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  #151  
Old Saturday, September 26, 2009
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Default Mutation

Mutation

Mutation is a change in DNA, the hereditary material of life. An organism’s DNA affects how it looks, how it behaves, and its physiology—all aspects of its life. So a change in an organism’s DNA can cause changes in all aspects of its life.

Mutations are random.
Mutations can be beneficial, neutral, or harmful for the organism, but mutations do not "try" to supply what the organism "needs." In this respect, mutations are random—whether a particular mutation happens or not is unrelated to how useful that mutation would be.
Not all mutations matter to evolution.
Since all cells in our body contain DNA, there are lots of places for mutations to occur; however, not all mutations matter for evolution. Somatic mutations occur in non-reproductive cells and won’t be passed onto offspring.

he only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations.

A single germ line mutation can have a range of effects:

1. No change occurs in phenotype.
Some mutations don't have any noticeable effect on the phenotype of an organism. This can happen in many situations: perhaps the mutation occurs in a stretch of DNA with no function, or perhaps the mutation occurs in a protein-coding region, but ends up not affecting the amino acid sequence of the protein.

2. Small change occurs in phenotype.
Cat with curled-ear mutation

A single mutation caused this cat’s ears to curl backwards slightly.

3. Big change occurs in phenotype.
Some really important phenotypic changes, like DDT resistance in insects are sometimes caused by single mutations1. A single mutation can also have strong negative effects for the organism. Mutations that cause the death of an organism are called lethals—and it doesn't get more negative than that.
There are some sorts of changes that a single mutation, or even a lot of mutations, could not cause. Neither mutations nor wishful thinking will make pigs have wings; only pop culture could have created Teenage Mutant Ninja Turtles—mutations could not have done it.

The Causes of Mutations
Mutations happen for several reasons.

1. DNA fails to copy accurately.
Most of the mutations that we think matter to evolution are "naturally-occurring." For example, when a cell divides, it makes a copy of its DNA—and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation.


2. External influences can create mutations.
Mutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural—even in the most isolated and pristine environments, DNA breaks down. Nevertheless, when the cell repairs the DNA, it might not do a perfect job of the repair. So the cell would end up with DNA slightly different than the original DNA and hence, a mutation.


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  #152  
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Default Types of Mutations

Mutation

Types of Mutations

Knowing a few basic types of mutations can help you understand why some mutations have major effects and some may have no effect at all.

Substitution
A substitution is a mutation that exchanges one base for another (i.e., a change in a single "chemical letter" such as switching an A to a G). Such a substitution could:
1. change a codon to one that encodes a different amino acid and cause a small change in the protein produced. For example, sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.
2. change a codon to one that encodes the same amino acid and causes no change in the protein produced. These are called silent mutations.
3. change an amino-acid-coding codon to a single "stop" codon and cause an incomplete protein. This can have serious effects since the incomplete protein probably won’t function.

Insertion
Insertions are mutations in which extra base pairs are inserted into a new place in the DNA.


Deletion
Deletions are mutations in which a section of DNA is lost, or deleted.


Frameshift
Since protein-coding DNA is divided into codons three bases long, insertions and deletions can alter a gene so that its message is no longer correctly parsed. These changes are called frameshifts.
For example, consider the sentence, "The fat cat sat." Each word represents a codon. If we delete the first letter and parse the sentence in the same way, it doesn’t make sense.
In frameshifts, a similar error occurs at the DNA level, causing the codons to be parsed incorrectly. This usually generates proteins that are as useless as "hef atc ats at" is uninformative.

