HUMAN MOLECULAR GENETICS. N7-2006 L. Duroux Slides assembled from diverse sources. Recommended reading list - textbooks. Human Molecular Genetics 3 Strachan & Read Garland Publishing, ISBN 0-8153-4182-2 Principles of Medical Genetics Gelehrter, Collins & Ginsburg
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HUMAN MOLECULAR GENETICS N7-2006 L. Duroux Slides assembled from diverse sources
Recommended reading list - textbooks • Human Molecular Genetics 3 • Strachan & Read • Garland Publishing, ISBN 0-8153-4182-2 • Principles of Medical Genetics • Gelehrter, Collins & Ginsburg • Lippincott, Williams & Wilkins, ISBN 0683034456 • Genetics in Medicine • Nussbaum, McInnes & Willard • Elsevier, ISBN 0721602444
Journals • Nature Genetics • http://www.nature.com/ng/index.html • Nature Reviews Genetics • http://www.nature.com/nrg/index.html • Trends in Genetics • http://www.trends.com/tig/default.htm
Lecture Plan • Examples of genetic diseases in Humans • Meiosis & Recombination • Mendelian Genetics • Modes of Heredity • Glossary and Standards
Role of Genes in Human Disease • Most diseases / phenotypes result from the interaction between genes and the environment • Some phenotypes are primarily genetically determined • Achondroplasia • Other phenotypes require genetic and environmental factors • Mental retardation in persons with PKU • Some phenotypes result primarily from the environment or chance • Lead poisoning
Struck by lightning 100% Environmental Infection Weight Hair Colour Cancer Diabetes Height 100% Genetic Sex, Down syndrome, achondroplasia
Medical genetics in the health service A Medical Genetics Unit Clinical Genetics Consultant Molecular Genetics Lab Cytogenetics Lab • Clinical diagnosis • Genetic counselling • Risk assessment • Prenatal & presymptomatic diagnosis
Types of Genetic Disorders • Chromosomes and chromosome abnormalities • Single gene disorders • Polygenic Disorders • Mutation and human disease
Chromosomal disorders • Addition or deletion of entire chromosomes or parts of chromosomes • Typically more than 1 gene involved • 1% of paediatric admissions and 2.5% of childhood deaths • Classic example is trisomy 21 - Down syndrome
Down Syndrome KARYOTYPE
Single gene disorders • Single mutant gene has a large effect on the patient • Transmitted in a Mendelian fashion • Autosomal dominant, autosomal recessive, X-linked, Y-linked • Osteogenesis imperfecta - autosomal dominant • Sickle cell anaemia - autosomal recessive • Haemophilia - X-linked
Neonatal fractures typical of osteogenesis imperfecta, an autosomal dominant disease caused by rare mutations in the type I collagen genes COL1A1 and COL1A2 A famous carrier of haemophilia A, an X-linked disease caused by mutation in the factor VIII gene Sickle cell anaemia, an autosomal recessive disease caused by mutation in the b-globin gene
Polygenic diseases • The most common yet still the least understood of human genetic diseases • Result from an interaction of multiple genes, each with a minor effect • The susceptibility alleles are common • Type I and type II diabetes, autism, osteoarthritis
a b c DNA genes unreplicated pair of homologs Chromosomes & Genes • Are long stable DNA strands with many genes. • Occur in pairs in diploid organisms. • The two chromosomes in a pair are called “homologs” • Homologs usually contain the same genes, arranged in the same order • Homologs often have different alleles of specific genes that differ in part of their DNA sequence.
The number of chromosomes per cell varies in different species From Griffiths et al. Introduction to Genetic Analysis W. H. Freeman 2000
Chromosome Structure sister chromatids telomeres centromere unreplicated replicated chromosome chromosome Each chromatid consists of a very long strand of DNA. The DNA is roughly colinear with the chromosome but is highly structured around histones and other proteins which serve to condense its length and control the activity of genes. a a
Key chromosomal regions Centromere A region within chromosomes that is required for proper segregation during meiosis and mitosis. Telomeres Specialized structures at chromosome ends that are important for chromosome stability.
Two types of cell divisions Mitosis Goal is to produce two cells that are genetically identical to the parental cell. Meiosis Goal is to produce haploid gametes from a diploid parental cell. Gametes are genetically different from parent and each other.
Homologs and Sisters Sister chromatids unreplicated replicated homologs homologs a
Mitosis 4n 2n 2n In mitosis the homologs do not pair up. Rather they behave independently. Each resultant cell receives one copy of each homolog. a
Meiosis I II 2n 4n 2n 1n In meiosis the products are haploid gametes so two divisions are necessary. Prior to the first division, the homologs pair up (synapse) and segregate from each other. In the second meiotic division sister chromatids segregate. Each cell receives a single chromatid from only one of the two homologs. a
Meiosis/perfect linkage P L P L P L P L P L P L P L p l p l p l p l p l p l p l only parental-type gametes a a
Meiosis w/recombination P L P L P l P L P L P l p l p l p L p L p l p l In some meiotic divisions these recombination events between the genes will occur resulting in recombinant gametes. a a
Mitosis vs Meiosis • One Division • Homologues do not pair • Centromeres divide • Each cell inherits both homologues • Mitosis is conservative producing daughter cells that are like parental cell. • Two Divisions • Homologues Pair up • In meiosis I, centromeres do not divide • Homologues segregate from each other. • Meiosis is not conservative, rather it promotes variation through segregation of chromosomes and recombination
3. Mendelian Genetics The laws of heridity
While assigned to teach, he was also assigned to tend the gardens and grow vegetables for the monks to eat. Augustinian Monk at Brno Monastery in Austria (now Czech Republic) Gregor Mendel: “Father of Genetics” Not a great teacher but well trained in math, statistics, probability, physics, and interested in plants and heredity. Mountains with short, cool growing season meant pea (Pisum sativum) was an ideal crop plant.
http://academic.evergreen.edu/v/vivianoc/homunculus.gif Contributions in 1860s (US Civil War Era) • Discovered Genes as Particles of Inheritance • Discovered Patterns of Inheritance • Discovered Genes Come from Both Parents • Egg + Sperm = Zygote • Nature vs Nurture • Sperm means Seed (Homunculus) • Discovered One Form of Gene (Allele) Dominant to Another • Discovered Recessive Allele Expressed in Absence of Dominant Allele
Mendel worked with peas (Pisum sativum) • Good choice for environment of monastery • Network provided unusual varieties for testing • Obligate self-pollination reproductive system • Permits side-by-side genetic barriers • Cross-pollinations require intentional process • Crosses meticulously documented • Crosses numerically/statistically analyzed • Work lost in journals for 50 years! • Rediscovered in 1900s independently by 3 scientists • Recognized as landmark work!
One Example of Mendel’s Work Dwarf Tall x Phenotype P dd Genotype DD Homozygous Dominant Homozygous Recessive All Tall Clearly Tall is Inherited… What happened to Dwarf? F1 Dd Tall is dominant to Dwarf Use D/d rather than T/t for symbolic logic Heterozygous F1 x F1 = F2 possible gametes Punnett Square: D d 3/4 Tall 1/4 Dwarf F2 D Tall DD Tall Dd possible gametes Dwarf is not missing…just masked as “recessive” in a diploid state… there IS a female contribution. d Tall Dd Dwarf dd
Two fundamental laws derived from Mendel’s work 1. The Law of Segregation: Genes exist in pairs and alleles segregate from each other during gamete formation, into equal numbers of gametes. Progeny obtain one determinant from each parent. 2. The Law of Independent Assortment Members of one pair of genes (alleles) segregate independently of members of other pairs.
After rediscovery of Mendel’s principles, an early task was to show that they were true for animalsAnd especially in humans
Problems with doing human genetics:Can’t make controlled crosses!Long generation timeSmall number of offspring per crossSo, human genetics uses different methods
Chief method used in human genetics is pedigree analysisI.e., the patterns of distribution of traits in kindreds
Pedigrees give information on:Dominance or recessiveness of allelesRisks (probabilities) of having affected offspring
Autosomal DominantFirst pedigree of this type:Farabee 1903Brachydactyly
Autosomal DominantMost dominant traits of clinical significance are very rareSo, most matings that produce affected individuals are of the form:Aa x aa
Autosomal Dominant Requirements for ideal auto. dom. pedigree: Every affected person must have at least 1 affected parent
Autosomal Dominant Requirements for ideal auto. dom. pedigree: Both males and females are affected and capable of transmitting the trait
Autosomal Dominant Requirements for ideal auto. dom. pedigree: No skipping of generations
Autosomal Dominant Requirements for ideal auto. dom. pedigree: No alternation of sexes: we see father to son, father to daughter, mother to son, and mother to daughter
Autosomal Dominant Requirements for ideal auto. dom. pedigree: In the usual mating, expect 1/2 affected, 1/2 unaffected
Example: Achondroplasia • Short limbs, a normal-sized head and body, normal intelligence
Caused by mutation in the FGFR3 gene • Fibroblast growth factor receptor 3 • Inhibits endochondral bone growth by inhibiting chondrocyte proliferation and differentiation • Mutation causes the receptor to signal even in absence of ligand