Heredity is something which can not be separated with the life of all organisms in this world. Each organism must do a passing of trait from parents to their offspring through genes in the sperm and eggs when they are proliferating, and it is known as a process of heredity. The functional unit structure which acts as a basic of this heredity is gene. A gene is a portion of DNA that determines the characteristic which will be degraded to the next generation. Through meiosis and reproduction, gene can be transmitted from one generation to another. The study of genes, how genes produce characteristics and how the characteristic are inherited is a field of biology called genetics. The first person to systematically study inheritance and formulate laws about how characteristics are passed from one generation to the next was an Augustinian monk named Gregor Mendel (1822-1884). Mendel’s work was not generally accepted until 1900, when three men, working independently, rediscovered some of the ideas that Mendel had formulated more than 30 years earlier. Because of his early work, the study of pattern of inheritance that follows the laws formulated by Gregor Mendel is often called Mendelian Genetics.
Nowadays, the science is developing in an extremely quick manner. Consequently, the development of genetics as a field of biology is also developing speedily. It is important for us to participate in developing the science, especially the development of genetics. However, the effort of developing biotechnology (genetics technique, cloning gene, etcetera) is impossible to be done without an understanding about genetics, as the gene is a vital substance of heredity. That’s why, the most important thing to be done today is to built our understanding about the genes and the all the mechanisms of inheriting. However, by forming a good understanding about the mendelian genetics as a basic topic of heredity, it will be able to open a chance for us in order that we are able to participate an effort to develop biotechnology and also another activity or research which are aimed to give a big advantage for humans. That’s why, it is extremely useful for us to arranged this letter.
1.What is the importance of gene in the process of inheritance?
2.How can Mendel’s law of inheritance explains the way of parent’s trait to be degraded to their offspring?
This letter is written to built an understanding about the importance of gene in the process of inheritance, and also to explain the way of a trait to be passed to the offspring.
The benefits which are able to be found from this letter are as follows:
1.We will be able to understand about the function of gene in the heredity process.
2.We will be able to understand about the mechanism of heredity process.
3.We will be able to understand about the law of Mendel.
If we are talking about The Law of Mendel, the first question which ought to be answered is “What is the Law of Mendel?”, it is a simple but also an important question. The law of mendel is a basic theory which is used as a basic to understand about how the characteristics of parents can be degraded to the next generation. However, before discussing more about mendelian genetics, the first thing that we have to understand is about the definition of gene, DNA, chromosome, chromatid, chromatin, and all the things which always be used to explain about heredity.
Gene is a unit of genetic. Gene is firstly exclaimed by W.Johanssen (1909). It is a portion of DNA that determines a characteristic. A gene contains a series of genetic information that determines the characteristic which will be passed from parents to their offspring. Gen is a result of a specific coding of nucleotide. Gene is located in a specific pace in chromosome which is called locus. The distance of this locus is measured from the posit5ion of centromere.
Generally, chromosome is found in a pair. Hence, gene is always be described in a pair. For instance, autosome and gonosome of Drosophila melanogaster are arranged by the pairs of chromosome. The pair of chromosome which has the same shape, size, and also the type of gene which is contained in it is called the pair of Homolog Chromosome. Meanwhile, in a homolog chromosome, there is a pair of locus which is placed in a horizontal line, it is an equal locus.
In an equal locus, if there two the same gene, so the genes will be given the same symbol. Capital symbol is used to sign the dominant gene, meanwhile noncapital symbol is used to sign the recessive gene. For instance, gene which determines the pigment of our skin is gene A which is dominant, so it makes our skin has a pigment. Meanwhile, its couple is a gene which is recessive, so it makes the skin does not have a pigment and finally causes an albino skin. Gene A and Gene a are the same gene because both of them determine the pigmentation of skin, but they just have a different capability to determine it.
The tiniest structural and functional unit of living thing is the cell. The center of cell is called nucleus. At nucleus, there are smooth threads which is able to absorb colour, it is called chromatin. When the cell is replicating, chromatins will form a solid product which is finally called chromosome. So, chromosome exactly is a group of chromatin which is unified and be solid. A chromosome is also an organized of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements, and other nucleotide sequence.