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  #153  
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Default Evidence for Evolution

Evidence for Evolution


Paleontology

One piece of evidence offered by Darwin is found in the science of paleontology. Paleontology deals with locating, cataloging, and interpreting the life forms that existed in past millennia. It is the study of fossils—the bones, shells, teeth, and other remains of organisms, or evidence of ancient organisms, that have survived over eons of time.
Paleontology supports the theory of evolution because it shows a descent of modern organisms from common ancestors. Paleontology indicates that fewer kinds of organisms existed in past eras, and the organisms were probably less complex. As paleontologists descend deeper and deeper into layers of rock, the variety and complexity of fossils decreases. The fossils from the uppermost rock layers are most like current forms. Fossils from the deeper layers are the ancestors of modern forms.
Comparative anatomy

More evidence for evolution is offered by comparative anatomy (Figure 1 ). As Darwin pointed out, the forelimbs of such animals as humans, whales, bats, and other creatures are strikingly similar, even though the forelimbs are used for different purposes (that is, lifting, swimming, and flying). Darwin proposed that similar forelimbs have similar origins, and he used this evidence to point to a common ancestor for modern forms. He suggested that various modifications are nothing more than adaptations to the special needs of modern organisms.




Darwin also observed that animals have structures they do not use. Often these structures degenerate and become undersized compared with similar organs in other organisms. The useless organs are called vestigial organs. In humans, they include the appendix, the fused tail vertebrae, the wisdom teeth, and muscles that move the ears and nose. Darwin maintained that vestigial organs may represent structures that have not quite disappeared. Perhaps an environmental change made the organ unnecessary for survival, and the organ gradually became nonfunctional and reduced in size. For example, the appendix in human ancestors may have been an organ for digesting certain foods, and the coccyx at the tip of the vertebral column may be the remnants of a tail possessed by an ancient ancestor.
Embryology

Darwin noted the striking similarity among embryos of complex animals such as humans, chickens, frogs, reptiles, and fish. He wrote that the uniformity is evidence for evolution. He pointed out that human embryos pass through a number of embryonic stages inherited from their ancestors because they have inherited the developmental mechanisms from a common ancestor. These mechanisms are modified in a way that is unique to an organism's way of life.
The similarities in comparative embryology are also evident in the early stages of development. For example, fish, bird, rabbit, and human embryos are similar in appearance in the early stages. They all have gill slits, a two-chambered heart, and a tail with muscles to move it. Later on, as the embryos grow and develop, they become less and less similar.
Comparative biochemistry

Although the biochemistry of organisms was not well known in Darwin's time, modern biochemistry indicates there is a biochemical similarity in all living things. For example, the same mechanisms for trapping and transforming energy and for building proteins from amino acids are nearly identical in almost all living systems. DNA and RNA are the mechanisms for inheritance and gene activity in all living organisms. The structure of the genetic code is almost identical in all living things. This uniformity in biochemical organization underlies the diversity of living things and points to evolutionary relationships.
Domestic breeding

From observing the domestic breeding experiments of animal and plant scientists, Darwin developed an idea about how evolution takes place. Domestic breeding brings about new forms that differ from ancestral stock. For example, pigeon fanciers have developed many races of pigeons through domestic breeding experiments. In effect, evolution has taken place under the guidance of human hands. The development of new agricultural crops by farmers and botanists provides more evidence for directed evolution.
Geographic distribution

Darwin was particularly interested in the life forms of the Galapagos Islands. He noticed how many of the birds and other animals on the islands were found only there. The finches were particularly puzzling because Darwin found 13 species of finches not found anywhere else in the world, as far as he knew. He concluded that the finches had evolved from a common ancestor that probably reached the island many generations earlier. In the isolation of the Galapagos Islands, the original finches had probably evolved into the 13 species.
Other geographic distributions also help to explain evolution. For instance, alligators are located only in certain regions of the world, presumably because they have evolved in those regions. The islands of Australia and New Zealand have populations of animals found nowhere else in the world because of their isolated environments.


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  #154  
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Default Evidence for Evolution

Evidence for Evolution

The fact that evolution has taken place can be established by taking several kinds of evidences that are available. These evidences can be either 'direct' or 'indirect'. Direct evidences interestingly are provided by organisms, which have now become extinct, while indirect evidences are available from the study of organisms that are existing today - extinct organisms.Evidences from Palaeontology

The branch of biology, which deals with the study of extinct organisms that are now available only in the form of fossils, is called palaeontology. The study of fossils indicates the structural features of organisms that existed then. A comparison of this with the existing forms gives a clear indication as to how evolution has taken place.
Fossils are the organic remains of plants and animals that existed long, long ago. An entire organism or a part of its body may have become buried in the deeper sediments of the earth resulting in the formation of fossils.

