Kuldeep Project 2

8/6/2019 Kuldeep Project 2

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INTRODUCTION

HeredityHeredity is the passing of traits to offspring (from its parent or ancestors). This isthe process by which an offspring cell or organismacquires or becomes

predisposed to the characteristics of its parent cell or organism. Through heredity,variations exhibited by individuals can accumulate and causesome species to evolve. The study of heredity in biology is called genetics, whichincludes the field ofepigenetics.

O verview I n humans, eye color is an inherited characteristic and an individual might inheritthe "brown-eye trait" from one of the parents. [1] I nherited traits are controlled

by genes and the complete set of genes within an organism's genome is calledits genotype. [2]

The complete set of observable that make up the structure and behaviour of anorganism is called itsphenotype. These traits come from the interaction of itsgenotype with the environment. [3] As a result, many aspects of an organism's

phenotype are not inherited. For example, suntanned skin comes from theinteraction between a person's genotype and sunlight; thus, suntans are not passedon to people's children. However, some people tan more easily than others, due todifferences in their genotype; a striking example are people with the inherited traitof albinism, who do not tan at all and are very sensitive to sunburn. [4]

Heritable traits are known to be passed from one generation to the next via DNA,a molecule that encodes genetic information. [2] DNA is a long polymer composedof four types of bases. The sequence of bases along a particular DNA molecule

specify the genetic information, in a manner similar to a sequence of lettersspelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA moleculethat specify a single functional unit are called genes; different genes have differentsequences of bases. Within cells, the long strands of DNA form condensedstructures called chromosomes. The specific location of a DNA sequence within a



chromosome is known as a locus. I f the DNA sequence at a locus varies betweenindividuals, the different forms of this sequence are calledalleles. DNA sequencescan change through mutations, producing new alleles. I f a mutation occurs within agene, the new allele may affect the trait that the gene controls, altering the

phenotype of the organism. [5] However, while this simple correspondence between an allele and a trait works insome cases, most traits are more complex and are controlled by multipleinteracting genes within and among organisms. [6][7] Developmental biologistssuggest that complex interactions in genetic networks and communication amongcells can lead to heritable variations that may underlay some of the mechanicsin developmental plasticity and canalization. [8]

Recent findings have confirmed important examples of heritable changes thatcannot be explained by direct agency of the DNA molecule. These phenomena areclassed as epigenetic inheritance systems that are causally or independentlyevolving over genes. Research into modes and mechanisms of epigeneticinheritance is still in its scientific infancy, however, this area of research hasattracted much recent activity as it broadens the scope of heritability andevolutionary biology in general. [9] DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference, and the threedimensional conformation of proteins (such as prions) are areas where epigenetic

inheritance systems have been discovered at the organismic level.[10][11]

Heritabilitymay also occur at even larger scales. For example, ecological inheritance throughthe process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effect that modifies andfeeds back into the selection regime of subsequent generations. Descendants inheritgenes plus environmental characteristics generated by the ecological actions of ancestors. [12] O ther examples of heritability in evolution that are not under thedirect control of genes include the inheritance of cultural traits, group heritability,and symbiogenesis. [13][14][15] These examples of heritability that operate above thegene are covered broadly under the title of multilevel or hierarchical selection,which has been a subject of intense debate in the history of evolutionary science.



Relation to theory of evolution When Charles Darwin proposed his theory of evolution in 1859, one of its major

problems was the lack of an underlying mechanism for heredity. Darwin believed

in a mix of blending inheritance and the inheritance of acquired traits (pangenesis).Blending inheritance would lead to uniformity across populations in only a fewgenerations and thus would remove variation from a population on which naturalselection could act. This led to Darwin adopting some Lamarckian ideas in later editions of On the Origin of Species and his later biological works. Darwin's

primary approach to heredity was to outline how it appeared to work (noticing thattraits could be inherited which were not expressed explicitly in the parent at thetime of reproduction, that certain traits could be sex-linked, etc.) rather thansuggesting mechanisms.

Darwin's initial model of heredity was adopted by, and then heavily modified by,his cousin Francis Galton, who laid the framework for the biometric school of heredity. Galton rejected the aspects of Darwin's pangenesis model which relied onacquired traits.

The inheritance of acquired traits was shown to have little basis in the 1880swhen August Weismann cut the tails off many generations of mice and found thattheir offspring continued to develop tails.

History The ancients had a variety of ideas about heredity: Theophrastus proposed thatmale flowers caused female flowers to ripen; Hippocrates speculated that "seeds"were produced by various body parts and transmitted to offspring at the time of conception, and Aristotle thought that male and female semen mixed atconception.Aeschylus, in 458 BC, proposed the male as the parent, with the female

as a "nurse for the young life sown within her."[citation needed ]

Various hereditary mechanisms were envisaged without being properly tested or quantified. These included blending inheritance and the inheritance of acquiredtraits. Nevertheless, people were able to develop domestic breeds of animals aswell as crops through artificial selection. The inheritance of acquired traits alsoformed a part of early Lamarckian ideas on evolution.