Chromosomes vary widely between different organisms. In eukaryotes, nuclear chromosomes are packed by proteins into a condensed structure called chromatin. This allow the very long DNA molecules to fit into the cell nucleus. Chromosomes are essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated. Unduplicated chromosomes are single linear strands, whereas duplicated chromosomes contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure. Chromosomal recombination plays a vital role n genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may aberrantly evade apoptosis leading to cancer. In practice “chromosome” is rather loosely defined term. However, a large body of work uses the term chromosome regardless of chromatin content.
(1)Chromatid (2) Centromere – the point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.
The genome is a set of all the genes necessary to specify an organism’s complete list of characteristics. The term genome is used in two ways. It may refer to the diploid (2n) or haploid (n) number of chromosomes in a cell.
The genotype of an organism is a listing of genes present in that organism. It consists of the cell’s DNA code, therefore we can not see the genotype of an organism. It is not yet possible to know the complete genotype of most organisms, but it is often possible to figure out the genes present that determine a particular characteristic. For example, there are three possible genotype combination of the two alleles for earlobe shape. Genotype are typically represented by uppercase and lowercase letters. In the case of earlobes trait, the alleles for free earlobe is designated “E”, whereas that for attached earlobes is “e.” A person’s genotype could be (1) two alleles for attached earlobes, (2) one allele for attached earlobe and one allele for free earlobes, or (3) two earlobes for free earlobes.
How would individuals with each of these three genotypes look? The way each combination of alleles expresses its self is known as the phenotype of the organism. The phrase gene expression refers to the degree to which a gene goes through transcription and translation to show itself as an observable feature of the individual. A person with two alleles for attached earlobes that do not hang free. A person with one allele for attached earlobe and one allele for free earlobe will have a phenotype that exhibits free earlobes. And an individual with two alleles for free earlobes will also have free earlobes. Simply, it should be understood that there are three genotypes, but only two phenotypes. It may happen because the person who has free-earlobe phenotype can have different genotypes.
Allele is the genes which is located at an equal locus in the homolog chromosome, but it gives an influence in a different way. Allele is an important factor which is used in the series experiment of Mendel. The expression of some genes is directly influenced by the presence of other alleles. For any particular pair of alleles in an individual, the two alleles from the two parents are either identical or not identical person are homozygous for a trait when they have the combination of two identical alleles for that particular characteristic, for example, EE and ee. A person with two alleles for freckles is said to be homozygous for that trait. A person with two alleles for no freckles is also homozygous. If an organism is homozygous, the characteristic expresses itself in a specific manner. A person homozygous for free earlobes has free earlobes, and a person homozygous for attached earlobes has attached earlobes.
Individuals are designated as heterozygous when they have two different allelic forms of a particular gene, for example, Ee. The heterozygous individual received one form of the gene from one parent and a different allele from the other parent. For instance, a person with one allele for freckles and one allele for no freckles is heterozygous. If an organism is heterozygous, these two different alleles interact to determine a characteristic. A carrier is any person who is heterozygous for a trait. In this situation, the recessive allele is hidden, that is, does not express itself enough to be a phenotype.
Often, one allele in the pair expresses itself more than the other. A dominant allele masks the effect of other alleles for the trait. For example, if a person has one allele for free earlobes and one allele for attached earlobes, that person has a phenotype of free earlobes. We say the allele for free earlobes is dominant. A recessive allele is one that, when present with another allele, has its actions overshadowed by the other; it is masked by the effect of the other allele. Having attached earlobes is the result of having a combination of two recessive characteristics. A person with one allele for free earlobes and one allele for attached earlobes has a phenotype of free earlobes. The expression of recessive alleles is only noted when the organism is homozygous for the recessive alleles. If you have attached earlobes, you have two alleles
for that trait. Don’t think that recessive alleles are necessarily bad. The term recessive has nothing to do with the significance or value of the allele—it simply describes how it can be expressed. Recessive alleles are not less likely to be inherited but must be present in a homozygous condition to express themselves. Also, recessive alleles are not necessarily less frequent in the population. Sometimes the physical environment determines whether or not dominant or recessive genes function. For example, in humans genes for freckles do not show themselves fully unless a person’s skin is exposed to sunlight.H.Codominance
In cases of dominance and recessiveness, one allele of the pair clearly overpowers the other. Although this is common, it is not always the case. In some combinations of alleles, there is a codominance. This is a situation in which both alleles in a heterozygous conditions express themselves.