Fossil Formation

Since traces of animals are found deeply embedded in the solid rock, the study of fossils includes the study of how these were formed. There are three large classes of rocks
Sedimentary rocks

, which were formed by packing together of sediment such as, mud and sand.



Igneous rocks

which were at one time the molten lava
Metamorphic rocks

, which were at one time sedimentary or igneous, but distorted by heat and pressure of overlying rocks.




Fossils are available only in sedimentary rocks.
Since more recent rock layers are on the top, the oldest fossils are found in the deepest rocks. In many places old rocks have become exposed due to erosion and weathering. Although sedimentary rocks are our major sources of fossil information, they are not the only source. Fossils have also been found in the tarpits, in amber (the fossilised resins of evergreen trees) and in ice. Excellently preserved insects have been found in amber while the entire Woolly Mammoths have got preserved in ice.
By arranging the fossils in the order in which they have appeared on earth many fossil series have been established. Such series demonstrate as to how various forms of life have changed gradually over a period of millions of years. As a result, the evolution of a number of animals such as horse, camel and elephant, has been clearly understood. Plenty of fossil evidence is now available so as to establish human evolution too. Wherever it is possible to determine the age of a rock, the age of a fossil enclosed in it has also been established. Conversely, where the age of the fossil is known, the age of the rocks can be directly determined. This process of finding out the age of a fossil is known as dating of fossils. The most effective way of dating is to measure the amount of radioactive material in the rock or soil. Half-life periods are calculated on the basis of the ratio of the radioactive material and its products of decay. Dating of fossils has provided an accurate determination of the age. The most commonly used radioactive materials are uranium and carbon-14. Even potassium-argon is being used.
Through extensive study of fossils, scientists, geologists and palaentologists have been able to construct a story of life in the form of a geological time scale. It lists the major geological periods of time and the types of life forms that existed then. These major periods of time are called eras. The eras have been subdivided into periods and periods have been subdivided into epochs. The following table summarises the major geological conditions and the biological features of different eras.





A well-known example of a fossil is the ancient bird Archaeopteryx. It showed a number of features like wings, feathers and beak seen in the present day birds. It also showed certain features like presence of teeth, presence of a long tail, and scales on the body, characteristics that are seen in reptiles. Thus, Archaeopteryx represents an extinct link between reptiles and birds.



Types of fossils

  • Unaltered
  • Petrified
  • Moulds and casts
  • Prints

Evidences from Homologous Organs

One of the indirect evidences for evolution comes from the study of homologous organs in organisms that exist today. Homologous organs are those, which have the same basic structure, but different functions. For example, an examination of the forelimb skeleton in different groups of land vertebrates reveals that the number and arrangement of bones remains the same. However, the forelimbs are used for diverse functions in different groups. In frogs, forelimbs are not generally used for locomotion or swimming. In a lizard, forelimbs are as prominently used for locomotion as the hind limbs. In birds, they are modified into wings. In mammals, they show further diversity. A bat's patagium, a man's arm, a horse's fore leg and a whale's flippers serve totally different functions, but possess the same basic structure. Such a structural homology indicates a probable common ancestry for the different groups.





Evidences from Embryology

Embryology is another branch, which offers some indirect evidences for evolution.
An embryologist by name Haeckel, proposed the idea 'ontogeny recapitulates phylogeny' which means that the evolutionary history (phylogeny) of a species is indicated by the developmental stages (ontogeny) that it passes through. In the course of development from a zygote into an adult, the various embryonic stages are thought to represent the various ancestral stages of that species. In the development of frog, the tadpole stage almost resembles a fish, indicating its fish ancestry. In birds, the young chick at the time of hatching has sedimentary teeth, which are lost subsequently, indicating the reptilian ancestry. Similarly, human embryos in their various stages of development resemble fish, chick, rabbit and monkey embryos. All mammals develop gills in their embryonic stages and lose them later. Even a whale loses them although they live in water. This is suggestive of a fish-like ancestry for all these groups.
Evidences from vestigial Organs

There are some structures in the body of organisms, which have no apparent use. Such structures are described as 'vestigial'. Vestigial organs indicate that they must have been present in a form in which they were highly functional in the ancestral forms. The human body is described as 'a living museum of vestigial organs'. More than 100 vestigial structures have been identified. A nictitating membrane, the vermiform appendix, the vestigial tail, the external ear muscles, to name a few, are vestigial structures in the human body. Flightless birds have vestigial flight muscles. The python has vestigial hind limbs. All these and many more clearly indicate that such structures have lost their significance in the course of evolution.