During the 18th century, Dutch microscopist Antonie van Leeuwenhoek (1632± 1723) discovered "animalcules" in the sperm of humans and other animals. Somescientists speculated they saw a "little man" (homunculus) inside each sperm.These scientists formed a school of thought known as the "spermists." They

contended the only contributions of the female to the next generation were thewomb in which the homunculus grew, and prenatal influences of the womb. Anopposing school of thought, the ovists, believed that the future human was in theegg, and that sperm merely stimulated the growth of the egg. O vists thoughtwomen carried eggs containing boy and girl children, and that the gender of theoffspring was determined well before conception.

G regor M endel : fa ther of m odern genetics The idea of particulate inheritance of genes can be attributed tothe Moravian [17] monk Gregor Mendel who published his work on pea plants in1865. However, his work was not widely known and was rediscovered in 1901. I twas initially assumed the Mendelian inheritance only accounted for large(qualitative) differences, such as those seen by Mendel in his pea plants²and theidea of additive effect of (quantitative) genes was not realised until R.A. Fisher's(1918) paper, "The Correlation Between Relatives on the Supposition of Mendelian I nheritance."

M odern develop m ent of genetics a nd

heredity

I n the 1930s, work by Fisher and others resulted in a combination of Mendelianand biometric schools into the modern evolutionary synthesis. The modernsynthesis bridged the gap between experimental geneticists and naturalists; and

between both and palaeontologists, stating that: [18][19]

1. All evolutionary phenomena can be explained in a way consistent withknown genetic mechanisms and the observational evidence of naturalists.

2. Evolution is gradual: small genetic changes, recombination ordered bynatural selection. Discontinuities amongst species (or other taxa) are



explained as originating gradually through geographical separation andextinction (not saltation).

3. Selection is overwhelmingly the main mechanism of change; even slightadvantages are important when continued. The object of selection is

the phenotypein its surrounding environment. The role of genetic drift isequivocal; though strongly supported initially by Dobzhansky, it wasdowngraded later as results from ecological genetics were obtained.

4. The primacy of population thinking: the genetic diversity carried in natural populations is a key factor in evolution. The strength of natural selection inthe wild was greater than expected; the effect of ecological factors such asniche occupation and the significance of barriers to gene flow are allimportant.

5. I n palaeontology, the ability to explain historical observations byextrapolation from micro to macro-evolution is proposed. Historicalcontingency means explanations at different levels may exist. Gradualismdoes not mean constant rate of change.

The idea that speciation occurs after populations are reproductively isolated has been much debated. I n plants, polyploidy must be included in any view of speciation. Formulations such as 'evolution consists primarily of changes inthe frequencies of alleles between one generation and another' were proposedrather later. The traditional view is that developmental biology ('evo-devo') playedlittle part in the synthesis, but an account of Gavin de Beer's work by Stephen JayGouldsuggests he may be an exception. [20]

Almost all aspects of the synthesis have been challenged at times, with varyingdegrees of success. There is no doubt, however, that the synthesis was a greatlandmark in evolutionary biology. I t cleared up many confusions, and was directlyresponsible for stimulating a great deal of research in the post-World War II era.

Trofim Lysenko however caused a backlash of what is now called Lysenkoism in

the Soviet Union when he emphasised Lamarckian ideas on the inheritance of acquired traits. This movement affected agricultural research and led to foodshortages in the 1960s and seriously affected the USSR.



T ypes of heredity

D om in a nt a nd recessive

An allele is said to be dominant if it is always expressed in the appearance of anorganism (phenotype). For example, in peas the allele for green pods, G, isdominant to that for yellow pods, g. Since the allele for green pods is dominant,

pea plants with the pair of alleles GG (homozygote) or Gg (heterozygote) will havegreen pods. The allele for yellow pods is recessive. The effects of this allele areonly seen when it is present in both chromosomes, gg (homozygote).

The description of a mode of biological inheritance consists of three main

categories:1. Numb er of involved loci

Monogenetic (also called "simple") ± one locus O ligogenetic ± few loci Polygenetic ± many loci

2 . I nvolved chro m oso m es

Autosomal ± loci are not situated on a sex chromosome Gonosomal ± loci are situated on a sex chromosome

X -chromosomal ± loci are situated on the X -chromosome (the morecommon case)

Y -chromosomal ± loci are situated on the Y -chromosome Mitochondrial ± loci are situated on the mitochondrial DNA

3 . C orrel a tion genotype±phenotype

Dominant

I ntermediate (also called "codominant") Recessive

These three categories are part of every exact description of a mode of inheritance in the above order. Additionally, more specifications may

be added as follows:



4 . C oincident a l a nd environ m ent a l inter a ctions

Penetrance Complete

I ncomplete (percentual number) Expressivity

I nvariable Variable

Heritability (in polygenetic and sometimes also in oligogenetic modes of inheritance)

Maternal or paternal imprinting phenomena (also see epigenetics)5 . S ex-linked inter a ctions

Sex-linked inheritance (gonosomal loci) Sex-limited phenotype expression (e.g., cryptorchism) I nheritance through the maternal line (in case of mitochondrial

DNA loci) I nheritance through the paternal line (in case of Y -chromosomal loci)

6 . L ocu s±loc u s inter a ctions

Epistasis with other loci (e.g., overdominance) Gene coupling with other loci (also see crossing over) Homozygotous lethal factors Semi-lethal factors Determination and description of a mode of inheritance is primarily

achieved through statistical analysis of pedigree data. In case theinvolved loci are known, methods of molecular genetics can also beemployed.



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