Partial Exam 2 BIOL - Dimuth Siritunga PDF

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BIOL 3300

Prof. Dimuth Siritunga

Partial Exam #2

Chapter 4: Modification of Mendelian Genetics Variation among the effect of mutation:  The chance alteration of the genetic material, known as mutations, arise spontaneously in nature. (it is a random occurrence)  Once they occur in gamete-producing cells (sex cells), the mutations are faithfully inherited. o Only mutations that occur in gametic cells will be passed on to the next generation. (Although, it would require a big number of mutated gametes)  Genes are identified by mutations that alter the phenotype. o There are many genes and each gene can be mutated in different ways  In nature, mutations provide the raw material for evolution o Mutations keep happening because our DNA dependent polymerase keeps messing up. But because of that, if it gets passed down for millions of years, we have evolution. o If mutations stop, evolution stops. Types of Mutations / Evolution:  The mutations that affect the outward visible properties of an organism (such as shape, color) are called visible mutations  The mutations that limit reproduction are called sterile mutation  The mutations that lead to the death of the organism are called lethal mutations Question:  Mendelian Genetics occurs but produces a lethal gene instead.

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However, most genes are pleiotropic, affecting more than one phenotypic character. (one gene, two phenotypes) o The Ay allele produces effects on two characters: coat color and survival o For example, the wide-ranging symptoms of sickle-cell disease are due to a single gene. Considering the intricate molecular and cellular interactions responsible for an organism’s development, it is not surprising that a gene can affect a number of an organism’s characteristics. (genes having more than one effect)

BIOL 3300

Prof. Dimuth Siritunga

Partial Exam #2

Epistasis: (two genes, one phenotype, no specific ratio)  In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus.  It is identified through its phenotype, when you don’t get 9:3:3:1. (May change because of lethal mutations, or epistasis if there is a non-Mendelian ratio) First example: Mice Coat Color  In mice and many other mammals, coat color depends on two genes o One, the epistatic gene, determines whether pigment will be deposited in hair or not.  Presence (C) is dominant to absence (c) o The second determines whether the pigment to be deposited is black (B) or brown (b)  The black allele is dominant to the brown allele o An individual that is cc has a white (albino) coat regardless of the genotype of the second gene.  In this example, now the phenotypic ratio has changed to 9:3:4, with 16 genotypes still. Second Example: Flower Color  At the two-low side, at least one of the alleles in each has to be dominant in order to become purple.  The purple comes from anthocyanin, which requires two chemical compounds a precursor (White) and intermediate (White) which requires having a big C. Third Example: Triangle or Oval  15 of them are triangle, 1 of them is oval. The epistatic relationship is having at least one dominant allele. (The A low side and B low side can react in the same way) Fourth Example:  To be white you need a dominant C, to be green you need a dominant G and to be blue yo need to be homozygous recessive. Enzymes:  May be inhibitory instead of progressive.  The dominant’s effect is to block, instead of progressing. Some characters do not fit the basis that Mendel Studied.  Quantitative characters vary in a population along a continuum (where there are members at every single quantifiable point of it)  These are usually due to polygenic inheritance (many genes coming together to give you one complex phenotype, such as height), the additive effects of two or more genes on a single phenotypic character. o For example, skin color in humans is controlled by at least three different genes (it is truly at least 8 or 11 genes).