Evidences from geographical distribution

The distribution of plants and animal species in different geographical areas of the world is also a form of indirect evidence to show that evolution has taken place. For example, Australia was, until recently, populated only with marsupial mammals and it had no placental mammals. The inference is that Australia was once connected with the rest of the world and got separated (continental drift) after mammals had begun to evolve, but before placental mammals had emerged.
Biochemical Evidences for Evolution

One of the strong indirect evidence for evolution comes from the study of Biochemistry of living organisms. Bio-chemically living organisms show considerable similarity. All organisms contain the same type of molecules, and almost all organisms control themselves through information contained in DNA. Not only that, the genetic code that could take infinite forms, is the same in every organism studied. The mechanism of protein synthesis is largely the same in all living organisms studied so far. These and other biochemical studies strongly suggest a common ancestry for all forms of life.
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  #155  
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Default Variations

Variations

Variations can be defined as the differences that occur in the characteristics between members of the same species. Variations occur with reference to every character. In the absence of variations, every species would have continued to exist in the same form and no new species would have arisen from the existing one. Thus, variations are the raw materials for organic evolution.Types of Variations

Variations can be classified into the following types:
Somatogenic Variations

These are variations that are restricted to the somatic cells of the organism. Such variations appear anytime during the life of an organism and such variations die along with the organism. Hence, these variations are non-heritable.
Blastogenic Variations

These are variations that are found in the gene pool of an organism. They may be already present in the ancestors or may occur any time. These variations are heritable and form the raw materials for evolution.
Continuous Variations

These are graded variations that are always found amongst the members of a species with reference to certain characters. Individuals exhibit continuous variations with reference to characteristics like colour, shape, size and body configurations. Such variations have an adaptive value but they cannot bring about formation of a new species.
Discontinuous Variations

These are distinct variations that are caused by genetic changes brought about by environmental changes.
Sources of Variations

Variations are caused due to several factors listed below.
Mutations

Mutations are sudden heritable changes brought about by the composition of a gene or a chromosome. Accordingly, it can be a gene mutation or a chromosomal mutation. Sometimes, there may be just a change in the chromosome number causing a variation. It is called as ploidy or genomatic mutation. All such mutations bring about change in the genetic information of the cell and in turn the traits of the organism.
Recombination

These are genetic variations brought about due to shuffling of parental genes into new combinations. Thus, the resulting offspring exhibits genotypic variations. Genetic recombinations occur due to:
  • Independent assortment of chromosomes during meiosis
  • Reciprocal recombination of genes due to crossing over and
  • Randomness in the fusion of gametes during fertilization
Genetic Drift

It is the change brought about in the gene frequency of a population by various factors. It may be involved in the elimination of genes pertaining to certain characters.
Natural Selection

It is primarily a process of differential reproduction. Some members of a population may have genes, which enable them to grow up and reproduce at a higher rate. This results in the formation of offspring having better chances of survival. Those organisms that produce large number of viable offsprings contribute to a greater percentage of genes to the gene pool of the next generation. If differential reproduction of this kind occurs for several generations, genes of the individuals, which produce more offspring, becomes predominant in the gene pool of that population leading to a change in the gene frequency.
Migration

Sometimes it is possible that members of two different demes of a species, which were separated from each other, come together due to migration of individuals. In such a situation the genes that are unique to each population (deme) intermingle due to inbreeding resulting in variations in the offspring.
Hybridisation

Variation could arise through a process of hybridisation, sometimes even in interspecific hybrids.
Genetic variability is necessary for a species to increase its chances of survival. The environment is constantly undergoing changes due to geological and biological processes. Hence, it is necessary for organisms to develop variations. The occurrence of variations in a population ensures that at least a few individuals in a population are able to adapt and survive in the changing environment.
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  #156  
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Default Isolation

