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2. From Mendel to modern genetics 2 © Zanichelli editore 2015

3. Mendel and the laws of inheritance 3 © Zanichelli editore 2015

4. Mendel’s experiments 4 Gregor Mendel performed a series of observations and experiments on the inheritance of characteristics using the common pea plant (Pisum sativum). © Zanichelli editore 2015

5. Pea plants as a model 5 Pea plants have some characteristics that made them a good model for Mendel’s experiments: they are easy to grow and they mature quickly; they have a short generation time and produce many offspring; they perform self-pollination; it is easy to control their reproduction. © Zanichelli editore 2015

6. The first experiments: seven traits 6 Mendel noticed that pea plants could assume two different variants of some traits. He examined seven traits. Traits Seed shape Seed color Pod shape Pod color Plant height Flower position Flower color Dominant trait RoundYellowInflatedGreenTallAxialPurple Recessive trait WrinkledGreenConstrictedYellowShortTerminalWhite © Zanichelli editore 2015

7. The first experiments: pure lines 7 Pure lines are plants that show the same trait generation after generation. Mendel used pure line plants in his first experiments. He crossed two pure lines that differed for only one trait, for example seed colour (P generation). The result was called F 1 generation. P generation F 1 generation © Zanichelli editore 2015

8. The first experiments: F 1 generation 8 Mendel then crossed the individuals of the F 1 generation. In the F 2 generation, he noticed that 3 out of 4 offspring showed one trait. Phenotypic ratio 3 : 1 F 2 generation F 1 generation © Zanichelli editore 2015

9. Law of segregation 9 The law of segregation, or Mendel’s first law, states that when any individual produce gametes, the two copies of “factors” segregate, so that offspring receive one factor from each parent. Each gamete contains only one factor from each pair. © Zanichelli editore 2015

10. Dominant and recessive traits 10 What Mendel called “factors” are now known as genes. A gene can occur in alternative variants, called alleles. The alleles for a gene can be the same (then the organism is homozygous for the trait) or different (then the organism is heterozygous for the trait). When two different alleles are present for one trait, one is expressed (dominant), while the other is not (recessive). © Zanichelli editore 2015

11. Genotype Phenotype YY Homozygous dominant Yellow seeds YyHeterozygousYellow seeds yy Homozygous recessive Green seeds Genotype and phenotype 11 © Zanichelli editore 2015 The genotype of an organism is the gene composition and arrangement. The phenotype corresponds to the expression of the gene as a trait.

12. Dihybrid cross /1 12 In a second series of experiments, Mendel selected plants that differed for two traits: dihybrid cross. P generation Round, yellow seeds YY, RR Wrinkled, green seeds yy, rr F 1 generation ? © Zanichelli editore 2015

13. Dihybrid cross /2 13 YRYryRyr YR Yr yR yr F 1 gametes A Punnett square is a diagram that can be used to predict the probability of genotypes and phenotypes in the next generation. © Zanichelli editore 2015

14. Law of independent assortment 14 The law of independent assortment, or Mendel’s second law, states that each pair of factors assort independently: the inheritance of alleles for one trait does not influence the inheritance of alleles for another trait. Each gamete can contain all possible factor combinations. © Zanichelli editore 2015

15. Testcross /1 15 A testcross can be used to determine whether an individual with a dominant phenotype is homozygous dominant or heterozygous. Unknown genotype (YY or Yy ?) Homozygous recessive (yy) It is the cross between an individual with dominant phenotype (but unknown genotype) and an individual with a homozygous recessive genotype. © Zanichelli editore 2015

16. Testcross /2 16 © Zanichelli editore Offspring If the offspring show only the dominant phenotype, the unknown individual is homozygous dominant. It half the offspring show the dominant and the other half the recessive phenotype, the unknown individual is heterozygous. Offspring Homozygous dominant Heterozygous

17. Human genetics 17 © Zanichelli editore 2015

18. Humans and genetic pedigrees 18 © Zanichelli editore 2015 The genetic pedigree is a technique that can be used to reveal the patterns by which a trait is inherited. It involves the representation of a family tree in which the presence or absence of a specific trait or disease is indicated for each family member. It makes use of standard symbols: = female = male = unaffected = affected

19. Autosomal recessive disorder /1 19 © Zanichelli editore 2015 Carrier Carriers

20. Autosomal recessive disorder /2 20 © Zanichelli editore 2015 An autosomal recessive disorder is expressed when an individual has two copies of an altered gene, that is when he or she is homozygous for the altered gene. An individual who is heterozygous for the affected gene is unaffected, but he or she is a carrier for the disorder. Two affected parents will always have affected children. Examples: cystic fibrosis, sickle-cell anaemia.

21. Autosomal dominant disorder /1 21 © Zanichelli editore 2015

22. Autosomal dominant disorder /2 22 An autosomal dominant disorder can appear also when an individual has just one copy of an altered gene, that is when he or she is heterozygous for the altered gene. Two heterozygous affected parents can have unaffected children (if they inherit two copies of the unaltered gene). Two unaffected parents will not have affected children (unless a new mutation occurs). Examples: Huntington disease, achondroplasia. © Zanichelli editore 2015

23. Incomplete dominance 23 Incomplete dominance occurs when the heterozygous phenotype is an intermediate between its homozygous parents. F 1 generation P generation F 2 generation © Zanichelli editore 2015

24. Multiple alleles and blood types 24 © Zanichelli editore Most genes exist in a large number of allelic forms (multiple alleles). PhenotypeGenotype AI A I A, I A i BI B I B, I B i ABIAIBIAIB 0ii One example is blood type. There are three different alleles for blood type (A, B and 0). A and B are dominant over 0. A and B are both co-dominant.

25. X-linked inheritance 25 Genes located on the X chromosome have a special pattern of inheritance, called X-linked inheritance. Some human genetic conditions, like color blindness and hemophilia, follow this pattern of inheritance. In X-linked recessive disorders, a woman will be unaffected but a carrier of the condition if she is heterozygous for the altered gene, whereas a male will be affected if he inherits the altered gene from the mother. © Zanichelli editore 2015