BIOL 3300

Prof. Dimuth Siritunga

Partial Exam #2

o Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance. o An AABBCC individual is dark and aabbcc is light. Phenotype depends on environment and genes  An organism’s phenotype reflects its overall genotype and unique environmental history. o A single tree has leaves that vary in size, shape, and greenness, depending on exposure to wind and sun. o For humans, nutrition influences height, exercise alter build, sun-tanning darkens the skin and experience improves performance on intelligence tests. o Even identical twins (genetic equals) accumulate phenotypic differences as a result of their unique experiences.  The relative importance of genes and the environment in influencing human characteristics is a very old and hotly contested debate. o By changing an environment, you may change a phenotype. o We really don’t know how an environment plays a role in the phenotype. Environmental effects on the expression of animal genes  PKU (phenylketonuria) is a recessive disorder of amino acid metabolism in humans o Infants homozygous for the mutant alleles accumulate toxic substances in their brains. o Amino acid phenylalanine is metabolized into toxic compounds in the affected infants leading to mental retardation.  The extra phenylalanine is deposited in their brains, as they are homozygous recessive for their phenotypes. o However, infants that are fed a low-phenylalanine diet will mature without serious mental deformities o An environmental effect – diet – is changed in order to modify a phenotype in humans.  This is how you can alter an environment to alter a phenotype.  You may have the genetic probability of an environmental effect on cardiovascular disease and lung cancer, but you may reduce the chance of getting them by changing your environment. Temperature influenced the coat color pattern of Siamese cats  These domestic cats are homozygous for allele Siamese, one of the many alleles of a gene that encodes an enzyme which catalyzes the production of the dark pigment melanin  This allele does not function at the cat’s normal body temperature (its heart temperature, thus the enzyme does not work closer to the heart and its fur will be white, unlike its extremities where the enzyme works, and its fur is darker)  It becomes active only at the lower temperatures found in the cat’s extremities, where it promotes the production of melanin  This darkens the animal’s ears, nose, paws and tail o It is a huge scientific endeavor to figure out: Phenotype = Genotype x(interacts with) Environment

BIOL 3300

Prof. Dimuth Siritunga SECTION 2: Sex, Chromosomes and Genes

Chapter 5 (pages 84-95) Partial Exam #2 Chapter 6 (pages 99-117) Chapter 7 (pages 120-131) Chapter 8 (pages 143-157) Chapter 15 (pages 280-291)

Introduction:  Chromosomes were discovered by Waldeyer in late 19th century by applying dyes to dividing cells. (This lets them visualize the chromosomes)  Chromosome number is (almost) always a even multiple number of the basic number n (23 in humans)  The basic number (n) is a set of chromosomes called the haploid genome: o Cells with 2 sets (2n) = Diploid o Cells with 3 sets (3n) = Triploid o Cells with 4 sets (4n) = Tetraploid  The basic number of chromosomes varies between species and is unrelated to the size and complexity of the species.  In some species, the X and Y sex chromosomes distinguish the cells of males and females o XX in females o XY in males  In some other species, XX is female and XO is male. o The XY system is not the only system available for determining sex.  Meiosis: homologous chromosomes line up, so how does and XY line up? o The tips of and X and a Y chromosome are genetically similar, but the middle is different.  Y chromosome is morphologically different from X chromosome  The genetic material common to the human X and Y chromosome is limited, mainly near the segments near the ends of the chromosomes. o Duchenne’s Muscular Dystrophy (the valuable chromosome is on the X and is not covered up by another X in the males that are XY) The chromosomal basis of inheritance  It was not until 1900 that biology finally caught up with Gregor Mendel  Independently, Karl Correns, Erich von Tscermak and Hugo de Vries all found that Mendel had explained the same results 35 years before. o These men had microscopes where they could actually see mitosis and meiosis, therefore contributing to what Mendel did.  Mendel’s hereditary factors are the genes located on chromosomes. (Visualized Chromosomes, Mendel called them Hereditable Factor)  Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors. o Chromosomes and genes are both present in pairs in diploid cells. o Homologous chromosomes separate and allele segregate during meiosis o Fertilization restores the paired condition for both chromosomes and genes.  Around 1902, other noted these parallels and a chromosome theory of inheritance began to take form. Sex Chromosomes  The chromosomal basis of sex is rather simple and varies with the organism  In humans, and other mammals there are two varieties of sex chromosomes, X and Y.

BIOL 3300   

Prof. Dimuth Siritunga

Partial Exam #2

In the X-Y System, Y and X chromosomes behave as homologous chromosomes during meiosis. o In reality, they are only partially homologous and rarely undergo crossing over. In both testes (XY) and ovaries (XX), the two sex chromosomes segregate during meiosis and each gamete receives one. (FALTA INFORMACION) Because of this, each conception has about a fifty-fifty chance of producing a particular sex. o The male has the choice of producing a daughter or a son.

The X-O System: (insects)  XX: female  XO: male

The ZW System: (chickens)  Mother determines the sex of the son  ZZ: male  ZW: female

The Haplo-Diploid system: (bees)  Haploid: male  Diploid: female

Tracing of a gene to a specific chromosome:  Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome in the really 20th century.  Used Drosophila melanogaster, a fruit fly species that eats fungi on fruit. (Very abundant system. They produce their children quickly) o Fruit flies are prolific breeders and have generation time of two weeks. o Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males) normally.  Morgan spent a year looking for variant individuals among the flies he was breeding. (He tried to follow an individual gene’s effect) o He discovered a single male fly with white eyes instead of the usual read. (Because he had microscopes, he could look at the individual and tell that is was a male). Essentially, the eye gene had mutated.  The normal character phenotype is the wild type.  Alternative traits are mutant phenotypes.  When Morgan crossed his white-eyed male with a red-eyed female, all the F1 offspring had red eyes. o The red allele appeared dominant to the white allele.  Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring. (This occurs in Mendel’s Monohybrid crosses, which means that the red eye color is dominant and monogenic)  Surprisingly, the white-eyed trait appeared only in males. o All the females and half the males had red eyes.  Morgan concluded that a fly’s eye color was linked to its sex. Morgan deduced that the gene with the white-eyed mutation is on the X chromosome alone, a sex-linked gene.  To get a white fly, he needs a heterozygous mother and a homozygous father. (White eyed father x Red eyed mother with white eye allele) Inheritance patterns of sex-linked genes:

Morgan’s Knowledge:  Red is dominant to White  Eye color is due to a single gene  Males are XY vs Females are XY

BIOL 3300  

Prof. Dimuth Siritunga

Partial Exam #2

In addition to their role in determining sex, the sex chromosomes especially the X chromosome, have genes for many characters unrelated to sex. These sex-linked genes follow the same pattern of inheritance as the white-eyed Drosophila.

Several serious human disorders are Sex - linked = X - linked  Duchenne muscular dystrophy: o Affects one in 3,500 males born in the United States. o Affected individuals rarely live past their early 20’s. o The disorder is characterized by a progressive weakening of the muscles and a loss of coordination.  The biggest muscle that fails is the heart. o A Duchenne affected male dies and never passes their trait to a daughter, thus there will never be females with the disease / mutation of the single gene.  Human Color blindness (X-linked vision disorder) o Color perception is controlled by 2 light-absorbing proteins – blue, red and green light. o Color blindness involving faulty judgment of red-green light follows and X-linked pattern of inheritance  5-10% of males are red-green color blind  1 = Female o X:A ratio < 0.5 = Male o X:A ratio is between 0.5 – 1 = Intersex (have characteristics of both sexes)  X = number of X compared to pairs of nonsex  A drosophila may have 3 and 4 X’s, unlike humans. Sex determination of other animals:  ZW system: male determines the sex of the child. o Male: Heterogametic o Female: Homogametic  Honeybees: o Female: Fertilized Egg (Diploid) o Male: Unfertilized Egg (Haploid) Inactivation of X-linked Genes: (only in females, since males have only one X)  Although female mammals inherit two X chromosomes, only one X chromosome is active.

BIOL 3300 

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Prof. Dimuth Siritunga

Partial Exam #2

Therefore, males and females have the same effective dose (one copy) of genes on the X chromosome. o During female development, one X chromosome per cell condenses (tightens so much, that it is not able to transcribe the X chromosome) into a compact object, a Barr body. o This inactivates most of its genes. Mary Lyon, a British geneticist, has demonstrated that the selection of which X chromosome to form the Barr body occurs randomly and independently in embryonic cells at the time of X inactivation. As a consequence, females consist of a mosaic (mixed pattern) of cells, some with an active paternal X, others with an active maternal X. o After Barr body formation, all descendent cells have the same inactive X. The orange and black pattern on tortoiseshell cats: o Their fur is determined by the X chromosome. For some event, some cells condense, and orange is expressed (even though it is not dominant).

