1. Genetics
Chapter 16

2. Traits, Genes, Alleles Genetics Genes Characters
Study of genes and ways they are inherited
Genes
Internal factors
Provide instructions to plant cells
How to grow and develop
How to respond to environment
Characters
Traits such as flower shape and color, stem length, leaf shape and arrangement, fruit type, seed shape

3. Gregor Mendel Discovered basic principles of genetics
Used garden peas as experimental organism
Published report in 1866
Work rediscovered around 1900

4. Gregor Mendel
Reasoned that factors for characters came in more than one form
Trait
Variant
Genes
Factors
Alleles
Alternate forms of a gene

5. DNA Code Reflected in Traits
Genes
Sequences of nucleotides in DNA
Watson and Crick provided valuable information when they discovered helical structure of DNA
Recognized how sequence of bases in DNA molecule act as code to specify sequence of amino acids in protein

6. DNA Code Reflected in Traits
Mutation
Changes in base sequence of DNA
Raw material for evolution through natural selection
Altered DNA passed from mutant organism to its progeny
Mutation spreads through population
Population evolves

7. Comparison of Wild-type Form and Mutant Form in Corn (Zea mays)
Alternate (Mutant) Form
Name of Gene
Filled endosperm
Shrunken endosperm (lacks sucrose synthase) sh
Yellow endosperm
White endosperm y
Colored (red) endosperm R
Normal endosperm
Waxy endosperm (altered starch-synthesizing enzyme) wx
Dormant seed
Viviparous (germinates on cob) Vp
Has isocitrate dehydrogenase
Lacks isocitrate dehydrogenase
idh

8. Comparison of Wild-type Form and Mutant Form in Garden Peas (Pisum sativum)
Alternate (Mutant) Form
Name of Gene
Yellow cotyledons
Green cotyledons I
Red flower petals
White flower petals A1
Smooth seed surface
Wrinkled seed surface R
Tall (more than 20 internodes)
Short (10-20 internodes) T
Green foliage
Yellow-green foliage O
Axillary flowers
Terminal flowers Fa
Straight pod
Curved pod Cp
Tendrils
No tendrils N

9. Chromosomes DNA in nucleus combined with proteins to form chromatin
Chromatin divided into chromosomes
Each chromosome is single linear strand of DNA
Gene
Has particular position (locus) on chromosome
Portion of DNA composed of between 300 and 3,000 bases
Separated from adjacent genes by stretches of DNA thought to be nonfunctional

10. Chromosomes Genome Number of chromosomes in plant varies with species
All the genes in an organism
Number of chromosomes in plant varies with species
Smallest number in plant is four
Coast redwood and some ferns have several hundred

11. Chromosomes Mitosis Vegetative reproduction
Every allele found in original cell will also be present in all the cells of plant
Vegetative reproduction
Also mitotic division
Progeny have same alleles as parent plants

12. Meiosis
Mechanism that offers greater genetic variety than mutation alone
Union of haploid egg and sperm result in diploid zygote

13. Meiosis Carries out two rounds of cell division Meiosis I Meiosis II
Converts original 2n cell to two 1n cells with different combinations of parental genes
Meiosis II
Mitotic division that separates sister chromatids and converts two 1n (haploid) cells to four 1n (haploid) cells

14. Meiosis I Stage Description Prophase I
Chromatids condense, synapsis occurs  each homologous pair of chromosomes comes together, pairing makes it easy for cell to divide in a way that it produces haploid cells, crossing over  allow homologous chromosomes to trade segments, synapsis and crossing over give chromosomes new combinations of parental genes, spindle forms
Metaphase I
Pause for checking for missing links between chromosomes and spindle, chromosome pairs move to cell’s equator
Anaphase I
Spindle pulls each chromosome with its two sister chromatids to one of the poles
Telophase I and Cytokinesis
Creates new nuclear envelopes, cells divide into 2 haploid cells, each cell has different combination of parental genes, each chromosome still has two sister chromatids

