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

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

4 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel and the laws of inheritance Before Gregor Mendel ( ) a “blending model of inheritance” was accepted. Mendel performed his experiments very carefully using the common pea plant (Pisum sativum).

5 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

6 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.

7 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel examined seven different traits. Each trait had only two possible variations. How did he performed the first experiments?

8 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel crossed two pure-lines that differed for only one trait (P generation) The result was the F 1 generation How did he performed the first experiments?

9 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel then crossed the individuals of the F 1 generation Three out of four offsprings in the F 2 generation showed one trait phenotypic ratio 3:1 How did he performed the first experiments?

10 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 “Inheritance of one trait is determined by pairs of ‘factors’ that segregate in the gamete formation” Law of Segregation Each gamete contains only one factor from each pair

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

12 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.

13 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.

14 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 II experiment: Mendel selected plants that differed for two traits. DIHYBRID CROSS P generation Tall plants Green peas Short plants Yellow peas The inheritance of one trait influences a different trait?

15 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Dihybrid cross P F1F1 F 1 gametes

16 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 “Each pair of factors assort independently: inheritance of alleles for one trait does not affect the inheritance for another trait” Law of Independent Assortment Each gamete can contain all possible factor combinations

17 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 The probability of genotypes and phenotypes in the final generation can be predicted using a Punnett square. Mendel’s and the laws of probability Alleles for earlobe shape: Free (E) Attached (e)

18 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s and the laws of probability Genotypic probabilities EE=½ x ½=¼ Ee=½ x ½=¼ eE=½ x ½=¼ ee=½ x ½=¼ Phenotypic ratio Free earlobe3 Attached earlobe1

19 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 dominantheterozygous

20 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 = male = unaffected = affected Autosomal recessive disorder Autosomal dominant disorder = union

21 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

22 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Mendel’s laws apply to humans Autosomal dominant disorder Ex. Neurofibromatosis Huntington disease Achondroplasia

23 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

24 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. 2. In F 2 generation all three genotypes appear 1. Pink flowers = Incomplete dominance

25 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. IAIA Antigens A on RBCDominant IBIB Antigens B on RBCDominant iNo antigens on RBCRecessive PhenotypeGenotype AI A I A, I A i BI B I B, I B i ABIAIBIAIB 0ii

26 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Polygenic inheritance Several genes and the environment can influence a single multifactorial trait. Black points = dominant alleles Color intensity = environmental influence Continuous variations of phenotypes result in a bell-shaped curve.

27 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.

28 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

29 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.

30 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 X-linked alleles have a unique inheritance pattern as they do not have a corresponding allele on the Y chromosome. X-linked inheritance Human X-linked disorders: Color blindness Muscular dystrophy Hemophilia

31 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 Genetic linkage is the tendency of genes that are located proximal to each other on a chromosome to be inherited together. Genetic linkage group

32 Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012 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. Mapping the chromosomes Gene map of chromosome 2 in Drosophila

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