1. Mendel’s Experimental, Quantitative Approach
When Mendel began his experiments in 1856, two main facts about inheritance were widely recognized:
All hybrid offspring of the same parents are similar in appearance.
When hybrids are mated, they do not breed true: some look like their parents, and some have features of their grandparents.
Mendel was the first to notice a pattern in the way parental traits reappear in the offspring of hybrids.

2. Mendel’s Experimental, Quantitative Approach
Mendel chose to work with peas because they were available in many varieties and because he could strictly control which plants mated with which.
Mendel chose to track only those characters that varied in an “either-or” manner.
Mendel also made sure that he started his experiments with varieties that were “true-breeding”.

3. Mendel’s Experimental, Quantitative Approach
Crossing pea plants
At what point after each cross could Mendel start quantifying F1 characteristics?

4. Essential Genetic Vocabulary
Genotype
Phenotype
Dominant allele
Recessive allele
Codominant alleles
Locus
Homozygous
Law of Segregation
Monohybrid
Heterozygous
Carrier
Test cross
Hybridization
P generation
F1 generation
F2 generation
Law of Independent Assortment
Dihybrid
Can you describe everything that is going on in this genetic cross using all of these terms?

5. Remember, alternative versions of genes account for variations in inherited characters, which are called alleles.
When choosing symbols for alleles, the first letter of the dominant phenotype is used. P p

6. The Testcross
In pea plants with purple flowers the genotype is not immediately obvious.
A testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype.
Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait.

7. 3 - T 1 - t Monohybrid Cross:
Is 154 Tall (TT or Tt) to 72 short (tt) an acceptable result of a monohybrid cross with independent assortment, segregation, and no crossing over?

8. 3 - T 1 - t Monohybrid Cross:
Is 154 Tall (TT or Tt) to 72 short (tt) an acceptable result of a monohybrid cross with independent assortment, segregation, and no crossing over?
crit (1 df, α = 0.05) = 3.84
obs = 5.67
Reject H0 that the observed and expected are equal and any differences occurred by chance. The differences are real and the results are not those of a monohybrid cross.

9. Incomplete Dominance:
When a true-breeding (therefore, homozygous) white flowered snapdragon (CWCW) is mated with a true- breeding (therefore, homozygous) red flowered snapdragon (CRCR), all the offspring have pink flowers. When the pink hybrids (heterozygotes) are allowed to self- pollinate what are the phenotype ratios of the F2 generation?

10. The Law of Independent Assortment
A dihybrid cross illustrates the inheritance of two characters and independent assortment.
It produces four phenotypes in the F2 generation.

11. Are these the results of a dihybrid cross?

12. Can we accept these as the results from a Dihybrid Cross?

13. The ABO blood group in humans is determined by multiple alleles.
Range of genotypes:
IAIA
IBIB or or
IAi
IAIB
IBi ii
Blood
Types: A AB B O

14. The ABO blood group in humans is determined by multiple alleles.
Reacts (clumps) when RBC’s from groups below are added to blood serum from groups at left.
A B AB O NO
YES
Antibodies Present
Anti-B
Anti-A
A Antigens on surface of RBC’s
B Antigens on surface of RBC’s
A and B Antigens on surface of RBC’s
No Antigens on surface of RBC’s

15. Extending Mendelian Genetics for Two or More Genes
Some traits may be determined by two or more genes. Ex: Albinism
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus.
Epistasis = standing above

16. Polygenic Traits Tend to Produce Normal Distributions

17. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations.
Connection to Hardy-Weinberg?
In the pedigrees to the right, the shaded individuals either have Widow’s peak or attached earlobes. Which trait is dominant? Recessive? Explain.

18. Mating of Close Relatives
Matings between relatives can increase the probability of the appearance of a genetic disease and are called consanguineous matings. They DO NOT guarantee genetic disorders or catastrophic morphological change.

19. Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype:
Includes its physical appearance, internal anatomy, physiology, and behavior.
Reflects its overall genotype and unique environmental history.
Hydrangea
(Hydrangea macrophylla)
flowers of the same genotype range from blue-violet to pink, depending on soil acidity.

20. Recessively Inherited Autosomal Disorders
Many genetic disorders are inherited in a recessive manner.
Why do recessively inherited disorders show up only in individuals homozygous for the allele?
Why are heterozygous carriers of recessive alleles phenotypically normal?

21. Sickle-Cell Disease is also autosomal recessive
Sickle-cell disease is caused by a base-pair substitution (point mutation) in one of the hemoglobin genes and the substitution of a single amino acid in the hemoglobin protein in red blood cells
It affects one out of 400 African-Americans.
Symptoms include physical weakness, pain, organ damage, and even paralysis.

22. Cystic Fibrosis is an autosomal recessive disorder
Mutation in the gene that codes for the cystic fibrosis transmembrane conductance regulator protein.
A deletion of three nucleotides results in a loss of the amino acid phenylalanine at the 508th position on the protein.
Symptoms of cystic fibrosis include:
Mucus buildup in the some internal organs, e.g. lungs.
Abnormal absorption of nutrients in the small intestine (impaired pancreas).
Causes impaired chloride transport
Why aren’t deleterious recessive alleles removed from populations by natural selection?