15. Meiosis II Stage Description Prophase II
No synapsis, no crossing over, each cell forms new spindle that links each sister chromatid of each chromosome
Metaphase II
Chromosome moved separately to equator, cell pauses to check for spindle linkage
Anaphase II
In each cell, spindle pulls the two sister chromatids of each chromosome to opposite poles
Telophase II and Cytokinesis
Each cell divides into two cells, each cell is haploid with different combinations of parental genes

16. Meiosis Special events of prophase I Synapsis Crossing over
Homologous chromosomes come together to form pairs
Crossing over
Chromatids of homologous chromosomes may exchange corresponding pieces with each other
Cross formed by chromatids during exchange  chiasma
Results in rearranged chromatids with fragments from both of the homologous chromosomes

17. Meiosis Recombination
New combinations of alleles resulting from crossing over

19. Key Terms for Understanding Genetics
Phenotype
Visible traits of an organism
Genotype
Collection of alleles of an organism
Homozygous
Two copies of the same allele
Example: TT or tt
Heterozygous
Different alleles of a gene
Example: Tt

20. Key Terms for Understanding Genetics
Dominant
Will be expressed (Tt or TT) condition
Overshadows recessive allele
Recessive
Expressed only in homozygous condition (tt)
Codominant or incompletely dominant
Plant shows trait that is intermediate between those of parents
Example: cross between red and white flowers yields progeny with pink flowers

21. Punnett Square Named for professor who popularized it
Way to keep track of combinations of alleles formed during fertilization
Shows expected genotypes of progeny
Also shows probability of expected genotypes

22. Gamete Possibilities TT  T Tt  T, t tt  t TTRR  TR TtRR  TR, tR
TtRr  TR, Tr, tR, tr TT T T
TtRr
BB  B
Bb  B, b
bb  b
BBSS  BS
BbSS  BS, bS
BBSs  BS, Bs
BbSs  BS, Bs, bS, bs tr TR Tr tR

23. Cross Involving Single GeneT t Tt
Egg
Pollen
T = tall t = dwarf
In a cross between a homozygous tall plant and a homozygous dwarf plant, all the progeny will be heterozygous tall plants.

24. Cross Involving Single GeneT t TT Tt tt
Egg
Pollen
T = tall t = dwarf
When two heterozygous tall plants are crossed, the expected progeny are as follows:  homozygous tall,  heterozygous tall,  dwarf. The genotypic ratio is 1:2:1 and the phenotypic ratio is 3:1.

25. Codominance or Incomplete DominanceRr
Egg
Pollen
RR = red flower rr = white flower Rr = pink flower
Crossing a homozygous red flower and a white flower results in progeny with all pink flowers (Rr).

26. Codominance or Incomplete DominanceRR Rr rr
Egg
Pollen
RR = red flower rr = white flower Rr = pink flower
A cross between two pink flowers yields expected progeny as follows:  red flowers (RR),  pink flowers (Rr),  white flowers (rr). The expected genotypic ratio is 1:2:1 and the expected phenotypic ratio is 1:2:1.

27. Cross Involving Two Genes
Overall pattern determined by combinations of alleles of several genes
Cross involving two genes
Dihybrid cross

28. Dihybrid Cross TR tr TtRr Egg Pollen
TTRR = tall plant with round seeds ttrr = dwarf plant with wrinkled seeds
A cross between a homozygous tall plant with round seeds and a dwarf plant with wrinkled seeds yields progeny that are predicted to all be tall plants with round seeds.

29. Dihybrid Cross TR Tr tR tr TTRR TTRr TtRR TtRr TTrr Ttrr ttRR ttRr
Egg
Pollen
T = tall plant t = dwarf plant R = round seeds r = wrinkled seeds
In a cross between two heterozygous tall plants with round seeds (TtRr x TtRr), the expected phenotypic ration of the progeny is 9:3:3:1.
9 tall plants with round seeds
3 tall plants with wrinkled seeds
3 dwarf plants with round seeds
1dwarf plant with wrinkled seeds

30. Test Cross
Used to determine genotype of organism with dominant phenotype
For example, tall plant could have genotype TT or Tt
Cross plant in question with recessive plant in order to determine genotype
Examine phenotypes of progeny to determine genotype of parent with dominant phenotype.

