Chapter 12 - Mendel and Heredity PDF

Chapter 12 Mendel & Heredity Be able to: Describe Mendel’s experiments showing that different alleles of a genetic trait can segregate into different gametes. Explain how Mendel’s results demonstrated that alleles of a genetic trait behaved as discrete particles and were not blended togeth...

Chapter 12 Mendel & Heredity Be able to: Describe Mendel’s experiments showing that different alleles of a genetic trait can segregate into different gametes. Explain how Mendel’s results demonstrated that alleles of a genetic trait behaved as discrete particles and were not blended together. Distinguish between the genotype and phenotype of an organism. Use a test cross to determine the genotype of an organism. Explain how a dihybrid cross demonstrates that alleles of one gene can sort independently of alleles for another, unlinked gene. Use probability rules to predict the outcomes of monohybrid and dihybrid crosses. Explain how the concept of dominance, codominance, incomplete dominance and multiple alleles influences phenotype. Use pedigree analysis to determine inheritance patterns of genes. Be able to: Explain how chromosome behavior in meiosis reflects inheritance patterns seen by Mendel (segregation of maternal and paternal alleles and independent assortment of different genes into gametes). Describe how genes on sex chromosomes have a different pattern of inheritance than genes on autosomes. CHAPTER 12: Mendel and Heredity What genetic principles account for the transmission of traits from parents to offspring?  One possible explanation of heredity is a “blending” hypothesis – The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green  An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea – Parents pass on discrete heritable units, genes  Gregor Mendel – Documented a particulate mechanism of inheritance through his experiments with garden peas  Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity – By breeding garden peas in carefully planned experiments Mendel’s Experimental, Quantitative Approach  Mendel’s Experimental, Quantitative Approach Mendel chose to work with peas – Because they are available in many varieties – Because he could strictly control which plants mated with which – You can eat peas! Crossing pea plants 1 Removed stamens from purple flower APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of 2 inheritance. In this example, Mendel crossed pea plants Transferred sperm- that varied in flower color. bearing pollen from stamens of white flower to egg- bearing carpel of purple flower TECHNIQUE TECHNIQUE Parental generation (P) Stamens Carpel (male) (female) 3 Pollinated carpel matured into pod 4 Planted seeds from pod TECHNIQUE RESULTS When pollen from a white flower fertilizes 5 Examined eggs of a purple flower, the first-generation hybrids all have purple offspring: flowers. The result is the same for the reciprocal cross, the transfer First all purple of pollen from purple flowers to white flowers. generation flowers offspring (F1) Vocabulary Some genetic vocabulary – Character: a heritable feature, such as flower color – Trait: a variant of a character, such as purple or white flowers  Mendel chose to track – Only those characters that varied in an “either-or” manner Mendel also made sure that – He started his experiments with varieties that were “true- breeding” Eg. White X White = white  In a typical breeding experiment – Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents – Are called the P generation  The hybrid offspring of the P generation – Are called the F1 (Filial) generation When F1 individuals self-pollinate – The F2 generation is produced P x F1 F2 P (Self cross) The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants – All of the offspring were purple When Mendel crossed the F1 plants – Many of the plants had purple flowers, but some had white flowers  Mendel discovered – A ratio of about three to one, purple to white flowers, in the F2 generation P Generation  (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation 705 purple 224 white  Mendel reasoned that – In the F1 plants, only the purple flower factor was affecting flower color in these hybrids – Purple flower color was dominant, and white flower color was recessive  Mendel observed the same pattern – In many other pea plant characters Mendel’s Model Mendel developed a hypothesis – To explain the 3:1 inheritance pattern that he observed among the F2 offspring Four related concepts make up this model 1. First, alternative versions of genes – Account for variations in inherited characters, which are now called alleles 2. Second, for each character – An organism inherits two alleles; one from each parent – A genetic locus is actually represented twice 3. Third, if the two alleles at a locus differ – Then one, the dominant allele, determines the organism’s appearance – The other allele, the recessive allele, has no noticeable effect on the organism’s appearance 4. Fourth, the Law of Segregation – The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes  Mendel discovered – A ratio of about three to one, purple to white flowers, in the F2 generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- P Generation  pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers RESULTS Both purple-flowered plants and white- flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white. F2 Generation 705 purple 224 white Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? – We can answer this question using a Punnett square  Mendel’s law of segregation, probability and the Punnett square P = Purple allele P Generation  