Download Important Genetics Terms

Notes to Teachers 6 th International Junior Science Olympiad (IJSO) • Learning objectives – Gregor Mendel’s experiments and Mendel’s Laws (1 hr) – Monohybrid and Dihybrid Crosses (1 hr) – Probability and Genetics (0.5 hr) – Beyond Mendelism (1 hr) – Human Genetics and pedigree charts (0.5 hr) – Human genetic disorders (1 hr) Life Goes On Dr. Kwok Cheong CHUNG Department of Biology The Chinese University of Hong Kong • Time allocation: 5 hours 1 Learning Outcomes 2 Father of Genetics: Gregor Mendel After studying this topic students will be able to: • Austrian Monk who worked on selective breeding of pea to track the inheritance pattern of certain characters • Established two laws of genetics – The Law of Segregation – The Law of Independent Assortment • Understand the basis of Mendelain genetics • Use grids to predict variation in offspring characteristics in monohybrid and dihybrid crosses • Recognise the scope of genetics beyond Mendelian genetics • Recognise variation in human characteristics • Describe the role of genes and chromosomes in human inheritance • Use family trees to determine the features of family members • Be able to calculate the chance of children being born male or female using model 3 Important Genetics Terms: • Character: Any heritable feature of an organism (e.g. flower color) • Trait: the possible variants of a character (e.g. for the character flower color in pea plants the possible traits are purple flowers and white flowers) • True­breeding plants: When self pollinated, these plants always produce offspring of the same quality • Hybridization: The crossing of two different, true­ breeding plants • P generation: The parental generation ­ initial cross of a genetic breeding • F1 generation: The offspring (filial generation) produced by the P generation (each subsequent generation in a breeding line is numbered after F1. e.g. F2, F3, etc) 4 Important Genetics Terms: 5 • Allele: different genes which affect the nature of a particular character • Dominant trait: The variant that is expressed in an organism that carries both alleles for a character • Recessive trait: The variant that is not expressed in an organism that carries both alleles for a character • Genotype: The composition of alleles that an organism inherits from its parents. (e.g. Pp, PP, pp) • Phenotype: The way a character appears (or is expressed) as a result of its genotype (e.g. Genotype PP = phenotype purple flowers, genotype pp = phenotype white flowers) • Homozygous (homozygote): Organism that has inherited the same variant of allele from its parent (e.g. PP or pp) • Heterozygous (heterozygote): Organism that has 6
inherited different alleles for the same character. (e.g. Pp) Dihybrid Cross F1 Generation F2 Generation • The Law of Segregation: The two alleles from a parent will be separated into two different gametes Dihybrid Cross 7 8 Probability and Genetics F2 Genotypes • F1: YyRr x YyRr • Four possibilities for gametes : YR, Yr, yR, and yr • F2 Phenotype ratio = 9:3:3:1 9 • There is a equal chance of Probability P
p
punnett a gamete receiving either square 1/2 1/2 one of a parent’s homologous chromosomes • Probability of the gamete to P
PP Pp have either allele is the 1/2 1/2x1/2= 1/2x1/2= same 1/4 1/4 • The law of multiplication ­ by multiplying the p
Pp pp probabilities of two or more 1/2 1/2x1/2= 1/2x1/2= independent events you 1/4 1/4 can determine the The law of addition ­ overall probability of the events occurring at the same time probability of a event obtained by adding the probabilities of the • ¼ PP: ½ Pp: ¼ pp 10 different ways an event can occur Probability and Genetics Probability and Genetics • Probabilities for more complex genetics crosses can be obtained by combining the laws of multiplication and addition without having to create a punnett square Ex.1 Trihybrid cross : PpYyRr x Ppyyrr What is the probability of getting an offspring with the genotype ppyyRr? pp: (1/2 x 1/2) = 1/4 yy: (1/2 x 1) = 1/2 Rr: (1/2 x 1) = 1/2 • Inheritance of two characters Y = yellow seeds R = round seeds y = green seeds r = wrinkled seeds P generation YYRR x yyrr YyRr F 1 generation YyRr x YyRr (dihybrid cross) • The Law of Independent assortment: The alleles for different characters will segregate independent of one another into gametes Ex.2 What is the probability of getting an offspring with the phenotype: purple flowers with yellow and wrinkled seeds? (PpYyRr x Ppyyrr) Step1: Find out the possible genotypes from this cross that will satisfy the desired phenotype Answer: PPYyrr or PpYyrr Step2: Figure out the probabilities of getting each genotype PPYyrr: ( ½ x ½ ) x ( ½ x 1) x ( ½ x 1) = 1/16 PpYyrr: [( ½ x ½ ) + ( ½ x ½ )] x ( ½ x 1) x ( ½ x 1) = + 2/16 overall probability of desired phenotype: 3/16 The overall probability of getting this genotype = ¼ x ½ x ½ = 1/16 11 12
Beyond Mendelism ­ Incomplete Dominance • Mendel's experiments ­ very simple genetic system: each gene had 2 alleles, dominant and recessive, and genes did not interact with each other • Actually it could be more complex – e.g. neither allele is completely dominant over the other • Resulting phenotype of the heterozygote ­ intermediate or blended phenotype of the two alleles • E.g. flower color of Snapdragon • Condition where both alleles are L M L N dominant and are equally expressed L M L M L M L M L N • The heterozygous genotype L N L M L N L N L N carries the results of both phenotypes • NOT a blending or intermediate L M L M L M L N L N L N phenotype as in incomplete dominance Anti­N X clot clot • E.g. Human M and N blood groups ­ antigens on the surface Anti­M clot clot X of red blood cells + respective antigens = clotting 13 • Multiple alleles: When the phenotype of a character has more than two allele types • E.g. Human ABO blood groups Agglutination (bad!) • Alleles possible: I A , I B , and i Universal recipient: • Blood group inheritance: • AB blood is also an example of codominance 15 Multiple Alleles in Humans • Eye color – Determined by two genes, one controls texture of the iris which refracts light to make blue – A second gene determines the relative abundance of melanin • Hair color – Vary on an almost continuous scale from black to brown to blond to red – The brown and black pigment is melanin – The red pigment is an iron containing molecule 14 Multiple alleles Beyond Mendelism Universal donor : Codominance 16 Beyond Mendelism • Pleiotropy: one gene can affect more than one characteristic in an organism’s phenotype 17 18
Beyond Mendelism Beyond Mendelism • Epistasis: one gene determines whether another gene will be expressed • Also known as a “stopping gene” • E.g. Mouse coat color: the mouse must have at least one dominant allele (C) for coat color to have the color expressed • Color of fur determined by another gene where B = black fur (dominant) and b = brown fur (recessive) • Polygenic inheritance: Where the phenotype of one character is determined by an additive effect of two or more genes • E.g. human skin color is thought to be controlled by three separate genes (some references said >100 genes!!) • Interaction among those genes determines what a person’s skin color is (amount of melanin) 19 Human Genetics 20 Human Genetics ­ Pedigree Analysis • The genes of humans behave in the same way as genes of other organisms • Of the estimated 100,000 human genes, most are identical in all humans • The relatively small number of “polymorphic” genes in humans account for only part of the variability that we see between humans • While each human (except for identical twins) has a unique set of genetic information, variation between humans also results from differences in the environment • Data mapped on a family tree • Reveals Mendelian patterns in human inheritance • Code: P 21 More Symbols in Pedigree Charts 22 Human Genetics ­ Family Trees • Since it is not practical (or ethical) to do controlled breeding experiments in humans we can use pedigrees to track characters through human families P F1 23 24
Autosomal Dominant Genes • Autosomal: Gene is on one of the autosomes (Chromosomes 1­22) • Male and female offspring equally likely to inherit trait • Dominant: Affected individuals can appear in every generation • A trait that appears in successive generations is normally due to a dominant allele • E.g. affected male with 2 normal and 2 affected children Autosomal Recessive Genes • In this example, two unaffected parents each carry one copy of a gene mutation for an autosomal recessive disorder • They have one affected child and three unaffected children, two of which carry one copy of the gene mutation 25 Example of Autosomal Recessive Inheritance • Blood diseases of thalassanemia (地中海貧血
症）, a potentially fatal hemoglobin deficiency • Sickle cell anemia （鐮刀型貧血症）