A classic example of codominance in plants involves the color of the petals of snapdragons. There are two alleles for the color of these flowers. Because neither allele is recessive, we cannot use the traditional capital and small letters as symbols for these alleles. Instead, the allele for white petals is given the symbol FW, and the one for red petals is given the symbol FR. There are three possible combinations of these two alleles:Genotype
From the explanation above, it should be noticed that there are only two different alleles, red and
white, but there are three phenotypes, red, white, and pink. It because both the red-flower allele and the white-flower allele partially express themselves when both are present, and this results in pink. This condition is called the codominance.
If we want to understand about the basic mechanism of heredity, we should also understand about the x-linked genes. Pairs of alleles located on nonhomologous chromosomes separate independently of one another during meiosis when the chromosomes separate into sex cells. Because each chromosomes has many genes on it, these genes tend to be inherited as a group. Genes located on the same chromosome that tend to be inherited together are called a “linkage group.” The process of crossing-over, which occurs during prophase I of meiosis, may split up these linkage groups. Crossing-over happens between homologous chromosomes donated by the mother and the father and results in a mixing of genes. The closer two genes are to each other on a chromosome, the more probable it is that they will be inherited together. People and many other organisms have two types of chromosomes. Autosomes (22 pairs) are not involved in sex
determination and have the same kinds of genes on both members of the homologous pair of chromosomes. Sex chromosomes are a pair of chromosomes that control the sex of
an organism. In humans, and some other animals, there are two types of sex chromosomes—the X chromosome and the Y chromosome. The Y chromosome is much shorter than
the X chromosome and has fewer genes for traits than found on the X chromosome. One genetic trait that is located on the Y chromosome contains the testis-determining gene—SRY. Females are normally produced when two X chromosomes are present. Males are usually produced when
one X chromosome and one Y chromosome are present. Genes found together on the X chromosome are said to be X-linked. Because the Y chromosome is shorter than the
X chromosome, it does not have many of the alleles that are found on the comparable portion of the X chromosome. Therefore, in a man, the presence of a single allele on his only X chromosome will be expressed, regardless of whethe it is dominant or recessive. A Y-linked trait in humans is the SRY gene. This gene controls the differentiation of the embryonic gonad to a male testis. By contrast, more than 100 genes are on the X chromosome. Some of these X-linked genes can result in abnormal traits such as color deficiency, hemophilia, brown teeth, and at least two forms of muscular dystrophy (Becker’s and Duchenne’s).
J.The history of mendel’s law of heredity
Heredity problem are concerned with determining which allele s are passed from the parents to the offspring and how likely it is that various types of offspring will be produced. The first person to develop a method of predicting the outcome of inheritance pattern was mendel, who performed experiments concerning the inheritance of certain chracteristics in the garden pea (Pisum sativum) plants. From his work, mendel conclude which traits were dominant and which were recessive.
The laws of inheritance were derived by Johann Gregor Mendel, a 19th century monk conducting hybridization experiments in garden peas (Pisum sativum). Between 1856 and 1863, he cultivated and tested some 29,000 pea plants. From these experiments he deduced two generalizations which later became known as Mendel's Laws of Heredity or Mendelian inheritance. He described these laws in a two part paper, Experiments on Plant Hybridization that he read to the Natural History Society of Brno on February 8 and March 8, 1865, and which was published in 1866.
Mendel's conclusions were largely ignored. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major block to understanding their significance was the importance attached by 19th Century biologists to the apparent blending of inherited traits in the overall appearance of the progeny, now known to be due to multigene interactions, in contrast to the organ-specific binary characters studied by Mendel. In 1900, however, his work was "re-discovered" by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has been somewhat debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries's paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work, or came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.
Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was William Bateson, who coined the term "genetics", "gene", and "allele" to describe many of its tenets. The model of heredity was highly contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits. Many biologists also dismissed the theory because they were not sure it would apply to all species, and there seemed to be very few true Mendelian characters in nature. However later work by biologists and statisticians such as R.A. Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed. Thomas Hunt Morgan and his assistants later integrated the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and create what is now known as classical genetics, which was extremely successful and cemented Mendel's place in history.
Mendel's findings allowed other scientists to predict the expression of traits on the basis of mathematical probabilities. A large contribution to Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding. He also only measured absolute (binary) characteristics, such as color, shape, and position of the offspring, rather than quantitative characteristics. He expressed his results numerically and subjected them to statistical analysis. His method of data analysis and his large sample size gave credibility to his data. He also had the foresight to follow several successive generations (f2, f3) of his pea plants and record their variations. Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters. Without his hard work and careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics.
K.Mendel’s law of heredity
Mendel stated that each individual has two factors for each trait, one from each parent. The two factors may or may not contain the same information. If the two factors are identical, the individual is called homozygous for the trait. If the two factors have different information, the individual is called heterozygous. The alternative forms of a factor are called alleles. The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. An individual possesses two alleles for each trait; one allele is given by the female parent and the other by the male parent. They are passed on when an individual matures and produces gametes: egg and sperm. When gametes form, the paired alleles separate randomly so that each gamete receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals the only allele that is expressed is the dominant. The recessive allele is present but its expression is hidden. Mendel summarized his findings in two laws; the Law of Segregation and the Law of Independent Assortment.
a.Law of Segregation (The "First Law")
The Law of Segregation states that when any individual produces gametes, the copies of a gene separate, so that each gamete receives only one copy. A gamete will receive one allele or the other. The direct proof of this was later found when the process of meiosis came to be known. In meiosis the paternal and maternal chromosomes get separated and the alleles with the characters are segregated into two different gametes.
b.Law of Independent Assortment (The "Second Law")
The Law of Independent Assortment, also known as "Inheritance Law", states that alleles of different genes assort independently of one another during gamete formation. While Mendel's experiments with mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios (Fig. 2). But the 9:3:3:1 table shows that each of the two genes are independently inherited with a 3:1 ratio. Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cat's color and tail length. This is actually only true for genes that are not linked to each other.
Independent assortment occurs during meiosis I in eukaryotic organisms, specifically metaphase I of meiosis, to produce a gamete with a mixture of the organism's maternal and paternal chromosomes. Along with chromosomal crossover, this process aids in increasing genetic diversity by producing novel genetic combinations.
Of the 46 chromosomes in a normal diploid human cell, half are maternally-derived (from the mother's egg) and half are paternally-derived (from the father's sperm). This occurs as sexual reproduction involves the fusion of two haploid gametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During gametogenesis - the production of new gametes by an adult - the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy.
In independent assortment the chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 223 or 8,388,608 possible combinations. The gametes will normally end up with 23 chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny.
Simply, we can make a brief summary that Mendel formulated several genetic laws to describe how characteristics are passed from one generation to the next and how they are expressed in an individual as follows:
1.Mendel’s law of dominance When an organism has two different alleles for a given trait, the allele that is expressed, overshadowing the expression of the other allele, is said to be dominant. The gene whose expression is overshadowed is said to be recessive.
2.Mendel’s law of segregation When gametes are formed by a diploid organism, the alleles that control a trait separate from one another into different gametes, retaining their individuality.
3.Mendel’s law of independent assortment Members ofone gene pair separate from each other independently of the members of other gene pairs.
L. Steps in Solving Heredity Problems: Single-Factor Crosses
The first type of problem which we will try to consider is the single-factor cross. A single-factor cross (sometimes called a monohybrid cross: mono = one; hybrid = combination) is a genetic cross or mating in which a single characteristic is followed from one generation to the next generation. For example, in humans, the allele for Tourette syndrome (TS) is inherited as an autosomal dominant allele. For centuries, people displaying this genetic disorder were thought to be possessed by the devil since they displayed such unusual behaviors.
