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MEIOSIS Why is Meiosis Important ? • Meiosis is important because it prevents the increase, or decrease, in the number of chromosomes (amount of DNA) in an offspring …. Abnormal chromosome numbers, extra chromosomes or fewer chromosomes than normal, can seriously effect the traits and survival chances of the offspring. Karyotype Photograph of Chromosomes Non-disjunction Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over) • Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. • Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring. Non-disjunctions usually occur in one of two fashions. The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set. Common Non-disjunction Disorders Down’s Syndrome – Trisomy 21 Turner’s Syndrome – Monosomy 23 (X) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18 Patau’s Syndrome – Trisomy 13 Down’s Syndrome – Trisomy 21 Down syndrome causes mental retardation, a characteristic face, and multiple malformations. Down syndrome is a relatively common birth defect. The chromosome abnormality affects both the physical and intellectual development of the individual. Turner’s Syndrome – Monosomy 23 (X) A congenital condition of females associated with a defect or absence of an X-chromosome, characterized by short stature, sexual underdevelopment, and other physical abnormalities. Kleinfelter’s Syndrome – Trisomy 23 (XXY) Affects males Edward’s Syndrome – Trisomy 18 Results in physical abnormalities such as low birth weight; a small, abnormally shaped head; small jaw; small mouth; low-set ears; and clenched fists with overlapping fingers. Those with Edward's syndrome also have heart defects, and other organ malformations such that most systems of the body are affected. The survival rate for Edward's Syndrome is still incredibly low. Only 5 - 10% of liveborn infants will survive their first year of life. Patau’s Syndrome – Trisomy 13 Patau's Syndrome is usually fatal, with most of the babies dying before birth and those who do make it to birth typically living only a few days. However, a small number of babies (<10%) live at least one year. 3 Ways of assessing the risk of passing on possible Genetic Disorders 1. Genetic Counseling * Helps identify parents at risk of passing on genetic disorders. * Helps parents with deciding whether of not they should have children. * Use family pedigrees to trace family histories. 2. Pedigree a diagram that shows the occurrence of genetic traits over several generations of a family 3. Amniocentesis An Amniocentesis is a procedure a pregnant woman can have in order to detect some genetics disorders…..such as nondisjunction. Amniocentesis Amniotic fluid withdrawn Chromosome Types Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male Sex Chromosomes “Sex Chromosomes” …….the 23rd set This person has 2 “X” chromosomes… and is a female. 23 Sex Chromosomes “Sex Chromosomes” …….the 23rd set 23 This person has 1 “X” chromosome and 1 “Y” chromosome… this is a male. Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22 Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus (position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”) Paternal Chromosome Maternal Chromosome eye color locus eye color locus hair color locus hair color locus Paternal Maternal Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue Homologous set #1 Homologue Organisms that reproduce Sexually are made up of two different types of cells. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. 2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes. Diploid Cells (symbolized as 2n) • Diploid cells (2n) have 2 copies of every chromosome, forming what is called a “Homologous” chromosome pair. In Diploid cells, one pair of chromosomes comes from the mother and the other pair comes from the father. The diploid number for humans is 46, or 2n = 46,…. Or you could say…..Humans have 23 homologous pairs, or…. Humans have 92 chromatids. n = number of chromosomes in a set… so….2n means 2 chromosomes in each set …. Polyploid cells have more than two chromosomes in each set … example: 3n (3 chromosomes in each set ) Development of Gametes The Male Gamete is the Sperm and is produced in the male gonad the Testes. The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries. During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the Zygote. (fertilized egg) Sperm + Ovum (egg) (n) + (n) fertilization Zygote (2n) Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n) Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. How Meiosis Works • In organisms that reproduce Sexually, meiosis produces gametes (sex cells). The male gamete (the sperm) and the female gamete (the ovum, egg) produced contain only ½ the number of chromosomes as the normal somatic cells. • Later, when the gametes unite (through “fertilization”) the normal number of chromosomes is restored. This produces an offspring with the normal number of chromosomes (DNA). Meiosis Meiosis is Two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes. During Prophase I “Crossing Over” occurs. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction) • During Crossing over chromosomes exchange segments of their DNA. Crossing Over creates variation (diversity) in the offspring’s traits. nonsister chromatids chiasmata: site of crossing over Tetrad variation Question: • A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: • 10 chromosomes (haploid) Meiosis in males is called Spermatogenesis and produces sperm. Meiosis in females is called Oogenesis and produces ova. Spermatogenesis Secondary Spermatocyte n=23 human sex cell 2n=46 sperm n=23 Primary Spermatocyte n=23 Secondary Spermatocyte haploid (n) n=23 diploid (2n) n=23 4 sperm cells are produced from each primary spermatocyte. meiosis I n=23 meiosis II Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte. Sexual Reproduction Sexual Reproduction involves the union of gametes (sperm and ovum (egg) from two separate individuals. Advantages of Sexual Reproduction There is greater genetic variation of the offspring and therefore, greater chance of survival in changing environments. Disadvantages of Sexual Reproduction Energy is expended finding a mate in many organisms. However some organisms have both male and female reproductive organs that are able to produce gametes simultaneously. Classical Mendelian Genetics (along with some other variations) • Genetics is the study of Heredity…………. Heredity is the passing of traits from one generation to the next. In February 1865, Mendel presented his findings to the Natural History Society of Brünn and published them under the title "Experiments in Plant Hybridization" in a scientific journal. No one seemed interested in Mendel's findings., his discoveries went unappreciated. Gregor Mendel died on January 6, 1884 at the age of 62. Sixteen years later, Mendel's findings were rediscovered. Gregor Mendel In 1900, three botanists in different parts of Europe came across his papers. They praised Mendel's research and achieved similar results in their own studies. Mendel's principles of heredity began to be referred to as the Mendelian Laws, and these laws are considered to be the foundation of the modern study of genetics. Your “Traits” are your characteristics! Examples would be: The color of your hair, your eyes, whether or not you are a taster, tall stem, short stem, stripes, no stripes…..etc. Why did Mendel use Sweet peas? 1. The traits of the sweet pea seem to “Breed True”………. Tall plants were “Tall”… short plants were “short”….. Etc… 2. Pollination was easy to control. (transfer of pollen from the male part of the plant to the female part of the plant) Flowers are the reproductive structures of the plant. - Attract pollinators, and protects the reproductive structures Pistil - Protects young flower Site where the eggs are produced base of the flower - The Male Reproductive Structures are called the Stamens. They consist of two parts: • The anther -- a small case in which the pollen grains (sperm) form. • The filament -- a slender stem that supports the anther. The Female Reproductive Structure is called the Pistil. It consists of three parts: • The stigma -- the pollen grains stick to this small sticky pad • The style -- the pollen grains grow down through this stem-like cylinder • The ovary -- this is where the young seeds (eggs) wait for the chromosomes in the pollen (sperm), and where they grow into mature seeds…. Which contain the embryos. also called a Carpel Pollination is the transfer of pollen from the male part of the plant to the female part of the plant. Self- Pollination is the transfer of pollen from the male part of the plant to the female part of the same plant. Pollen from plant A pollinates plant A… the offspring now is composed of the DNA from only one parent. Selfpollination ensures genetic continuity….. every generation is genetically identical. Cross- Pollination is the transfer of pollen from the male part of one plant to the female part of different plant. Pollen from plant B is transferred to plant A… the offspring now is composed of the DNA of both parents. Crosspollination creates diversity. Mendel prevented his sweet peas from selfpollinating by removing the anthers from the flowers. Some Vocabulary You’re Going to Need Genes • A Gene is a segment of DNA which codes for a particular trait. • Your traits are determined by your Genes. Your Genes are located on your chromosomes. Genes always occur in pairs (sets of 2). You inherit one set of genes from your Father, and one set of genes from your Mother. Allele An Allele is one of the possible options for a given characteristic or trait. • Human Examples: - you can have an allele for brown eyes or an allele for blue eyes - you can have an allele for brown hair or an allele for blonde hair Pea Plant Examples: – An allele for flower color: purple or white – An allele for height: tall or short – An allele for flower position: terminal or axial Genotype • What genes you inherited for that trait • Example: If you have Brown hair it is because you have inherited a gene for Brown Hair. Phenotype • The actual expressed trait • What you see when you look at an organism…. Blue eyes, brown hair ??? Dominant Trait • An allele that can mask or hide the expression of another allele for the same trait. • Is represented by a capital (big) letter • Example: In Humans, Brown eyes is a Dominant trait • The dominant trait will always be expressed in the next generation. Recessive Trait • An allele that is masked or hidden when present with another allele for the same trait • Is only expressed when two are inherited • Is represented by a lower case (little) letter • In Humans: Blue eyes is a Recessive trait Homozygous If an organism is homozygous it has either two dominant genes (homozygous dominant)for a trait or two recessive genes (homozygous recessive) for a trait • When the two alleles inherited are the same • Homozygous dominant – Both are dominant alleles Example: TT = tall + tall = a tall plant Key T = Tall, t = short • Homozygous recessive – Both are recessive alleles Example: tt = short + short = a short plant Heterozygous If an organism is heterozygous it has a dominant gene for one trait and a recessive gene for the other trait • When the two alleles inherited are different • Only the dominant allele is expressed or visible Example: Tt = tall + short = a tall plant Key T = Tall, t = short Individuals • Purebreds = have only one allele for a trait Example: TT, tt, PP,pp (They are either Homozygous Dominant or Homozygous Recessive) • Hybrids = have different alleles for a trait. • Example: Tt or Pp Probability Is the likelihood that a specific event will occur A probability may be expressed as a decimal percent age or a fraction Use the following equation to determine probability Probability = Number of times an event is expected to happen Number of times an event could happen Sample: A trait occurs in 35 individuals out of a total of 105 individuals occurs with a probability of? Solution: 35/105 = 1/3 or 33% or 0.33 Terminology and Symbols used by Mendel • P1 = parent generation • F1 = first generation of children • F2 = second generation of children “F” stands for filial, or sons and daughters Mendel’s Laws Law of Segregation • Each pair of alleles separate during the formation of the gametes…. traits could disappear and then reappear. NB: The Law of Segregation describes meiosis… which had not been discovered yet. Example TT x tt P1 Gametes T T t t Alleles have separated to form gametes Law of Independent Assortment • Each pair of alleles separate independent from other pairs of alleles during the formation of the gametes. • One trait has no effect on the inheritance of the others! In other words… the big ”G” and the big “W” don’t also have to go together into the same gametes. likewise… the little ”g” and the little “w” don’t have to go together into the same gametes. Types of Crosses (One) Monohybrid Cross = cross one trait, TT x tt (Two) Dihybrid Cross = cross for two traits, TTpp x ttPP • Yes!… there are trihybrid crosses…. etc…. But we won’t cover them in this class Monohybrid Cross • A cross between two individuals • Examines only one trait • Parents have contrasting traits Let P = Purple, p = white P1 PP Purple Flower x pp White Flower Punnett Squares (named after RC Punnett) An easy (nonmathematical) way to calculate the probability of the traits for the offspring of two specific individuals. Website that performs genetic Punnett squares http://www.athro.com/evo/gen/punexam.html Problem 1 Is it possible for two brown-eyed parents to have a blue-eyed child? Let B = Brown eyes, b = blue eyes “B” = is a capital letter, so…. “B” (brown eyes) is dominant. “b” = is lower case, so… blue eyes are recessive. Possible Genotypes are: BB – Homozygous Dominant = Brown eyes Bb - Heterozygous = Brown eyes bb – Homozygous Recessive = blue eyes ***A heterozygous male marries a heterozygous female. Give the “genotypic” and “phenotypic” ratios for the F1 generation. Solution to Problem 1 P1 gametes Bb B x b F1 Bb B b B b B BB Bb b Bb bb Genotypic ratio – 1 BB:2 Bb: 1bb (or 1:2:1) Phenotypic ratio - 3 Brown eyes: 1 blue eyes (3:1) *** Is it possible for two brown-eyed parents to have a blue-eyed