Chapter 4 Mendel and the Gene Idea AP Biology
Overview: Drawing from the Deck of Genes The blending hypothesis was the most widely favored explanation of heredity in the 800s Idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) Predicts that a freely mating population will give rise to a uniform population of individuals over many generations This hypothesis contradicts many everyday observations and the results of breeding experiments with plants and animals Ex)Traits reappear after skipping a generation The particulate hypothesis is an alternative to the blending model Idea that parents pass on discrete heritable units (genes) Gregor Mendel documented a particulate mechanism through his experiments with garden peas
Concept 4.: Mendel used the scientific approach to identify two laws of inheritance
Pea Plants as Subjects for Genetic Study There are many advantages of using pea plants for genetic study: There are many varieties with distinct heritable features, or characters (ex: flower color); Character variants (such as purple or white flowers) are called traits Large numbers of offspring are produced during each mating in a short generation time Mating of plants can be controlled Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another Self-pollination can also be forced
Mendel s Experiment Mendel chose to track only those characters that varied between 2 distinct alternatives Ex) Purple vs. white flowers He also used varieties that had been allowed to self-fertilize over many generations, producing true-breeding populations Fig. 4-2 These plants produce offspring of the same variety when they self-pollinate TECHNIQUE Mendel then cross-pollinated 2 contrasting truebreeding pea varieties in a typical breeding experiment Crossing of 2 true-breeding varieties is called hybridization True-breeding parents are referred to as P (parental) generation Hybrid offspring are known as F ( st filial) generation Allowing F hybrids to self-pollinate produces an F2 (2 nd filial) generation Parental generation (P) RESULTS First filial generation offspring (F ) 2 Stamens Carpel 3 4 5
The Law of Segregation In his experiments, Mendel crossed contrasting, true-breeding white and purple flowered pea plants All of the F hybrids were purple When Mendel crossed the F hybrids, many of the F 2 plants had purple flowers, but some had white Fig. 4-3-3 Mendel discovered a ratio of about 3 purple: white in F 2 generation He reasoned that the heritable factor for white flowers was not destroyed in F generation but was somehow masked when purple flower factor was present Purple flower color is dominant trait EXPERIMENT P Generation (true-breeding parents) F Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers White flower color is recessive trait 705 purple-flowered plants 224 white-flowered plants
Patterns of Inheritance in Other Pea Plant Characters Mendel observed the same pattern of inheritance in six other pea plant Table 4- characters, each represented by two traits What Mendel called a heritable factor is what we now call a gene
Mendel s Model Mendel developed a hypothesis to explain the 3: inheritance pattern he observed in F 2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes
Alleles The first concept is that alternative versions of genes account for variations in inherited characters Ex) the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome Fig. 4-4 Alleles arise due to slight variations in nucleotide sequence at these loci Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
Inheritance of Alleles The second concept is that an organism inherits 2 alleles for each character, one from each parent Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical Ex) True-breeding plants of Mendel s P generation Fig. 4-4 Alternatively, the two alleles at a locus may differ Ex) F hybrids Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
Dominant vs. Recessive Alleles The third concept is that if the two alleles at a locus differ, then: One allele (the dominant allele) determines the organism s appearance The other allele (the recessive allele) has no noticeable effect on appearance Ex) In the case of flower color, the F plants had purple flowers because the allele for that trait is dominant
