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Immagini e concetti della biologia Sylvia S. Mader Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

A7 - Mendel: laws of inheritance 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). 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 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. 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. 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 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 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 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. 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. 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 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Dihybrid cross F1 gametes P F1 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 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. Attached (E) Free (e) Alleles for earlobe shape: Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Genotypic probabilities Mendel’s and the laws of probability Genotypic probabilities EE = ½ x ½ ¼ Ee eE ee Phenotypic ratio Free earlobe 3 Attached earlobe 1 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Testcrosses A one-trait testcross determine whether a dominant phenotype is homozygous dominant or heterozygous. homozygous dominant heterozygous 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 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Autosomal recessive disorder Mendel’s laws apply to humans Autosomal recessive disorder Ex. Tay-Sachs disease Cystic fibrosis Phenylketonuria Sickle-cell disease Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Autosomal dominant disorder Mendel’s laws apply to humans Autosomal dominant disorder Ex. Neurofibromatosis Huntington disease Achondroplasia 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 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 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 i No antigens on RBC Recessive Phenotype Genotype A IAIA , IAi B IBIB , IBi AB IAIB ii Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Polygenic inheritance Several genes and the environment can influence a single multifactorial traits. 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

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. Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

Genes position in fruit fly Drosophila Genes are carried by chromosomes One gene influences multiple phenotypic traits Genes position in fruit fly Drosophila 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. 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 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. 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 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

1% recombinants = 1 map unit Mapping the chromosomes A direct relation exists between recombinant phenotypes and the distance between alleles 1% recombinants = 1 map unit Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012