These motor and verbal behaviors or tics are involuntary and range from mild (e.g.leg tapping, eye blinking, face twitching) to the more violent forms such as the shouting of profanities, head jerking, spitting, compulsive repetition of words, or even barking like a dog. The symptoms result from an excess production of the brain messenger, dopamine. If both parents are heterozygous (have one allele for Tourette and one allele for no Tourette syndrome) what is the probability that they can have a child without Tourette syndrome? With Tourette syndrome?
The steps in Solving Heredity Problems by using the Single-Factor Crosses is able to make us understand about the way of a trait to be passed to the next generation. There are five basic steps which are involved in this single-factor crosses to solve the heredity problem. Those steps are as follows:.
Step 1: Assign a Symbol for Each Allele.
Usually a capital letter is used for a dominant allele and a small letter for a recessive allele. Use the symbol T for Tourette and t for no Tourette.
Allele Genotype Phenotype
T = Tourette TT Tourette syndrome
t = normal Tt Tourette syndrome
Step 2: Determine the Genotype of Each Parent
and Indicate a Mating.
Because both parents are heterozygous, the male genotype is Tt. The female genotype is also Tt. The × between them is used to indicate a mating. Tt × Tt
Step 3: Determine All the Possible Kinds of Gametes Each
Parent Can Produce.
Remember that gametes are haploid; therefore, they can have only one allele instead of the two present in the diploid cell. Because the male has both the Tourette syndrome allele and the normal allele, half his gametes will contain the Tourette syndrome allele and the other half will contain the normal allele. Because the female has the same genotype, her gametes will be the same as his. For genetic problems, a Punnett square is used.
A Punnett square is a box figure that allows you to determine the probability of genotypes and phenotypes of the progeny of a particular cross. Remember, because of the process of meiosis, each gamete receives only one allele for each characteristic listed. Therefore, the male will produce sperm with either a T or a t; the female will produce ova with either a T or a t. The possible gametes produced by the male parent are listed on the left side of the square and the female gametes are listed on the top. In our example, the Punnett square would show a single dominant allele and a single recessive allele from the male on the left side. The alleles from the female would appear on the top.
Male Possible female gametes T & t
Step 4: Determine All the Gene Combinations That Can Result When These Gametes Unite.
To determine the possible combinations of alleles that could occur as a result of this mating, simply fill in each of the empty squares with the alleles that can be donated from each parent. Determine all the gene combinations that can result when these gametes unite.
Step 5: Determine the Phenotype of Each Possible Gene Combination.
In this problem, three of the offspring, TT, Tt, and Tt, have Tourette syndrome. One progeny, tt, is normal. Therefore, the answer to the problem is that the probability of having offspring with Tourette syndrome is 3⁄4; for no Tourette syndrome, it is 1⁄4. Take the time to learn these five steps. All single-factor problems can be solved using this method; the only variation in the problems will be the types of alleles and the number of possible types of gametes the parents can produce. Now let’s consider a problem in which one parent is heterozygous and the other is homozygous for a trait.
M.Double Factor Cross
Besides that single-factor croos, there also A double-factor cross which is able to be used to solve heredity problem. The double factor cross is a genetic study in which two pairs of
alleles are followed from the parental generation to the offspring. Sometimes this type of cross is referred to as a dihybrid (di = two; hybrid = combination) cross. This problem is solved in basically the same way as a single-factor cross. The main difference is that in a double-factor cross you are working with two different characteristics from each parent. It is necessary to use Mendel’s law of independent assortment when considering double-factor problems. Recall that
according to this law, members of one allelic pair separate from each other independently of the members of other pairs of alleles. This happens during meiosis when the chromosomes egregate. (Mendel’s law of independent assortment applies only if the two pairs of alleles are located on separate chromosomes. We will assume this is so in double-factor crosses.) In humans, the allele for free earlobes is dominant over the allele for attached earlobes. The allele for dark hair dominates the allele for light hair. If both parents are heterozygous for earlobe shape and hair color, what types of offspring can they produce, and what is the probability for each type?
Use the symbol E for free earlobes and e for attached earlobes.
Use the symbol D for dark hair and d for light hair.
