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Chapter 9 Patterns of Inheritance BIOLOGY: Today and Tomorrow, 4e starr evers starr 9.1 Menacing Mucus Cystic fibrosis, the most common fatal genetic disorder in the US, is caused by a deletion in the CFTR gene The CF allele persists at high frequency despite devastating effects Only those homozygous for the CF allele have the disorder Victims of Cystic Fibrosis ANIMATED FIGURE: Crossing garden pea plants To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 9.2 Tracking Traits Mid-1800s: Genes and chromosomes were unknown; Gregor Mendel’s experiments with pea plants established principles of inheritance Breeding Garden Peas carpel anther A) The flowers of garden pea plants have reproductive parts called anthers and carpels. Pollen grains that form in anthers produce male gametes; female gametes form in carpels. Breeding Garden Peas B) Experimenters can control the transfer of hereditary material from one pea plant to another by snipping off a flower’s anthers (to prevent the flower from self-fertilizing), and then brushing pollen from another flower onto its carpel. In this example, pollen from a plant that has purple flowers is brushed onto the carpel of a white-flowered plant. Breeding Garden Peas C) Later, seeds develop inside pods of the crossfertilized plant. An embryo in each seed develops into a mature pea plant. Breeding Garden Peas D) Every plant that arises from the cross has purple flowers. Predictable patterns such as this are evidence of how inheritance works. Inheritance in Modern Terms Organisms breed true for a trait because they carry identical alleles of genes governing that trait Homozygous Having identical alleles of a gene Heterozygous Having two different alleles of a gene Inheritance in Modern Terms The particular set of alleles an individual carries is the individual’s genotype Gene expression results in phenotype – an individual’s observable traits An allele is dominant when its effect masks that of a recessive allele paired with it Genotype gives rise to phenotype genotype PP (homozygous for dominant allele P) pp (homozygous for recessive allele p) Pp (heterozygous for alleles P and p) phenotype 9.3 Mendelian Inheritance Patterns A cross (mating) between heterozygous individuals can reveal dominance relationships among the alleles under study Monohybrid cross Cross in which individuals with different alleles of a gene are crossed Dominant trait will have a 3:1 phenotype ratio Segregation of Genes When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate Each gamete that forms carries only one of the two genes of a pair Segregation of Genes 1 2 DNA replication meiosis I meiosis II gametes (P) gametes (p) zygote (Pp) 3 DNA replication meiosis I 2 1 meiosis II 3 gametes (P) gametes (p) zygote (Pp) Stepped Art Figure 9-4 p153 Punnett Squares Punnett squares are used to calculate the probability of the genotype and phenotype of offspring of crosses In a testcross, an individual with a dominant trait (but an unknown genotype) is crossed with an individual known to be homozygous for the recessive allele The pattern of traits among offspring can reveal whether the tested individual is heterozygous or homozygous Punnett Squares: A Monohybrid Cross Monohybrid cross: First generation parent plant homozygous for purple flowers parent plant homozygous for white flowers Pp hybrid two types of gametes A) All of the F1 (first generation) offspring of a cross between two plants that breed true for different forms of a trait are identically heterozygous (Pp). These offspring make two types of gametes: P and p. Monohybrid cross: Second generation B) A cross between two of the identically heterozygous F1 offspring is a monohybrid cross. In this example, the phenotype ratio among the F2 (second generation) offspring is 3:1 (three purple to one white). Dihybrid Crosses Mendel’s dihybrid crosses showed inheritance of one trait did not affect inheritance of other traits Dihybrid cross Experiment in which individuals with different alleles of two genes are crossed (9:3:3:1 ratio) Independent assortment A gene tends to be distributed independently of how other genes are distributed Dihybrid Cross: First generation parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems 2 PpTt dihybrid 3 four types of gametes 1 Dihybrid Cross: Second generation 4 The Contribution of Crossovers Two genes located close together on the same chromosome tend to be inherited together When two genes on the same chromosome are far apart, crossing over occurs more frequently between them; they tend to assort independently A Human Example: Skin Color Variations in skin color depend on the kinds and amounts of melanins produced More than 100 gene products affect production and deposition of melanins Independent assortment of these genes produces a wide variety of phenotypes Variation in Skin Color ANIMATED