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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
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Genetics
Chapter 16
Traits, Genes, Alleles
• Genetics– 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
Gregor Mendel
• Discovered basic principles of genetics• Used garden peas as experimental
organism• Published report in 1866
– Work rediscovered around 1900
Gregor Mendel
• Reasoned that factors for characters came in more than one form
• Trait – Variant
• Genes – Factors
• Alleles– Alternate forms of a gene
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
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
Comparison of Wild-type Form and Mutant Form in Corn (Zea mays)Wild-type Form Alternate (Mutant) Form Name of Gene
Filled endosperm Shrunken endosperm (lacks sucrose synthase) sh
Yellow endosperm White endosperm y
Colored (red) endosperm Yellow endosperm R
Normal endospermWaxy endosperm (altered starch-synthesizing enzyme)
wx
Dormant seed Viviparous (germinates on cob) Vp
Has isocitrate dehydrogenase
Lacks isocitrate dehydrogenase idh
Comparison of Wild-type Form and Mutant Form in Garden Peas
(Pisum sativum)Wild-type Form 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
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
Chromosomes
• Genome– 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
Chromosomes
• Mitosis– 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
Meiosis
• Mechanism that offers greater genetic variety than mutation alone
• Union of haploid egg and sperm result in diploid zygote
Meiosis
• Carries out two rounds of cell division– Meiosis I
• 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
Meiosis IStage 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 IPause for checking for missing links between chromosomes and spindle, chromosome pairs move to cell’s equator
Anaphase ISpindle 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
Meiosis IIStage Description
Prophase IINo synapsis, no crossing over, each cell forms new spindle that links each sister chromatid of each chromosome
Metaphase IIChromosome moved separately to equator, cell pauses to check for spindle linkage
Anaphase IIIn 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
Meiosis
• Special events of prophase I– Synapsis
• 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
Meiosis
– Recombination• New combinations of alleles resulting from
crossing over
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
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
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
Gamete Possibilities• TT T • Tt T, t• tt t• TTRR TR• TtRR TR, tR• TTRr TR, Tr• TtRr TR, Tr, tR, tr
TT
T T
TtRr
TR Tr tR tr
Cross Involving Single Gene
T T
t Tt Tt
t Tt 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.
Cross Involving Single Gene
T t
T TT Tt
t 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.
Codominance or Incomplete Dominance
R R
r Rr Rr
r Rr Rr
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).
Codominance or Incomplete Dominance
R r
R RR Rr
r 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.
Cross Involving Two Genes
• Overall pattern determined by combinations of alleles of several genes
• Cross involving two genes– Dihybrid cross
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.
Dihybrid CrossTR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr TtRr TtrrtR TtRR TtRr ttRR ttRr
tr 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
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.
Test Cross
T T
t Tt Tt
t Tt Tt
T t
t Tt tt
t Tt tt
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.
Egg Egg
Pollen Pollen
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%)
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
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
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
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
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
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
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
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
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)
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
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
Polyploidy
• Polyploidy can also result from interbreeding between different species of plants– Original progeny sterile– Can become fertile if cells become polyploid
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