Meiosis and the Sexual Life Cycle

Meiosis and the Sexual Life Cycle Link to Slides – Ch 13 AP – must know that there are 2 stages of Meiosis… Must refer to 1) meiosis 2) meiosis Must know similarities and differences of mitosis and meiosis and how they connect to each other Interphase wont happen 2 times Gametogenesis = meiosis 22 autosomes 1 sex pair rH factor - ph factor in blood measuring whether or not u have protein Ex O- = no protein O+ = protein Chi squared table Only use it if question ASKS. ★​ If youre working with organisms you round down in the END. ★​ Always use p=0.05 unless said otherwise ★​ Must figure out what null hypothesis is and either reject or accept it. Right = reject ●​ Values betw. observed and expected are statistically significant & not due to chance ●​ If x^2 is equal to or greater than crit value, then you reject null hypothesis. Left = fail to reject (accept) ●​ Values betw. observed and expected are NOT statistically significant & due to chance Degrees of freedom= total outcomes-1 x^2: (o-e)^2/e Null Hypothesis (H0): There is NO statistically significant difference between the observed values and the expected values. The difference is due to chance. 12/1 Variations on a Theme ●​ Offspring resemble their parents more than they do unrelated individuals ●​ Heredity is the transmission of traits from one generation to the next ●​ Variation is demonstrated by the differences in appearance that offspring show from parents and siblings ●​ Genetics is the scientific study of heredity and variation Offspring acquire genes from parents by inheriting chromosomes ●​ In a literal sense, children do not inherit particular physical traits from their parents ●​ It is the variation of genes that are actually inherited Inheritance of Genes ●​ Genes are the units of heredity and are made up of segments of DNA ●​ Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) ●​ Most DNA is packaged into chromosomes ●​ Humans have 46 chromosomes in the nuclei of their somatic cells, all cells of the body except gametes and their precursors ●​ A gene’s specific position along a chromosome is called its locus (loci - plural) Comparison of Asexual and Sexual Reproduction ●​ In asexual reproduction (mitosis), a single individual passes all of its genes to its offspring without the fusion of gametes ●​ A clone is a group of genetically identical individuals from the same parent ●​ In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Behavior of Chromosome Sets in Human Life Cycle ●​ Fertilization is the union of gametes (the sperm and the egg) ●​ The fertilized egg is called a zygote and has one set of chromosomes from each parent ●​ The zygote produces somatic cells by mitosis and develops into an adult ●​ At sexual maturity, the ovaries and testes produce haploid gametes ●​ Gametes are the only types of human cells produced by meiosis, rather than mitosis ●​ Meiosis results in one set of chromosomes in each gamete ●​ Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Sets of Chromosomes in Human Cells ●​ The sex chromosomes, which determine the sex of the individual, are called X and Y ●​ Human females have a homologous pair of X chromosomes (XX) ●​ Human males have one X and one Y chromosome ●​ The remaining 22 pairs of chromosomes are called autosomes ●​ A gamete (sperm or egg) contains a single set of chromosomes and is thus a haploid cell (n) ●​ For humans, the haploid number is 23 (n = 23) ●​ Each set of 23 consists of 22 autosomes and a single sex chromosome (X or Y) ●​ In an unfertilized egg (ovum), the sex chromosome is X ●​ In a sperm cell, the sex chromosome may be either X or Y ●​ Each pair of homologous chromosomes includes one chromosome from each parent ●​ The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father ●​ A diploid cell (2n) has two sets of chromosomes ●​ For humans, the diploid number is 46 (2n = 46) ●​ Gametes are (n) one set of chromosomes. For humans, the haploid number is 23 (n=23) ●​ In a cell in which DNA synthesis has occurred, each chromosome is replicated ●​ Each replicated chromosome consists of two identical sister chromatids Genetics: Genotype and Phenotype ●​ An organism’s genotype is the set of genes that it carries. ○​ Ex. BB, bb, Bb ●​ An organism’s phenotype is all of its observable characteristics — which are influenced both by its genotype and by the environment. ○​ Ex. Blue eyes, Brown eyes ●​ Differences in the genotypes can produce different phenotypes. Reminder: ●​ Attached sister chromatids = 1 chromosome ●​ Separated sister chromatids = 2 chromosomes Sets of Chromosomes in Human Cells ●​ Human somatic cells have 23 pairs of chromosomes. ○​ 22 pairs of Autosomes (non -sex chromosomes) ○​ 1 pair of sex chromosomes (XX or XY) ●​ A karyotype is an ordered display of the pairs of chromosomes from