1. Fundamentals of Genetics
Chapter 9

2. Mendel’s Legacy
Section 9.1

3. Genetics The field of biology dedicated to understanding heredity…
Heredity- the passing of traits from parents to offspring
Was founded by the work of Gregor Johann Mendel, the “Father of Genetics”

4. Gregor Mendel Austrian monk & science teacher
Studied heredity and statistics
Worked with pea plants in his garden
Mid 1800s

5. Mendel’s Peas: Studied 7 characteristics of peas
Each characteristics had 2 different traits
ex) flower color
Page 176

6. Mendel’s Experiments:
1. Started off with 2 pure (true breeding) plant
groups for a particular characteristic
P generation (parent)
2. Cross pollinated these plant groups (sexual reproduction) to produce the next generation of plants
F1 generation (first filial)
he counted & recorded the traits of the offspring

8. Then he allowed the F1 generation plants self pollinate (sexual reproduction with self) to produce the next generation of plants
F2 generation (second filial)
he counted & recorded the traits of the offspring.

9. Mendel’s Results: Mendel named this the dominant factor
The F1 generation always showed only 1 of the 2 traits for the characteristic
Mendel named this the dominant factor
The F2 generation always showed a 3:1 (or a 75% to a 25%) ratio between the 2 traits
Mendel named the second the recessive factor

10. Purple is dominant
White is recessive F1 F1 F2

11. Notice that the predicted ratios & the actual ratios are not exact!

12. Mendel’s Laws of Inheritance
1) Law of Segregation
Mendel stated that… a pair of factors is segregated, or separated, during the formation of gametes.
So, What does this mean?
Each egg and sperm receives only one factor from each parent.

13. 2. Law of Independent Assortment
Mendel also stated that … factors for different characteristics are distributed to gametes independently.
So, What does this mean?
The factors for different characteristics are not connected.

15. Molecular Genetics Mendel called them “factors” Abbreviations:
The study of the structure & function
of chromosomes & genes
Allele: alternate form of a gene
Mendel called them “factors”
Abbreviations:
Dominant allele = capital letter (B)
Recessive allele = lower case letter (b)

17. Genetic Crosses
9.2

18. Vocab: Genotype: gene combination for a trait (BB, Bb, bb)
Phenotype: the physical feature resulting from a genotype (Black, white)

20. Genotype Vocab: Homozygous: “same genes”
When the organism has the same alleles for the characteristic (also called pure)
Ex.) BB = homozygous dominant
bb = homozygous recessive
Heterozygous: “different genes”
When the organism has different alleles for the characteristic (also called hybrid)
Ex.) Bb

21. Examples:

23. Genetic Probability:
The likelihood (probability) of offspring of known parents can be determined by Punnett squares

24. Types of Genetic Crosses:
Monohybrid Cross-
cross involving a single trait
ex.) flower color

25. 2) Dihybrid Cross
Cross involving 2 traits

26. Let’s Practice Punnett Squares!
Monohybrid Cross
Dihybrid Cross

27. How do you find an unknown genotype?
Testcross
Example) Is the red flower a pure (RR) or hybrid (Rr) ?
Cross the unknown flower with a pure recessive flower (rr).
This will allow the recessives to show up in the next generation.

28. The results:
If the flower is hybrid, the offspring will be 50% red and 50% white.
If the flower is pure, the offspring will be all red.
Result if flower is hybrid
Result if flower is pure

29. Complications! Incomplete Dominance and Codominance
Incomplete Dominance: occurs when the F1 offspring has a phenotype between that of the parents.
neither allele is completely dominant

30. Example) Japanese Four o’clock flowers
The flowers may be red or white in the purebred form, but the hybrid form of the flowers is pink. Q- What would happen if you crossed a red four o’clock flower with a white four o’clock flower? A- a pink flower!

31. 2) Codominance
Occurs when both alleles are expressed in a heterozygous offspring.
Neither allele is dominant or recessive
Example)
Q- What would happen if you crossed a horse with a white coat (rr) with a horse with a red coat (RR)?
A- A horse with a roan coat (Rr) ~ both red and white hairs

32. Question:
In rabbits, the allele for black coat color (B) is dominant over the allele for brown coat color (b). Predict the results of a cross between a rabbit heterozygous for black coat color and a rabbit homozygous for brown coat color.
What is the genotypic ratio?
What is the phenotypic ratio?

33. Question:
Albinism is a recessive disorder (aa) that results in a lack of pigment in the eyes, skin, and hair.
What are the genotypes and phenotypes of the parents if they have 2 children with albinism and 2 children normally pigmented.
Use a Punnett square to complete this problem

34. Brown hair color is dominant over blonde hair, and brown eyes are dominant over blue eyes.
Q- If 2 heterozygous parents for both traits mate, what is the phenotypic ratio of their offspring?
__________ Brown hair, Brown eyes
__________ Brown hair, Blue eyes
__________ Blonde hair, Brown eyes
__________ Blonde hair, Blue eyes

35. Incomplete Dominance Q:
Scottish fold cats are heterozygotes whose ears fold down onto their heads several days after birth. The homozygote genotype, rr, for this trait causes death. Construct a Punnett square to determine the genotypic and phenotypic ratios of a cross between 2 cats with folded ears.

36. B. Complications: Multiple alleles
ABO blood groups
Dominant
Dominant
Codominant
Recessive

37. C. Complications: Pleiotropy
- One gene affects many characters
- Sickling allele of hemoglobin

38. D. Complications: Polygenic Inheritance and Quantitative Characters
- One trait determined by multiple genes
- Converse of pleiotropy
- e.g., skin color: at least 3 genes

39. Complications: Epistasis
- Expression of one gene depends on another
- Mouse coat color:
B - black coat
b - brown coat
C - pigment
c - no pigment

40. What are my chances? Obtain 2 pennies.
Flip each coin simultaneously 50 times. Record the results of each set of flips.
Total the number of combinations of your results:
__ Heads-Heads
__ Heads-Tails
__ Tails-Tails

41. Record your results on the board.
Calculate your percent chance of each combination.
(# of combos) 50
Calculate the class average percent chance of each combination.