheritance
- Essay on Multiple Alleles
- Essay on the Chromosomal Theory of Inheritance
- Essay on Linkage and Crossing Over
- Essay on Mutations
1. Essay on the Meaning of Genes:
The term ‘gene’ was coined by Danish botanist Wilhelm Johannsen in 1909. It is the basic physical and functional unit of heredity. Heredity is the transfer of characters from parents to their offspring that is why children resemble their parents. A hereditary unit consists of a sequence of DNA (except in some viruses that contain RNA, instead) that occupies a specific location on a chromosome and determines a particular characteristic in an organism. DNA is a vast chemical information database that carries the complete set of instructions for making all the proteins that a cell will ever need.
Each gene contains a particular set of instructions, usually coding for a particular protein. Genes achieve their effects by directing protein synthesis. The sequence of nitrogenous bases along a strand of DNA determines the genetic code. When the product of a particular gene is needed, the portion of the DNA molecule that contains that gene splits, and a complementary strand of RNA, called messenger RNA (mRNA), forms and then passes to ribosomes, where proteins are synthesized.
A second type of RNA, transfer RNA (tRNA), matches up the mRNA with specific amino acids, which combine in series to form polypeptide chains, the building blocks of proteins. Experiments have shown that many of the genes within a cell are inactive much or even all of the time, but they can be switched on and off.
DNA resides in the core, or nucleus, of each of the body’s trillions of cells. Every human cell (with the exception of mature red blood cells, which have no nucleus) contains the same DNA. Each human cell has 46 molecules of double-stranded DNA. Human cells contain two sets of chromosomes, one set inherited from the mother and one from the father. (Mature sperm and egg cells carry a single set of chromosomes). Each set has 23 single chromosomes – 22 autosomes and an X or Y sex chromosome. (Females inherit an X from each parent, while males get an X from the mother and a Y from the father.) In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases.
The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes. Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features. Genes carry information that determines the traits, the characteristics we inherit from our parents. The branch of biology that deals with heredity, especially the mechanisms of hereditary transmission and the variation of inherited characteristics among similar or related organisms is known as genetics.
2. Essay on Mendelian Genetics:
Sir Gregor Johann Mendel (1822 to 1884) was Austrian monk who used garden pea (Pisum sativum) for his experiments and published his results in 1865. His work, however, was rediscovered in 1900, long after Mendel’s death, by Tschermak, Correns and DeVries. Mendel was the first to suggest principles underlying inheritance. He is regarded as the founder or father of genetics. He developed the concept of the factors to explain results obtained while cross breeding strains of garden peas. He identified physical characteristics (phenotypes), such as plant height and seed colour, which could be passed on, unchanged, from one generation to another.
The hereditary factor that predicted the phenotype was later termed a “gene”. The genetic constitution of an organism is known as genotype. Mendel hypothesized that genes were inherited in pairs, one from the male and one from the female parent. Plants that bred true had inherited identical genes (homozygotes) from their parents, whereas plants that did not breed true inherited alternative copies (hybrids, or heterozygotes) of the genes (alleles) from one parent that were similar, but not identical, to those from the other parent.
Alleles are the alternative forms of the same gene which determine contrasting characters. One chromosome might contain a version of the eye colour gene that produces blue eyes, and other chromosome might contain a version that produces brown eyes. If an individual has both versions of the gene, the individual is heterozygous for the eye colour trait. If an individual has the same version of the eye colour gene on both chromosomes, the individual is homozygous for the eye colour trait. In case plants the allelic character of height are the tall (T) and dwarf (t).
Alleles are one alternative of a pair or group of genes that could occupy a specific position on a chromosome. Genes are composed of sequences of nucleotides, and a variation in this sequence can affect the protein made from that gene. A change in the manufacture of a protein in an organism often leads to an observable result. There are many different alleles for the gene that manufactures protein to give humans their unique eye colour. There are two alleles for flower colour in the common garden pea.
Some of these alleles had a greater effect on the phenotypes of hybrids than others. For example, if a single copy of a given allele was sufficient to produce the same phenotype seen in homozygous organisms, that gene is termed a “dominant”. Conversely, if the allele could only be detected in the minority of the offspring of hybrid parents that were homozygous for that “weaker” allele, the gene is termed a “recessive”. Dominant and recessive are relative terms. Consider a plant with a gene for red flower colour and a gene for blue flower colour.