a cell ●​ The two chromosomes in each pair are called homologous chromosomes, or homologs ●​ Chromosomes in a homologous pair are the ~ same length and shape and carry genes controlling the same inherited characters \ Diagnosis Turner’s syndrome - nondisjunction (failure to separate) 12/3 Meiosis reduces the number of chromosome sets from diploid to haploid ●​ Gametes do not produce new gametes. A GERMLINE cell divides to make gametes. This process is referred to as gametogenesis. ●​ Gametogenesis (gamete formation) is the process of formation and development of specialized generative cells, gametes (oocytes/sperms), from primordial germ cells. ●​ In humans, the primordial germ cell (PGC) is the primary undifferentiated stem cell type that will differentiate towards gametes: spermatozoa or oocytes. Meiosis ●​ Like mitosis, meiosis is preceded by the replication of chromosomes ●​ Meiosis takes place in two consecutive cell divisions, called meiosis I and meiosis II ○​ Only 1 interphase, but not called Interphase I. (Has prophase I, Anaphase I, etc) ●​ The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis ●​ Each daughter cell has only half as many chromosomes as the parent cell Same genes but diff variations - blue and red genes Stages of meiosis ●​ Growth in Meiosis I is called Interphase (just like mitosis). Interphase does not occur in Meiosis II ●​ Division in meiosis I occurs in four phases: ○​ prophase I ■​ In early prophase I, each chromosome pairs with its homolog and crossing over occurs ■​ X-shaped regions called chiasmata are sites of crossovers ○​ metaphase I ■​ In metaphase I, pairs of homologs line up at the metaphase plate, with one chromosome facing each pole ■​ Microtubules from one pole are attached to the kinetochore of one chromosome of each pair ■​ Microtubules from the other pole are attached to the kinetochore of the other chromosome ○​ anaphase I ○​ telophase I and cytokinesis Crossing Over and Synapsis During Prophase I ●​ After interphase, the sister chromatids are held together by proteins called cohesins ●​ During synapsis, DNA breaks are repaired, joining DNA from one nonsister chromatid to the corresponding segment of another ●​ This process leads to genetic variation between organisms Crossing Over ●​ Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent. This adds to diversity of organisms. ●​ Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome ●​ In humans, an average of one to three crossover events occurs per chromosome Evolutionary significance of genetic variation within populations ●​ Natural selection results in the accumulation of genetic variations favored by the environment ●​ Sexual reproduction contributes to the genetic variation in a population, which originates from mutations ●​ Animals that always reproduce asexually are quite rare Comparison of Mitosis and Meiosis ●​ Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell ●​ Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell ●​ Three events are unique to meiosis, and all three occur in meiosis I ○​ Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information ○​ Homologous pairs at the metaphase plate ○​ Separation of homologs during anaphase I 1️⃣ Male gametogenesis (spermatogenesis) ●​ One diploid germline cell undergoes meiosis → produces 4 sperm. ●​ All 4 sperm are functional gametes and can potentially fertilize an egg. 2️⃣ Female gametogenesis (oogenesis) ●​ One diploid germline cell undergoes meiosis → produces: ●​ 1 ovum (egg) → functional/viable, can be fertilized ●​ 3 polar bodies → small, non-functional, usually disintegrate ➔​ Why? The egg keeps most of the cytoplasm to nourish the embryo; polar bodies get very little cytoplasm and serve no further role. ●​ All 4 gametes are not always usable. ●​ In females, only 1 ovum per meiosis is functional/viable; the other 3 polar bodies are “null.” and will shrink. ●​ In males, all 4 sperm are functional. Stages of Meiosis Contd ●​ Cytokinesis separates the cytoplasm ●​ At the end of meiosis, there are four daughter cells, each with a haploid set (non-identical) of unreplicated chromosomes ●​ Each daughter cell is genetically distinct from the others and from the parent cell ●​ Meiosis is referred to as a reduction division Genetic Variation produced in sexual life cycles contributes to evolution ●​ Mutations (changes in an organism’s DNA–Nucleotides) are the original source of genetic diversity ●​ Mutations create different versions of genes called alleles ●​ Reshuffling of alleles during sexual reproduction produces genetic variation Origins of Genetic Variation among offpring ●​ The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation ●​ Three mechanisms contribute to genetic variation: ○​ Independent assortment of chromosomes ○​ Crossing over ○​ Random fertilization