Heredity Essay, Research Paper
The male of many animals has one chromosome pair, the sex chromosomes, consisting of unequal members called X and Y. At meiosis the X and Y chromosomes first pair, then disjoin and pass to different cells. One-half of the gametes formed contain the X and the other half the Y chromosome. The female has two X chromosomes, and all egg cells normally carry a single X. The eggs fertilized by X-bearing spermatozoa give females (XX), and those fertilized by Y-bearing spermatozoa give males (XY).
The genes located in the X chromosomes exhibit what is known as sex-linkage or crisscross inheritance. This is due to a crucial difference between the paired sex chromosomes and the other pairs of chromosomes. The members of the autosome pairs are truly homologous; that is, each member of a pair contains a full complement of the same genes (albeit, perhaps, in different allelic forms). The sex chromosomes, on the other hand, do not constitute a homologous pair, as the X chromosome is much larger and carries far more genes than does the Y. Consequently, many recessive alleles carried on the X chromosome of a male will be expressed just as if they were dominant, for the Y chromosome carries no genes to counteract them. The classic case of sex-linked inheritance, described by Morgan in 1910, is that of the white eyes in Drosophila. White-eyed females crossed to males with the normal red eye colour produce red-eyed daughters and white-eyed sons in the F1 generation and equal numbers of white-eyed and red-eyed females and males in the F2 generation. The cross of red-eyed females to white-eyed males gives a different result: both sexes are red eyed in F1, the females in the F2 generation are red eyed, half of the males are red eyed, and the other half white eyed. As interpreted by Morgan, the gene that determines the red or white eyes is borne on the X chromosome, and the allele for red eye is dominant over that for white eye. Since a male receives its single X chromosome from his mother, all sons of white-eyed females also have white eyes. A female inherits one X chromosome from her mother and the other X from her father. Red-eyed females may have genes for red eyes in both of their X chromosomes (homozygotes) or may have one X with the gene for red and the other for white (heterozygotes). In the progeny of heterozygous females one half of the sons will receive the X chromosome with the gene for white and will have white eyes, and the other half will receive the X with the gene for red eyes. The daughters of the heterozygous females crossed with white-eyed males will have either two X chromosomes with the gene for white and hence white eyes or will have one X with white and the other X with the gene for red eyes and will be red-eyed heterozygotes.
In humans, the red-green colour blindness and hemophilia are among many traits showing sex-linked inheritance and consequently are due to genes borne in the X chromosome.
In some animals–birds, butterflies and moths, some fish, and at least some amphibians and reptiles–the chromosomal mechanism of sex determination is a mirror image of that described above. The male has two X chromosomes and the female an X and Y chromosome. Here the spermatozoa all have an X chromosome; the eggs are of two kinds, some with X and others with Y chromosomes, usually in equal numbers. The sex of the offspring is then determined by the egg rather than by the spermatozoon. Sex-linked inheritance is altered correspondingly. A male homozygous for a sex-linked recessive trait, crossed to a female with the dominant one, gives in the F1 generation daughters with the recessive trait and heterozygous sons with the corresponding dominant trait. The F2 generation has recessive and dominant females and males in equal numbers. A male with a dominant trait crossed to a female with a recessive trait gives uniformly dominant F1 and a segregation in a ratio of 2 dominant males : 1 dominant female : 1 recessive female.