This plant bears red flowers, although it has a gene for blue flower colour, too. Red flower colour is the dominant trait, while blue flower colour is the recessive trait. The red colour gene in a sense overpowers the blue colour gene. In order for the plant to have blue flowers, it would need to completely lack the gene for red flower colour. Dominant traits are normally represented by uppercase letters, such as R. The corresponding recessive trait would be represented by a lowercase letter, r. A plant with genotype Rr will have red flowers, as would a plant with genotype RR. But a plant with genotype rr would have blue flowers.
Mendelian genetics, also known as classical genetics, is the study of the transmission of inherited characteristics from parent to offspring. Gregor Mendel actually calculated the ratios of observable characteristics in the common garden pea plant Pisum sativum. Mendel studied seven characteristics in peas including seed texture, seed colour, flower colour, flower position; stem length, pod shape and pod colour (Fig. 6.1). Peas were a good model system, because he could easily control their fertilization by transferring pollen with a small paintbrush.
This pollen could come from the same flower (self-fertilization), or it could come from another plant’s flowers (cross-fertilization). Because the seven pea plant characteristics tracked by Mendel were consistent in generation after generation of self- fertilization. These parental lines of peas could be considered pure-breeders (or, in modern terminology, homozygous for the traits of interest). Mendel and his assistants eventually developed 22 varieties of pea plants with combinations of these consistent characteristics. He applied mathematics and statistics to analyze the results obtained by him.
Mendel started his pea breeding program by allowing certain pea plants to repeatedly self-fertilize. Peas are able to fertilize their own flowers which are called selfing. If pea selfing continues over many generations the pea plants will be homozygous or have an identical pair of genes for a certain characteristic. These plants will contain either two identical recessive genes (homozygous recessive) for a characteristic or two identical dominant genes (homozygous dominant) for the same characteristic and are considered pure-breeding for those characteristics.
For example, purple flower colour in peas is dominant and white flower colour in peas is recessive. When a white flowered (homozygous recessive) pea plant is crossed with a purple flowered (homozygous dominant) pea plant, the resulting offspring all has purple flower colour.
The gene composition (genotype) for the flower genes in each of these types of pea plants is represented as shown below:
Before Mendel’s work, the most popular theory of inheritance stated that the qualities of the parents blended to form the qualities of the child. Under this theory, one tall parent and one short parent would produce a child of medium height. Most ordinary observations seemed to support this hypothesis, which rejected the notion of discrete units of inheritance (i.e., genes). However, this theory was poorly equipped to deal with such phenomena as two brown-eyed parents giving birth to a blue-eyed baby. Like that, when Mendel cross- pollinated one variety of pure bred plant with another, these crosses would yield offspring that looked like either one of the parent plants, not a blend of the two.
In another instance, when Mendel cross-fertilized plants with wrinkled seeds to those with smooth seeds, he did not get progeny with semi-wrinkled seeds. Instead, the progeny from this cross had only smooth seeds. In general, if the progeny of crosses between pure bred plants looked like only one of the parents with regard to a specific trait, Mendel called the expressed parental trait the dominant trait.
Mendel used characteristics of pea plants and four o’clock flowers (Mirabilis jalapa) to analyze the hereditary patterns of these traits. His historic experiments led him to the conclusion that inherited characteristics were carried in discrete, independent units (later named genes). In Mendel’s interpretation, hereditary characteristics occurred in pairs of factors that had specific relationships. Mendel first crossbred one tall, true-breeding plant with one short, true-breeding plant.
Contrary to the blending theory, all the offspring were tall. In terms of genotype, the original tall plant was TT (two dominant alleles; homozygous), the short plant was tt (two recessive alleles; homozygous), and the second-generation plants were Tt (one dominant and one recessive allele; heterozygous). When Mendel next allowed these plants to self-fertilize, he found that the short trait reappeared in the third generation. The ratio of short to tall plants was almost exactly 3:1. Their genotypes were as follows -1 short (tt) : 2 tall (Tt): 1 tall (TT). Based on these observations (Fig. 6.2), Mendel formulated a series of laws that are the basis of what we now term “Mendelian” inheritance patterns.
