1. Fundamentals of Genetics
Chapter 9
Fundamentals of Genetics

2. Mendel’s Legacy Mendel observed seven characteristics of pea plants
Ex: flower color
Each characteristic occurred in two contrasting traits
Trait: genetically determined variant of a characteristic
Ex: yellow flower color

3. Mendel’s Legacy The seven characteristics:
Plant height (long and short)
Flower position along stem (axial and terminal)
Pod color (green and yellow)
Pod appearance (inflated and constricted)
Seed texture (round and wrinkled)
Seed color (yellow and green0
Flower color (purple and white)

4. Mendel’s Experiments
He initially studied each characteristic and its contrasting traits individually
Began growing true-breeding plants
True-breeding: pure; always produced offspring with that trait when they self-pollinate
Produced true-breeding plants by self-pollinating pea plants until he had 14

5. Mendel’s Experiments
He then cross-pollinated pairs of plants that were true-breeding for contrasting traits of a single characteristics
Ex: he crossed a plant with purple flowers and a plant with white flowers
This was called the P (parent) generation

6. Mendel’s Experiments
When the plants matured, he recorded the number of each type of offspring produced by each cross
Called the offspring the F1 generation

7. Mendel’s Experiments
He then allowed the F1 generation to self-pollinate, and the next offspring generation was called the F2 generation
He performed hundreds of crosses and documented every result

10. Mendel’s Results and Conclusions
In one of his experiments, Mendel crossed a plant true-breeding for green pods with a plant that was true-breeding for yellow pods
The F1 generation had all green pods
He then let the F1 generation self-pollinate
The F2 generation had ¾ green pods and ¼ yellow pods

11. Mendel’s Results and Conclusions
These results made Mendel believe that each characteristic is controlled by factors
A pair of factors must control each trait

12. Mendel’s Results and Conclusions
Mendel got these results through the thousand of crosses
F1 generation: one trait disappeared
F2 generation: trait reappeared in a 3:1 ratio

13. Mendel’s Results and Conclusions
Mendel hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it masked the other trait
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

14. Support for Mendel’s Conclusions
Most of Mendel’s findings agree with what biologists now know about molecular genetics
Molecular genetics: the study of the structure and function of chromosomes and genes

15. Support for Mendel’s Conclusions
Allele: each of two or more alternative forms of a gene
Mendel’s factors are now called alleles
Letters are used to represent alleles
Dominant alleles: represented by a capital letter
Recessive alleles: represented by a lowercase letter

16. Genotype and Phenotype
Genotype: an organism’s genetic makeup
Consists of the alleles that the organism inherits from its parents
Ex: flower color
Purple flowers: either PP or Pp
White flowers: pp
P is the dominant allele
P is the recessive allele

17. Genotype and Phenotype
Phenotype: an organism’s appearance
Since PP and Pp are dominant genotypes, they will have purple flowers
Since pp is a recessive genotype, they will have the recessive phenotype, which is white flowers

18. Genotype and Phenotype
Homozygous: when both alleles of a pair are alike
An organism may be homozygous dominant or homozygous recessive
Ex: PP or pp
Heterozygous: when the two alleles in the pair are different
Ex: Pp

19. Probability
Probability: the likelihood that a specific event will occur
May be expressed as a decimal, a percentage, or a fraction
Determined by the following equation:
P= # of times an event is expected to happen
# of times an event could happen

20. Probability
For example, the dominant trait of yellow seed color appeared in the F2 generation 6,022 times
The recessive trait of green seed color appeared 2,001 times
The total number of individuals was 8,023

21. Probability Probability that the dominant trait will appear: 6,022
= or 75% or ¾ or 3:1
8,023

22. Probability
The results predicted by probability are more likely to occur when there are many traits

23. Probability Probability that the recessive trait will appear: 2,001
= 0.25 or 25% or ¼ or 1:3
8,023

24. Predicting Results of Monohybrid Crosses
Monohybrid cross: a cross in which only one characteristic is tracked
The offspring are called monohybrids
Biologists use a Punnett square to aid them in predicting the probable distribution of inherited traits in the offspring

