Meiosis Notes November 14, 2012
Meiosis Notes November 14, 2012 1st…. A Review of Chromosomes • http://learn.genetics.utah.edu/content/begin/ scale/ Chromosome Numbers • In a human, a body cell contains 46 chromosomes. Chromosome Numbers • Chromosomes come in homologous pairs, thus genes come in pairs. Homologous pairs – matching genes – one from female parent and one from male parent • When homologous chromosomes are paired together, this is referred to as a tetrad. • Example: Humans have 46 chromosomes or 23 pairs. One set from dad 23 in sperm One set from mom 23 in egg Chromosome Numbers • Cells with both sets of homologous chromosomes are called diploid. These cells are represented by 2N. In humans, the diploid number is 46, so 2N=46. • In sexually reproducing organisms, the gametes (sex cells – sperm and egg) contain one set of chromosomes. They are called haploid, and are represented by the symbol N. In humans, the haploid is 23, so N=23. Match the Numbers! 23 CHROMOSOMES HAPLOID GAMETES DIPLOID N SEX CELLS 2N SOMATIC CELLS BODY CELLS ASEXUAL REPRODUCTION 46 CHROMOSOMES SEXUAL REPRODUCTION MEIOSIS PHASES OF MEIOSIS • Meiosis is a process of reduction division where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to produce 4 haploid cells. • Meiosis is divided into two process – Meiosis I – Meiosis II Meiosis makes gametes (haploid cells) *illustrate in notes MEIOSIS MITOSIS 2N 2N 2N ? ? ? ? ? ? ? ? ? PHASES OF MEIOSIS I MEIOSIS I • The end result is two haploid cells, each with half the number of chromosomes as the original cell. The chromosome exists as sister chromatids at this point. PHASES OF MEIOSIS I PRE-MEIOSIS - INTERPHASE • During interphase, DNA and organelles are copied. • 46 chromosomes 92 chromosomes (diploid) • Don’t forget….centrioles are also copied! PHASES OF MEIOSIS I PROPHASE I • Spindle fibers appear; nuclear membrane disappears. • Homologous chromosomes are paired, forming a tetrad – Each tetrad contains 4 sister chromatids • Crossing over occurs. Portions of chromatids are exchanged, contributing to genetic variation in offspring CROSSING OVER • As homologous chromosomes pair up and form tetrads in Meiosis I, they actually exchange portions of their chromatids. • Crossing over is the reason for genetic diversity! It produces new combinations of genes. PHASES OF MEIOSIS I METAPHASE I • Tetrads/homologous chromosomes line up in the middle of the cell • Spindle fibers attach PHASES OF MEIOSIS I ANAPHASE I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell PHASES OF MEIOSIS I TELOPHASE I • Cytokinesis occurs; two new nuclear envelopes form • End result is two daughter cells, each with 46 chromosomes PHASES OF MEIOSIS II PROPHASE II • Spindle fibers appear; nuclear envelope disappears. • Question of the day: why is there not another interphase? PHASES OF MEIOSIS II METAPHASE II • Chromosomes line up in the middle of the cell PHASES OF MEIOSIS II ANAPHASE II • Sister chromatids separate and move toward opposite ends of the cell PHASES OF MEIOSIS II TELOPHASE II • Cytokinesis occurs; four new nuclear envelopes form • End result is four daughter cells, each with 23 haploid chromosomes PHASES OF MEIOSIS II - CYTOKINESIS MALES • In males, cytokinesis makes four equal cells that develop heads and tails – sperm – called spermatogenesis FEMALES • In females, cytokinesis is unequal; one cell gets most of the cytoplasm to become the mature egg, while the other three usually die and are known as polar bodies. Called oogenesis. • Egg and Sperm are called gametes. FEMALES http:www.cellsalive.com/meiosis.htm Chromosome Problems A. Failure of the homologous chromosomes to separate normally during meiosis -- nondisjunction 1. missing a chromosome – monosomy (45 chromosomes) 2. having an extra chromosome – trisomy (47 chromosomes) 3. most embryos fail to survive, but some do a. short, round face, upper eyelids cover the inner part of the eye, mental retardation – trisomy 21 -three #21 chromosomes (47 chromosomes) – Down Syndrome b. male with longer-than-average limbs, sterile – XXY Klinefelter’s Syndrome c. female with short stature, webbed neck, sterile – XO Turner’s Syndrome B. Parts of chromosomes break off and reattach in different ways, occurs in meiosis: deletion / translocation / duplication / inversion Deletion Duplication Inversion Translocation C. Detecting chromosome