3. Essay on the Punnett Square:
Mendel worked by observing characteristics (phenotypes) and calculating the ratios of each type to form his principles of inheritance. However we can predict the ratios of phenotypes by using Mendel’s principles. One of the most common methods of determining the possible outcome of a cross between two parents is called a Punnett square. To perform a Punnett square one must first figure out all the possible combinations of the alleles to be studied for each parent.
The possible gametes for one parent go on the X axis and the possible gametes for the other parent go on the Y axis (one allele in each cell of the upper row (traditionally the mother) and rightmost column (traditionally the father). The gamete combinations are then paired in the squares below and to the side of each type, i.e. the offspring’s genotypes are then calculated by observing the intersection of the mother’s and father’s individual alleles (much like a multiplication table).
Example 1:
Eye colour in human is much more complex. A mother and father, both having the brown eye phenotype, have a child. We know that both parents carry the gene for blue eye colour and therefore are heterozygous for this trait. These parents can either donate a dominant B to the gamete or a recessive b to the gamete (Fig.6.3).
The outcome of this cross shows that 3 times out of 4 (75%) the child will have brown eyes and 1 out of 4 times the child will have blue eyes (25%). The probability that the child’s genotype will be heterozygous, for eye colour alleles, is 50%. The probability is 25% for either the homozygous recessive or dominant genotype.
Example 2:
X-linked characteristic: colour blindness in human
There are several known X-linked characteristics in humans but few, if any, Y-linked characteristics are usually reported. Females have two X chromosomes with one or the other X chromosome remaining active in a mosaic pattern in a tissue. Males have only one X chromosome so if the X chromosome of a male has a defective allele there is no companion X chromosome to compensate for the deficiency. A female must have the same defective allele on both her X chromosomes to demonstrate any deficiencies (Fig. 6.4).
4. Essay on the Mendelian Principles:
During Mendel’s time DNA had not been identified as the substance of heredity and it was unknown how offspring obtained certain characteristics from their parents. Since Mendel’s work elucidated dominant and recessive characteristics his study supported the particulate theory of inheritance. Mendel accomplished this work by calculating the ratios of observable characteristics of the offspring from known parental types.
The first parental types were homozygous recessive and homozygous dominant pure breeding types. The parental generation or P generation, by definition, is always homozygous recessive and homozygous dominant for the traits to be studied. The offspring which results from the mating of parental types (P generation) will always be heterozygous for the characteristic.
a. Mendel’s Law of Dominance:
The first law of Mendel states that “In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the progeny, in other words factors retain their identity from generation to generation and do not blend in the hybrid”. In other words it says that, if two plants that differ in just one trait are crossed, then the resulting hybrids will be uniform in the chosen trait. Depending on the traits is the uniform features either one of the parents’ traits (a dominant-recessive pair of characteristics) or it is intermediate.
When two pure breeding organisms of contrasting characters are crossed, only one character of the pair appears in the F1 generation, known as the dominant character (example- tallness) and the other unexpressed or hidden character is known as the recessive character (example- dwarfness). When Mendel crossed a true breeding red flowered plant with a true breeding white flowered one, the progeny was found to be red coloured. The white colour suppressed and the red colour dominated.
Mendel’s law of dominance is generally true, but there are many exceptions to the law. For each of the seven pairs of characters examined, it was observed that one allelomorph dominated over the other, so that F1 exhibits one or the other alternative phenotypes represented in the parents. Some inherited traits do not exhibit strict Mendelian dominant/ recessive relationships. The simplest example of this phenomenon is called codominance, or incomplete dominance.
This pattern is displayed in the colours of four o’clock flowers. When a white and a red flower are cross-fertilized, the second generation is all pink. However, when a pink flower is allowed to self-fertilize, the white and red attributes return. The colour ratios for this third-generation cross are – 1 white: 2 pink: 1 red. This pattern is due to the fact that three alleles, instead of the usual two, determine colour in four o’clock flowers. If red colour is designated R and white colour r, then pink colour (not red or white) is the phenotypic effect of genotype Rr. (This is one type of pattern formerly used in support of the blending theory of inheritance). Thus in certain cases the hybrid offsprings resemble one parent much more closely than the other but does not resemble it exactly, so the dominance is incomplete. This is termed as incomplete dominance (Fig. 6.5).