Isolation
Isolation is the segregation or separation of populations by certain barriers, which prevent interbreeding. As a result gene flow between populations is prevented. Each population on isolation, develops genetic divergence independently leading to the formation of new species. The factor which brings about isolation is called 'isolating mechanism'. The main types of isolating mechanisms are as given below.
Geographical Isolation

It is the isolation in space of the two populations of the same species, due to geographical barriers such as the sea, deserts, mountains and rivers, for land plants and animals.
Geographical isolation plays an important role in 'allopatric speciation'. Allopatric species are the related groups of individuals occupying different geographical areas. A single common gene pool gets split into two gene pools due to the barriers such as a river, or a mountain. Each such isolated population is affected by separate environmental factors. This leads to development of genetic divergence. Once the genetic divergence is achieved, the two populations cannot interbreed even if the barrier disappears. Hence, the two isolated populations can be recognised as two separate species. The southern elephant seal Micounga leonica occurs in the cold waters of the southern coasts of South America, South Africa, Australia and New Zealand. A close relative of this is the northern elephant seal Micounga argostiastics. It is found in the cold waters along the coast of western North America. The two forms are very much similar to each other. However, the two forms are separated by about 5000 km of tropical seas. Hence, the two forms cannot interbreed.



Reproductive Isolation

It is the isolation brought about by genetically determined agency, which prevents interbreeding. Following are the various mechanisms by which reproductive isolation can be brought about.
Premating or Prezygotic Isolation

These come into play in the early stages of the reproductive process itself and prevents the formation of a fertilized egg (zygote). They can be as follows:
Ecological or Habitat Isolation

The differences found in the habitats occupied by the two populations of the same species, may prevent interbreeding between them. Habitat isolation may not give any opportunity to meet and mate, to the isolated populations.
Seasonal Isolation

It is the isolation brought about due to the differences in the breeding season of the population.
Ethological or Behavioural Isolation

In some species, differences in the sexual behaviour may prevent inter breeding between the populations.
Mechanical or Morphological Isolation

It is the isolation brought about by the morphological differences, particularly with reference to reproductive organs (external genital organs).
Physiological Isolation

The physiological differences between individuals may also sometimes bring about isolation.
Gametic Mortality Isolation

In some cases of inter specific mating, the gametes get destroyed in the genital tract due to antigenic reactions.






Postmating or Postzygotic Isolation

These are isolating mechanisms that operate after mating occurs in the individuals. Following are some of them:
Cytological Isolation

It is the situation where after mating, fertilization fails to occur due to differences in the chromosomal number.
Zygote Mortality Isolation

In some cases, even if successful fertilization occurs following an interspecific mating, the zygote may not survive. It may die at any stage of development.
Hybrid Inviability Isolation

Here the hybrid organism resulting from an interspecific breeding fails to survive.
Hybrid Sterility Isolation

Sometimes viable hybrids may be formed but may become sterile, failing to produce young ones. e.g., Mule.
Hybrid Breakdown Isolation

It refers to the inviability or adaptive inferiority of the hybrids in several filial generation or hybrids in back cross.
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  #157  
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Default Epistasis

I
Epistasis
is the interaction between genes. Epistasis takes place when the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic, while the phenotype altered or suppressed is said to be hypostatic. Epistasis should be distinguished from Dominance, which is an interaction between alleles at the same gene locus.
In general, the fitness increment of any one allele depends in a complicated way on many other alleles; but, because of the way that the science of population genetics was developed, evolutionary scientists tend to think of epistasis as the exception to the rule. In the first models of natural selection devised in the early 20th century, each gene was considered to make its own characteristic contribution to fitness, against an average background of other genes. Some introductory college courses still teach population genetics this way.
Epistasis and genetic interaction refer to different aspects of the same phenomenon. The term epistasis is widely used in population genetics and refers especially to the statistical properties of the phenomenon, and does not necessarily imply biochemical interaction between gene products.
Examples of tightly linked genes having epistatic effects on fitness are found in supergenes and the human major histocompatibility complex genes. The effect can occur directly at the genomic level, where one gene could code for a protein preventing transcription of the other gene. Alternatively, the effect can occur at the phenotypic level. For example, the gene causing albinism would hide the gene controlling color of a person's hair. In another example, a gene coding for a widow's peak would be hidden by a gene causing baldness. Fitness epistasis (where the affected trait is fitness) is one cause of linkage disequilibrium.
Studying genetic interactions can reveal gene function, the nature of the mutations, functional redundancy, and protein interactions. Because protein complexes are responsible for most biological functions, genetic interactions are a powerful tool.