mutations: 1. Picture of individual’s chromosomes – karyotype 2. Amniotic fluid surrounding an embryo is removed for analysis (done 3½ to 4 months of pregnancy) – amniocentesis 3. Analysis of chorionic villi which grows between mother’s uterus and placenta (done 2 months of pregnancy – chorionic villi sampling (CVS) COMPARISONS! MITOSIS • Occurs in ALL body cells • Chromosomes are duplicated • Chromosomes are pulled apart by spindle fibers • An identical, complete set of chromosomes moves towards each side of cell • Cytokinesis • End product are two IDENTICAL cells • Daughter cells are diploid – 2N = 46 MEIOSIS • Occurs in sex cells ONLY • During first meiotic division, crossing over occurs • During second meiotic division, chromosome duplication does not occur • Produces four cells, each with half a set of complete chromosomes • Cells are haploid (N = 23) • Cells are GENETICALLY DIFFERENT from each other GENETICS THE PROCESS OF GETTING US HOW WE ARE. Genetics Notes Who is Gregor Mendel? “Father of Genetics” A. Mendel stated that physical traits are inherited as “particles” B. Mendel did not know the “particles” were actually chromosomes and DNA Traits • Genetics – study of how traits are passed from parent to offspring • Traits are determined by the genes on the chromosomes. A gene is a segment of DNA that determines a trait. • One pair of Homologous Chromosomes: Gene for eye color (blue eyes) Homologous pair of chromosomes Gene for eye color (brown eyes) Alleles – different genes (possibilities) for the same trait – ex: blue eyes or brown eyes Gene Linkage •Genes located on same chromosome – gene linkage •Genes located on same chromosome cannot go through-- independent assortment •Chromosomes are distributed to gametes randomly or independently Package of genes inherited together Law of Independent Assortment 1. Principle of Independent Assortment – genes for different traits can segregate independently during the formation of gametes. Therefore, the inheritance of one trait has no effect on the inheritance of another. • Example: Hair color and Eye color These genes segregate independently and do not influence each other’s inheritance (i.e. not all people with blonde hair will have blue eyes). NOVEMBER 15, 2013 • WHY DO SOME SIBLINGS HAVE SIMILAR TRAITS WHILE OTHERS DO NOT? Dominant and Recessive Genes • Gene that prevents the other gene from “showing” – dominant • Gene that does NOT “show” even through it is present – recessive • Symbol – Dominant gene – upper case letter – T Recessive gene – lower case letter – t Dominant color Recessive color Example: Straight thumb is dominant to hitchhiker thumb T = straight thumb t = hitchhikers thumb (Always use the same letter for the same alleles— No S = straight, h = hitchhiker’s) Straight thumb = TT Straight thumb = Tt Hitchhikers thumb = tt * Must have 2 recessive alleles for a recessive trait to “show” • Both genes of a pair are the same – homozygous or purebred TT – homozygous dominant tt – homozygous recessive • One dominant and one recessive gene – heterozygous or hybrid Tt – heterozygous BB – Black Bb – Black w/ white gene bb – White Law of Segregation • During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other (during MEIOSIS) • Alleles for a trait are then “recombined” at fertilization, producing the genotype for the traits of the offspring. Genotype and Phenotype • Combination of genes an organism has (actual gene makeup) – genotype Ex: TT, Tt, tt • Physical appearance resulting from gene make-up – phenotype Ex: hitchhiker’s thumb or straight thumb Law of Dominance • In a cross of parents that are homozygous dominant and homozygous recessive, only one form of the trait will appear in the next generation. • All the offspring will be heterozygous and express only the dominant trait. • RR x rr yields all Rr (Round Seeds) Generation “Gap” • Parental P1 Generation = the parental generation in a breeding experiment. • F1 generation = the first – generation offspring in a breeding. (1st filial generation) – From breeding individuals from the P1 generation • F2 generation = the second – generation offspring in a breeding experiment – 2nd filial generation – From breeding individuals in the F1 generation Punnett Square and Probability • Used to predict the possible gene makeup of offspring – Punnett Square • Cross involving one trait -- monohybrid • Example: Black fur (B) is dominant to white fur (b) in mice 1. Cross a heterozygous male with a homozygous recessive female. Black fur (B) Heterozygous male White fur (b) White fur (b) Homozygous recessive female White fur (b) Male = Bb X Female = bb b Male gametes - N (One gene in sperm) B b b Bb Bb bb bb Female gametes – N (One gene in egg) Possible offspring – 2N Write the ratios in the following orders: Genotypic ratio = 2 Bb : 2 bb 50% Bb : 50% bb Genotypic ratio homozygous : heterozygous : homozygous dominant recessive Phenotypic ratio = 2 black : 2 white 50% black : 50% white Phenotypic ratio dominant : recessive Cross 2 hybrid mice and give the genotypic ratio and phenotypic ratio. B b B BB Bb b Bb bb Bb X Bb Genotypic ratio = 1 BB : 2 Bb : 1 bb 25% BB : 50% Bb : 25% bb Phenotypic ratio = 3 black : 1 white 75% black : 25% white Example: A man and woman, both with brown eyes (B) marry and have a blue eyed (b) child. What are the genotypes of the man, woman and child? What is the probability of them having a blue eyed child? Bb X Bb Man = Bb B b B BB Bb b Bb bb Woman = Bb 3 brown : 1 blue (2 carriers) ¼ or 25% blue NOVEMBER 18, 2013 DIHYBRID CROSSES • HOW TO: AABB x CCDD Crossing involving 2 traits – Dihybrid crosses • Example: In rabbits black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross 2 hybrid rabbits and give the phenotypic ratio for the first generation of offspring. Possible gametes: BbHh X BbHh BH BH Gametes Bh Bh bH bH BH bh bh Phenotypes - 9:3:3:1 9 black and straight 3 black and curly 3 brown and straight 1 brown and curly BH Bh bH bh BBHH BBHh BbHH BbHh Bh BBHh BBhh BbHh Bbhh bH BbHH BbHh bbHH bbHh bh BbHh Bbhh bbHh bbhh • Example: In rabbits black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross a rabbit that is homozygous dominant for both traits with a rabbit that is homozygous dominant for black coat and heterozygous for straight hair. Then give the phenotypic ratio for the first generation of offspring. BBHH X BBHh Possible gametes: BH BH Bh BH Phenotypes: 100% black and straight BH BBHH Bh Gametes BBHh Gametes (Hint: Only design Punnett squares to suit the number of possible gametes.) Gamete Formation Practice: • Black fur (B) in guinea pigs is dominant to White Fur (b), and rough fur (R) in guinea pigs is dominant to smooth (r). Cross a heterozygous parent (BbRr) with a heterozygous parent (BbRr). BbRr BbRr – Possible Gametes: Parent 1: BR, Br, bR, br Parent 2: BR, Br, bR, br BR Br bR br BR Br bR br • Brown eyes (B) are dominant to blue eyes (b), and brown hair (H) is dominant to blonde hair (h). Cross a blued eyed, homozygous brown haired parent with a blue eyed, blonde haired parent. – Possible Gametes: Parent 1: bH Parent 2: bh bbHH bH bH bH bH bbhh bh bh bh bh Sex Determination 1. What is the probability of a husband and wife having a boy or girl baby? Chance of having female baby? 50% male baby? 50% X X X XX XX Y XY XY Who determines the sex of the child? father Incomplete dominance and Codominance • When one allele is NOT completely dominant over another (they blend) – incomplete dominance Example: In carnations the color red (R) is incompletely dominant over white (r). The hybrid color is pink. Give the genotypic and phenotypic ratio from a cross between 2 pink flowers. Rr X Rr R r R RR Rr r Rr rr Genotypic = 1 RR : 2 Rr : 1 rr Phenotypic = 1 red : 2 pink : 1 white • When both alleles are expressed – Codominance Example: In certain species of chickens black feathers (FB) are codominant with white feathers (FW). Heterozygous chickens have black and white speckled feathers. Show the F1 from crossing 2 hybrid chickens. Give the genotypic and phenotypic ratio. FBFB = black FWFW = white FBFW = speckled FB FW FB F BF B F BF W FW F BF W FWFW F BF W X F B F W Genotypic = 1 FBFB : 2 FBFW : 1 FWFW Phenotypic = 1 black : 2 speckled: 1 white Sex – linked Traits • Genes for these traits are located only on the X chromosome (NOT on the Y chromosome) • X linked alleles always show up in males whether dominant or recessive because males have only one X chromosome • Examples of recessive sex-linked disorders: 1. colorblindness – inability to distinguish between certain colors (most common red/green) You should see 58 (upper left), 18 (upper right), E (lower left) and 17 (lower right). Color blindness is the inability to distinguish the differences between certain colors. The most common type is red-green color blindness, where red and green are seen as the same color. 