The Law of Segregation The fourth concept states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Known as the law of segregation Ex) Egg or sperm gets only one of the two alleles that are present in the somatic cells of the organism Segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis If organism has identical alleles for a character (true-breeding), than that allele will be present in all gametes If different alleles are present (F hybrids), 50% of gametes receive dominant allele and 50% receive recessive allele
Punnett Squares Mendel s segregation model accounts for the 3: ratio he observed in the F 2 generation of his numerous crosses Fig. 4-5-3 Possible combinations of sperm and egg can be shown using a Punnett square P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p Diagram for predicting the results of a genetic cross between individuals F Generation of known genetic makeup Appearance: Genetic makeup: Purple flowers Pp Capital letter represents a Gametes: / 2 P / 2 p dominant allele F 2 Generation P Sperm p Lowercase letter represents a recessive allele Eggs P p PP Pp Pp pp 3
Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character Homozygous organisms are true-breeding because all their gametes contain the same allele May be homozygous dominant (PP purple) May be homozygous recessive (pp-white) An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes, heterozygotes are not true-breeding Produce gametes with different alleles (P or p) Heterozygotes display the dominant trait
Genotype vs. Phenotype An organism s traits do not always reveal its genetic composition Due to the different effects of dominant and recessive alleles Therefore, we distinguish between an organism s: Fig. 4-6 Phenotype Genotype Purple PP (homozygous) Phenotype = physical appearance 3 Purple Purple Pp (heterozygous) Pp (heterozygous) 2 Genotype = genetic makeup White pp (homozygous) Ex) PP and Pp plants have Ratio 3: Ratio :2: the same phenotype (purple) but different genotypes
The Testcross Individuals displaying the dominant phenotype can have one of 2 genotypes Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous Fig. 4-7 A testcross can be carried out to determine genetic makeup of this mystery individual In this cross, the mystery individual is bred with homozygous recessive individual TECHNIQUE Predictions If any offspring display the recessive phenotype, the mystery parent must be heterozygous Eggs Dominant phenotype, unknown genotype: PP or Pp? P P Recessive phenotype, known genotype: If PP If Pp or Sperm Sperm p p p p Pp Pp Pp Pp Eggs P p Pp pp Pp pp pp RESULTS All offspring purple or / 2 offspring purple and / 2 offspring white
Monohybrid vs. Dihybrid Crosses Mendel derived the law of segregation by following a single character (ex: flower color) The F offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Mendel identified a second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F generation, heterozygous for both characters Crossing between F dihybrids (dihybrid cross) can determine whether two characters are transmitted to offspring as a package or independently
The Law of Independent Assortment Using a dihybrid cross, Mendel developed the law of independent assortment States that each pair of alleles segregates independently of each other pair of Fig. 4-8 alleles during gamete formation EXPERIMENT P Generation YYRR yyrr Ex) Inheritance of pea color will Gametes YR yr not affect inheritance for F Generation YyRr pea shape Predictions Hypothesis of dependent assortment Hypothesis of independent assortment This law applies only to genes on different, nonhomologous chromosomes Genes located near each other on Predicted offspring of F 2 generation Eggs / 2 / 2 YR yr Sperm / YR 2 / 2 yr YYRR YyRr / 4 YyRr yyrr or Eggs / 4 / 4 / 4 YR Yr yr Sperm / YR 4 / Yr 4 / yr 4 / 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr 3 / 4 the same chromosome tend to be Phenotypic ratio 3: / 4 yr YyRr Yyrr yyrr yyrr inherited together RESULTS 35 08 0 32 9 / 6 3 / 6 3 / 6 / 6 Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3:
Concept Check 4. ) A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a Punnett square for this cross. Assume pollen comes from the ii plant. 2) Pea plants heterozygous for flower position an stem length (AaTt) are allowed to self-pollinate, and 400 of the resulting seeds are planted. Draw a Punnett square for this cross. How many offspring would be predicted to have terminal flowers and be dwarf (see Table 4., pp. 265)? 3) List the different gametes that could be made by a pea plant heterozygous for seed color, seed shape, and pod shape (YyRrIi; see Table 4.). How large a Punnett square would you need to predict the offspring of a self-pollination of this trihybrid?