Another example of codominance is the ABO blood typing system used to determine the type of human blood. It is common knowledge that a blood transfusion can only take place between two people who have compatible types of blood. Human blood is separated into different classifications on the basis of presence and absence of specific antigens or proteins in the red blood cells.
The protein’s structure is controlled by three alleles; i, IA and IB. The first allele is, i, the recessive of the three, and IA and IB are both co-dominant when paired together. If the recessive allele i is paired with IB or IA, its expression is hidden and is not shown. When the IB and IA are together in a pair, both proteins A and B are present and expressed.
The ABO system is called a multiple allele system for there are more than two possible allele pairs for the locus. The individual’s blood type is determined by which combination of alleles he/she has. There are four possible blood types in order from most common to most rare- O, A, B and AB. The O blood type represents an individual who is homozygous recessive (ii) and does not have an allele for A or B (Table 6.2).
Blood types A and B are co-dominant alleles. Co-dominant alleles are expressed even if only one is present. The recessive allele i for blood type O is only expressed when two recessive alleles are present. Blood type O is not apparent if the individual has an allele for A or B. Individuals who have blood type A have a genotype of IAIA or IAi and those with blood type B, IBIB or IBi, but an individual who is IAIB has blood type AB.
b. Mendel’s Law of Segregation:
The law of segregations is a law of inheritance proposed by Mendel in 1866. According to this law, “each organism is formed of a bundle of characters. Each character is controlled by a pair of factors (genes). During gamete formation, the two factors of a character separate and enter different gametes”. This law is also called law of purity of gametes. At formation of gametes, the two chromosomes of each pair separate (segregate) into two different cell which form the gametes.
This is a universal law and always during gamete formation in all sexually reproducing organisms, the two factors of a pair pass into different gametes. Each gamete receives one member of a pair of factors and the gametes are pure. That is two members (alleles) of a single pair of genes are never found in the same mature sperm or ovum (gamete) but always separate out (segregate).The factors of inheritance (genes) normally are paired, but are separated or segregated in the formation of gametes (eggs and sperm), i.e., it states that the individuals of the F2 generation are not uniform, but that the traits segregate.
Depending on a dominant-recessive crossing or an intermediate crossing are the resulting ratios 3:1 or 1:2:1. This concept of independent traits explains how a trait can persist from generation to generation without blending with other traits. It explains, too, how the trait can seemingly disappear and then reappear in a later generation. The principle of segregation was consequently of the utmost importance for understanding both genetics and evolution.
Monohybrid Cross:
The crossing of two plants differing in one character is called monohybrid cross. Mendel carried out monohybrid experiments on pea plants and based on the results of monohybrid experiment, he formulated the law of segregation. Mendel selected two pea plants, one with a tall stem and the other with a dwarf or short stem. These plants were considered as parental plants (P) and were pure breed. A pure plant is one that breeds true in respect of a particular character for a number of generations. The pure-bred tall and dwarf plants were treated as parents and were crossed.
Seeds were collected from these plants. These seeds were sown and a group of plants were raised. These plants constituted the first filial generation (F1 generation). All the F1 plants were tall and were inbred. The seeds were collected and the next generation (F2) was raised. In the F2 generation, two types of plants were found. They were tall and dwarf. Mendel counted the number of tall and dwarf plants. Of the 1064 plants of F2 generation, 787 plants were tall and 277 plants were dwarf (75% were tall plants and 25% were dwarf plants). Thus the tall plants occurred in the ratio 3: 1 (Fig. 6.6).
c. Mendel’s Principle of Independent Assortment:
The Principle of Independent Assortment describes how different genes independently separate from one another when reproductive cells develop. Mendel formulated the Principle of Independent Assortment from the observations he got from the dihybrid crosses, which are crosses between organisms that differ with regard to two traits.
It is now known that this independent assortment of genes occurs during meiosis in eukaryotes. Meiosis is a type of cell division that reduces the number of chromosomes in a parent cell by half to produce four reproductive cells called gametes. In humans, diploid cells contain 46 chromosomes, with 23 chromosomes inherited from the mother, while a second similar set of 23 chromosomes inherited from the father. Pairs of similar chromosomes are called homologous chromosomes. During meiosis, the pairs of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment of homologous chromosomes is random. This means that all the maternal chromosomes will not be separated into one cell, while all the paternal chromosomes are separated into another. Instead, after meiosis occurs, each haploid cell contains a mixture of genes from the organism’s mother and father.