2. hemophilia – blood won’t clot (blood clotting factor VIII defective) 3. Duchenne muscular dystrophy – wasting away of skeletal muscles • Example: A female that has normal vision but is a carrier for colorblindness marries a male with normal vision. Give the expected phenotypes of their children. N = normal vision n = colorblindness XN Xn X XN Y XN Xn XN XNXN XNXn XNY XnY Y Phenotype: 1normal vision female 1 normal vision female (carrier) 1 normal vision male 1 colorblind male Take out meiosis review and notes • HAPPY TUESDAY – NO WARM UP TODAY Meiosis Review • Meiosis Animation • Fertilized egg Zygote embryo Pedigrees • Graphic representation of how a trait is passed from parents to offspring • Tips for making a pedigree 1. Circles are for females 2. Squares are for males 3. Horizontal lines connecting a male and a female represent a marriage 4. Vertical line and brackets connect parent to offspring 5. A shaded circle or square indicates a person has the trait 6. A circle or square NOT shaded represents an individual who does NOT have the trait 7. Partial shade indicates a carrier – someone who is heterozygous for the trait • Example: Make a pedigree chart for the following couple. Dana is color blind; her husband Jeff is not. They have two boys and two girls. HINT: Colorblindness is a recessive sex-linked trait. XnXn Has trait XNY Can pass trait to offspring Multiple Alleles • 3 or more alleles of the same gene that code for a single trait • In humans, blood type is determined by 3 alleles – IA, IB, and iO BUT genes come in pairs, so each human can only inherit 2 alleles 1. Codominant – IA, IB Recessive – iO 2. Blood type – A = IAIA or IAiO B = IBIB or IBiO AB = IAIB O = iOiO Example: A woman homozygous for type B blood marries a man who is heterozygous type A. What will be the possible genotypes and phenotypes of their children? IBIB X IA iO IB IB IA IAIB IAIB iO IBiO IBiO Genotypic = 2 IAIB : 2 IBiO Phenotypic = 2 AB : 2 B Polygenic Inheritance • Effect of 2 or more genes on a single phenotypic characteristic • Examples: eye color, skin color, height, body mass • Skin color: dark is dominant to light, genes are not linked Ex: aabbcc = very light AABBCC = very dark AaBbCc = medium color – 3 dominant, 3 recessive AABbcc = medium color – 3 dominant, 3 recessive • Disorders that are polygenic: autism, diabetes, cancer Gel Electrophoresis Mutations • Mutation – sudden genetic change (change in base pair sequence of DNA) • Can be : Harmful mutations – organism less able to survive: genetic disorders, cancer, death 5 – 8 genes in humans results in death – lethal mutation Beneficial mutations – allows organism to better survive: provides genetic variation Neutral mutations – neither harmful nor helpful to organism • Mutations can occur in 2 ways: chromosomal mutation or gene/point mutation • Only mutations in sex cells are passed from parent to offspring Chromosomal mutation: • less common than a gene mutation • more drastic – affects entire chromosome, so affects many genes rather than just one • caused by failure of the homologous chromosomes to separate normally during meiosis (nondisjunction) • chromosome pairs no longer look the same – too few or too many genes, different shape • Examples: Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21 Turner’s syndrome – only 45 chromosomes, missing a sex chromosome (X) Klinefelter’s syndrome – 47 chromosomes, extra X chromosomes (XXY) Gene or Point Mutation • most common and least drastic • usually only one gene is altered • Examples: Recessive gene mutations: Sickle cell anemia – red blood cells are sickle shaped instead of round and get stuck in the blood vessels – can cut off blood supply to organs – heterozygous condition protects people from malaria Cystic fibrosis – mucus clogs lungs, liver and pancreas Tay-Sachs Disease – deterioration of the nervous system – early death Phenylketonuria (PKU) – an amino acid common in milk cannot be broken down and as it builds up it causes mental retardation – newborns are tested for this Dominant gene mutations: Huntington’s disease – gradual deterioration of brain tissue, shows up in middle age and is fatal Dwarfism – variety of skeletal abnormalities
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