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

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

4. 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).
Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. Dihybrid cross F1 gametes P F1
Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012

16. 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

17. 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

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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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