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1. Chapter 9: Fundamentals of Genetics
2. 9.1 Mendels Legacy Genetics is the field of biology devoted to understanding how characteristics are transmitted from parents to offspring.
3. Gregor Mendel In 1843, at the age of 21, Gregor Mendel entered a monastery in Brunn, Austria. His task of tending the garden gave him time to think and to observe the growth of many plants.
Heredity is the transmission of characteristics from parents to offspring.
4. Mendels Garden Peas Mendel observed 7 characteristics of pea plants. Each characteristic occurred in two contrasting traits: plant height (long and short stems), flower position along stem (axial or terminal), pod color (green or yellow), pod appearance (inflated or constricted), seed texture (smooth or wrinkled), seed color (yellow or green), and flower color (purple or white).
6. Mendels Garden Peas Read last paragraph on page165. Mendel collected seeds...for such variations.
7. Mendels Methods Male reproductive parts of a flower: anthers
Female reproductive parts of a flower: stigma.
8. Mendels Methods Mendel was able to document the traits of each generations parents by carefully controlling how the pea plants were pollinated.
Pollination occurs when pollen grains produced in the anthers are transferred to the stigma.
9. Mendels Methods Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or a flower on the same plant.
Cross-pollination involves flowers of two separate plants.
Pea plants normally reproduce through self-pollination.
10. Mendels Experiments He began by growing plants that were pure for each trait. Plants that are pure for a trait always produce offspring with that trait.
The term strain denotes plants that are pure for a specific trait.
He called each strain a parental generation or P1 generation
12. Mendels Experiments Read 2nd to last paragraph on page 166. Mendel the..seeds to develop.
When the plants matured, he recorded the number of each type of offspring produced by each P1 plant. Mendel called the offspring produced by each P1 generation the first filial generation, or F1 generation.
14. Mendels Experiments He then allowed the flowers from the F1 generation to self-pollinate and collected seeds. Mendel called the plants in this generation the second filial generation, or F2 generation.
16. Mendels Results and Conclusions Punnet Squares
17. Recessive and Dominant Traits Whenever Medel crossed strains, on of the P1 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 other factor for a specific characteristic.
18. Recessive and Dominant Traits The trait that did not appear in the F1 generation but reappeared in the F2 generation was thought to be controlled by a recessive factor.
THEREFORE, a trait controlled by a recessive factor had/has no observable effect on an organisms appearance when it was/is paired with a trait controlled by a dominant factor.
19. The Law of Segregation Mendel concluded that the paired factors separate during the formation of reproductive cells. This means that each reproductive cell, or gamete, receives only one factor(s) of each pair.
When two gametes combine during fertilization, the offspring have two factor(s) controlling a specific trait.
20. The Law of Segregation The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes.
21. The Law of Independent Assortment Mendel also crossed plants that differed in two characteristics such as in flower color and seed color. The data from these more complex crosses showed that traits produced by dominant factors do not always appear together.
Mendel concluded that the factors for different characteristics are not connected.
22. The Law of Independent Assortment The law of independent assortment states that factors for different characteristics are distributed to gametes independently.
23. Chromosomes and Genes Most of Mendels findings agree with what biologists now know about molecular genetics. Molecular genetics is the study of the structure and function of chromosomes and genes.
A gene is the segment of DNA on a chromosome that controls a particular hereditary trait. Because chromosomes occur in pairs, genes also occur in pairs.
24. Chromosomes and Genes Each of several alternative forms of a gene is called an allele. Mendels factors are now called alleles.
Letters are used to represent alleles.
25. Chromosomes and Genes Capital letters refer to dominant alleles, and lowercase letters refer to recessive alleles. For example, the dominant allele for the trait of green pod color may be represented by G, and the recessive allele for the trait of yellow pod color may be represented by g.
26. Review Define the terms dominant and recessive.
27. Review Differentiate genes from alleles.
28. Review What happens during meiosis that would allow genes located on the same chromosome to separate independently of one another?
29. 9.2 Genetic Crosses
30. Genotype and Phenotype The genetic makeup of an organism is its genotype. The genotype consists of the alleles that the organism inherits from its parents.
For example, the genotype of the white-flowering pea plant consists of two recessive alleles for the white flower color, represented as pp. The genotype of a purple-flowering pea plant may be either PP or Pp.
31. Genotype and Phenotype The appearance of an organism as a result of its genotype is called its phenotype. The phenotype of a PP or a Pp pea plant is purple flowers, whereas the phenotype of a pp pea plant is white flowers.
Human phenotypes can appear to be altered by behavior. Hair dye, contact lenses of varying colors, and plastic surgery can all change an individuals appearance, but they do not alter the individuals true phenotype or genotype.
32. Genotype and Phenotype When both alleles of a pair are alike, the organism is said to be homozygous for that characteristic. An organism may be homozygous dominant or homozygous recessive.
For example, a pea plant that is homozygous dominant for flower color would have the genotype PP. A pea plant that is homozygous recessive for flower color would have the genotype pp.
33. Genotype and Phenotype When the two alleles in the pair are different, the organism is heterozygous for that characteristic. A pea plant that is heterozygous for flower color would have the genotype Pp.
34. Probability Probability is the likelihood that a specific event will occur. A probability may be expressed as a decimal, a percentage, or a fraction.
# of times an event is expected to happen
Probability = -------------------------------------------------
# of opportunities for an event to happen
Examples:
35. Predicting Results of Monohybrid Crosses A cross between individuals that involves one pair of contrasting traits is called a monohybrid cross.
36. Example 1: Homozygous * Homozygous
37. Example 3: Heterozygous * Heterozygous The ration of the genotypes that appear in offspring is called the genotypic ratio. The probable genotypic ratio of the monohybrid cross represented in Figure 9-7 is 1BB, 2Bb, 1bb.
The ration of the offsprings phenotypes is called the phenotypic ratio. The probable phenotypic ratio of the cross represented in Figure 9-7 is 3black, 1 brown.
38. Example 3: Heterozygous * Heterozygous In bunny rabbits:
B = dominant, black
b = recessive, brown
39. Example 4: Testcross How might you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)? You could perform a test cross, in which an individual of unknown genotype is crossed with a homozygous recessive individual.
40. Example 5: Incomplete Dominance Recall that in Mendels pea-plant crosses, one allele was completely dominant over another, a relationship called complete dominance.
In complete dominance, heterozygous plants and dominant homozygous plants are indistinguishable in phenotype.
Sometimes the F1 offspring will have a phenotype in between that of the parents, a relationship called incomplete dominance.
41. Example 5: Incomplete Dominance Incomplete dominance occurs when two or more alleles influence the phenotype, resulting in a phenotype in between the dominant trait and the recessive trait.
In four oclock flowers,
red = R
white = r
42. Example 6: Codominance Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring. In Codominance, neither allele is dominant or recessive, nor do the alleles blend in the phenotype.
RR = red coat color
RR = white coat color
RR = roan color
44. Ex: Red Coat - RR
White Coat RR
White & Red Mixed RR (roan)
46. Predicting Results of Dihybrid Crosses A dihybrid cross is a cross between individuals that involves two pairs of contrasting traits.
Predicting the results of a dihybrid cross is more complicated than predicting the results of a monohybrid cross because there are more possible combinations of alleles to work out.
Review