Another feature of independent assortment is recombination. Recombination occurs during meiosis and is a process that breaks and recombines the pieces of DNA to produce new combinations of genes. Recombination scrambles pieces of maternal and paternal genes, which ensures that genes assort independently from one another. It is important to note that there is an exception to the law of independent assortment for genes that are located very close to one another on the same chromosome because of genetic linkage.
Dihybrids Cross between Two Heterozygous Individuals:
A dihybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in two traits. Mendel determined what happens when two plants that are each hybrid for two traits are crossed. Mendel therefore decided to examine the inheritance of two characteristics at once. Based on the concept of segregation, he predicted that traits must sort into gametes separately. By extrapolating from his earlier data, Mendel also predicted that the inheritance of one characteristic did not affect the inheritance of a different characteristic.
Mendel tested the idea of trait independence with more complex crosses. First, he generated plants that were pure bred for two characteristics, such as seed colour (yellow and green) and seed shape (round and wrinkled). These plants would serve as the Pi generation for the experiment. In this case, Mendel crossed the plants with Round and Yellow seeds (RRYY) with plants with wrinkled and green seeds (rryy). From his earlier monohybrid crosses, Mendel knew which traits were dominant- round and yellow.
So, in the F1 generation, he expected all round, yellow seeds from crossing these pure bred varieties, and that is exactly what he observed. Mendel knew that each of the Fi progeny were dihybrids; in other words, they contained both alleles for each characteristic (RrYy). He then crossed individual Fi plants (with genotypes RrYy) with one another. This is called a dihybrid cross. Mendel’s results from this cross were present in a 9:3:3:1 ratio. The outcome shows a phenotypic ratio of 9 of the offspring having yellow round peas, 3 having yellow wrinkled peas, 3 having green round peas and 1 having green wrinkled peas. This is a classic 9:3:3:1 phenotypic ratio which is always the result in a dihybrid cross between two heterozygotes with unlinked traits.
The proportion of each trait was still approximately 3:1 for both seed shape and seed colour. In other words, the resulting seed shape and seed colour looked as if they had come from two parallel monohybrid crosses; even though two characteristics were involved in one cross, these traits behaved as though they had segregated independently. From these data, Mendel developed the third principle of inheritance- the principle of independent assortment i.e. alleles at one locus segregate into gametes independently of alleles at other loci. Such gametes are formed in equal frequencies (Fig. 6.7).
Trihybrid Cross:
A trihybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in three traits (Fig. 6.8).
5. Essay on the Test Cross:
A test cross is a way to explore the genotype, the genetic makeup of an organism. Early use of the test cross was as an experimental mating test used to determine what alleles are present in the genotype. Consequently, a test cross can help to determine whether a dominant phenotype is homozygous or heterozygous for a specific allele.
Diploid organisms, like humans, have two alleles at each genetic locus, or position, and one allele is inherited from each parent. Different alleles do not always produce equal outward effects or phenotypes. One allele can be dominant and mask the effect of a second recessive allele in a heterozygous organism that carries two different alleles at a specific locus. Recessive alleles only express their phenotype if an organism carries two identical copies of the recessive allele, meaning it is homozygous for the recessive allele. This means that the genotype of an organism with a dominant phenotype may be either homozygous or heterozygous for the dominant allele. Therefore, it is impossible to identify the genotype of an organism with a dominant trait by visually examining its phenotype.
A test cross is the means by which a scientist can determine whether an individual with a dominant phenotype has a homozygous (AA) or heterozygous (Aa) dominant genotype. The test cross involves mating the individual with the dominant phenotype to an individual with a recessive (aa) phenotype and observing the offspring produced. If the individual being tested is homozygous dominant, then all offspring will have a dominant phenotype, since all the offspring will have at least one A (dominant) allele.
If the tested individual is heterozygous dominant, then half of the offspring will show the dominant phenotype, while the other half shows the recessive phenotype.
6. Essay on the Backcrossing:
It is the crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent or it is the crossing of a heterozygous organism and one of its homozygous parents. It is used in horticulture, animal breeding and in production of gene knockout organisms (Fig. 6.9).