31. Test Cross T t Tt Egg T t Tt tt Egg Pollen Pollen
If all the progeny from the cross are tall plants, then the organism in question was homozygous (TT).
If there are any dwarf plants that result from the cross, then the organism in question was heterozygous (Tt). The expected genotypic ratio in this case is 1:1 and the phenotypic ratio is 1:1.

32. Linked Traits
If genes migrate as a unit rather than independently during gamete formation, then genes are described as linked
Crossing over may result in formation of recombinant combinations of linked alleles
Farther apart two genes are, the more likely crossing over will occur between them
No matter how far apart the two genes are, the fraction of gametes with recombinant combinations is never greater than the fraction of gametes with parental combinations of alleles (never greater than 50%)

33. Maternal Inheritance Involving Organellar Chromosomes
Most genes in plant cell located in nuclear chromosomes
Also chromosomes of DNA in plastids and mitochondria
Organellar chromosomes
Smaller than nuclear chromosomes
Do contain genes which can mutate

34. Maternal Inheritance Involving Organellar Chromosomes
During fertilization, only chloroplasts and mitochondria from egg are incorporated into zygote
Chloroplasts and mitochondria from sperm cells either do not enter egg or degenerate during fertilization
All chloroplast and mitochondria genes in zygote come from egg and all alleles of these genes show maternal inheritance

35. Plant Breeding Earth’s population keeps growing
Acres of land under cultivation has decreased
Food production/person is at least as great as it was in the 1950s
Results mainly due to breeding of new, more productive plants
Easier to grow or harvest
Resistance to disease or stress
Edible parts that are more attractive or nutritious

36. Mating Plants Combines Useful Traits
Example: want to breed tomato variety that is fungus resistant
Mate successful but fungus-susceptible commercial variety with wild variety that shows fungus resistance
Progeny will be resistant but probably have inedible fruit
Mate progeny with the commercial variety (back cross) and test progeny of mating for resistance

37. Mating Plants Combines Useful Traits
By chance, some of resistant progeny will have acquired genes needed for edible fruit
Most resistant progeny mated again with commercial variety and most resistant progeny again selected
After several cycles, strain of commercial fruit with resistance to fungus results

38. Multiple Genes Some traits vary continuously within a certain range
Size of harvested organ, sugar content, firmness of fruit
Factors that lead to continuous variation
Involvement of multiple genes
Individually have small effect on phenotype
Collectively combine to provide wide range of variation

39. Multiple Alleles Sometimes gene has multiple alleles
Each has different degree of activity
Increases the number of possible phenotypic forms
Environmental effects may alter form of phenotype
Randomness of environmental effects tends to blur distinction among genotypes

40. Heterosis Hybrid vigor
Progeny from mating two inbred (highly homozygous) strains
Larger and healthier than parents
Special problem
Hybrids do not breed true  produce both heterozygous and homozygous progeny

41. Heterosis
Solution
Produce new hybrid plants through vegetative reproduction
Produce hybrid seed by mating two homozygous strains
Develop strains that are male-sterile (do not produce anthers or viable pollen)

42. Polyploidy Refers to having more than two sets of chromosomes
May occur spontaneously
Cell replicates DNA and separates chromatids but fails to complete cell division
Commonly occurs in last stages of development of tracheary elements and storage tissues
Less common in meristem

43. Polyploidy
If polyploid plant fertilizes itself, progeny will also be polyploid
Polyploid plants often larger and more vigorous than parental types
Polyploids seem more tolerant of environmental stresses such as short, cool growing seasons, aridity, or high temperatures

44. Polyploidy
Polyploidy can also result from interbreeding between different species of plants
Original progeny sterile
Can become fertile if cells become polyploid