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University of South Carolina
ANTH 101

Print - Biology-Online Genetics and Evolution Meiosis - The Genetics of Reproduction As mentioned in previous pages, the genetic information found in DNA is essential in creating all the characteristics of an organism. This remains the case when passing genetic information to offspring, that can occur via a process called meiosis where four haploid cells are created from their diploid parent cell. For a species to survive, and genetic information to be preserved and passed on, reproduction must occur. This can be done by passing on the information found in the chromosomes via the gametes that are created in meiosis. Chromosome Complement Humans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another. Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete created by a human is haploid possesses 23 chromosomes. Tetraploid organisms possess more than 3 sets of a particular chromosome. Reproduction Reproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringing together half the genetic information from the parents into one new cell, that is now genetically different from both its parents. This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways, explained soon. The Process of Meiosis The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again to form 4 haploid cells. These can be subdivided into four distinct phases which are a continuous process 1st Division Prophase - Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of a chromosome) where crossing over can occur. Crossing offer can increase genetic variation. Metaphase - Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, through chance, increasing genetic variation via independent assortment. Anaphase - The homologous chromosomes move to opposing poles from the equator Telophase - A new nuclei forms near each pole alongside its new chromosome compliment. At this stage two haploid cells have been created from the original diploid cell of the parent. 2nd Division Prophase II - The nuclear membrane disappears and the second meiotic division is initiated. Metaphase II - Pairs of chromatids line up at the equator Anaphase II - Each of these chromatid pairs move away from the equator to the poles via spindle fibres Telophase II - Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote. Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species as a whole. Each of these gametes possess unique genetic information due to situations in meiosis where genetic diversity is increased, all of which is elaborated upon on the next page. Independent Assortment and Crossing Over The previous page investigates the process of meiosis, where 4 haploid gametes are created from the parent cell. Half the genetic information from a parent is present in these haploids, which fuse with gametes of the opposite sex to create a zygote, with a complete chromosome compliment that 1 of 14 4/11/14 8:51 PM Print - Biology-Online will create offspring after prolonged growth. The process of meiosis increases genetic diversity in a species. The sex organs which produce the haploid gametes are the site of many occurrences where genetic information is exchanged or manipulated. Independent Assortment of Chromosomes Alleles for a particular phenotype determine what characteristic an organism will express, as with the following example where Chromosome 1 contains an allele for blonde hair Chromosome 2 contains an allele for brown hair Chromosome 3 contains an allele for blue eyes Chromosome 4 contains an allele for brown eyes The top assortment to the left produces 2 blonde hair/blue eyes gametes while the below produces 2 brown hair/brown eyes gametes The top assortment on the right produces 2 blonde hair/brown eyes gametes while the below produces 2 brown hair/blue eyes gametes The above indicates that even though the two homologous chromosomes contain the same genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each of the 4 gametes produced. With 23 chromosomes in a human gamete, their are 2 23 combinations (8388608 combinations) Crossing Over During meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair. This point of contact is deemed the chiasmata, and can allow the exchange of genetic information between chromosomes. This further increases genetic variation. There are also many other ways in which genetic variation is increased in a species gene pool, all of which are described in the following pages. The next page investigates the work of Gregor Mendel, an Austrian monk famous for his work involving monohybrid and dihybrid crossing, alongside the continuation into looking at genetic diversity through meiosis and genetics in general. Crossing Over and Genetic Diversity Gregor Mendel, an Austrian monk, is most famous in this field for his study of the phenotype of pea plants, including the shape of the peas on the pea plants. Gregor Mendel's Work Mendel's goal was to have a firm scientific basis on the relationship of genetic information passed on from parents to offspring. In light of this he focused on how plant offspring acquired the phenotype of their seeds. In this example, there are two choices, round and wrinkled seeds. The plants that were used in the experiment had to be true breeding, i.e. those plants with round seeds must have had parents with round seeds, who in turn had parents producing round seeds etc. This is done to increase the accuracy of results. After