7. Essay on the Limitations of Mendelian System:
The simple system of Mendelian genetics is very powerful and serves to explain the inheritance patterns of numerous traits. However, many traits are controlled by many genes acting in tandem, and thus do not obey strict Mendelian patterns (although their constituent genes may). Furthermore, many human traits are strongly influenced by the environment as well, and therefore their phenotypes cannot be said to be Mendelian (though the genetic components may be). In sum, Mendelian patterns are important, but cannot be applied universally. Individual traits must be researched to find out if they obey typical Mendelian patterns.
8. Essay on the Polygenic or Quantitative Inheritance:
When a trait (feature or character) is controlled by a single gene it is termed monogenic inheritance. Many traits or features are controlled by a number of different genes. For example, the skin colour of humans and the kernel colour of wheat results from the combined effect of several genes, none of which are singly dominant. Polygenes affecting a particular trait are found on many chromosomes. Each of these genes has equal contribution and cumulative the total effect. Three to four genes contribute towards formation of the pigment in the skin of humans.
So there is a continuous variation in skin colour from very fair to very dark. Such inheritance controlled by many genes is termed quantitative inheritance or polygenic (poly meaning due many genes) inheritance. In polygenic inheritance, each dominant gene controls equally the intensity of the character. The effect of the dominant genes in cumulative and the intensity of character or trait depend upon the number of dominant genes (Fig. 6.10).
9. Essay on Multiple Alleles:
Alleles are located in corresponding parts of homologous chromosomes, only one member of a pair can be present in a given chromosome and only two are present in a cell of a diploid. Alleles are genes that are members of the same gene pair, each kind of allele affecting a trait differently than the other. A diploid organism has, by its definition, only two alleles at one time, yet exceptions to the rules do appear. Many examples were found where more than two alternative alleles, also called multiple alleles, are present.
In these cases two or more different mutations must have taken place at the same locus but in different individuals or at different times. Multiple alleles are alternative states at the same locus. The different alleles of a series are usually represented by the same symbol. Subscripts and superscripts are used to identify different members of a series of alleles. Most alleles produce variations of the same trait, but some produce very different phenotypes.
The most famous example of multiple alleles was discovered in rabbits. It was known that Albino rabbits were produced on occasion in variously coloured rabbit populations. After conducting a monohybrid cross between a coloured and Albino rabbit, it was discovered that the members of a pair of alternative genes, either ca or C, must be responsible for coloured or albino rabbits. A cross of homozygous coloured (CC) and albino (ca ca) rabbits were made and the F1 generation was all coloured, while the F2 generation had three coloured and one albino. This showed that one pair of alleles was involved, the wild C and the mutant allele ca. It was determined that C was dominant over ca (Fig. 6.11).
Figure 6.11: Inheritance of skin colour
10. Essay on the Chromosomal Theory of Inheritance:
Sutton and Boveri in 1902 observed by that maternal (from mother) and paternal (from father) character come together in the progeny which is diploid or2n and has chromosomes in pairs and later on segregate during the formation of gametes. The gametes have a single chromosome from each pair and are haploid or n. Chromosomes from two parents come together in the same zygote as a result of the fusion of two gametes and again separate out during the formation of gametes. Chromosomes are filamentous bodies present in the nucleus and seen only during cell division. The above two observations proved that there is a remarkable similarity between the behavior of character during inheritance and that of chromosomes during meiosis.
This led Sutton and Boveri to propose ‘chromosomal theory of inheritance’ and its salient features are as follows:
a. The somatic (body) cells of an organism, which are derived by the repeated division of zygote have two identical sets of chromosomes, i.e., they are diploid. Out of these, one set of chromosomes is received from the mother (maternal chromosomes) and one set from the father (paternal chromosomes). Two chromosomes of one type (carrying same genes) constitute a homologous pair. Humans have 23 pairs of chromosomes.
b. The chromosomes of homologous pair separate out during meiosis at the time of gamete formation.
c. The behavior of chromosomes during meiosis indicates that Mendelian factors or genes are located linearly on the chromosomes. With progress in molecular biology it is now known that a chromosome is made up of a molecule of DNA and segments of DNA are the genes.