child? YES Problem 2 Let B = Brown eyes, b = blue eyes “B” = is a capital letter, so…. “B” (brown eyes) is dominant. “b” = is lower case, so… blue eyes are recessive. Possible Genotypes are: BB – Homozygous Dominant = Brown eyes Bb - Heterozygous = Brown eyes bb – Homozygous Recessive = blue eyes ***A homozygous recessive male marries a heterozygous female. Give the “genotypic” and “phenotypic” ratios for the F1 generation. Solution to Problem 2 P1 gametes bb b x b F1 Bb B b B b b Bb bb b Bb bb Genotypic ratio - 2 Bb: 2 bb (reduced to 1Bb:1bb) Phenotypic ratio - 2 Brown eyes: 2 blue eyes (reduced to 1Brown:1Blue) B B Gametes P ? p Pp ?p p Pp ?p Test Cross • A cross used to determine what genes (genotype) a parent has. • Cross the unknown individual (PP or Pp) with an individual that is homozygous recessive for that trait (pp). • The Children will show the second unknown gene. Dihybrid Cross • A cross between two individuals • Examines two traits at the same time • Parents have contrasting traits Let P = Purple, p = white T = Tall, t = short P1 = PPTT Purple Flower, Tall Gametes PT Pt x pptt White Flower, short pT pt Determining the Sex of the Offspring • The sex of the Offspring is determined by the male’s sperm cell. Females only have “X” chromosomes which code for females. Males have “X” chromosomes and “Y” chromosomes which code for males. If the ”X” sperm fertilizes the egg the offspring will be a female. If the ”Y” sperm fertilizes the egg the offspring will be a male. P1 gametes Male Female XY XX X Y X F1 X Y XX XY X X X XX XY Results 50% XX, 50% XY 50% female, 50% male Some Human Sex-linked Traits are traits that are coded for on the “X” chromosomes … never the “Y” chromosomes Examples include: Hemophilia Red-green color blindness Baldness Sex-linked traits are rarely found in women because they would require an afflicted male to reproduce with a carrier female. Hemophilia sometimes called the “Bleeders” disease, Hemophilia is an inherited disease. It is caused by a defect in one of the genes that determine how the body makes blood clotting factors. The major signs and symptoms of hemophilia are: Bleeding and Bruising. • Internal bleeding is common in those with severe hemophilia. • The extent of bleeding depends on the type and severity of the disease: • Children with mild to moderate disease may not have any symptoms at birth. • Boys with severe disease may bleed heavily after circumcision. • • • • In most children, the first signs are: 1. Heavy bruising and bleeding from the gums as they cut their baby teeth 2. Bumps and bruises from frequent falls as they learn to walk 3. Swelling and bruising from bleeding in the joints and muscles. Normal Mother + Father with Hemophilia Case 1 Each pregnancy has a 50% chance of resulting in a female carrier and a 50% chance of resulting in a normal male. Sons of hemophiliac fathers and normal mothers will not have hemophilia. Carrier Mother + Normal Father Case 2 Each pregnancy has a 25% chance of resulting in a normal female, a 25% chance of resulting in a female carrier, a 25% chance of resulting in a normal male, and a 25% chance of resulting in a male with hemophilia. Red-green color blindness (can you find the number?... If not you may be Red-Green color blind) Multiple Alleles • Three or more alleles for a given trait. • Example: blood types you can have A, B, O or AB type blood. Codominance (can produce 3 phenotypes) • Both alleles are completely expressed • Neither hides the other • Example: Let R = Red Flower, W = White Flower Possible Offsprings RR = Red Flower WW = White Flower RW = Red and White Stripes P1 RR Gametes R F1 WW R W W W R RW RW R RW RW Results Genotypic ratio: 100% RW x Phenotypic ratio: 100% Red and White Striped W Incomplete Dominance • Neither Allele is dominant • Both alleles are expressed, or blended Example: Let R = Red Flower, W = White Flower Possible Offsprings RR = Red Flower WW = White Flower RW = Pink P1 Gametes RR R F1 WW R W W W R RW RW R RW RW Results Genotypic ratio: 100% RW x Phenotypic ratio: 100% Pink W Fraternal Twins • Fraternal (Dizygotic) Twins form when two separate eggs are fertilized by separate sperm. Fraternal twins are no more genetically similar than siblings born apart. Identical Twins • Identical twins occur when a single egg is fertilized by a single sperm to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb. Pleiotropy • One gene influences many phenotypes. (traits) • Example in peas: One gene determines round or wrinkled… The Same gene affects starch metabolism and water absorption. Polygenic Inheritance • Many genes determine a single phenotype (trait) • Not just 2 or 3 varieties, but a continuous variation or range of phenotypes • The opposite of pleiotropy • Example: Human Skin Color