E = free earlobes D = dark hair
e = attached earlobes d = light hair
Determine the genotype for each parent and show a mating. The male genotype is EeDd, the female genotype is EeDd, and the × between them indicates a mating. So, EeDd × EeDd
Determine all the possible gametes each parent can produce and write the symbols for the alleles in a Punnett square. Because there are two pairs of alleles in a double-factor cross, each gamete must contain one allele from each pair— one from the earlobe pair (either E or e) and one from the hair color pair (either D or d). In this example, each parent can produce four different kinds of gametes. The four squares on the left indicate the gametes produced by the male; the four on the top indicate the gametes produced by the female. To determine the possible gene combinations in the gametes, select one allele from one of the pairs of alleles and match it with one allele from the other pair of alleles. Then match the second allele from the first pair of alleles with each of the alleles from the second pair. This may be done as follows:
Determine all the gene combinations that can result when these gametes unite. Fill in the Punnett square.
Determine the phenotype of each possible gene combination. In this double-factor problem there are 16 possible ways in which gametes can combine to produce offspring. There are four possible phenotypes in this cross. They are representedin the following chart.
EEDD or EEDd or EeDD or EeDD or EeDd
Free earlobes/dark hair
EEdd or Eedd
Free earlobes/light hair
eeDD or eeDd
Attached earlobes/dark hair
Attached earlobes/light hair
So, we can see its result as follows:
From the table above, we are able to see clearly about the probability the phenotype of the offsring as follows:
1.9⁄16 free earlobes, dark hair
2.3⁄16 free earlobes, light hair
3.3⁄16 attached earlobes, dark hair
4.1⁄16 attached earlobes, light hair
Briefly, the comparison of the possible phenotyphe wich will be appear at the offspring is 9:3:3:1.
N. Alternative Inheritance Situations
So far we have considered a few straightforward cases in which a characteristic which is determined by a simple dominance and recessiveness between two alleles. Other situations, however, may not fit these patterns. Some genetic characteristics are determined by more than two alleles; moreover, some traits are influenced by gene interactions and some traits are inherited differently, depending on the sex of the offspring.
So far we have also discussed only traits that are determined by two alleles, for example, A, a. However, there can be more than two different alleles for a single trait. All the various
forms of the same gene (alleles) that control a particular trait are referred to as multiple alleles. However, one person can have only a maximum of two of the alleles for the characteristic.
A good example of a characteristic that is determined by multiple alleles is the ABO blood type. There are three alleles for blood type:
IA = blood has type A antigens on red blood cell surface
IB = blood has type B antigens on red blood cell surface
i = blood type O has neither type A nor type B antigens onsurface of red blood cell
In the ABO system, A and B show codominance when they are together in the same individual, but both are dominant over the O allele. These three alleles can be combined as pairs
in six different ways, resulting in four different phenotypes:
Blood type A
Blood type A
Blood Type B
Blood type B
Blood type AB
Blood Type O
Multiple-allele problems are worked as single-factor problemsPolygenic Inheritance
Thus far we have considered phenotypic characteristics that are determined by alleles at a specific, single place on homologous chromosomes. However, some characteristics are determined by the interaction of genes at several different loci (on different chromosomes or at different places on a single chromosome). This is called polygenic inheritance. The fact that a phenotypic characteristic can be determined by many different alleles for a particular characteristic is referred to as genetic heterogeneity. A number of different pairs of alleles
may combine their efforts to determine a characteristic. Skin color in humans is a good example of this inheritance pattern. According to some experts, genes for skin color are located at a minimum of three loci. At each of these loci, the allele for dark skin is dominant over the allele for light skin. Therefore a wide variety of skin colors is possible depending on how many dark-skin alleles are present . Polygenic inheritance is very common in determining characteristics that are quantitative in nature. In the skincolor example, and in many others as well, the characteristics cannot be categorized in terms of either/or, but the variation in phenotypes can be classified as how much or what amount . For instance, people show great variations in height. There are not just tall and short people—there is a wide range. Some people are as short as
1 meter, and others are taller than 2 meters. This quantitative trait is probably determined by a number of different genes. Intelligence also varies significantly, from those who