Essay on Sex-Linked Characteristics:
In animals the sex is determined by the presence or absence of the Y chromosome. The X and Y chromosomes are not homologous but are completely different chromosomes which carry unique information. No human can exist without at least one X chromosome. There is a viable human phenotype that has one X chromosome and no companion X or Y. These individuals are said to have the Turner syndrome. Turner syndrome (X 0) individuals are females who are of normal to above intelligence and usually have few deficiencies considering their lack of an entire chromosome. One major deficiency of Turner syndrome is sterility and non-development of secondary sexual characteristics.
Certain traits in humans and other organisms can demonstrate sex-linked inheritance of characteristics. This means that the inherited traits are present on the sex determining chromosomes the X or the Y. Since there appears to be more information on the X chromosome than on the Y chromosome of humans, most known sex-linked characteristics are actually X- linked characteristics.
In sex-linked traits, such as colour-blindness, the gene for the trait is found on the X chromosome (a sex chromosome). Sex-linked traits affect primarily males, since they have only one copy of the X chromosome (male genotype: XY). Females, who have two copies of the X chromosome, are affected only if they are homozygous for the trait. Females can, however, be carriers for sex-linked traits, passing their X chromosomes on to their sons. Sex-linked inheritance works as follows- if a female carrier and a normal male give birth to a daughter, she has a 1 in 2 chance of being a carrier of the trait (like her mother). If the child is a son, he has a 1 in 2 chance of being affected by the trait. If a female carrier and an affected male give birth to a daughter, she will either be affected or be a carrier. If the child is a son, he will either be affected or be entirely free of the gene.
Another example of a sex-linked trait is haemophilia, made famous by the “Queen Victoria pedigree” of the European nobility. Beginning with Queen Victoria of England (in whom it was probably a spontaneous mutation), the haemophilia gene spread quickly throughout the European rulers (who intermarried as a matter of course). The disease, which prevents blood from clotting properly and renders a minor injury a life-threatening event, claimed several young men of the royal line. Especially since male heirs were preferred over female as successors to the thrones of Europe, the spread of such a debilitating disease was a major problem.
11. Essay on the Linkage and Crossing Over:
The fact behind Mendel’s success was the genes encoding his selected traits did not reside close together on the same chromosome. If they had, his dihybrid cross results would have been much more confusing, and he might not have discovered the law of independent assortment. The law of independent assortment holds true as long as two different genes are on separate chromosomes. When the genes are on separate chromosomes, the two alleles of one gene (A and a) will segregate into gametes independently of the two alleles of the other gene (B and b). Equal numbers of four different gametes will result- AB, aB, Ab, ab. But if the two genes are on the same chromosome, then they will be linked and will segregate together during meiosis, producing only two kinds of gametes.
For instance, if the genes for seed shape and seed colour were on the same chromosome and a homozygous double dominant (yellow and round, RRYY) plant was crossed with a homozygous double recessive (green and wrinkled, rryy), the F1 hybrid offspring, as usual, would be double heterozygous dominant (yellow and round, RrYy). However, since in this example the R and Y are linked together on the chromosome inherited from the dominant parent, with r and y linked together on the other chromosome, only two different gametes can be formed- RY and ry.
Therefore, instead of 16 different genotypes in the F2 offspring, only three are possible: RRYY, RrYy, rryy and instead of four different phenotypes, only the original two will exist. Notice that the inheritance pattern now resembles that seen in a monohybrid cross, with a 3:1 phenotypic ratio, rather than the 9:3:3:1 ratio expected from the dihybrid cross. If physically linked on a single chromosome, the round and yellow alleles would segregate together, and the wrinkled and green alleles would segregate together, no round green seeds or wrinkled yellow seeds would ever appear.
The above explanation, however, neglects the influence of the crossing over of genetic material that occurs during meiosis. The farther away two genes are from one another, the more likely an exchange point for crossing over will form between them. At these exchange points, the alleles of one gene switch to the opposite homologous chromosome, while the other gene alleles remain with their original chromosomes. When alleles switch places like this, the resulting gametes are called recombinant. In the example above, the original parental gametes would be RY and ry, while the recombinant gametes would be Ry and rY. Thus four different kinds of gametes will be formed, instead of only two formed when the genes were linked (Fig. 6.12).
If two genes are extremely close together, crossing over will almost never occur between them, and the recombinant gametes will almost never form. If they are very far apart on the chromosome, crossing over will almost certainly occur between them, and recombinant gametes will form just as often as if the genes were on different chromosomes (50 percent of recombinant). If the genes are at an intermediate distance from each other, crossing over may sometimes occur between them and sometimes not (Fig. 6.13).