II

Epistasis
, the interaction between genes, is a topic of current interest in molecular and quantitative genetics. A large amount of research has been devoted to the detection and investigation of epistatic interactions. However, there has been much confusion in the literature over definitions and interpretations of epistasis. We note that the degree to which statistical tests of epistasis can elucidate underlying biological interactions may be more limited than previously assumed. Epistasis, defined generally as the interaction between different genes, has become a hot topic in complex disease genetics in recent years. For complex traits such as diabetes, asthma, hypertension and multiple sclerosis, the search for susceptibility loci has, to date, been less successful than for simple Mendelian disorders. This is probably due to complicating factors such as an increased number of contributing loci and susceptibility alleles, incomplete penetrance, and contributing environmental effects. The presence of epistasis is a particular cause for concern, since, if the effect of one locus is altered or masked by effects at another locus, power to detect the first locus is likely to be reduced and elucidation of the joint effects at the two loci will be hindered by their interaction. If more than two loci are involved, the situation is likely to be further complicated by the possibility of complex multiway interactions among some or all of the contributing loci.


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Default Epistasis

Epistasis is the best example for gene interaction. It is a pattern of inheritance where a pair of genes situated at one locus, prevent the expression of a pair of genes situated at another locus. Such genes are called inhibiting genes or epistatic genes (Epi = above/over gene, static = standing). It is an intergenic or non allelic form of gene interaction. The basic genes, the expression of which is prevented by the epistatic genes, are called as hypostatic genes. Epistasis reduces the number of phenotypes in the F2 generation of a dihybrid cross.
A classical example of epistasis is seen in the white fowls. There are two varieties of white fowls white leg horn and white plymouth rock. A cross between a homozygous, dominant, white leghorn and a homozygous recessive white Plymouth-rock results in a progeny of F1 generation containing dihybrid white progeny and of the F2 generation consists of white and coloured fowls in the ratio of 13:3 in place of the normal phenotypic ratio of 9:3:3:1. In addition, there is a reduction in the number of phenotypes to just two.

P1 Phenotype: White Leg Horn Fowl x Plymouth Rock Fowl







CCII WHITE -- - CCIi WHITE-- --CcII WHITE-- ----CcIi WHITE
CCIi WHITE ----ccii COLOURED---- CcIi WHITE -- --CciiCOLOURED
CcII WHITE-- -- CcIiWHITE --------CcII WHITE-- ----- cCIi WHITE
CcIi WHITE ----- Ccii COLOURED -- CcIi WHITE --- cciiWHITE

WHITE FOWL: COLOURED FOWL13 : 3

Here, the basic gene C produces colour in the feathers. But the inhibiting gene I prevents the appearance of colour. Gene I interacts with gene C in such a way to suppress its expression. As a result the leg horn fowl is white CCII. The plymouth rock fowl is white since it has recessive genes ccii. In the F1 generation all the progeny are white, but heterozygous (CcIi) when these fowls are allowed to inbreed, in the F2 generation, the genotypes having gene I along with gene C will produce white fowls. The genotypes which do not have gene I give rise to coloured fowls.

Gene Interaction Examples

Phenotype------ Phenotypic ratio
1. Coat colour in mouse -- 9:3:4 Black Albino
2. Fruit colour in squash ---12: 3: 1 White: Yellow: Green
3. Flower colour in Pea ---9: 7 Purple: White
4. Fruit shape in squash--- 9:6:1 Disc:Circu!ar:Long
5. Fruit shape in shepherds purse ---15:1 Trianular: Ovoid
6. Feather colour in Fowls---- 3:3 White: Coloured
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Default Mendel's Law of Independent Assortment

Mendel's Law of Independent Assortment

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel's law of segregation, states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization. Mendel arrived at this conclusion by performing monohybrid crosses. These were cross-pollination experiments with pea plants that differed in one trait, for example pod color.

