- LAWS OF INHERITANCE
- MOLECULAR BASIS OF INHERITANCE
- LINKAGE AND RECOMBINATION
- SEX DETERMINATON
Introduction of Genetics
Introduction of Genetics
Genetics is the branch of science where we study about genes and their characteristics, that deals with the transmission of information from parents to the offsprings. Genes are the specific sequences of nucleotides that can code for particular proteins. Through the process of meiosis cell division and sexual reproduction, genes are transmitted from one generation to the next generation.
Example, a children usually seem like their parents because they have inherited their parents' genes. In the branch of genetics we tries to find which traits are inherited, and then explain how these traits are transferred from one generation to the next generation. So, we can say genetics is the branch of science that finds why an elephant always gives birth only to a elephant and why not birth some other animal. Members in a family become like to each other and genetics is the reason for it.
This branch of biology look for which features are inherited in the next generation. A feature of an organism is known as trait in genetics. It aslo explains that how these features are transfer from one generation to the next generation. DNA is a large molecule which carries the genetic information of an organism, which is copied and inherited across generations. DNA carries the traits in as instructions for building and handleing an organism. Genes are the segments of DNA which carries these instructions.
LAWS OF INHERITANCE
Father and Founder of genetics
The scientist, Gregor Johann Mendel is acknowlwdge as the “Father and Founder of genetics”. How inherited traits are transfer between generations comes from principles which was first proposed by Gregor Johann Mendel in 1866. We came to know about laws of inheritance or Mendel’s laws of inheritance from the results of Mendel’s experiment on pea plants.
The principles of Mendel's law is apply to the traits of plants, also in traits of the animals. The principles of Mendel's law also can explain how anyone inherit their parent's eye colour, hair colour etc. In next we study about the experiments performed by Mendel, which are very important to understand before learning about Mendel’s laws of inheritance.
Mendel’s Experiments on Pea Plant:
Mendel selected peas(Pisum sativum) for his experiment for many reasons:
- The pea plants are easy to grow and many generations can be shown each year.
- It is very easy to maintain them.
- Pea flowers contains both male and female parts,called stamen and stigma.
- The Peas can be cross-pollinated but also can be naturally self-pollinating.
Traits in the pea plants:
In pea plants Mendel followed the inheritance of 7 traits in the plants, and each of trait had 2 forms. Mendel identified pure-breeding pea plants that consistently showed one form of a trait after generations of self-pollination.
He studied from his experiment and Mendel made a list of contrasting characters:
Then he crossed these pure-breeding lines of the plants and recorded the traits of the hybrid progeny. Then he found that all of the first-generation (F1) hybrids looked like one of the parent plants.
Laws of Inheritance:
Mendel proposed three laws:
- Law of Dominance
- The Law of Segregation
- Law of independent assortment
#Law of Dominance:
In the law of Dominance we about to know that, “In a cross of pure parents are for contrasting traits, only one form of the trait will occur in the next generation. The offspring which are hybrid for a trait will have only the dominant trait in the phenotype.”
So, if there exists two contrasting traits, one of the traits will always suppress the other, there by expressing itself. T suppresses t, thus making the offspring plant tall(T). This traits are known as Dominating trait. The suppressed trait is known as Recessive trait. The recessive trait freely express itself in the absence of the dominant state,
#The Law of Segregation:
The law of of segregation express that "In the process of gamete formation, each of gene separates from each other, so that each gamete carries only one allele for each gene.”
This is the second law of inheritance . This law explains that the pair of alleles separate from each other during meiosis cell division process or gamete formation so that only one allele will be present in each gamete.
The law of segregations supports the phenotypic ratio of 3:1 which is homozygous dominant : heterozygous recessive . homozygous dominant offspring shows dominant traits while the homozygous recessive shows the recessive trait.
#Law of independent assortment:
Law of independent assortment express that “the alleles of two more genes get sorted into gametes independent of each other. The allele received for one gene does not impact the received allele for another gene.”
The experiments Mendel always described that the combinations of traits of the offspring are always different from their parental traits. Based on this statement,he created the Law of Independent Assortment.
This law also says that at the process of gamete and zygote formation, the genes are independently transfered on to the parents from the offspring.endent assortment
MOLECULAR BASIS OF INHERITANCE
Molecular Basis of Inheritance
DNA is a double helical structure of which was cracked by Watson and Crick based. It carries all the genetic information. DNA helix made up of strands. Each strand is made with repeating units of the nucleotides. The nucleotide consists of 3 components which are, ribose or deoxyribose sugar, nitrogenous base (purines or pyrimidines) and phosphate. Due to the presence of negatively charged phosphate groups DNA is negatively charged and this negatively charged phosphate groups are stabilized by basic protein known as histone.
DNA is made up of 6 molecular structures that comprise of one phosphate molecule and five carbon sugar,deoxyribose. Nucleotide is a basic building block of a DNA. A nucleotide is made up of one of the 4 bases,which are sugar molecule, and phosphate molecule.A sugar-phosphate chain is the backbone of a DNA molecule and bases are on the inside. Nucleotide subunits are attached together to create a DNA strand which providing polar stability.The DNA structure is three-dimensional which arises from chemical and structural features of 2 polynucleotide chain. A purine base pairs up with pyrimidine base. For instance guanine pairs with cytosine. hydrogen bond hold the two strands that are complementary to each other and the direction of the strands are antiparallel.
DNA Replication is a process by two new identical copies of DNA are from a single DNA molecule. DNA is a self-replicative molecule. It is a process of biological inheritance. The replication time in prokaryotes takes only a few minutes but in eukaryotes it takes hours. DNA is a double helix which is made up of two DNA strands and two strands are complementary to each other. At the time of replication these two strands of a helix separate to form two new DNA molecules. After the formation of the DNA strands, one strand is similar to one of the strand and the other strand is complementary to the parent strand. This type of replication is called semi-conservative replication.
In DNA replication, DNA polymerase in the most important enzyme, which is involved.
Central Dogma explains about how the genetic information transfer from generation to generation occurs in a bologycal system. This also explains about DNA replication process and then gets converted into messenger RNA with the help of transcription process. Then this mRNA or messenger RNA is translated to form proteins.Crick was first introduce about the central dogma. In retroviruses, the transfer process of genetic information is opposite,RNA to DNA to mRNA to Protein.
The process starts when The negatively charged DNA is wrapped around special positively charged protein molecules called histones. A nucleosome is a combined loop of DNA and protein. The packaged nucleosomes into a thread sometimes called as "beads on a string" . The end result is a fiber known as chromatin of eukaryotes.
In coding, decoding, regulation and expression of genes Ribonucleic acid or RNA is a polymeric essential molecule in various biological roles.In some viruses RNA replaces DNA or deoxyribonucleic acid as a carrier of genetic codes and It functions in cellular protein synthesis. Adenine, guanine, cytosine, and uracil are the 4 nitrogenous bases in RNA.
Human Genome Project:
The Human Genome Project or HGP was an international scientific research project. The aim of this project to determining the base pairs that creates the human DNA, Also to indentify and mapping all of the genes of the human genome from both a physical and a functional standpoint. The HGP project remains the world's largest collaborative biological project. The advances of genetic engineering techniques have made this project possible. In 1984 the idea was picked up by the US government, then the planning started, after that in 1990 the project launched and on April 14, 2003 was announced completed.
Evolution is a process by which the genetic material of a population changes over time. Evolution cause the adaptations of organisms to their changing environments and can result of changing genes characteristics new traits, and new species. Evolutionary processes rely on the changes in genetic variability and on the changes in allele sequences over time.
Microevolution cause changes over time in DNA sequences and allele frequencies within a species. The changes occurs may be because of mutations, which is the process to introduce new alleles into a population. In addition, also by gene flow new alleles can be introduced in a population by which is occurs during breeding between two populations that carry unique alleles. In contrast with microevolution, it cause large changes at the species level, which is result from the accumulation of numerous small changes on the microevolutionary scale. The evolution of a new species an example of macroevolution.
- The evolution implies change with respect to time.
- The genetics is a major factor in evolution.
- Hardy-Weinberg formula state that population remain constant.
Here we discuss about the main 4 process in evolution:
- Natural Selection.
It supplies the genetic variation to population that makes possible the new genotypes, that may be necessary for the evolation success of the population, that is mutation are unexpected heritable changes in the genetic imformation make up of an organism. Mutation are stable.
- Mutation is the only mechanism by which we can know about genetic variability.
- Mutation provides the raw material for evolution.
- The 1st important factor of evolution is mutation.
In process of Genetic recombination, the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent takes place. In eukaryotes, the genetic recombination process during meiosis cell division can lead to a new set of genetic information that can be transfered from the parents to the progeny. Most of recombination is occurring naturally.
Recombations the 2nd factor considerd important for evolution.
# Natural Selection:
Natural selection is the process by which organisms can better adapted to their environment to survive and to produce more progeny. Charles Darwin was first fully introduced about the theory of its action and now it is admited as be the main process that brings about the evolution. Natural selection is the 3rd factor of evolution.