Therefore, the percentage of recombinant gametes (reflected in the percentage of recombinant offspring) correlates with the distance between two genes on a chromosome. By comparing the recombination rates of multiple different pairs of genes on the same chromosome, the relative position of each gene along the chromosome can be determined. This method of ordering genes on a chromosome is called a linkage map.
12. Essay on Mutations:
Mutations are errors in the genotype that create new alleles and can result in a variety of genetic disorders. In order for a mutation to be inherited from one generation to another, it must occur in sex cells, such as eggs and sperm, rather than in somatic cells. The best way to detect a genetic disorder is karyotyping.
i. Autosomal Mutations:
There are certain human genetic diseases which are inherited in a Mendelian fashion such as disease phenotype will have either a clearly dominant or clearly recessive pattern of inheritance, similar to the traits in Mendel’s peas. Such a pattern will usually only occur if the disease is caused by an abnormality in a single gene. The mutations that cause these diseases occur in genes on the autosomal chromosomes, the chromosomes that determine bodily characteristics and exist in all cells, both sex and somatic, as opposed to sex-linked diseases.
ii. Recessive Disorders:
Genetic disorders are initially arises as a new mutation that changes a single gene so that it no longer produces a protein that functions normally. A disease resulting from a mutation that an allele which produces a non-functional protein will be inherited in a recessive fashion so that the disease phenotype will only appear when both copies of the gene carry the mutation, resulting in a total absence of the necessary protein. If only one copy of the mutated allele is present, the individual is a heterozygous carrier, showing no signs of the disease but able to transmit the disease gene to the next generation.
Albinism is an example of a recessive illness, resulting from a mutation in a gene that normally encodes a protein needed for pigment production in the skin and eyes. Many recessive illnesses occur with much greater frequency in particular racial or ethnic groups that have a history of intermarrying within their own community. For example, Tay-Sachs disease is especially common among people of Eastern European Jewish descent. Other well-known autosomal recessive disorders include sickle-cell anaemia and cystic fibrosis.
iii. Dominant Disorders:
Usually, a dominant phenotype results from the presence of at least one normal allele producing a protein that functions normally. In the case of a dominant genetic illness, there is a mutation that results in the production of a protein with an abnormal and harmful action. Only one copy of such an allele is needed to produce disease, because the presence of the normal allele and protein cannot prevent the harmful action of the mutant protein. Huntington’s disease, which killed folksinger Woody Guthrie, is a dominant genetic illness. A single mutant allele produces an abnormal version of the Huntington protein; this abnormal protein accumulates in particular regions of the brain and gradually kills the brain cells.
iv. Chromosomal Disorders:
Mutation of a single gene results in recessive and dominant characteristics. Some genetic disorders result from the gain or loss of an entire chromosome. Normally, paired homologous chromosomes separate from each other during the first division of meiosis. If one pair fails to separate, an event called non-disjunction, then one daughter cell will receive both chromosomes and the other daughter cell will receive none. When one of these gametes joins with a normal gamete from the other parent, the resulting offspring will have either one or three copies of the affected chromosome, rather than the usual two.
(a) Trisomy:
A single chromosome contains hundreds to thousands of genes. A zygote with three copies of a chromosome (trisomy), instead of the usual two, generally cannot survive embryonic development. Chromosome 21 is a major exception to this rule; individuals with three copies of this small chromosome (trisomy 21) develop the genetic disorder called Down syndrome. People with Down syndrome show at least mild mental disabilities and have unusual physical features including a flat face, large tongue, and distinctive creases on their palms. They are also at a much greater risk for various health problems such as heart defects and early Alzheimer’s disease.
(b) Monosomy:
The absence of one copy of a chromosome (monosomy) causes even more problems than the presence of an extra copy. Only monosomy of the X chromosome is compatible with life.
(c) Polyploidy:
Polyploidy occurs when a failure occurs during the formation of the gametes during meiosis. The gametes produced in this instance are diploid rather than haploid. If fertilization occurs with these gametes, the offspring receive an entire extra set of chromosomes. In humans, polyploidy is always fatal, though in many plants and fish it is not.