Mendel began to wonder what would happen if he studied plants that differed in two traits. Would both traits be transmitted to the offspring together or would one trait be transmitted independently of the other? From his experiments Mendel developed the principle now known as Mendel's law of independent assortment.

Mendel's Law of Independent Assortment

Mendel performed dihybrid crosses (mating of parent plants that differ in two traits) in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive.

The resulting offspring or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

Fig A



Mendel then allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 9:3:3:1 ratio. About 9 of the F2 plants had green pods and yellow seeds, 3 had green pods and green seeds, 3 had yellow pods and yellow seeds and 1 had a yellow pod and green seeds.

Fig B



Mendel performed similar experiments focusing on several other traits like seed color and seed shape, pod color and pod shape, and flower position and stem length. He noticed the same ratios in each case. From these experiments Mendel formulated what is now known as Mendel's law of independent assortment. This law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.

Genotype and Phenotype

In Mendel's experiment with pod color and seed color (Figure A) we see that the genotype or genetic makeup of the F1 plants is GgYy. The phenotypes or expressed physical traits are green pod color and yellow seed color. Both of these traits are dominant.

The F2 generation pea plants (Figure B) show two different phenotypes for each trait. Pod color is either green or yellow and seed color is either yellow or green. There are nine different genotypes:

F2 Genotypes------------------------------F2 Phenotypes
GGYY, GGYy, GgYY, GgYy --------Green pod, Yellow seeds
GGyy, Ggyy---------------------------Green pod, Green seeds
ggYY, ggYy----------------------------Yellow pod, Yellow seeds
ggyy --------------------------------------Yellow pod, Green seeds
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Default Multiple Alleles

Multiple Alleles
The term multiple allele is a condition where more than two genes occupy the same locus, on the same pair of homologous chromosomes, in different organisms. Each of these genes expresses a totally different character. The inheritance of A B O blood groups in man is an example of multiple alleles. The four blood groups A, B, AB and O are due to the presence of three genes occupying the same locus on the same pair of chromosomes in different human beings. The discovery of blood groups dates back to the year 1900. A German doctor by name Carl Landsteiner discovered the four blood groups in man. He was examining the reason for the instant death of many persons immediately after a blood transfusion. He isolated the plasma and RBC from the blood samples of different persons. He found that whenever the plasma was mixed with the RBC of the same person, the mixture was smooth. However, when the plasma and RBC belonged to different persons, the mixture was found to be smooth in some cases and clumped in others. Detailed analysis conducted by Landsteiner showed that the human blood contained two specific substances called antigens, which are responsible for either smooth mixing or clumping. He named these antigens as antigen A and antigen B. Based on the presence or absence of these antigens, he classified the human blood into four groups namely A, B, AB and O.
The following table represents the antigens and the corresponding antibodies found in the four blood groups of man.

Blood Group----- Antigen in RBC--------Antibody in plasma
A----------only antigen A -------------------Antibody-b
B-----------only antigen in B------------Antibody-a
AB------ Both antigens A and B ---------------------------Nil
O------- Neither-----------------------------------Both antibody a and b





Blood Transfusion

The transfer of blood from one person to another is called blood transfusion. In all cases of blood transfusion, it is necessary to match the blood group of the recipient with the blood group of donor. The following table represents the blood group matching.








From the table it is clear that persons with blood group AB can receive blood from any other person. Hence they are commonly described as universal recipients persons with blood group O can donate blood to any other person. Hence, they are commonly described as universal donors.

Blood groups............ Genotypes
A ........ IAIb or IAIO
B ......... Ibb or IbIO
AB ...... IAIb
O ....... IOIO



The fruit-fly, Drosphila melanogaster has 15 alleles for eye colour. In rabbits, there are 4 alleles for colour. In all these cases, at any given time, only two of the alleles can occupy the same locus on a pair of homologous chromosomes.
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