Concept 4.2: The laws of probability govern Mendelian inheritance
The Rules of Probability Mendel s laws of segregation and independent assortment reflect the rules of probability Probability scale ranges from 0- An event that is certain to occur has a probability of An event that will never occur has a probability of 0 One independent event has no impact on the outcome of the next independent event Ex) When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene s alleles
The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule: the probability that two or more independent events will occur together is the product of their individual probabilities Ex) Probability of coming up with 2 heads on 2 separate coin tosses is ½ X ½ = ¼ Fig. 4-9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Probability in an F monohybrid cross can be determined using the multiplication rule / 2 R Sperm / 2 r Ex) Probability of obtaining a white flowered pea plant (pp) from the cross Pp x Pp = ½ X ½ = ¼ / 2 R Eggs / 2 r 2 R R R r r / 4 / 4 r R r 2 / 4 / 4
The Addition Rule Addition rule: the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities Can be used to figure out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous Dominant allele can come from egg OR sperm (not both) in a heterozygote Fig. 4-9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Ex) Probability of obtaining an F2 Sperm heterozygote from the cross / 2 R / 2 r Rr x Rr is ¼ + ¼ / 2 R R R = ½ R r Eggs / 4 / 4 / 2 r r R r r / 4 / 4
Solving Complex Genetics Problems with the Rules of Probability Multiplication and addition rules can be used to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously Fig. 4-UN In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together
Concept Check 4.2 ) For any gene with a dominant allele C and a recessive allele c, what proportions of the offspring from a CC X Cc cross are expected to be homozygous dominant, homozygous recessive, and heterozygous? 2) An organism with the genotype BbDD is mated to one with the genotype BBDd. Assuming independent assortment of these 2 genes, write the genotypes of all possible offspring from this cross and use the rules of probability to calculate the chance of each genotype occurring. 3) Three characters (flower color, seed color, and pod shape) are considered in a cross between 2 pea plants (PpYyIi x ppyyii). What fraction of offspring would be predicted to be homozygous recessive for at least 2 of the 3 characters?
Concept 4.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Complex Patterns of Inheritance The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes
Degrees of Dominance Alleles can show different degrees of dominance and recessiveness in relation to each other Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, neither allele is completely dominant Fig. 4-0-3 The phenotype of F hybrids is somewhere between the phenotypes of the two parental P Generation Red C R C R White C W C W varieties Gametes C R C W Ex) Red snapdragon X White snapdragon = Pink snapdragons F Generation Pink C R C W In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Ex) Human blood type F 2 Generation C R Gametes / 2 / 2 C Eggs R C R C W Sperm / C R C W 2 / 2 / C R 2 C R C W / C W 2 C R C W C W C W
Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don t interact Alleles are simply variations in a gene s nucleotide sequence Ex) Pea seed shape: one dominant allele results in enough enzyme to make adequate amounts of branched starch For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive (need 2 recessive alleles to inherit disease) At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant (/2 the normal enzyme activity is sufficient to prevent lipid accumulation in brain) At the molecular level, the alleles are codominant (heterozygotes produce equal numbers of normal and dysfunctional enzymes)
Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles Ex) Some cases of polydactyly (having extra fingers or toes) are caused by presence of dominant allele Only one baby out of 400 in the United States is born with extra fingers or toes
Multiple Alleles Most genes exist in more than two allelic forms Ex) The four phenotypes of the ABO blood group in humans are determined by three alleles Fig. for 4- the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate The enzyme encoded by the I B allele adds the B carbohydrate Allele I A I B Carbohydrate i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype A B Red blood cell appearance Phenotype (blood group) I A I A or I A i A I B I B or I B i B The enzyme encoded by the I A I B AB i allele adds neither ii (b) Blood group genotypes and phenotypes O
Pleiotropy, Epistasis, and Polygenic Inheritance Most genes have multiple phenotypic effects, a property called pleiotropy Ex) Responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Some traits may also be determined by two or more genes Include 2 different situations: Epistasis Polygenic inheritance
Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus Ex) In mice and many other mammals, coat color depends on two genes One gene determines the pigment color : Fig. 4-2 B = black b = brown The other gene determines whether the pigment will be deposited in the hair C = color c = no color The gene for pigment deposition (C/c) is said to be epistatic to the gene that codes for pigment color (B/b) Eggs / 4 BC / 4 bc / 4 Bc / 4 bc Sperm BbCc BbCc / 4 / 4 / 4 / 4 BC bc Bc bc BBCC BbCC BBCc BbCc BbCC bbcc BbCc bbcc BBCc BbCc BBcc Bbcc BbCc bbcc Bbcc bbcc 9 : 3 : 4
Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance Fig. 4-3 Controlled by at least 3 separately inherited genes AaBbCc AaBbCc Sperm / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Black circles represent dark-skin alleles (A,B,C) / 8 / 8 / 8 White circles represent light-skin alleles (a,b,c) Eggs / 8 / 8 / 8 One dark-skin allele contributes one / 8 / 8 unit of darkness to phenotype Phenotypes: Number of / 64 6 / 64 5 / 64 20 / 64 5 / 64 6 / 64 / 64 dark-skin alleles: 0 2 3 4 5 6
The Environmental Impact on Phenotype The phenotype for a character may also depend on environment as well as genotype Genotypes are generally not associated with a rigidly defined phenotype but rather a range of phenotypes due to environmental influences The norm of reaction is the phenotypic range of a genotype influenced by the environment Norms of reactions are usually broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively Fig. 4-4influence phenotype Ex) Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Concept Check 4.3 ) Incomplete dominance and epistasis are both terms that define genetic relationships. What is the most basic distinction between these terms? 2) If a man with type AB blood marries a woman with type O blood, what blood types would you expect in their children? 3) A rooster with gray feathers is mated with a hen of the same phenotype. Among their offspring, 5 chicks are gray, 6 are black, and 8 are white. What is the simplest explanation for the inheritance of these colors in chickens? What phenotypes would you expect in the offspring of a cross between a gray rooster and a black hen?
