1. Fundamentals of Genetics
CHAPTER 9
Fundamentals of Genetics

2. Mendel’s Legacy
Genetics is the field of biology devoted to understanding how characteristics are transmitted from parents to offspring.
Genetics was founded with the work of Gregor Johann Mendel.

3. Gregor Mendel Known as the “Father of Genetics”
Born in 1822 to a poor family in the Czech Republic
Mendel excelled in school and eventually became a monk.
In 1843, at the age of 21, Mendel entered a monastery in Brunn, Austria (Brunn is now in the Czech Republic).

5. Gregor Mendel
In addition to his monastery duties, he taught high school science.
Mendel also was in charge of the monastery garden.

6. Site of Gregor Mendel’s experimental garden in the Czech Republic
Mendelian Genetics
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Site of Gregor Mendel’s experimental garden in the Czech Republic
Fig. 5.co

7. Gregor Mendel
Mendel worked with garden peas and studied how their characteristics are passed in a very predictable manner to their offspring.
Between 1856 and 1863, Mendel cultivated and tested about 28,000 pea plants!
He found that the plants' offspring retained traits of the parents.

8. Why Peas? Mendel used peas to study inheritance because:
Peas are easy to grow
Peas produce many offspring
Peas have many easy to observe traits including:
Seed color - Green or yellow
Seed shape - Round or wrinkled

9. Why Peas? Pod color - Green or yellow
Pod shape - Smooth or constricted
Flower color - White or purple

10. Why Peas? Flower position - Axial or terminal
Plant size - Tall or short

11. Why Peas?
Mendel was able to control how the pea plants were pollinated.
Mendel knew that
The male part of each flower produces pollen, (containing sperm).
The female part of the flower produces egg cells.
Pollination occurs when pollen grains produced in the male reproductive parts called the anthers are transferred to the female reproductive part called the stigma.

12. Why Peas?
Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or a flower from the same plant.

13. Why Peas?
Cross-pollination involves fertilization from the flowers of separate plants.
Cross-pollination

14. Why Peas?
Pea flowers are constructed in such a way that they typically self-fertilize.
If scientists snip off the anthers, they can prevent the pea plant from self-fertilizing, and place pollen from another plant on the stigma to cross-pollinate the pea plant.
This gives scientists the ability to control which pea plants will pollinate with each other.
Anthers
Pea flower
Pea flower
Stigma

15. Why Peas?
By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas.

16. Why Peas?
By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas.
Snip

17. Why Peas?
By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. .

18. Why Peas?
By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. .

19. Why Peas?
By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. .

20. Mendel’s Experiments
Mendel began by growing plants that were true-breeding for each trait. Plants that are true-breeding, or purebred, for a trait always produce offspring with that trait when they self-pollinate.
For example: Pea plants that are true-breeding for the purple flower color self-pollinate to produce offspring with purple flowers. Purebred white flowered plants self-pollinate to produce offspring with white flowers.

21. Mendel’s Experiments
Mendel produced true-breeding plants for each of the 14 contrasting traits that he observed by self-pollinating the pea plants for several generations. He called these true-breeding plants the P generation.
P generation – parent generation
F1 (first filial) generation – offspring of the P generation
F2 (second filial) generation – offspring of the F1 generation

22. Mendel’s Experiments
The next part of Mendel’s experiment involved cross-pollinating true-breeding P generation plants with contrasting forms of a trait.
For example: He cross-pollinated a pure purple flowered plant with a pure white flowered plant.
What do you think the F1 generation looked like?

23. Mendel’s Experiments
He observed that all of the F1 offspring had purple flowers despite the fact that one parent was true-breeding for white flowers!

24. Mendel’s Experiments
Mendel repeated this procedure for the other six traits. This table shows Mendel’s crosses and results.

25. Mendel’s Experiments
Next, Mendel allowed the F1 plants to self-pollinate.
What do you think the F2 generation looked like?
He observed that 75% of the F2 generation had purple flowers and 25% had white flowers!

27. Mendel’s Results and Conclusions
Mendel’s observations led him to hypothesize that something within the pea plants controlled the characteristics observed.
He called these controls factors.
Mendel hypothesized that each trait was inherited by means of a separate factor.
Because the characteristics studied had two alternative forms, he reasoned that a pair of factors must control each trait.
Mendel did not know that the “factors” were actually genes.

