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2 Laws of Heredity - Single Gene Disorders Mendel (1865), Experiments in Plant Hybridization - no impact, paper essentially ignored, rediscovery in 1900 Mendel’s key idea: “elements” – now called genes are the basic units of heredity. Laws based on observation. 3 Mendel’s experiments: In cross-pollinating plants that either produce yellow or green peas exclusively, Mendel found that the first offspring generation (f1) always has yellow peas. However, the following generation (f2) consistently has a 3:1 ratio of yellow to green. 4 Mendel’s conclusions 1. 2. 3. Inheritance of each trait is determined by "units" that are passed on to descendents unchanged (these units are now called genes) An individual inherits one such unit from each parent for each trait A trait may not show up in an individual but can still be passed on to the next generation. In this experiment, the starting parent plants were homozygous for pea color. The plants in the f1 generation were all heterozygous 5 Mendel’s first law: the principle of segregation For any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter of chance. We now know that this segregation of alleles occurs during the process of sex cell formation (meiosis). 6 Mendel’s second law: principle of independent assortment Different pairs of alleles are passed to offspring independently of each other. The result is that new combinations of genes present in neither parent are possible. Today, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes. 7 Family tree (pedigree) Male Female Genes … Alleles … Mating Parents Children Affected Homozygous YY or GG Heterozygous YG Plomin, 2000 Fig 2.1 (Pg. 6) 8 Practice … Draw the family tree for a YY male and a GG female! f1: all heterozygotes! heterozygotes! Draw the family tree for a YG male and a YG female! 9 The concept of dominance With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other. Which is to say, it masked the presence of the other allele. For example, when the genotype for pea color is YG (heterozygous), the phenotype is yellow. However, the dominant yellow allele does not alter the recessive green one in any way. Both alleles can be passed on to the next generation unchanged. 10 Mendel’s Laws: SUMMARY Law of Segregation: …alleles separate, one allele passes from each parent … Law of Independent Assortment alleles pass independently of each other Principles: 1. 2. 3. Inherited features governed by a pair of “elements” One element inherited from each parent Elements can dominate in their expression 11 Terminology TODAY Mendel’s “elements” are now called genes Genes come in alternative forms, called alleles Genotype – an individual’s combination of alleles Phenotype – the observable trait Homozygous – two copies of the same allele (AA, aa) Heterozygous – one copy of each allele (Aa) Mendelian diseases are diseases that are the result of a single gene, they generally have a large effect on behavior & distinctive patterns of familial transmission Examples from psychiatry: Genetic disorders with Mendelian Patterns of Inheritance 13 Huntington’s Disease Onset: mid-adulthood Prevalence: 1 in every 20,000 Starts with: personality changes, forgetfulness Next 15-20 years: complete loss of motor function and intellect No treatment has been found to stop or delay the decline Consistent pattern of Heredity: AUTOSOMAL DOMINANT 14 Heredity of Huntington disease Male Draw the pedigree of a Huntington disease family with one HD parent! Female Mating Parents Children Affected Plomin, 2000 Fig 2.1 (Pg. 6) Affected individuals have one parent with the disease, in the above case, the affected parent is heterozygous. In this case approx. half of the children 15 develop the disease. Phenylketonuria Onset: early childhood Prevalence: 1 in every 10,000 Severe problems in early neural development leading to mental retardation Due to disturbance in the metabolism of phenylalanine (an essential amino-acid) Treatment: special diet in early childhood lacking phenylalanine “Runs in families”, but typically NO affected parents: AUTOSOMAL RECESSIVE 16 Heredity of Phenylketonuria Male Draw the pedigree of a PKU family! Female Mating Parents Children Affected Carriers Plomin, 2000 Fig 2.1 (Pg. 6) PKU individuals do not typically have parents with PKU. If one child has PKU, the risk for other siblings is 25%. Parents are carriers in such cases. 17 Color blindness Most common form: red-green color blindness (more frequent 8% in males) SKIP-A-GENERATION PHENOMENON: If mother is colorblind, & the father is not: all of the sons but none of the daughters are affected! However, half the daughter’s sons are likely to be affected! Consistent pattern of Heredity: recessive allele on the X chromosome! Sex-linked inheritance Males are XY and females are XX Two sex chromosomes are not genetically equivalent (Y is about ¼ the size of the X) Traits associated with genes on the X chromosome - X-linked Traits associated with genes on Y chromosome - Y-linked X-linked traits Males Females One X chromosome Inherited from mother Two possible genotypes X+Y XmY Have trait/do not have trait Hemizygous Males Two X chromosomes Inherited from both parents Three possible genotypes X +X + X +X m XmXm Heterozygotes are carriers of recessive traits. Females transmit their X randomly to either their sons or daughters transmit their X to their daughters, Y to their sons Males are more likely to be affected than females regarding X-linked recessive traits Color blindness: an x-linked recessive trait Male Female Draw the pedigree of a color blind mother and an unaffected father! cc Mating C Parents Children c cC c cC Affected Carriers Heterozygotes for autosomal trait X-linked carrier Carrier of X-linked recessive trait Plomin, 2000 Fig 2.1 Meiosis Process of gametic cell production (gametogenesis) in which genetic material is reduced by half (from diploid to haploid) Problem! Things can go wrong in meiosis: e.g. nondisjunction of chromosomes: new egg and sperm should have only member of each chromosome pair (haploid set). When the division does not occur properly, an egg (or sperm) may have both members of the chromosome resulting in trisomy! Chromosomal abnormalities responsible for more than half of spontaneous abortions (miscarriages) Some fetuses with chromosomal anomalies survive, though with developmental abnormalities Most common: Down’s syndrome Trisomy 21 - Down’s syndrome Down’s syndrome Clinical features: growth retardation, mental retardation, distinct head and facial characteristics, heart problems, premature aging 95% of the time, nondisjunction occurred in mother 1/1000 birth increased rates with advanced maternal age (>35) Nondisjunction is more likely to occur as female grows older and activates immature eggs that have been dormant for decades Chromosome 21 is the smallest Chapter 3 (pages 28-37) Complex phenotypes (depression, intelligence) also seem to run in families, but they don’t show straightforward patterns of inheritance What’s going on? Lifetime expectancy of Schizophrenia Prevalence: 1/100 NO consistent pattern of heredity: morbidity risk estimate: Resemblance of Cognitive Ability (IQ) NO consistent pattern of Heredity: correlation increases with genetic relatedness Complex, quantitative traits do NOT violate Mendel’s laws! IQ height pea size schizophrenia blood pressure Single-locus Completely Additive Model: Genotype: Phenotype: A1 A 1 71” A1 A2 70” A2A2 69” Single-Locus Phenotypic Distribution A2A2 69” A2A1 70” A1A1 71” Two-Locus Phenotypic Distribution A2A1B2B1 A2A2B2B1 A2A2B2B2 68” A2A1B2B2 69” A2A2B1B1 A1A1B2B1 A1A1B2B2 A2A1B1B1 70” 71” A1A1B1B1 72” Three-Locus Phenotypic Distribution 67” 68” 69” 70” 71” 72” 73” Infinite Number of Loci Model Quantitative genetics Polygenic Model: a quantitative phenotype is influenced by many loci (genes) all of which have a small (equal) and additive effect. Types of Genetic Influence: Monogenic (Single-gene, Mendelian) Multigenic (polygenic, multifactorial) Chromosomal • Mendel's experiments and laws of heredity • Family trees of – autosomal dominant (Huntington’s) – autosomal recessive (Phenylketonuria) – and x-linked traits. • Chromosomal anomalies - Down's syndrome • Complex traits and quantitaive genetics 38 Selected readings Textbook: Ch 2. pg. 6.-12. Ch 3. pg. 20.-25. and pg. 28.-37. The outline from this lecture presentation will be available at the course website: Practice for exam: Draw the pedigree of a color blind father and an unaffected (non-carrier) mother! 41 4 Sex-Linked Traits: NORMAL: A: 29, B: 45, C: --, D: 26 Red-Green mix: A: 70, B: --, C: 5, D: -Red blind: A: 70, B: --, C: 5, D: 6 Green blind: A: 70, B: --, C: 5, D: 2 http://waynesword.palomar.edu/colorbl1.htm