Independent Assortment of Chromosomes ●​ Number of combinations possible when chromosomes assort independently into gametes is (2^n) where n is the haploid number ●​ For humans (n=23), there are more than 8 million chromosomes. (2^33) possible combinations of 1/14/26 Chapter 14 - Mendel and the Gene Idea Mendel used the scientific approach to identify two laws of inheritance ●​ Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments ●​ Mendel became known as the “Father of Genetics” Mendel’s Experimental, Quantitative Approach ●​ Mendel’s approach allowed him to deduce principles that had remained elusive to others ●​ A heritable feature that varies among individuals (such as flower color) is called a character ●​ Each variant for a character, such as purple or white color for flowers, is called a trait ●​ Peas were available to Mendel in many different varieties ​ ​ ​ ​ ​ ●​ Mendel cross pollinated each flower. ●​ First generation is (P) Parental generation. ●​ P1 xP2 = Truebreeding - Self-pollination & duplication of the same species. White = white ; purple = purple. ●​ Truebreeding -> First filial generation (F1) ●​ Mendel chose to track only those characters that occurred in two distinct alternative forms ●​ He also started with varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) ●​ In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization ●​ The true-breeding parents are the P generation ●​ The hybrid offspring of the P generation are called the F1 generation (First filial) ●​ When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced The Law of Segregation ●​ When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple ○​ Crossed (white)P1xP2(Purple) = got all Purple (Hybrids) ●​ When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white ●​ Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation ●​ Always start with true breeding; guarantee that there will be numbers and variation. What happened: (White)P1xP2(purple)[TRUEBREEDING] -> crossed pollination -> F1 Hybrids (Purple) 100% -> F2 Hybrids (3 Purple(75%)), 1 white(25%)) Purple (Dominant) White (Recessive). Dominant doesn’t equal a chance to get the trait. Purple is only dominant in this case because it is expressed over white. No matter what you do, if started with cross pollination with 2 true breeding plants, population will approximate 75% dominant (purple) and 25% recessive (white) every time. ●​ Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids ●​ Mendel called the purple flower color a dominant trait and the white flower color a recessive trait ●​ The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation ●​ Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits ●​ What Mendel called a “heritable factor” is what we now call a gene ●​ Whatever’s expressed in the F1 generation is dominant. Mendel’s Ideas ●​ First: Alternative versions of genes account for variations in inherited characters ●​ For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers ●​ These alternative versions of a gene are called alleles ●​ Each gene resides at a specific locus on a specific chromosome Allele = variation of a trait ●​ Second: for each character, an organism inherits two alleles, one from each parent ●​ Mendel made this deduction without knowing about chromosomes ●​ The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation ●​ Or the two alleles at a locus may differ, as in the F1 hybrids ●​ Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance (Law of Dominance) ●​ In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant ●​ Fourth (the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes ●​ Thus, an egg or a sperm gets only one of the two alleles that are present in the organism ●​ This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis ●​ The model ALWAYS accounts for the 3:1 ratio observed in the F2 generation of Mendel’s crosses (TRUEBREEDING–Phenotype). Its genotype ratio is 1:2:1. ○​ Genotypes: PP, Pp, pp ●​ Possible combinations of sperm and egg can be shown using a Punnett square ●​ A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele The Testcross ●​ An individual with the dominant phenotype could be either homozygous dominant or heterozygous ●​ To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual ●​ If any offspring display the recessive phenotype, the mystery parent must be heterozygous Figure out its phenotype and its parents The Law of Independent Assortment ●​ Mendel derived the law of segregation by following a single character ●​ The F1 offspring produced in this cross were monohybrids, heterozygous for one character ●​ A cross between such heterozygotes is called a monohybrid cross Law of dominance - dominant gene is expressed and crossed over a recessive (check) Law of Independent Assortment - handful of variations Law of Segregation - make diff gametes. Each gamete only has 1 of the 2 chromosomes that made up a homologous pair. Independent Assortment and Dihybrid Cross (Two Traits) ●​ Mendel identified his second law of inheritance by following two characters at the same time ●​ Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters ●​ A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently NON MENDELIAN GENETICS Inheritance patterns are often more complex than predicted by simple Mendelian genetics ●​ The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied ●​ Many heritable characters are not determined by only one gene with two alleles ●​ However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance Extending Mendelian genetics for a Single Gene ●​ Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: ○​ When alleles are not completely dominant or recessive ○​ When a gene has more than two alleles ○​ When a gene produces multiple phenotypes Degrees of Dominance ●​ Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical ●​ In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties. A third variation is the outcome. ○​ Ex. Red and White Rose = Pink rose ●​ In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways TWO MAIN DIFFERENCES TO LOOK OUT FOR BETW NONMENDELIAN AND MENDELIAN: ^ non mendelian bc its pink instead of red or white in F1 generation. Neither parent is dominant over the other (INCOMPLETE DOMINANCE), so outcome is a third variation (pink– 100%). Still does not lose the red and white variations. Superscript(exponent) in nonmendelian. The relationship between Dominance and Phenotype ●​ In the case of pea shape, the dominant allele codes for an enzyme that converts an unbranched form of starch in the seed to a branched form ●​ The recessive allele codes for a defective form of the enzyme, which leads to an accumulation of unbranched starch ●​ This causes water to enter the seed, which then wrinkles as it dries Frequency of Dominant Alleles ●​ Dominant alleles are not necessarily more common in populations than recessive alleles ●​ One baby out of 400 in the United States is born with extra fingers or toes ●​ This condition, polydactyly, is caused by a dominant allele, found much less frequently in the population than the recessive allele Another ex: red x white flower = red + white flower. NOT pink. Multiple alleles ●​ Most genes exist in populations in more than two allelic forms ●​ For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme that attaches A or B carbohydrates to red blood cells: I^A, I^B, and i ●​ The enzyme encoded by the I^A allele adds the A carbohydrate, whereas the enzyme encoded by the I^B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither AB is codominant Polygenic Inheritance ●​ Quantitative characters are those that vary in the population along a continuum ●​ Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype ●​ Height is a good example of polygenic inheritance: Over 180 genes affect height ●​ Skin color in humans is also controlled by many separately inherited genes SUMMARY GAMETOGENESIS (MAKING GAMETES) ●​ ●​ ●​ ●​ ●​ ●​ ●​ ●​ Female development of gametes during embryonic development Male development of gametes during puberty Egg is non-motile ( do not move) Male (sperm) have flagella and are motile Females produce one egg + polar bodies/germline cell Males produce 4 sperm cells, equal in size Eggs mature in menstrual cycle Sperms produce continuously Probability Laws Govern Mendelian Inheritance ●​ Mendel’s laws of segregation and independent assortment reflect the rules of probability ●​ When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss ●​ In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles The Multiplication of (AND) and Addition (OR) Rules Applied to Monohybrid Crosses ●​ The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities ●​ Probability in an F1 monohybrid cross can be determined using the multiplication rule ●​ Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele ●​ The addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities ●​ The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous Solving Complex Genetics Problems with the Rules of Probability ●​ We can apply the rules of probability to predict the outcome of crosses involving multiple characters ●​ A multi-character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously ●​ In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied Extending Mendelian Genetics for Two or More Genes ●​ Some traits may be determined by two or more genes ●​ In epistasis, one gene affects the phenotype of another due to interaction of their gene products ●​ In polygenic inheritance, multiple genes independently affect a single trait Epistasis ●​ In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus ○​ Locus = location in