25. Example 1: Homozygous x homozygous
PP and pp
All offspring are Pp
100% probability that the offspring will have the genotype Pp and thus the phenotype purple flower color

26. Example 2: Homozygous and Heterozygous
BB and Bb
Offspring are BB and Bb
The probability of an offspring having BB genotype is 2/4 or 50%
The probability of an offspring having Bb genotype is 2/4 or 50%
The probability of an offspring have the dominant black coat is 4/4 or 100%

27. Example 3: Heterozygous x Heterozygous
Bb and Bb
The probability of an offspring having a BB genotype is ¼ or 25%
The probability of an offspring having a Bb genotype is 2/4 or 50%
The probability of an offspring having a bb genotype is ¼ or 25%

28. Example 3: Heterozygous x Heterozygous
¾ or 75% of the offspring resulting from this cross are predicted to have a black coat
¼ or 25% of the offspring are predicted to have a brown coat (recessive phenotype)

29. Example 3: Heterozygous x Heterozygous
Genotypic ratio: the ratio of the genotypes that appear in offspring
The probable genotypic ratio of the monohybrid cross represented is 1BB: 2 Bb: 1 bb
Phenotypic ratio: the ratio of the offspring’s phenotypes
The probable phenotypic ratio of the cross is 3 black : 1 brown

30. Example 4: Testcross
In guinea pigs, both BB and Bb result in a black coat
How would you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)?
Perform a testcross
Testcross: an individual of unknown genotype is crossed with a homozygous recessive individual
Can determine the genotype of any individual whose phenotype expresses the dominant trait

31. Example 4: Testcross
If the black guinea pig of unknown genotype is homozygous black, all offspring will be black
If the individual with the unknown genotype is heterozygous black, about half will be black

32. Example 5: Incomplete Dominance
In Mendel’s pea-plant crosses, one allele was completely dominant over another
Called complete dominance
In complete dominance, heterozygous plants and homozygous dominant plants are indistinguishable in phenotype
Ex: PP and Pp produce purple flowered plants

33. Example 5: Incomplete Dominance
Sometimes, the F1 offspring will have a phenotype in between that of the parents
Called incomplete dominance
Incomplete dominance occurs when the phenotype of a heterozygote is between the phenotypes determined by the dominant and recessive traits

34. Example 5: Incomplete Dominance
Both the allele for red flowers (R) and the allele for white flowers (r) influence the phenotype
Neither allele is completely dominant
When red flowers are crossed with white flowers, all of the F1 offspring have pink flowers

35. Example 5: Incomplete Dominance
100% of the offspring have the Rr genotype
The probable genotypic ratio is 1RR: 2 Rr: 1 rr
Since neither allele is completely dominant, the phenotypic ratio is 1 red: 2 pink: 1 white

36. Example 6: Codominance
Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring
Neither allele is dominant or recessive, nor do the alleles blend in the phenotype
Example: blood types
Determined by two alleles

37. Predicting Results of Dihybrid Crosses
Dihybrid cross: a cross in which two characteristics are tracked
The offspring are called dihybrids

38. Homozygous X Homozygous
Ex: Predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds
Round seeds (R) is dominant over wrinkled seeds (r)
Yellow seeds (Y) is dominant over green seeds (y)

39. Homozygous X Homozygous
The Punnett Square used to predict the results of a cross between a parent of the genotype RRYY and a parent of the genotype rryy will contain 16 boxes
Alleles are carried by the male and female gametes

40. Homozygous X Homozygous
The genotype of all of the offspring of this cross will be heterozygous for both traits: RrYy
All have round, yellow seed phenotypes

41. Heterozygous X Heterozygous
Cross two pea plants heterozygous for round, yellow seeds
Offspring are likely to have nine different genotypes

42. Heterozygous X Heterozygous
These 9 genotypes will result in pea plants that have the following four phenotypes:
9/16 that have round, yellow seeds (genotypes RRYY, RRYy, RrYY, and RrYy)
3/16 that have round, green seeds (genotypes Rryy and Rryy)
3/16 that have wrinkled, yellow seeds (genotypes rrYY and rrYy)
1/16 that have wrinkled, green seeds (genotype rryy)