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

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

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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

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, 2018

17. 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

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, 2018

19. 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

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, 2018

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, 2018

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, 2018