FIGURE: Dihybrid cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATED FIGURE: Independent assortment To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATED FIGURE: Monohybrid cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATED FIGURE: Test Cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 9.4 Beyond Simple Dominance Codominant Refers to two alleles that are both fully expressed in heterozygous individuals Incomplete dominance Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes Codominance: Blood Type Genotypes: Phenotypes (blood type): AA or AO A AB A B BB or BO OO B O Incomplete Dominance homozygous parent (RR) X homozygous parent (rr) heterozygous offspring (Rr) A) Cross a red-flowered with a white-flowered snap-dragon, and all of the offspring will have pink flowers. Incomplete Dominance B) If two of the pink-flowered snapdragons are crossed, the phenotypes of their offspring will occur in a 1:2:1 ratio. Epistasis Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis Epistasis Effect in which a trait is influenced by the products of multiple genes Example: Labrador retriever coat color Epistasis Pleiotropy Products of pleiotropic genes influence two or more traits Mutations in pleiotropic genes are associated with sickle cell anemia, cystic fibrosis, and Marfan syndrome INTERACTION: Incomplete dominance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Chicken combs To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Dog color To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 9.5 Complex Variations in Traits Mutations, interactions among genes, and environmental conditions can affect one or more steps in a metabolic pathway, and contribute to variation in phenotypes Example: Seasonal changes affect production of pigments that color the skin and fur of many animals Example: Water flea phenotypes depend on whether the aquatic insects that prey on them are present Example: Genetically identical yarrow plants grow to different heights at different altitudes Snowshoe Hare in Summer and Winter A) The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left ); in winter, it is white (right ). The variation offers seasonally appropriate camouflage from predators. Water flea, with and without predators B) The body form of the water flea on the top develops in environments with few predators. A longer tail spine and a pointy head (bottom) develop in response to chemicals emitted by predatory insects. Environmental Effects on Plant Phenotypes Height (centimeters) 60 0 3060 1400 30 Elevation (meters above sea level) C) The height of a mature yarrow plant depends on the elevation at which it grows. Continuous Variation Continuous variation A range of small increments of phenotype in a trait that is influenced by the products of multiple genes The more genes and other factors that influence a trait, the more continuous the distribution of phenotype Bell curve Curve that results when range of variation in a continuous trait is plotted against frequency in a population Continuous Variation in Eye Color Continuous Variation in Height A) Male biology students at the University of Florida were divided into categories of one-inch increments in height and counted. Continuous Variation (Bell Curve) Number of individuals 20 15 10 5 0 63 64 65 66 67 68 69 70 71 72 73 74 75 76 B) Graphing the resulting data produces a bell-shaped curve, an indication that height varies continuously. 77 9.5 Human Genetic Analysis Inheritance patterns in humans are studied by following inherited genetic disorders in a family through generations and graphing results as a pedigree chart Pedigree analyses shows whether a trait is associated with a dominant or recessive allele, and whether the allele is on an autosome or a sex chromosome Pedigree: Polydactyly Types of Genetic Variation Single genes on autosomes or sex chromosomes govern more than 6,000 genetic abnormalities and disorders Genetic abnormality An uncommon version of a heritable trait that does not result in medical problems Genetic disorder A heritable condition that results in a syndrome of mild or severe medical problems ANIMATION: Coat color in the Himalayan rabbit To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Height Graph To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 9.6 Human Genetic Disorders Some dominant or recessive alleles on autosomes or the X chromosome are associated with genetic abnormalities or disorders An autosomal dominant allele is expressed in homozygotes and heterozygotes An autosomal recessive allele is expressed only in homozygotes Some Autosomal Dominant Traits Autosomal Dominant Inheritance normal mother affected father X meiosis and gamete formation A) A dominant allele on an autosome (red ) is fully expressed in heterozygous people affected child normal child disorder-causing allele (dominant) normal mother affected father meiosis