successfully producing two generations from these true breeding plants, the following was evident The first generation of plants produced all had a round seed phenotype. When these first generation plants were crossed, a ratio of 3 round seeds averaged every 1 wrinkled seed. The ratio of 3:1 was not exact, though this is because of the randomness of the processes that are executed to produce these plants. For example, independent assortment is completely random, as are mutations, therefore variable results occur producing a sampling error. Due to the scale of the experiment done by Gregor Mendel, the sampling error was smaller than that of a smaller scale experiment. Mendel successfully hypothesised that the reason for this trend in phenotypes from generation to generation was down to the fact that genetic information was being passed on from their parents. 2 of 14 4/11/14 8:51 PM Print - Biology-Online The fact that round seeds appeared more frequently than wrinkled seeds is due to round seeds being the dominant phenotype, which when present effectively 'masks' the phenotype of the recessive (wrinkled seed) gene. Dominant and Recessive Alleles All plant seeds produced in the first generation were round 3 out of every 4 plant seeds produced in the second generation were round The parents, one possessing wrinkled seeds the other possessing round were crossed together, for some reason in the first and second generation the presence of the round seed gene in offspring superceded the presence of the wrinkled seed. This is called dominance. The next page investigates this dominance, and how it can successfully be predicted. The following page also has examples of a monohybrid and dihybrid cross. Dominance and Crossing Over The previous page investigated Gregor Mendel and how he found trends in the phenotypes of offspring produced by true breeding parents Dominant and Recessive Alleles Mendel paved the way to discovering that alleles that code for a particular characteristic, such as the shape of the seeds produced are expressed in dominant and recessive genes. When dominant genes were present, they would supercede the presence of wrinkled and were deemed the dominant gene. For example; If the genotype for seeds was Rr (where R is dominant and r is recessive), R would supercede the recessive gene and the plant would express a round seed phenotype. If the genotype was rr (where both are recessive) there are no dominant genes therefore the recessive phenotype for wrinkled seed is expressed The previous page mentioned that in the first generation all offspring produced were round seeds, and in the second generation for every three that were round seeded there would be one wrinkled seed. This can be expressed in a Punnett square as illustrated below. All dominant genes are marked in red, and all recessive genes are marked in green. Whenever the dominant gene is present in an organism this will be expressed. We can summarise the above diagram in the following statements Parent 1 possesses 2 dominant genes in its genotype Parent 2 possesses 2 recessive genes in its genotype The gametes produced by parent 1 all contain the dominant gene while parent 2's gametes all possess the recessive gene. When parent 1 and 2's gametes are crossed, the genotype is Rr. Since all of them have the dominant gene passed on from parent 1, they all express the dominant phenotype The gametes from the first generation produce 50% R gametes and 50% r gametes The second generation produces 1 RR 2 Rr and 1 rr genotypes in offspring, resulting in 3 round seed phenotypes per 1 wrinkled seed This is an example of a monohybrid cross, where we are studying one respect of an animals genotype. The next page continues to look at dominance and examples of monohybrid and dihybrid crossing plus related info. 3 of 14 4/11/14 8:51 PM Print - Biology-Online Mendel's Law & Mendelian Genetics Previous pages have described how genetic information is passed along from parents to offspring. Mendel summarised this in his first law, the principle of segregation Mendel's First Law "The alleles of a gene exist in pairs but when gametes are formed, the members of each pair pass into different gametes. Thus each gamete contains only one allele of each gene." Incomplete Dominance When a particular gene possesses both dominant and recessive alleles, it is possible for incomplete dominance to occur, where the organism at hand expresses a phenotype morphed by the expression of both the dominant and recessive alleles. In essence, heterozygous (possessing opposing alleles Rr) organisms derived from homozygous (possessing the same alleles RR or rr) are created, they possess a phenotype different to that of both their parents. Some of the following examples of monohybrid and dihybrid crossing illustrate this incomplete dominance. Multiple Alleles Diploid organisms naturally have a maximum of 2 alleles for each gene expressing a particular characteristic, one deriving from each parent. In some cases, however, more than two types of allele can code for a particular characteristic, as is the case of genetic coding for blood type in humans. Their are up to 6 possible genotypes that code for the four blood groups, A, B, AB and O. Example of a Cross The following dihybrid cross involves two true breeding pea plants, where two factors are looked at, the shape of the seed and the colour of the seed. Summary of Mendelian Genetics The past few pages have elaborated on the work of Gregor Mendel and how his work has paved the way to predicting the characteristics of offspring. However, a degree of randomness is involved, when involving factors such as independent assortment during meiosis and the possibility of genetic mutations (explained in further pages). In light of this, Mendel's work allowed us to see that there is a degree of genetic inheritance from parents in offspring though modern biology indicates that more factors come into play to determine the final genotype and phenotype of an organism. 4 of 14 4/11/14 8:51 PM Print - Biology-Online Sticking to the subject of genetics, the next page looks at sex determination via chromosomes X and Y and some of the genetic traits inherited via these two chromosomes. Chromosomes X and Y and Sex Determination In a human, the normal chromosomes complement is 46, 44 of which are autosomes while 2 distinct chromosomes are deemed sex chromosomes, which determine the sex of an organism and various sex linked characteristics. In most animals, those who possess XX chromosomes are female while male animals possess an X and a Y chromosome. However, this is not true of all organisms, as it can be reversed in some species. Sex Determination A humans' sex is predetermined in the sperm gamete. The egg gamete mother cell is said to be homogametic, because all its cell possess the XX sex chromosomes. sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome. In light of this, there are two possibilities that can occur during fertilisation between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilises the egg) they are responsible for determining sex. Chromosomes X and Y Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the 'extra' X chromosomes as indicated below. The arrows indicate sex linked genes in the X chromosome. In this homologous pairing, all those genes are dominant, because there are no opposing genes in the Y chromosome to offer dominance. So when the organism has an XY chromosome compliment (i.e. a male), these sex linked genes are freely expressed in the organisms phenotype, an example being hairy ears developing in old age. Sex Linked Characteristics These sex linked genes on the X chromosome display a number of characteristics. The following are just some examples of phenotypes as a result of these genes in expression; Red-Green colour blindness Haemophilia - A condition which prevents the clotting of the blood Hairy ears in men through advancing age More information on sex linked characteristics and how they are passed on from generation to generation will be available in new areas of the site soon. 5 of 14 4/11/14 8:51 PM Print - Biology-Online The next page looks at genetic mutations and the consequences as a result of them. Chromosome Mutations It is natures intention that the exact genetic information from both parents will be seen in the offspring's DNA in the the critical stages of fertilisation. However, it is possible for this genetic information to mutate, which in most cases, can result in fatal or negative consequencies in the outcome of the new ogranism. Non-Disjunction and Down's Syndrome One well known example of mutation is non-disjunction. Non-disjunction is when the spindle fibres fail to seperate during meiosis, resulting in gametes with one extra chromosome and other gametes lacking a chromosome. If this non-disjunction occurs in chromosome 21 of a human egg cell, a condition called Down's syndrome occurs. This is because their cells possess 47 chromosomes as opposed to the normal chromosome compliment in humans of 46. The fundamental structure of a chromosome is subject to mutation, which will most likely occur during crossing over at meiosis. There are a number of ways in which the chromosome structure can change, as indicated below, which will detrimentally change the genotype and phenotype of the organism. However, if the chromosome mutation effects an essential part of DNA, it is possible that the mutation will abort the offspring before it has the chance of being born. The following indicates types of chromosome mutation where whole genes are moved: Deletion of a Gene As the name implies, genes of a chromosome are permanently lost as they become unattached to the centromere and are lost forever Normal chromosome before mutation Genes not attached to centromere become loose and lost forever New chromosome lacks certain genes which may prove fatal depending on how important these genes are Duplication of Genes In this mutation, the mutants genes are displayed twice on the same chromosome due to duplication of these genes. This can prove to be an advantageous mutation as no genetic information is lost or altered and new genes are gained Normal chromosome before mutation Genes from the homologous chromosome are copied and inserted into the genetic sequence New chromosome possesses all its initial genes plus a duplicated one, which is usually harmless The next page continues looking at these chromosome mutations and mutations that happen within genes that can prove to be more harmful to the organism at hand. The following pages also investigates polyploidy in species. 