1. Gregor Mendel Austrian monk
Studied science and mathematics at University of Vienna
Conducted breeding experiments with the garden pea Pisum sativum
Carefully gathered and documented mathematical data from his experiments
Formulated fundamental laws of heredity in early 1860s
Had no knowledge of cells or chromosomes
Did not have a microscope

2. Gregor Mendel

3. Fruit and Flower of the Garden Pea

4. Garden Pea Traits Studied by Mendel

5. Blending Inheritance Theories of inheritance in Mendel’s time:
Based on blending
Parents of contrasting appearance produce offspring of intermediate appearance
Mendel’s findings were in contrast with this
He formulated the particulate theory of inheritance
Inheritance involves reshuffling of genes from generation to generation

6. One-Trait Inheritance
Mendel performed cross-breeding experiments
Used “true-breeding” (homozygous) plants
Chose varieties that differed in only one trait (monohybrid cross)
Performed reciprocal crosses
Parental generation = P
First filial generation offspring = F1
Second filial generation offspring = F2
Formulated the Law of Segregation

7. Mendel’s Monohybrid Crosses: An Example

8. Law of Segregation

9. Law of Independent Assortment

10. Figure 11A

11. Modern Genetics View
Each trait in a pea plant is controlled by two alleles (alternate forms of a gene)
_____________ allele (capital letter) masks the expression of the _______ allele (lower- case)
Alleles occur on a homologous pair of chromosomes at a particular gene locus
____________ = identical alleles
____________ = different alleles

12. Homologous Chromosomes

13. Genotype Versus Phenotype
Refers to the two alleles an individual has for a specific trait
If identical, genotype is ____________
If different, genotype is ____________
Phenotype
Refers to the physical appearance of the individual

14. Punnett Square
Table listing all possible genotypes resulting from a cross
All possible sperm genotypes are lined up on one side
All possible egg genotypes are lined up on the other side
Every possible zygote genotypes are placed within the squares

15. Punnett Square Showing Earlobe Inheritance Patterns

16. Monohybrid Testcross
Individuals with recessive phenotype always have the homozygous recessive genotype
However, Individuals with dominant phenotype have indeterminate genotype
May be homozygous dominant, or
Heterozygous
Test cross determines genotype of individual having dominant phenotype

17. One-Trait Test Cross Unknown is Heterozygous

18. One-Trait Test Cross Unknown is Homozygous Dominant

19. Two-Trait Inheritance
Dihybrid cross uses true-breeding plants differing in two traits
Observed phenotypes among F2 plants
Formulated Law of Independent Assortment
The pair of factors for one trait segregate independently of the factors for other traits
All possible combinations of factors can occur in the gametes

20. Two-Trait (Dihybrid) Cross

21. Dihybrid Cross

22. Mendelian Inheritance Reflects Rules of Probability
Rules of Multiplication:
The probability that independent events will occur simultaneously is the product of their individual probabilities.

23. Mendelian Inheritance Reflects Rules of Probability
Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?
Answer:
Probability that an egg from the F1 (Pp) will receive a p allele = ½
Probability that a sperm from the F1 will receive a p allele = ½
Overall probability that 2 recessive alleles will unite at fertilization: ½ x ½ = ¼

24. Mendelian Inheritance Reflects Rules of Probability
Works for Dihybrid Crosses:
Question: For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR?
Answer:
Probability that an egg from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼
Probability that a sperm from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼
Overall probability of an F2 plant having the genotype YYRR: ¼ x ¼ = 1/16

25. Mendelian Inheritance Reflects Rules of Probability
Rules of Addition: The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.
Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote?
Answer:
There are 2 ways in which a heterozygote may be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg.

26. Mendelian Inheritance Reflects Rules of Probability
Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼
Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼
Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

27. Incomplete Dominance
Heterozygote has phenotype intermediate between that of either homozygote
Homozygous red has red phenotype
Homozygous white has white phenotype
Heterozygote has pink (intermediate) phenotype
Phenotype reveals genotype without test cross

28. Incomplete Dominance

29. Multiple Allelic Traits
Some traits controlled by multiple alleles
The gene exists in several allelic forms (but each individual only has two)
ABO blood types
The alleles:
IA = A antigen on red cells, anti-B antibody in plasma
IB = B antigen on red cells, anti-AB antibody in plasma
i = Neither A nor B antigens, both antibodies

30. Multiple Allelic Traits
Phenotype (Blood Type)
Genotype
A (actually AA or AO)
IAIA or IAi
B (actually BB or BO)
IBIB or IBi AB
IAIB
O (actually OO) ii

31. Inheritance of Blood Type

32. Polygenic Inheritance
Occurs when a trait is governed by two or more genes having different alleles
Each dominant allele has a quantitative effect on the phenotype
These effects are additive
Result in continuous variation of phenotypes

33. Height in Human Beings

34. Frequency Distributions in Polygenic Inheritance

35. Terminology Pleiotropy Codominance __________________________
______________________________
Sickle-cell (incomplete dominance)
Codominance
__________________________
ABO blood type (multiple allelic traits)
Epistasis
________________________________
Human skin color (polygenic inheritance)

36. Epistasis
Epistasis:
absence of expected phenotype as a result of masking expression of one gene pair by the expression of another gene pair.
The homozygous recessive condition masks the effect of a dominant allele at another locus.

40. Environment and Phenotype: Himalayan Rabbits
Both genotype and environment affect phenotype; relative importance of both influences vary.
Fur was plucked out & an ice pack applied. New fur grew in black.
Temperature can affect the phenotypes of some plants (e.g., primroses) and animals (e.g.,Siamese cats, Himalayan rabbits).
Conclusion: the enzyme that produces melanin (a dark pigment) in rabbits is active only at low temperatures.

41. Human Genetic Disorders
Autosome - Any chromosome other than a sex chromosome
Genetic disorders caused by genes on autosomes are called autosomal disorders
Some genetic disorders are autosomal dominant
An individual with AA has the disorder
An individual with Aa has the disorder
An individual with aa does NOT have disorder
Other genetic disorders are autosomal recessive
An individual with AA does NOT have disorder
An individual with Aa does NOT have disorder, but is a carrier
An individual with aa DOES have the disorder

42. Autosomal Recessive Pedigree Chart

43. Autosomal Dominant Pedigree Chart

44. Autosomal Recessive Disorders
Tay-Sachs Disease
Progressive deterioration of psychomotor functions
Cystic Fibrosis
Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous
Phenylketonuria (PKU)
Lack enzyme for normal metabolism of phenylalanine

45. Cystic Fibrosis Therapy

46. Autosomal Dominant Disorders
Neurofibromatosis
Tan or dark spots develop on skin and darken
Small, benign tumors may arise from fibrous nerve coverings
Huntington Disease
Neurological disorder
Progressive degeneration of brain cells
Severe muscle spasms
Personality disorders

47. A Victim of Huntington Disease

48. Human Genetic Disorders
Review
Blending Inheritance
Monohybrid Cross
Law of Segregation
Modern Genetics
Genotype vs. Phenotype
Punnett Square
Dihybrid Cross
Law of Independent Assortment
Human Genetic Disorders