Observations on pedigrees or experimental crosses show that certain traits exhibit sex-linked inheritance; the behaviour of the X chromosomes at meiosis is such that the genes they carry may be expected to exhibit sex-linkage. This evidence still failed to convince some skeptics that the genes for the sex-linked traits were in fact borne in certain chromosomes seen under the microscope. An elegant experimental proof was furnished in 1916 by the U.S. geneticist Calvin Blackman Bridges. As stated above, white-eyed Drosophila females crossed to red-eyed males usually produce red-eyed female and white-eyed male progeny. Among thousands of such “regular” offspring there are occasionally found exceptional white-eyed females and red-eyed males. Bridges constructed the following working hypothesis. Suppose that during meiosis in the female, gametogenesis occasionally goes wrong, and the two X chromosomes fail to disjoin. Exceptional eggs will then be produced carrying two X chromosomes and eggs carrying none. An egg with two X chromosomes coming from a white-eyed female fertilized by a spermatozoon with a Y chromosome will give an exceptional white-eyed female. An egg with no X chromosome fertilized by a spermatozoon with an X chromosome derived from a red-eyed father will yield an exceptional red-eyed male. This hypothesis can be rigorously tested. The exceptional white-eyed females should have not only the two X chromosomes but also a Y chromosome, which normal females do not have. The exceptional males should, on the other hand, lack a Y chromosome, which normal males do have. Both predictions were verified by examination under a microscope of the chromosomes of exceptional females and males. The hypothesis also predicts that exceptional eggs with two X chromosomes fertilized by X-bearing spermatozoa must give individuals with three X chromosomes; such individuals were later identified by Bridges as poorly viable “superfemales.” Exceptional eggs with no Xs, fertilized by Y-bearing spermatozoa, will give zygotes without X chromosomes; such zygotes die in early stages of development.
Chromosomal Aberrations
Two general types of chromosomal abnormalities occur: numerical and structural. Numerical aberrations result from nondisjunction; that is, from the failure of a pair of homologous chromosomes or a pair of sister chromatids to separate during cell division. As described above, when nondisjunction occurs during meiosis two types of germ cells will be formed, those with an extra chromosome and those with a missing chromosome. If one of the former combines with a normal germ cell, the new fertilized egg and all the cells of the individual it produces will have an extra chromosome ; if one of the latter combines with a normal germ cell, the fertilized egg will lack a chromosome. If nondisjunction occurs after fertilization, the resulting individual will be a mosaic and will have two or more populations of cells differing in chromosomal number.
Structural aberrations result from chromosome breakages. Chromosomes may break spontaneously, or they may be broken by such environmental agents as radiation, viruses, and toxic chemicals. If a chromosomal segment breaks off and is not rejoined, it may be lost entirely in the gametes or somatic cells that derive, respectively, from meiosis or mitosis. Such a loss is called a deletion. In other instances, the broken-off segment may rejoin its chromosome but with its position inverted 180?; such inversions can alter the sequence of genetic information along the chromosome. In other cases, the segment may become translocated; that is, it may become attached to a different chromosome. When such a rearrangement occurs between two nonhomologous chromosomes without net loss or gain of chromosomal material, it is called a balanced, or reciprocal, translocation, and the individual is not phenotypically affected. If, however, the translocation results in the deletion or duplication of chromosomal material in gametes or somatic cells, the effects may be severe. This is especially true in the event of gametes that carry autosomal translocations; such chromosomal aberrations often produce lethal phenotypic effects.
Другие работы по теме:
Love Essay Research Paper A Response to
Love Essay, Research Paper A Response to Rape Fantasies and Rape Rape is a strong word. It means power. Only from a victim s eyes can one truly comprehend this life-altering occurrence. It is a nightmare to go through as a victim fearing trust in people. It is an up most horrifying action that destroys a victims well being.
Power Vs Powerless Essay Research Paper The
Power Vs. Powerless Essay, Research Paper The Idea of Male Power vs. Female Powerlessness in ?The Young Housewife? ?The Young Housewife? is a poem by William Carlos Williams that deals with many issues. One of the main issues that this poem deals with is the conflict between male power and female powerlessness.
Man Sized Job Essay Research Paper ManSized
Man Sized Job Essay, Research Paper Man-Sized Job was written by Sharlot Hall (1870-1943). It is a poem that defines a woman?s work from a man?s point of view. Poems like this were uncommon in the late 1800?s ? early 1900?s, especially if women wrote them. The two aspects of this poem that I would like to analyze are the vocabulary used and the male chauvinistic point-of-view.
Male And Female Baldness Essay Research Paper
Baldness is the partial or total absence to; hair in the scalp. Some thinning of the scalp hair take place as a normal part of the process of aging. However, many types of abnormal hair loss also occur. Some
The Relationship Between Heredity And Environment Essay
, Research Paper The Relationship between Heredity and Environment February 22, 2000 William Lee People of the world believe that the way you turn out as a person depends on either your inherited genes or the environment around you. In the article, The Wild boy of Aveyron it talks about how a boy was found in the wild, and how he was in isolation from the human race for so long, that he didn t really have any human characteristics.
Genes Essay Research Paper Genes are the
Genes Essay, Research Paper Genes are the smallest units of heredity. The information from all the genes, taken together, makes up the blueprint or plan for the human body and its functions. A gene is a short segment of DNA which is interpreted by the body as a plan or template for building a specific protein.
Feminization Essay Research Paper Although anabolic
Feminization Essay, Research Paper . Although anabolic steroids are derived from a male sex hormone, men who take them may actually experience a ?feminization? effect along with a decrease in normal male sexual function.. Although anabolic steroids are derived from a male sex hormone, men who take them may actually experience a ?feminization? effect along with a decrease in normal male sexual function..
Daddy Essay Research Paper Plath
Daddy Essay, Research Paper Plath’s poem “Daddy” describes her feelings of oppression from her childhood and conjures the struggle many women face in a male-dominated society. The conflict of this poem is male authority versus the right of a female to control her own life and be free of male domination. Plath’s conflicts begin with her father and continue into the relationship between her and her husband.
Sex Vs Gender Essay Research Paper Group
Sex Vs. Gender Essay, Research Paper Group D: Sex Vs. Gender A person is born either male or female, however according to Mead the roles of males and females are conditioned by that person’s given culture. There were two articles studied by Group D, the first one tries to prove that men or women can be the aggressive sex, while the second article deals on which sex talks more and why.
The XYY Syndrome Essay Research Paper subject
The XYY Syndrome Essay, Research Paper subject = Intro to Biology title = XYY Syndrome papers = XYY Males Syndrome is a rare genetic disorder which affects males due to an extra Y
Power Essay Research Paper Does the world
Power Essay, Research Paper Does the world belong to the strong? As they are used in this sentence, belong and strong are two multifaceted words. They can each indicate different definitions, changing the meaning of the question.
Iroquois Essay Research Paper The Iroquois
Iroquois Essay, Research Paper The Iroquois The Iroquois is a tribe of the Eastern Woodland indians. The Iroquois is one tribe that is seperated into five groups. These groups are called , Seneca ,
Essence Of Time Essay Research Paper The
Essence Of Time Essay, Research Paper The Essence of Time The male species has a very creative mind. The creative mind becomes particularly active when the case involves the female species. In Andrew Marvell?s “To His Coy Mistress,” the author shows how his creative mind is put to use. Marvell, uses time in an attempt to manipulate his coy mistress.
Xmen Essay Essay Research Paper Discuss the
Xmen Essay Essay, Research Paper Discuss the construction and representation of gender in X-men As well as the films content, the films title can also be discussed, especially when discussing gender representation. X-Men implies mystery and secrecy, but the Men in the title is expected to cover the female members of the cast as well as the male.
Aids Epidemic Essay Research Paper The Aids
Aids Epidemic Essay, Research Paper The Aids epidemic continues to grow globally. According to this article “33.6 million people are expected to be living with virus this year.” Also an expected 2.6 million people will die from the virus this year. This is the most people that will die from the virus since the epidemic was discovered around two decades ago.
The Giver Essay Research Paper It is
The Giver Essay, Research Paper It is the future. There is no war, no hunger, and no pain. No one in The Community wants for anything. Everyone is provided for. Each Family Unit is entitled to one female and male child. Each member of The Community has their profession carefully chosen for them by the Committee of Elders, and they never make a mistake.
