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Immagini e concetti della biologia Sylvia S. Mader Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

A7 - Principles of inheritance Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

Dihybrid cross F1 gametes P F1 Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

Mendel and the laws of probability The probability of genotypes and phenotypes in the final generation can be predicted using a Punnett square. Free (E) Attached (e) Alleles for earlobe shape: Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

Testcrosses A one-trait testcross determines whether a dominant phenotype is homozygous dominant or heterozygous. homozygous dominant heterozygous Sylvia S. Mader, Concepts of Biology © Zanichelli editore, 2018

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, 2018

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, 2018

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, 2018