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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 endosperm Waxy endosperm (altered starchsynthesizing 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 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 Creates new nuclear envelopes, cells divide into 2 haploid Telophase I and cells, each cell has different combination of parental genes, Cytokinesis each chromosome still has two sister chromatids 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 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 Egg T T t Tt Tt t Tt Tt 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 Egg T t T TT Tt t Tt tt 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 Egg R R r Rr Rr r Rr Rr 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 Egg R r R RR Rr r Rr rr 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 Egg TR Pollen tr TTRR = tall plant with round seeds TtRr 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 Cross Egg TR Tr tR tr TR TTRR TTRr TtRR TtRr Tr TTRr TTrr TtRr Ttrr tR TtRR TtRr ttRR ttRr tr TtRr Ttrr ttRr ttrr 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 Egg Egg T T Pollen T t Pollen t Tt Tt t Tt tt 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. 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