a Dna ●​ For example, in Labrador retrievers and many ●​ other mammals, coat color depends on two genes ●​ One gene determines the pigment color (with alleles B for black and b for brown) ●​ The other gene (with alleles E for color and e for ●​ no color) determines whether the pigment will be deposited in the hair ●​ If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1 ●​ However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs ●​ Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1 ^ Varies with soil types, not only genes Nature and Nurture: Environmental Impact on Phenotype ●​ Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype ●​ The phenotypic range is broadest for polygenic characters ●​ Traits that depend on multiple genes combined with environmental influences are called multifactorial Pedigree Analysis ●​ A pedigree is a family tree that describes the interrelationships of parents and children across generations ●​ Inheritance patterns of particular traits can be traced and described using pedigrees ●​ Sex-linked (X-linked) is linked to the X chromsome and determines whether and individual will inherit the recessive traits. ○​ If question asks what percentage of a specific gender of offspring, it will be 2 out of 4 of the boxes. If asks for all offspring, it is 4 boxes. ○​ Y carries nothing The Behavior of Recessive Alleles ●​ Recessively inherited disorders show up only in individuals homozygous for the allele ●​ Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal ●​ Most individuals with recessive disorders are born to carrier parents ●​ Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Diseases ●​ Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain ○​ At the organismal level, the allele is recessive ○​ At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant ○​ At the molecular level, the alleles are codominant ●​ Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent ○​ The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell ○​ Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine ●​ Sickle-cell disease affects one out of 400 African-Americans. Genetic disorder w/ evolutionary implications ○​ It is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells ○​ In homozygous individuals, all hemoglobin is abnormal (sickle-cell) ○​ Symptoms include physical weakness, pain, organ damage, and even paralysis ●​ Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms ●​ About one out of ten African Americans has sickle-cell trait, an unusually high frequency ●​ Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common Dominantly Inherited Disorders ●​ Some human disorders are caused by dominant alleles ●​ Dominant alleles that cause a lethal disease are rare and arise by mutation ●​ Achondroplasia is a form of dwarfism caused by a rare dominant allele Fetal Testing ●​ In amniocentesis, the liquid that bathes the fetus is removed and tested ●​ In chorionic villus sampling (CVS), a sample of the placenta is removed and tested ●​ Other techniques, such as ultrasound, allow fetal health to be assessed visually in utero How to decipher x link and somatic - > father to son x linked Independent assortment = 933​ 1 ●​ AP UNIT 4.5 & 4.6 - Chapter 12 ●​ AP UNIT 5 - 5.1 & 5.2 ○​ Chapter 13 - Cell Division Meiosis ■​ Overview of Ch 12 as it relates to meiosis ■​ The Cell Cycle, Meiosis & Mitosis (cell division) ■​ Comparing Mitosis (Diploid) & Meiosis (Haploid) ■​ Reduction Division ■​ Crossing-Over ■​ Sexual Reproduction ■​ Fertilization ■​ Non-disjunction 1/26 Thomas Hunt Morgan’s Choice of Experimental organism ●​ Morgan chose to study Drosophila melanogaster - fruit fly species ●​ Several characteristics make fruit flies a convenient organism for genetic studies: ○​ They produce offspring ○​ Generation can be bred every two weeks ○​ They only have four pairs of chromosomes. ○​ Wild type - OR Normal (NO recombinants), phenotypes that were common in the fly population ○​ Traits alternative to wild type are called mutant phenotypes (anything recombinant). ■​ 1st mutant discovered - fly w white eyes instead of wild-type red eyes. Correlating behavior of genes alleles with behavior of a chromosome pair ●​ Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) ○​ F1 had all red eyes - dominant ○​ Crossed red x red = F2 showed 3:1 red to white eye ratio, but only males had white eyes ●​ Morgan reasoned that the white-eyed mutant allele must be located on the X chromosome because males only inherit one X chromosome ●​ Finding supported the chromosome theory of inheritance Wild type(normal) = W^+ -> PLUS SIGN WriTTEN The Chromosomal Basis of Sex ●​ Humans and other mammals have 2 types of sex chromosomes: a larger X chromosome and a smaller Y chromosome. ●​ A person with two X chromosomes develops as a female, while a male develops