• Spinal muscular atrophy （脊柱肌肉萎縮症) • Diabetes • There are also links between inbreeding and deafness and muteness 26 X­linked Dominant, Affected Father In this example, a man with an X­linked dominant condition has two affected daughters and two unaffected sons 27 X­linked Dominant, Affected Mother 28 X­linked Recessive, Affected Father • In this example, a woman with an X­linked dominant condition has an affected daughter, an affected son, an unaffected daughter, and an unaffected son • In this example, a man with an X­linked recessive condition has two unaffected daughters who each carry one copy of the gene mutation, and two unaffected sons who do not have the mutation 29 30
Examples of Dominant­Recessive Genes X­linked Recessive, Carrier Mother Dominant Traits Recessive Traits Eye color Vision • In this example, an unaffected woman carries one copy of a gene mutation for an X­linked recessive disorder • She has an affected son, an unaffected daughter who carries one copy of the mutation, and two unaffected children who do not have the mutation Hair 31 Examples of Dominant­Recessive Genes Appendages grey, green, hazel, blue normal vision nearsightedness night blindness color blindness* blonde, light, red hair red hair straight hair full head of hair widow’s peak baldness* normal hairline 32 *sex­linked characteristic Examples of Dominant­Recessive Genes Dominant Traits Recessive Traits Facial features Dimples 酒窩
brown eyes farsightedness normal vision normal vision normal vision dark hair non­red hair curly hair Dominant Traits Recessive Traits no dimples unattached earlobes attached earlobes Other freckles 雀斑
no freckles broad lips thin lips immunity to poison ivy susceptibility to poison ivy normal pigmented skin albinism normal blood clotting hemophilia* extra digits normal number normal hearing fused digits normal digits congenital deafness
先天性重度聽障
normal hearing and speaking normal – no PKU short digits normal digits fingers lack 1 joint normal joints limb dwarfing normal proportion clubbed thumb normal thumb double­jointedness normal joints 33 Human Genetic Disorders • Recessively inherited disorders: – follow the laws of simple dominance – homozygous recessive genotype results in a person having the disease or disorder – A person that is heterozygous for this character is considered a carrier of the disease – E.g. Cystic fibrosis 囊腫性纖維化 ­ prevalent in whites of European descent – Tay­Sachs disease 泰伊­薩克斯二氏病 ­ Jewish people that descended from central Europe – Sickle cell anemia ­ Africans and those of African descent 35 deaf mutism 聾啞
phenylketonuria 苯酮尿症
(PKU) *sex­linked characteristic 34 Human Genetic Disorders • Dominantly inherited disorders – Follow the laws of simple dominance – Homozygous dominant or heterozygous genotype results in a person having the disease or disorder – E.g. Huntington’s Disease 亨廷頓氏病
• Lethal disorder that continues to persist in the population • Carrier shows no evidence of the phenotype until they are 35­45 years of age and may have already passed the allele on to their offspring – Achondroplastic dwarfism 先天性軟骨發育不全 36
Genetic Testing Genetic Testing • Carrier testing: Persons that are high risk for certain disorders due to their heritage, familial affliction, etc, can be tested to see if they carry an allele for a disorder before they decide to produce offspring • E.g. Tests exist for Tay­Sachs, one form of cystic fibrosis, sickle cell disease, and Huntington’s disease 37 38 References for Further Studies Genetic Testing • Fetal testing for genetic disorders and conditions: • Chorinic Villus Sampling 絨毛膜
絨毛取樣 (CVS): Fetal tissue from the placenta is removed by suction. This tissue has enough cells present for immediate karyotyping and is much faster than amniocentesis • Ultrasound and fetoscopy 胎兒
鏡檢查: Less invasive ways to visualize fetus in utero and look for large, obvious disorders • Fetal testing for genetic disorders and conditions: • Amniocentesis 羊膜穿刺術檢
查: A small amount of amniotic fluid is removed from the uterus and karyotyped to look for known genetic abnormalities • Takes several weeks due to the small number of fetal cells recovered which need to be cultured to increase the amount of DNA present 39 • Mendelian inheritance ­ Wikipedia – http://en.wikipedia.org/wiki/Mendelian_inheritance • Mendelian Genetics – http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/me ndel1.htm • Monohybrid Cross Problem Set – http://www.biology.arizona.edu/mendelian_genetics/proble m_sets/monohybrid_cross/monohybrid_cross.html • Mendel and the Gene Idea – http://www.micro.utexas.edu/faculty/sjasper/bio212/mendel. html • http://www.emunix.emich.edu/~rwinning/genetics/beyond.htm • http://en.wikipedia.org/wiki/Nucleotide • Human Genetic Disorders 40
– http://www.humangeneticsdisorders.com/