1. Chapter 14: Mendel & the Gene Idea
AP Biology

2. 14.1
Mendel used the scientific approach to identify two laws of inheritance

3. Gregor Mendel Known as the “Father of Genetics”
Experimented with pea plants to develop principles of genetics
In the 1800s… before anyone even knew:
what DNA was, let alone how it worked (1940s-1950s)
What meiosis was or how it worked (1870s-1880s)

4. Gregor Mendel
Knew some “factor” caused organisms to have “characters” like their parents
Now we call them genes and traits

5. Mendel’s Experiments P generation – true breeding
F1 generation – heterozygous
F2 generation –
3:1 ratio; recessive trait reappears

6. New Vocabulary, courtesy of Mendel
Dominant: gene that is always expressed if present
Recessive: gene that only expressed when 2 copies are inherited
Homozygous/True-breeding/Pure: two of the same allele (ex: BB, bb)
Heterozygous/Hybrid: two different alleles (ex: Bb)
Phenotype: physical appearance of an organism (based on its genes)
Genotype: genetic makeup of an organism (letters)
P generation: parent generation (Mendel – true-breeding plants)
F1 generation: first generation made from crossing P generation
F2 generation: second generation made from crossing F1generation

7. Mendel’s Laws Law of Segregation
Two alleles of a gene separate during meiosis
Each parent can give one of each homologous chromosome (one copy of each gene) to offspring

8. Mendel’s Laws Law of Independent Assortment
Each allele pair segregates into gametes independently of other pairs (random)
How homologous chromosomes align in metaphase I

9. Genetic Crosses Monohybrid crosses cross 1 trait Ex: pea color
F2 generation shows 3:1 ratio

10. Genetic Crosses Dihybrid crosses cross 2 traits at once
Ex: pea color and shape
F2 generation shows 9:3:3:1 ratio

11. 14.2
The laws of probability govern Mendelian inheritance

12. Laws of Probability Rule of Multiplication
Probability that independent events will occur in sequence
Look for “and”

13. Laws of Probability, cont.
Rule of Addition
Probability that events will occur mutually exclusive of each other
Look for “or”

14. Bozeman Probability in Genetics: Multiplication & Addition Rules

15. 14.3
Inheritance patterns are often more complex than predicted by simple Mendelian genetics

16. Inheritance Patterns
Complete dominance (as shown in Mendel’s pea plants)
Dominant is always expressed over recessive
Recessive only shows if both alleles are recessive
RR = purple; Rr = purple; rr = white

17. Mendel was lucky – most inheritance follows other patterns…
Let’s look at the exceptions to the norms

18. Incomplete Dominance
One allele is not completely dominant over another
Heterozygous organisms display a blend between both phenotypes
Example: snapdragons

19. Codominance Alleles do NOT blend
Both traits show in heterozygous organism
Both alleles expressed dominantly
Neither allele is recessive
Ex: roan cattle

20. Multiple Alleles Many genes have more than 2 alleles Examples:
human blood type
fur color in rabbits

21. Polygenics Multiple genes affect single phenotype
Ex: skin color, height
The more dominant alleles inherited, the darker the skin, taller the offspring

22. Environmental Influence on Genes
DNA isn’t the only factor that influences a phenotype
Environment can influence genes too
In reality, phenotype is a combination of an organism’s genes, environment, as well as other factors
Examples:
Temperature
Light
Moisture
Minerals
Nutrients

23. Environmental Influence on Genes – Temperature
Snowshoe hare
Genes code for pigments that give its hair a brownish-grey color called agouti.
However, during cold winter months the alleles for pigment production are turned-off and the hare appears white in color.
In this case temperature regulates the expression of the coat color alleles.

24. Environmental Influence on Genes – Temperature
Himalayan rabbits
Genotype ch/ch should produce black pigments in their hair.
when the rabbit’s body temperature is above 37 o C, the ch alleles are turned-off and the rabbit’s hair appears white
if the rabbit’s body temperature falls below 37 o C, black pigments will appear in the rabbit’s hair

25. Environmental Influence on Genes – Soil pH
Hydrangeas
flower coloration is subject to the pH of the soil in which the hydrangea bush is growing.
In acidic soil, the flowers pink pigments are produced.
In basic soil, blue pigmentation is produced in flowers.

26. 14.4
Many human traits follow Mendelian patterns of inheritance

27. Pedigrees
Analyze inheritance of traits through family relationships

28. Cystic Fibrosis Autosomal recessive
Defective chloride channels in cells leads to build up of mucus in pancreas, lungs, digestive tract, etc.

29. Tay Sachs Autosomal recessive
Fatal neurological/brain degeneration beginning at around 6 months of age

30. Sickle Cell Disease Autosomal recessive
Causes abnormally shaped red blood cells, leads to anemia, clots

31. Achondroplasia
Autosomal dominant
Dwarfism

32. Huntington’s Disease Autosomal dominant
Progressive degeneration of brain/nervous system
Late onset (age 35-45)

33. Fetal Testing & Newborn Screening
Amniocentesis
Chorionic villus sampling
Blood testing
Newborn screening
Blood analysis for genetic disorders