Variation of Number and Structure of Chromosomes: Chapter 6: 99-117 Alterations of chromosome number:  Meiosis: occurs only in the gamete producing the sex cells.  Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells. o This may occur if tetrad chromosomes do not separate properly during Meiosis I o Alternatively, sister chromatids may fail to separate during Meiosis II. o Nondisjunction occurring during the First Meiotic Division is more devastating than in the Second Meiotic Division.  As a consequence of nondisjunction, some gametes receive two copies of the same type of chromosome and another gamete receives no copies. Terminology for Variation in Chromosome Numbers:  Aneuploidy (2n ± x chromosomes): you have one or maybe two messed up chromosomes. (Example: Down Syndrome)  Polyploidy (3n, 4n, 5n…): you have more than 2 copies of a chromosome. This does not exist for the animal kingdom. (The closest to us are amphibians) o Polyploidy is relatively common among plants and much less common among animals.  The spontaneous origin of polyploid individuals plays an important role in the evolution of plants.  Both fishes and amphibians have polyploid species  Offspring results from fertilization of a normal gamete (n) with one after nondisjunction will have an abnormal chromosome number or aneuploidy. (2n+1 or 2n-1) o Trisomic cells have three copies of a particular chromosome type and have 2n +1 total chromosomes. (Down Syndrome)

BIOL 3300

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Prof. Dimuth Siritunga

Partial Exam #2

o Monosomic cells have only one copy of a particular chromosome type and have 2n – 1 chromosome. If the organism survives, aneuploidy typically leads to a distinct phenotype.

Alterations of chromosome structure:  Deletion: removes a chromosomal segment  Duplication: repeats a segment  Inversion: reverses a segment within a chromosome  Translocation: moves a segment from one chromosome to another, non-homologous one. o A gene that wanted to be transcribed and low-side level, is now transcribed at high-level (and vice versa) o Each area of a chromosome has a different rate of transcription, and by switching them around it changes the rate of transcription. This leads to a genetic problem. Structural Changes:  Deletions and duplications are common in Meiosis (takes place in gametic cells) o Homologous chromatids may break and rejoin at incorrect places such that one chromatid will lose more genes than it receives.  A diploid embryo that is homozygous for a large deletion or male with a large deletion to tis single X chromosome is usually missing many essential genes and this leads to a lethal outcome. o Duplications and translocations are typically harmful.  Reciprocal translocation or inversion (switch) can alter phenotype because a gene’s expression is influenced by its location (changing its transcription rate) o Not as devastating as a deletion.  Several serious human disorders are due to alterations of chromosome number and structure.  Although the frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that embryos are aborted. (reason why there are no trisomic human beings) o These developmental problems result from an imbalance among gene products.  Certain aneuploid conditions (such as Down Syndrome) upset the balance less, leading to survival to birth and beyond. o These individuals have a set of symptoms – a syndrome – characteristic of the type of aneuploidy.  One aneuploid condition, Down Syndrome, is due to three copies of chromosome 21. o It affects one in 700 children born in the United States.  Although chromosome 21 is the smallest human chromosome, it severely alters an individual’s phenotype in specific ways.  Most cases of Down Syndrome results from nondisjunction during gamete production in one parent. o The mother is the most probable for giving the child the n + 1 (nondisjunction occurs more in the mother than in the father) because of age. As the mother ages, the spindle fiber checkpoints don’t function. Raising the probability of producing and n + 1 gamete.

BIOL 3300  

Prof. Dimuth Siritunga

Partial Exam #2

The frequency of Down Syndrome correlates with the age of the mother. o This is linked to some age-dependent abnormality in the spindle checkpoint during meiosis I, leading to nondisjunction. Trisomies of other chromosomes also increase in incidence with maternal age, but it is rare for infants with these autosomal trisomies to survive for long.

Patau – another aneuploid condition:  Known as Trisomia 13 (three copies of the chromosome 13)  Survivors rarely surpasses one ear of life  50% are dead within the 1st month and 70% are dead within months  This syndrome affects to 1 of each 10,000 births  The risk of having a baby with this syndrome increases with the age of the mother Trisomic Conditions in Sex Chromosomes are Less Harmful:  Klinefelter’s syndrome, an XXY male, occurs once in every 2000 live births o These individuals have male sex organs, but are sterile o There may be feminine characteristics o Their intelligence is normal  Males with an extra Y chromosome (XYY) tend to somewhat be taller than average.  Trisomy X (XXX), which occurs once in every 2000 live births, produces healthy females.  Monosomic X or T...