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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. 1 2 Mendel’s conclusions Mendel’s experiments: 1. 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. 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 4 3 Practice … Family tree (pedigree) Male Female Draw the family tree for a YY male and a GG female! Genes … Alleles … Mating f1: all heterozygotes!! heterozygotes Parents Children Affected Draw the family tree for a YG male and a YG female! Homozygous YY or GG Heterozygous YG Plomin, 2000 Fig 2.1 (Pg. 6) 5 6 1 Mendel’s second law: principle of independent assortment 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). 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. 8 7 Practice … The concept of dominance Draw the family tree for a AB male and a BB female! AB BB AB BB AB BB Draw the family tree for a Cc male and a Cc female! Cc Cc CC Cc Cc cc 10 9 Mendel’s Laws: SUMMARY The concept of dominance What will be the color if the Y allele is dominant? Genotype – 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. Law of Segregation: …alleles separate, Law of Independent Assortment alleles one allele passes from each parent … Phenotype pass independently of each other Principles: 1. 2. YG appears as yellow!!! f2: 3:1 ratio! Only GG will be green!!! 3. 11 Inherited features governed by a pair of “elements” One element inherited from each parent Elements can dominate in their expression 12 2 Examples from psychiatry: Genetic disorders with Mendelian Patterns of Inheritance 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 14 Heredity of Huntington disease Huntington’s Disease Male Onset: mid-adulthood Prevalence: 1 in every 20,000 Starts with: personality changes, Draw the pedigree of a Huntington disease family with one HD parent! Female Mating forgetfulness Next 15-20 years: complete loss of motor No treatment has been found to stop or delay the decline Consistent pattern of Heredity: AUTOSOMAL DOMINANT Parents function and intellect Children Affected Plomin, 2000 Fig 2.1 (Pg. 6) 15 Heredity of Phenylketonuria Phenylketonuria Male Onset: early childhood Prevalence: 1 in every 10,000 Female Severe problems in early neural development leading to mental retardation Mating Due to disturbance in the metabolism of phenylalanine (an essential amino-acid) Treatment: special diet in early childhood lacking phenylalanine Affected “Runs in families”, but typically NO affected parents: AUTOSOMAL RECESSIVE Carriers 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 16 develop the disease. Draw the pedigree of a PKU family! Parents Children 17 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. 18 3 Other examples of Autosomal Dominant Diseases Mendelian Patterns of Inheritance Autosomal dominant FH Familial Hypercholesterolemia Autosomal recessive X-linked recessive (next class…) Polydactyly 19 Other examples of Autosomal Dominant Diseases Neuro- From alleles to genotypes A – is the normal allele (wild type), fibromatosis 20 a – is the mutant allele Marfan’s syndrome Achondroplasia AA is the homozygous wild genotype aa is the homozygous mutant genotype Aa is the heterozygous genotype 21 Allele & genotype frequencies A – is the normal allele (wild type), a – is the mutant allele IF frequency of A allele is 50% = 0.5 IF frequency of a allele is 50% = 0.5 What is the frequency of the following genotypes: AA = 0.5 x 0.5 = 0.25 Aa = (both Aa & aA) = 2 x 0.5 x 0.5 = 0.5 aa = 0.5 x 0.5 = 0.25 Another example… R – a rolling tounge dominant allele NR – a non-rolling tongue recessive allel R = 60% NR = 40% Can roll: R-R, R-NR, NR-R Cannot roll: NR homozygotes 84% 16% 4 http://udel.edu/~mcdonald/mythtongueroll.html Not all mendelian traits are „true” autosomal… ☺ • Mendel's laws of heredity (Law of segregation, Principle of independent assortment, Mendelian diseases) • Family tree (pedigree) (gene, allele, homozygous, heterozygous) • Huntington Disease & Phenylketonuria (features and heredity) • Allele and genotype frequencies (calculations) 26 25 Selected readings Practice for exam: Textbook: Ch 2. pg. 6-12. The outline from this lecture presentation will be available at the course website! Calculate the percentage of carriers for an autosomal recessive trait, if the allele frequency of the dominant allele is 10%. 27 5