In 1859, we know about the theory of evolution by natural selection, which was first formulated in Darwin's book "On the Origin of Species" . Natural selection is the process by which organisms change over time and as a result of changes in heritable physical or behavioral traits.
- Isolation is the 4th factor in evolution.
- Isolation is the process of separation of two or more population to prevent the interbreeding.
- The genetics interested in evolution are often referred to as a Neo-Darwinians.
LINKAGE AND RECOMBINATION
T.H. Morgan, the scientist who was first studied the phenomenon of linkage with the help of common fruit fly or Drosophila melanogaster.
Types of Linkage and its significance:
In Drosophila, the genes of grey body(G) and long wings(L) are dominant over black body(g) and vestigial wings(l). If pure breeding grey bodied long winged Drosophila (GL) are crossed with black bodied vestigial winged Drosophila (gl), in F2 generation a 3 : 1 ratio of parental phenotypes occurs (3 is for grey body long winged and one is for black body vestigial winged).
Morgan choose Drosophila melanogaster for his experiment due to some important reasons:
- He noticed that a white-eyed male drosophila instead of the regular red eyes.
- Size of the drosophila was small.
- They have a short lifespan, therefore so many generations can be studied in a short time frame.
- Drosophia’s have a high reproduction rate.
Morgan crossed a pure-bred white eyed male with pure-bred red-eyed female drosophila. As expected following the Mendel’s laws, the F1 progeny were born with red eyes. When F1 generation was crossed among each other, ratio of red-eyed to white eyed progeny were 3:1. However, Morgan noticed that there was no white- eyed female in the F2 generation.
To understand further Morgan performed a cross between a heterozygous red-eyed female with a white-eyed male. This gave a ratio of 1:1:1:1 in the progeny (white eyed female:red eyed female: white eyed male: red eyed male). This experiment made Morgan to think about the linkage between the traits and sex chromosomes. He performed many cross experiments and identified that the gene is responsible for the eye color which was situated on the X chromosome.
Sex-determination process is a biological process that determines the development of sexual characteristics in an organism. Most of the organisms that creates their offspring with the help of sexual reproduction process have two sexes. Sex determination helps to determine the organism gender whether it will be male or a female, and these are the two most common sexes. Due to parthenogenesis some species have only one sex , they act of a female reproducing without fertilization.
In 1891 German scientist Hermann Henkingin was first studied about sex determination. he was studying in the insect when he first observed a different nuclear component in some of the male gametes. Chromosomal sex determination,is the most widely used technique. In this technique we know about sex chromosome of male which is X or Y chromosome. The sex chromosome of a male decides the gender of the offspring. In most cases male carries "XY" chromosome and female carries "XX" chromosome . In this process it is also possible to perform genetic tests to sort out any chromosomal or genetic disorders. Sex determination is a genetic process to identify the sex of the organism.
Sex Determination in Humans
Genetic inheritance is the process by which sex of human beings is determined. Genes inherited from the parents to identify the sex of the offspring whether it will be a boy or a girl.
All the genes that contains all the characters for a offspring, are linearly arranged on chromosomes. Genes for sexual characters also include in these chromosomes.
Usually, all the characters related to the reproductive system are known as sexual characters and those that are not related to are the reproductive system known as vegetative characters. Sex chromosomes, that carry genes for sexual characters and those that chromosomes carry genes for the vegetative characters are known as autonomies.
Y chromosome is the sex chromosome that carries the genes for male characters and the X chromosome that carries the genes for female characters.
We have a total number of 46 chromosomes. Half of these 46 chromosome come from the mother and the rest, from the father. 44 are autonomies and 2 are sex chromosomes out of these 46 chromosomes. The sex chromosomes are not always in a perfect pair. there are 44 autonomies and two X chromosomes In females , and there are 44 autonomies, one X chromosomei n males and one Y chromosomes. In female, there are 44 + XX, and other hand the chromosomes in man are 44 + XY.
In the process of gamete formation, the normal diploid chromosome number is halved. This is known as haploid condition. All eggs of a female have 22 + X chromosomes. A male produces two types of sperms—one type carries the 22 + X composition and the other 22 + Y chromosomes. So, in every 100 sperms, 50 have Y chromosomes and 50 have X chromosomes.
X or Y any one of this can fertilize the egg. If a Y sperm fertilizes the egg, then the zygote has the 44 + XY composition, and the result of it, the embryo develops into a boy. When a X sperm fertilizes the egg, and the result of it, the zygote has the 44 + XX composition, this embryo grows into a girl. From the mother all the children inherit one X chromosome.
So, the sex is always determined by the sex chromosome of the father. If one who inherits the Y chromosome of the father is a boy and if one who inherits the X chromosome of the father is a girl.