are severely retarded to those who are geniuses. Many of these traits may be influenced by outside environmental factors such as diet, disease, accidents, and social factors. These
are just a few examples of polygenic inheritance patterns. Pleiotropy Even though a single gene produces only one type of mRna during transcription, it often has a variety of effects on the
phenotype of the person. This is called pleiotropy. Pleiotropy (pleio = changeable) is a term used to describe the multiple effects that a gene may have on the phenotype. A good
example of pleiotropy has already been discussed, that is, PKU. In PKU a single gene affects many different chemical reactions that depend on the way a cell metabolizes the amino acid phenylalanine commonly found in many foods . Another example is Marfan syndrome , a disease suspected to have occurred in former U.S. president, Abraham Lincoln. Marfan syndrome is a disorder of the body’s connective tissue but can also have effects in many other organs including the eyes, heart, blood, skeleton, and lungs. Symptoms generally appear as a tall, lanky body with long arms and spider fingers, scoliosis, osteoporosis, and depression or protrusion of the chest wall (funnel chest/pectus excavatum or pigeon chest/pectus carinatum). In many cases these nearsighted people also show dislocation of the lens of the eye. The white of the eye
(sclera) may appear bluish. Heart problems include dilationof the aorta and prolapse of the heart’s mitral valve. Death may be caused by a dissection (tear) in the aorta from the
rupture in a weakened and dilated area of the aorta, called an aortic aneurysm.
O. Environmental Influences on Gene Expression
Maybe people assumed that the dominant allele would always be expressed in a heterozygous individual. It is not so simple! Here, as in other areas of biology, there are exceptions. For example, the allele for six fingers (polydactylism) is dominant over the allele for five fingers in humans. Some people who have received the allele for six fingers have a fairly complete sixth finger; in others, it may appear as a little stub. In another case, a dominant allele causes the formation of a little finger that cannot be bent like a normal little finger. However, not all people who are believed to have inherited that allele will have a stiff little finger. In some cases, this dominant characteristic is not expressed or perhaps only shows on one hand. Thus, there may be variation in the degree to which an allele expresses itself in an individual. Geneticists refer to this as variable expressivity. A good example of this occurs in the genetic abnormality neurofibromatosis type 1 (NF1) (figure 10.9). In some cases it may not be expressed in the population at all. This is referred to as a lack of penetrance. Other genes may be interacting with these dominant alleles, ausing the variation in expression. Both internal and external environmental factors can influence the xpression of genes. For example, at conception, a male receives genes that will eventually determine the pitch of his voice. However, these genes are expressed differently after puberty. At puberty, male sex hormones are released. It is an example of internal factor which influence the phenotype of a person. Besides that, there also external factor, such us the kinds of activity that always be done by the person. It is able to gve an big influence for the appearance phenotype.in other words, we can also say that the phenotype of someone is not only be influenced by the internal factor (such us its trait from their parents), but it also be influenced by the external factor (the factor from the environment).
The conclusions of this topic are as follows:
1.Gene has an important rule at the process os passing the traits from parents to their offspring, because gene is the unit which determine the traits which will be passed.
2.Mendel formulated several genetic laws to describe how characteristics are passed from one generation to the next and how they are expressed in an individual as follows:
a.Mendel’s law of dominance When an organism has two different alleles for a given trait, the allele that is expressed, overshadowing the expression of the other allele, is said to be dominant. The gene whose expression is overshadowed is said to be recessive.
b.Mendel’s law of segregation When gametes are formed by a diploid organism, the alleles that control a trait separate from one another into different gametes, retaining their individuality.
c.Mendel’s law of independent assortment Members ofone gene pair separate from each other independently of the members of other gene pairs.
1.If another writer intend to make a handing out like this one, it will be better to master about the topic of Mendelian Genetics.
2.To make the readers are easier to understand about this topic, firstly they should understand the definition of all the word which always be used to explain about heredity, such us gene, chromosome, allele, etcetera.
Anonima. 2009. “Mendelian Inheritance.” http://www.wikipedia.com.
Anonimb. 2009. “Chromosome.” http://www.wikipedia.com.
Karmana, Oman. 2007. “Biologi.” Bandung: Grafindo Utama.
Ross, enger. 2002. “Concepts In Biology Tenth Edition”. USA: The McGraw-Hill