Sea Lion Essay Research Paper The Sea
Sea Lion Essay, Research Paper The Sea Lion is found in the West Coast of North America and the Galapagos Islands. The name of the sea lion is the California Sea Lion or the Zaluphus Californianus. The phylum is the
Are Male Condoms Or Female Condoms A
Better Contraceptive? Essay, Research Paper The male condom is a thin layer made of latex or sheepskin that fits over the erect penis and stops sperm from entering the female’s body when ejaculating. They are the most commonly used contraceptive and you can purchase them at drug stores or supermarkets without a prescription.
Man And Woman Essay Research Paper The
Man And Woman Essay, Research Paper The differences between boys and girls are the result from the combination between nature and nurture. These two elements not only build up someone s behavior but also have different kind of influences. Both elements cannot be separated because it played in different times and different situation.
The Merchant Of Venice The Relationship Between
Antonio And Bassanio Essay, Research Paper The Merchant of Venice: The Relationship between Antonio and Bassanio Antonio, the protagonist of the story, is extremely good friends with Bassanio.
Pacific Disease Essay Research Paper There was
Pacific Disease Essay, Research Paper There was a terrible disease on an island far off the Pacific Ocean. There were five people, six totals including me. I was the only one who had the medicine that can cure the disease. Only problem is, the medicine is enough for one person and one person only. Who will I choose? Ok let?s just get this straight.
Gender Role Reversal Essay Research Paper Throughout
Gender Role Reversal Essay, Research Paper Throughout one’s life each of us face new obstacles due to the progression of the life cycle. Many of these so-called obstacles we encounter are from the many role demands that each one of us has to fulfill. As we get older our lives take on new roles, such as being a: wife, parent, school teacher, etc.
Intelligence Essay Research Paper Intelligence hereditary
Intelligence Essay, Research Paper Intelligence: hereditary or acquired? Is intelligence determined primary by heredity or by the environment? This issue has aroused intense debates because many believe it has a little bit of both, but to what extent? The ones that believe that believe that every person is born with a fixed amount of intelligence argue that there is little one can do to improve intelligence, so special education programs should not be expected to produce an increase in the children?s IQ.
Personality Essay Research Paper PersonalityWhat is personality
Personality Essay, Research Paper Personality What is personality? Personality is defined as a ingrained and relatively enduring patterns of thoughts, feeling and behavior. How can anybody be certain how his or her personality is going to turn out. I think people go through many different types an stages of personality throughout life.
Gender 2 Essay Research Paper GENDER
Gender 2 Essay, Research Paper GENDER Gender can be defined as the sex-role that a person takes on according to guidelines or standards instilled in us by society. One can be a male or female biologically, but still be perceived as the opposite sex due to the way one may think or present him self or her self. Whether or not we are born with certain biological traits different in male versus female is the nature versus nurture question that has been around for years.
Cell Reproduction Essay Research Paper Bio bugs
Cell Reproduction Essay, Research Paper Bio bugs The bio bugs were a great example of cell reproduction and heredity. In this essay I will discuss all the concepts of cell reproduction and heredity as they apply to the bio bugs.
Male Teachers Essay Research Paper It is
Male Teachers Essay, Research Paper It is not very often that we see or hear of male teachers in preschools and elementary schools. Often times, male teachers in preschools and elementary schools are perceived as gay or looked down upon as being ?wimpy? because of the amount of sensitivity required to teach young children.
Nature Vs Nurture Essay Research Paper The
Nature Vs. Nurture Essay, Research Paper The debate over the importance of nature verses the importance of nurture has been going on since the beginning of modern science. Both sides of the argument have undeniably good points proving that that influence is more important than the other. Both nature and nurture are important because they both help in shaping personality.
Miss Emily
’s Male Interaction In A Rose For Emily Essay, Research Paper Miss Emily’s male interaction in A Rose for Emily The way that a person is raised has a lot to do with their
Sexism Essay Research Paper SexismSo what exactly
Sexism Essay, Research Paper Sexism So what exactly is sexism? Sexism is the belief that rights and roles in ones society should be governed by ones sex. Historically, sexism has been male-driven and