Concept 4.4: Many human traits follow Mendelian patterns of inheritance
Human Genetics Humans are not good subjects for genetic research Generation time is too long (~20 years) Parents produce relatively few offspring Breeding experiments are unacceptable The study of human genetics continues to advance despite these constraints
Pedigree Analysis Geneticists must analyze results of matings that have already occurred Information about a family s history for a particular trait is assembled into a pedigree A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can thus be traced and described using pedigrees Pedigrees can also be used to make predictions about future offspring Can help calculate probability that a child will have a particular genotype and phenotype We can also use the multiplication and addition rules to predict the probability of specific phenotypes
Fig. 4-5b st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) Fig. 4-5a WW ww or Widow s peak Ww No widow s peak (a) Is a widow s peak a dominant or recessive trait? Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left)
Fig. 4-5c st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff or FF Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?
Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These disorders range in severity: Relatively mild: albinism (lack of skin pigmentation) Increases susceptibility to skin cancers and vision problems Fig. 4-6 Life-threatening: cystic fibrosis Parents Normal Normal Aa Aa Sperm Eggs A a A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino
The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal Most people with recessive disorders are born to parents that are carriers of the disorder If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele People with recent common ancestors are more likely to carry the same recessive alleles Indicated in pedigrees by double lines Most societies and cultures have laws or taboos against marriages between close relatives
Cystic Fibrosis Cystic fibrosis (CF) is the most common lethal genetic disease in the United States /25 (4%) people of European descent are carriers of CF allele Strikes one out of every 2,500 people of European descent Normal allele for this gene codes for a membrane protein that is involved in transport of chloride ions between cells and extracellular fluid The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes Results in abnormally high concentration of extracellular chloride Causes mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine If untreated, most children infected with CF die before the age of 5
Sickle-Cell Disease Sickle-cell disease is the most common inherited disorder among African Americans Affects one out of 400 African-Americans (/0 are carriers) Frequency of recessive allele can be explained by the observation that a single copy of sickle-cell allele reduces frequency and severity of malaria attacks Caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Sickled cells may clump and clog small blood vessels Symptoms include physical weakness, pain, organ damage, and even paralysis
Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Lethal dominant alleles often cause death of offspring prior to maturity and reproduction Fig. 4-7 Ex) Achondroplasia is a form of dwarfism caused by a rare dominant allele Occurs in /25,000 people Eggs d Parents Dwarf Dd Normal dd Sperm D d Dd dd Dwarf Normal d Dd Dwarf dd Normal
Huntington s Disease Lethal dominant alleles can escape elimination if they do not become apparent until more advanced ages Ex) Huntington s disease is an irreversible and fatal degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 45 years of age Affects /0,000 people in the US Geneticists have tracked Huntington s allele to a locus near the tip of chromosome 4 Sequencing of this gene has allowed development of a test to detect the presence of this allele
Multifactorial Disorders Many diseases have both genetic and environmental components Known as multifactorial disorders Include heart disease, diabetes, cancer, alcoholism, schizophrenia, and bipolar disorder Hereditary component is usually polygenic Ex) Many genes affect cardiovascular health, making some individuals more prone to heart attacks and strokes Little is understood about the genetic contribution to most multifactorial diseases The best public health strategy seems to be educating people about the importance of environmental factors and promoting healthy behaviors
Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders Ex: What is the probability that a couple will have a child with a recessively inherited disorder if both of their brothers died of this disorder? (2/3) x (2/3) x (/4) = /9 Ex: What is the probability that this same couple will have a child with the disorder if they already have a child with the disease? (/2) x (/2) = /4
Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately These tests are already available for: Tay-Sachs disease Sickle-cell disease Cystic fibrosis The tests allow people with family histories of genetic disorders to make informed decisions about having children These tests also pose potential problems Breaches of confidentiality Stigmatization of carriers Discrimination by health and life insurance companies, as well as employers
Fetal Testing - Amniocentesis Tests performed in conjunction with a technique called amniocentesis can determine if a developing fetus has a genetic disorder In amniocentesis, the liquid that bathes the fetus is removed and tested Can usually be performed during the 4 th -6 th week of pregnancy Some disorders can be detected from the presence of certain chemicals in the fluid Fig. 4-8 Fetus Amniotic fluid withdrawn Placenta Uterus Cervix Fluid Fetal cells (a) Amniocentesis Centrifugation Several hours Several weeks Several weeks Biochemical tests Placenta Karyotyping Chorionic villi Several hours Several hours Fetus Fetal cells Suction tube inserted through cervix (b) Chorionic villus sampling (CVS) Tests for other disorders require that the amniotic fluid be cultured to produce cells
Fetal Testing: Chorionic Villus Sampling In chorionic villus sampling (CVS), a sample of the placenta is removed and tested Fig. 4-8 Amniotic fluid withdrawn Physician inserts narrow tube through cervix and into the uterus and suctions out a tiny sample The cells of the chorionic villi of the placenta (the portion sampled) are derived from fetus Fetus Placenta Uterus Cervix Fluid Fetal cells (a) Amniocentesis Centrifugation Several hours Several weeks Several weeks Biochemical tests Placenta Karyotyping Chorionic villi Several hours Several hours Fetus Fetal cells Suction tube inserted through cervix (b) Chorionic villus sampling (CVS) These cells have the same genotype as the new individual These cells also divide rapidly, allowing for quicker karyotyping and analysis Can be performed as early as the 8 th -0 th week of pregnancy
Fetal Testing: Imaging Techniques Other techniques allow fetal health to be assessed visually in utero Ultrasound: sound waves are used to produce an image of the fetus Carries no known risk to mother or fetus Fetoscopy: a needle-thin tube containing a viewing scope and fiber optics (to transmit light) is inserted into uterus
Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States Include screening for phenylketonuria (PKU) Recessively inherited disorder affecting one out of every 0,000-5,000 children in the US Affected children cannot properly metabolize the amino acid phenylalanine Phenylalanine and its byproduct (phenylpyruvate) accumulate to toxic levels in blood, causing mental retardation If deficiency is detected in newborns, a special diet low in phenylalanine will usually allow normal development and prevent retardation
Concept Check 4.4 ) Beth and Tom each have a sibling with cystic fibrosis, but neither Beth nor Tom nor any of their parents have the disease. Calculate the probability that if this couple has a child, the child will have CF. What would be the probability if a test revealed that Tom is a carrier but Beth is not? 2) Joan was born with 6 toes on each foot, a dominant trait called polydactyly. Two of her 5 siblings and her mother, but not her father, also have extra digits. What is Joan s genotype for the number-of-digits character? Explain your answer. Use D and d to symbolize the alleles for this character. 3) What would you suspect if Peter was born with polydactyly, but neither of his biological parents had extra digits?
You should now be able to:. Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F generation, F 2 generation 2. Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype 3. Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F 2 generation Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
4. Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance 5. Define and give examples of pleiotropy and epistasis 6. Explain why lethal dominant genes are much rarer than lethal recessive genes 7. Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings