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1 Sylvia S. Mader Immagini e concetti della biologia 2 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 A7 - Mendel: laws of inheritance 3 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel and the laws of inheritance Before Gregor Mendel (1822-1884) a “blending model of inheritance” was accepted. Mendel performed his experiments very carefully using the common pea plant (Pisum sativum). 4 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Why the pea plant? • Easy to grow and mature quickly • Have a short generation time • Self-pollinate • Easy to control the reproduction • Produce many offsprings 5 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 How did he performed the first experiments? To begin his experiments, Mendel used plants that showed the same trait generation by generation, also called pure lines. 6 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 How did he performed the first experiments? Mendel examined seven different traits. Each trait had only two possible variations. 7 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 How did he performed the first experiments? Mendel crossed two pure-lines that differed for only one trait (P generation) The result was the F1 generation 8 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 How did he performed the first experiments? Mendel then crossed the individuals of the F1 generation Three out of four offsprings in the F2 generation showed one trait phenotypic ratio 3:1 9 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Law of Segregation “Inheritance of one trait is determined by pairs of ‘factors’ that segregate in the gamete formation” Each gamete contains only one factor from each pair 10 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s factors Mendel’s “factors” are the genes and a gene can occur in alternate “variations” called alleles. The position of a gene on the chromosome is called locus. 11 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Dominant or recessive? When two different alleles are present for one trait, one is expressed (dominant) while the other one is not (recessive). A homozygous organism has two copies of the same allele, a heterozygous organism has one of each type of allele at the gene locus. 12 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genotype Vs phenotype The gene composition and arrangement of an organism is its genotype. The expression of the genes into a trait is referred to as the phenotype. 13 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 The inheritance of one trait influences a different trait? II experiment: Mendel selected plants that differed for two traits. P generation DIHYBRID CROSS Tall plants Green peas Short plants Yellow peas 14 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Dihybrid cross F1 gametes F1 F1 gametes P 15 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Law of Independent Assortment “Each pair of factors assort independently: inheritance of alleles for one trait does not affect the inheritance for another trait” Each gamete can contain all possible factor combinations 16 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s and the laws of probability The probability of genotypes and phenotypes in the final generation can be predicted using a Punnett square. Free (E) Alleles for earlobe shape: Attached (e) 17 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s and the laws of probability Genotypic probabilities EE = ½x½ = ¼ Ee = ½x½ = ¼ eE = ½x½ = ¼ ee = ½x½ = ¼ Phenotypic ratio Free earlobe 3 Attached earlobe 1 18 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Testcrosses A one-trait testcross determines whether a dominant phenotype is homozygous dominant or heterozygous. homozygous dominant heterozygous 19 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s laws apply to humans Genetic pedigrees can reveal the patterns of inheritance. How to create a pedigree = female = unaffected = male = affected = union Autosomal recessive disorder Autosomal dominant disorder 20 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s laws apply to humans Autosomal recessive disorder Ex. Tay-Sachs disease Cystic fibrosis Phenylketonuria Sickle-cell disease 21 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s laws apply to humans Autosomal dominant disorder Ex. Neurofibromatosis Huntington disease Achondroplasia 22 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genetic disorders Autosomal disorders can be detected early on. Amniocentesis, embryonic test and egg test can be used to detect genetic disorders. Egg test used before in vitro fertilization 23 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Complex inheritance patterns Incomplete dominance: phenotype of the heterozygous is an intermediate between its homozygous parents. 1. Pink flowers = Incomplete dominance 2. In F2 generation all three genotypes appear 24 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Complex inheritance patterns Multiple alleles: most genes exist in a large number of allelic forms. A,B,0 blood groups: comprises three sets of alleles at the I locus. IA Antigens A on RBC Dominant IB Antigens B on RBC Dominant i No antigens on RBC Recessive Phenotype Genotype A IAIA , IAi B IBIB , IBi AB IAIB 0 ii 25 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Polygenic inheritance Several genes and the environment can influence a single multifactorial trait. Continuous variations of phenotypes result in a bell-shaped curve. Black points = dominant alleles Color intensity = environmental influence Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 26 Pleiotropy A single gene influences multiple phenotypic traits Sickle-cell anemia (SCA) Autosomal recessive blood disorder characterized by red blood cells that assume an abnormal, sickle shape. 27 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genes are carried by chromosomes One gene influences multiple phenotypic traits Genes position in fruit fly Drosophila 28 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genes of the X chromosome have a unique inheritance pattern X-linked inheritance means that the gene causing the trait or the disorder, i.e. white eyes, is located on the X chromosome. 29 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 X-linked inheritance X-linked alleles have a unique inheritance pattern as they do not have a corresponding allele on the Y chromosome. Human X-linked disorders: • Color blindness • Muscular dystrophy • Hemophilia 30 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genetic linkage group Genetic linkage is the tendency of genes that are located proximal to each other on a chromosome to be inherited together. 31 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mapping the chromosomes A linkage map is a genetic map of a species that shows the position of its genes relative to each other in terms of recombination frequency. Gene map of chromosome 2 in Drosophila 32 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mapping the chromosomes A direct relation exists between recombinant phenotypes and the distance between alleles 1% recombinants = 1 map unit 33 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012