28. Recessive and Dominant Traits
Whenever Mendel crossed strains, one of the P traits failed to appear in the F1 plants. In every case, that trait reappeared in a ratio of about 3:1 in the F2 generation.
Mendel hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it masked, or dominated, the factor for the other trait in the pair.
He thought that the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor.
Thus, a trait controlled by a recessive factor had no observable effect on an organism’s appearance when that trait was paired with a trait controlled by a dominant factor.
This is known as Mendel’s Law of Dominance or complete dominance.

29. Mendel’s Results and Conclusions
Trait - any characteristic that can be passed from parent to offspring
Heredity - passing of traits from parent to offspring
Genetics - study of heredity
Today, we know that these traits are controlled by sections of DNA called genes.

30. Designer “Genes”
Gene – a short segment of DNA that contains the instructions for a single trait
Alleles – two or more alternative forms of a gene (dominant & recessive)
Dominant - stronger of two genes that masks or hides the effect of the other gene; represented by a capital letter (R)
Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

31. More Terminology
Genotype - gene combination for a trait (e.g. RR, Rr, rr)
Phenotype - the physical feature resulting from a genotype (e.g. red, white)

32. Mendelian Genetics
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The Law of Dominance
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.
All the offspring will be heterozygous and will display only the dominant trait in the phenotype.
Example:
YY x yy Yy

33. Mendelian Genetics
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The Law of Dominance
In complete dominance, YY plants and Yy plants are indistinguishable in phenotype, that is, both have yellow seeds.

34. Genotype & Phenotype in Flowers
Mendelian Genetics
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Genotype & Phenotype in Flowers
Defining alleles: R = red flower r = yellow flower
All genes occur in pairs, so 2 alleles control a characteristic
Possible gene combinations are:
Genotypes _______ _______ _______
Phenotypes _______ _______ _______ RR Rr rr
RED
RED
YELLOW

35. Genotypes
Homozygous genotype - gene combination involving 2 dominant or 2 recessive alleles; also called true-breeding or pure 
Examples:
RR = homozygous dominant
rr = homozygous recessive
Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid

36. The Law of Segregation
The law of segregation states that a pair of alleles is segregated, or separated, during the formation of gametes.
Each gamete receives only one allele of each pair.

37. The Law of Segregation
When two gametes combine during fertilization, the offspring has two alleles for each characteristic (one allele from each parent).

38. The Law of Independent Assortment
The Law of Independent Assortment states that alleles for different traits are distributed to gametes independently of one another.

39. The Law of Independent Assortment
In other words, the genes are located on separate chromosomes, and therefore, these traits are not linked together.

40. Example of Incomplete Dominance in Snapdragon Flowers
Red = RR
White = WW
Pink = RW

41. Example of Codominance in Cattle
White = WW
Red = RR
Roan = RW

42. Example of Codominance in Cats
Tan = TT
Black = BB
Tabby = TB

43. Example of Codominance in Chicken
Black = BB
White = WW
Speckled with black and white feathers = BW

44. Incomplete Dominance – heterozygous offspring have phenotype between the dominant and recessive traits - The heterozygous individuals show a blend of the trait
Ex – Snapdragon Flower.

45. Codominance – when both alleles for a gene are expressed in a heterozygous offspring
Ex – AB blood type
Neither allele is dominant or recessive, No blending of phenotypes

46. Dihybrid Crosses – when two characteristics are tracked at the same time - offspring are called dihybrids P Generation: RRYY x rryy R = Round (dominant), r = wrinkled (recessive) Y = Yellow (dominant), y = green (recessive) ry ry ry ry RY
Phenotypic Ratio – All Round, Yellow
Genotypic Ratio – All RrYy (Heterozygous round, heterozygous yellow)
RrYy

47. Ex 2: P Generation RrYy x RrYy RY Ry rY ry RY Ry rY ry

48. Phenotypic Ratio – 9 Round, Yellow (genotypes RRYY, RRYy, ,RrYY, RrYy) 3 Round, Green (RRyy, Rryy) 3 Wrinkled, Yellow (rrYY, rrYy) 1 Wrinkled, Green (rryy)