6 of 14 4/11/14 8:51 PM Print - Biology-Online Genetic Mutations This page continues from the previous page investigating genetic mutations... Inversion of Genes This is where the order of a particular order of genes are reversed as seen below Normal chromosome un-altered The connection between genes break and the sequence of these genes are reversed The new sequence may not be viable to produce an organism, depending on which genes are reversed. Advantageous characteristics from this mutation are also possible Translocation of Genes This is where information from one of two homologous chromosomes breaks and binds to the other. Usually this sort of mutation is lethal An un-altered pair of homologous chromosomes Translocation of genes has resulted in some genes from one of the chromosomes attaching to the opposing chromosome Alteration of a DNA Sequence The previous examples of mutation have investigated changes at the chromosome level. The sequence of nucleotides on a DNA sequence are also susceptible to mutation. Deletion Here, certain nucleotides are deleted, which affects the coding of proteins that use this DNA sequence. If for example, a gene coded for alanine, with a genetic sequence of C-G-G, and the cytosine nucleotide was deleted, then the alanine amino acid would not be able to be created, and any other amino acids that are supposed to be coded from this DNA sequence will also be unable to be produced because each successive nucleotide after the deleted nucleotide will be out of place. Insertion Similar to the effects of deletion, where a nucleotide is inserted into a genetic sequence and therefore alters the chain thereafter. This alteration of a nucleotide sequence is known as frameshift Inversion Where a particular nucleotide sequence is reversed, and is not as serious as the above mutations. This is because the nucleotides that have been reversed in order only affect a small portion of the sequence at large Substitution A certain nucleotide is replaced with another, which will affect any amino acid to be synthesised from this sequence due to this change. If the gene is essential, i.e. for the coding of haemoglobin then the effects are serious, and organisms in this instance suffer from a condition called sickle cell anaemia. All of the genetic mutations looked at through the last 2 pages more or less have a negative impact and are undesired, however, in some cases they 7 of 14 4/11/14 8:51 PM Print - Biology-Online can prove advantageous. Genetic mutations increase genetic diversity and therefore have an important part to play. They are also the reason many people inherit diseases. The next page looks at polyploidy, a type of mutation that effects chromosome content of an organism, and also investigates the frequency of mutations and factors that play a part in this. Mutation Frequency and Polyploidy The previous two pages have investigated mutations, and this page continues with more information related to genetic mutations. Polyploidy Humans are diploid creatures, meaning for every chromosome in our body, there is another one to match it. Read the following Haploid creatures have one of each chromosome Diploid creatures have two of each chromosome Triploid creatures have three of each chromosome Polyploid creatures have three or more of each chromosome They can be represented by n where n equals haploid, 2n equals diploid and so on. It is possible for a species, particularly plant species, to produce offspring that contains more chromosomes than its parent. This can be a result of non-disjunction, where normally a diploid parent would produce diploid offspring, but in the case of non-disjunction in one of the parents, produces a polyploid. In the case of triploids, although the creation of particular triploids in species is possible, they cannot reproduce themselves because of the inability to pair homologous chromosomes at meiosis, therefore preventing the formation of gametes. Polyploidy is responsible for the creation of thousands of species in today's planet, and will continue to do so. It is also responsible for increasing genetic diversity and producing species showing an increase in size, vigour and an increased resistance to disease. Mutation Frequency This page and the previous two have investigated the different ways that mutations arise, and the following elaborates on the ways in which mutations are instigated. Barring all external factors, mutations occur very rarely, and are rarely expressed because many forms of mutation are expressed by a recessive allele. However there are many mutagenic agents that artificially increase the rate of mutations in an organism. The following are some factors that increase genetic mutations in organisms Members of species in a particular geographic area or ethnic origin are more susceptible to mutations High dosages of X-Rays or ultraviolet light can increase the likeliness of a mutation Radioactive substances increase the rate of mutations exponentially As mentioned previously, genetic mutations are a source of new variation in a species because it physically alters the sequence of nucleotides in a given sequence, therefore altering the genome in a unique way. The next pages investigate genetic diversity in more detail, an how certain alleles (perhaps mutations) are favoured over other alleles in natural selection... Theory of Natural Selection In the 19th century, a man called Charles Darwin, a biologist from England, set off on the ship HMS Beagle to investigate species of the island. After spending time on the islands, he soon developed a theory that would contradict the creation of man and imply that all species derived from common ancestors through a process called natural selection. Natural selection is considered to be the biggest factor
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