from a zygote with one X and one Y ZZ = male, ZW = female ●​ Short segments at ends of Y chromosomes are homologous with the X, allowing the two to behave like homologs during meiosis in males ●​ A gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo ●​ A gene that is located on either sex chromosome is called a sex-linked gene ●​ Genes on the Y chromosome are called Y-linked genes ●​ Only 78 genes, coding for about 25 proteins, have been identified on the human Y chromosome ●​ Genes on the X chromosome are called X-linked genes; the human X chromosome contains about 1,100 genes ●​ X linked recessive disorders are much more common in males than in females bv they only need to inherit ONE recessive allele to inherit the disorder ●​ Most x linked disorders happen to have a carrier mother X^CX^c crossing over XCY ●​ X linked if mom->son, dad->daughter Some disorders caused by recessive alleles on the X chromosome in humans: ●​ Color blindness (mostly X-linked) ●​ Duchenne muscular dystrophy ●​ Hemophilia X-Inactivation in Female Mammals ●​ In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development ●​ The inactive X condenses into a Barr body ●​ If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character ●​ If one X chromosome that is normal inactivates, person inherits a disorder. If one X chromosome that is mutated inactivates, person doesn’t inherit a disorder. ○​ Cells split and then Once inactivated, it condenses into a barr body ●​ Calico cats example Linked genes tend to be inherited together because they are located near each other on the same chromosome ●​ Each chromosome has hundreds or thousands of genes (except the Y chromosome) ●​ Genes that are located on the same chromosome tend to be inherited together and are called linked genes ●​ They have to have a certain distance to allow them to cross over. How Linkage Affects Inheritance ●​ Morgan did experiments with fruit flies to see how linkage affects inheritance of two characters ●​ Morgan crossed flies that differed in traits of body color and wing size ●​ The first cross was a P generation cross to generate F1 dihybrid flies ●​ The second was a testcross Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders. *Abnormal Chromosome Number ●​ In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis ●​ As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Nondisjunction betw mI and MII - OUTCOMES affected Meiosis I nondisjunction ●​ Homologous chromosomes fail to separate. ○​ Both members of a homologous pair go to the same daughter cell. ●​ Results after meiosis: ○​ 2 gametes with an extra chromosome (n+1) ○​ 2 gametes missing a chromosome (n–1) ○​ All four gametes are abnormal. Meiosis II nondisjunction ●​ Sister chromatids fail to separate. ○​ Happens in only one of the two cells produced by meiosis I. ●​ Results after meiosis: ○​ 1 gamete with n+1 ○​ 1 gamete with n–1 ○​ 2 normal gametes (n) ○​ Only half the gametes are abnormal Aneuploidy results from fertilization of gametes in which nondisjunction occurred Offspring have an abnormal number of a particular chromosome ●​ Monosomic zygote - one copy of a particular chromosome ●​ Trisomic zygote has three copies of a particular chromosome PolyPloidy is a condition in which organism has more than 2 complete sets of chromosomes ●​ Triploidy (3n) is three sets of chromosomes ●​ Tetraploidy (4n) is 4 sets of chromosomes Common in plants, but not animals Polyploids are more normal in appearance than aneuploids. Nondisjunction of sex chromosomes produces a variety of aneuploid conditions ●​ G1 stops the whole cycle in nondisjunction when it happens in mitosis and triggers apoptosis to prevent cell division of affected/mutated cells. Syndromes Klinefelter syndrome (MALES) - extra chromosome in a male, XXY individuals. These males don’t exhibit any syndrome. (One of X chromosomes is inactivated but still shows up as XXY) XXX females are healthy with no unusual physical features but are at risk for learning disabilities. Turner syndrome (FEMALES) - X0 females, who are sterile (can’t produce children); only known viable monosomy in humans Down syndrome (Trisomy 21): is an aneuploid condition that results from three copies of chromosome 21 It affects about one out of every 830 children born in the United States The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained Alterations in chromosomes (mutations) ●​ Breakage of a chromosome can lead to four types of changes in chromosome structure: ○​ Deletion removes a chromosomal fragment ○​ Duplication repeats a segment ○​ Inversion reverses orientation of a segment within a chromosome ○​ Translocation moves a segment from one chromosome to another Graphing for frq Growth over time - scatter plot with line Points independent of each other - scatter plot only groupings - bar graph?