and gamete formation affected child normal child disorder-causing allele (dominant) Stepped Art Figure 9-17a p163 ANIMATED FIGURE: Pedigree diagrams To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Some Autosomal Recessive Traits Autosomal Recessive Inheritance A) Only people homozygous for a recessive allele on an autosome have the trait associated with the allele. carrier mother carrier father X meiosis and gamete formation In this example, both parents are carriers (red). Each of their children has a 25 percent chance of inheriting two alleles, and being affected by the trait. affected child carrier child normal child B) The albino phenotype is associated with autosomal recessive alleles that cause a deficiency in melanin. disorder-causing allele (recessive) carrier mother carrier father meiosis and gamete formation affected child carrier child normal child disorder-causing allele (recessive) Stepped Art Figure 9-18a p163 Victim of Tay–Sachs disease X-Linked Recessive Disorders Alleles on the X chromosome are inherited and expressed differently in males and females Males cannot transmit a recessive X-linked allele to their sons Females pass X-linked alleles to male offspring Example: red-green color blindness Some X-Linked Recessive Disorders X-Linked Recessive Inheritance carrier mother normal father X meiosis and gamete formation A) In this example of X-linked inheritance, the mother carries a recessive allele on one of her two X chromosomes (red ). normal daughter or son carrier daughter affected son recessive allele on X chromosome carrier mother normal father meiosis and gamete formation normal daughter or son carrier daughter affected son recessive allele on X chromosome Stepped Art Figure 9-20a p165 Red–green color blindness B) A view of color blindness. The image on the left shows how a person with red– green color blindness sees the image on the right. The perception of blues and yellows is normal; red and green appear similar. Red–green color blindness You may have one form of red–green color blindness if you see a 7 instead of a 29 in this circle. C) Part of a standardized test for color blindness. A set of 38 of these circles is commonly used to diagnose deficiencies in color perception. You may have another form of red–green color blindness if you see a 3 instead of an 8 in this circle. INTERACTION: Autosomal-dominant inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE INTERACTION: Autosomal-recessive inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE INTERACTION: X-linked inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE VIDEO: Genetics, Sociology, and Breast Cancer 9.8 Changes in Chromosome Number Many flowering plants, and some insects, fishes and other animals are polyploid – having three or more of each type of chromosome characteristic of the species Chromosome number can change permanently, usually resulting from nondisjunction – the failure of chromosomes to separate normally during meiosis or mitosis Nondisjunction During Meiosis Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Stepped Art Figure 9-21 p166 Aneuploidy Aneuploidy A chromosome abnormality in which a cell has too many or too few copies of a particular chromosome (trisomy, monosomy) The most common aneuploidy in humans, trisomy 21, causes Down syndrome Some Disorders Caused by Aneuploidy Autosomal Change and Down Syndrome Trisomy 21 (Down syndrome) The only autosomal trisomy that allows humans to survive to adulthood Affected individuals tend to have certain physical features and impairments Nondisjunction leading to trisomy 21 increases with age of the mother Down Syndrome Change in Sex Chromosome Number Usually associated with learning difficulties, speech delays, and motor skill impairment Female sex chromosome abnormalities: Turner syndrome (XO) XXX syndrome Male sex chromosome abnormalities: Klinefelter syndrome (XXY) XYY syndrome 9.9 Genetic Screening Geneticists estimate the chance that a couple’s offspring will inherit a genetic abnormality or disorder Potential parents who may be at risk of transmitting a harmful allele to offspring have screening or treatment options Prenatal Diagnosis Obstetric sonography may reveal defects associated with a genetic disorder Other tests performed before birth carry risks of miscarriage or injury to fetus Amniocentesis Chorionic villi sampling (CVS) Fetoscopy Three ways of imaging a fetus C) Fetoscopy A) Conventional ultrasound B) 4D ultrasound The amniotic sac amniotic sac chorion Preimplantation Diagnosis A single cell taken from an embryo produced by in vitro fertilization is tested before implantation 9.10 Menacing Mucus (revisited) The cystic fibrosis (CF) allele is very common in some populations The CF allele is lethal in homozygotes, but offers heterozygotes some protection against bacterial diseases such as typhoid fever Digging Into Data: Cystic Fibrosis and Typhoid Fever