p = White allele Appearance (phenotype): Purple flowers Genetic makeup (genotype): PP White flowers pp Meiosis P p Gametes: Fertilization F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1/ 1/ 2 P 2 p F1 sperm P p F2 Generation P PP Pp Punnett F1 eggs square p Pp pp 3 : 1 Useful Genetic Vocabulary An organism that is homozygous for a particular gene – Has a pair of identical alleles for that gene – Exhibits true-breeding Ex. PP An organism that is heterozygous for a particular gene – Has a pair of alleles that are different for that gene Ex. Pp More Vocabulary… An organism’s phenotype – Is its physical appearance (eg. Purple flowers) An organism’s genotype – Is its genetic makeup (eg. PP )  Mendel’s law of segregation, probability and the Punnett square P = Purple allele P Generation  p = White allele Appearance (phenotype): Purple flowers Genetic makeup (genotype): PP White flowers pp Meiosis P p Gametes: Fertilization F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1/ 2 P 1/ 2 p F1 sperm P p F2 Generation P PP Pp Punnett F1 eggs square p Pp pp Offspring: 3 : 1 Phenotype versus genotype Phenotype Genotype Purple PP 1 (homozygous) Pp 3 Purple (heterozygous) 2 Pp (heterozygous) Purple pp 1 White (homozygous) 1 Ratio 3:1 Ratio 1:2:1 Hmmm… So I’ve got a pea plant with purple flowers. Do I know the genotype? How could you figure it out? The Testcross In pea plants with purple flowers – The genotype is not immediately obvious The Testcross A testcross – Allows us to determine the genotype of an organism with the dominant phenotype, but an unknown genotype – Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait P_ X pp The testcross Testcross  Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP? or Pp? pp If PP, If Pp, then all offspring then 1⁄2 offspring purple purple: and 1⁄2 offspring white: p p p p P P Pp Pp Pp Pp P p Pp Pp pp pp The Law of Segregation Mendel derived the law of segregation (the two alleles of a gene separate during gamete formation) – By following a single trait The F1 offspring produced in this cross – Were monohybrids, heterozygous for one character The Law of Independent Assortment Mendel identified his 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 F1 generation, heterozygous for both characters  How are two characters transmitted from parents to offspring? – As a package? – Independently?  A dihybrid cross – Illustrates the inheritance of two characters Produces four phenotypes in the F2 generation  Mendel’s law of segregation, probability and the Punnett square P = Purple allele P Generation  p = White allele Appearance (phenotype): Purple flowers Genetic makeup (genotype): PP White flowers pp Meiosis P p Gametes: Fertilization F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1/ 1/ 2 P 2 p F1 sperm P p F2 Generation P PP Pp Punnett F1 eggs square p Pp pp 3 : 1 Law of Independent Assortment Using the information from a dihybrid cross, Mendel developed the law of independent assortment – Each pair of alleles segregates independently from others during gamete formation  P Generation YYRR yyrr Gametes YR  yr F1 Generation YyRr Hypothesis of Hypothesis of dependent independent assortment assortment Sperm 1 ⁄4 YR 1 ⁄4 Yr 1 ⁄4 yR 1 ⁄4 yr Sperm 1⁄ Eggs 1⁄ YR 2 2 yr 1 ⁄4YR Eggs YYRR YYRr YyRR YyRr 1 ⁄ 2 YR F2 Generation YYRR YyRr (predicted 1 ⁄4 Yr YYRr YYrr YyRr Yyrr offspring) 1 ⁄2yr YyRr yyrr 1 ⁄4 yR YyRR YyRr yyRR yyRr 3 ⁄4 1 ⁄4 1 ⁄4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 ⁄16 3 ⁄16 3 ⁄16 1 ⁄16 Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 (64 boxes!) (256 boxes!) (1024 boxes!)  The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment – Reflect the rules of probability The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule – States that the probability that two or more independent events will occur together is the product of their individual probabilities  Probability in a Rr  Rr Segregation of Segregation of monohybrid cross alleles into eggs alleles into sperm – Can be Sperm determined r using this rule R 1⁄ 1⁄ 2 2 Probability of getting two R R R R r heads (homozygote = RR): 1⁄ 2 1⁄ 1⁄ 4 (1/2) X (1/2) = 1/4 Eggs 4 r r 1⁄ r R r 2 1⁄ 1⁄ 4 4  The rule of addition – States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities 1/2 1/2 R r Probability of a heterozygote (Rr): R RR Rr (1/2)(1/2) + (1/2)(1/2) = 1/2 or 1/2 1/4 1/4 (1/4) + (1/4) = 2/4 = 1/2 rR r r r 1/2 1/4 1/4 Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probability – 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 (ie. independent events) In calculating the chances for various genotypes from such crosses – Each character first is considered separately and then the individual probabilities are multiplied together The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four genes, what are the probabilities that F2 offspring will have the following genotypes? 1. aabbccdd 2. AaBBCCDd The Bad News… Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely simple  Extending Mendelian Genetics for a Single Gene The inheritance of characters by a single gene – May deviate from simple Mendelian patterns  Confounding Features of Inheritance (CFI): The Spectrum of Dominance Complete dominance – Occurs when the phenotypes of the heterozygote and dominant homozygote are identical Like purple pea flowers CFI: The Spectrum of Dominance In codominance – Two dominant alleles affect the phenotype in separate, but distinguishable, ways The human blood group MN – Is an example of codominance  P Generation In incomplete dominance Red  White CWCW CRCR – The phenotype of F1 hybrids Gametes CR is somewhere between the CW phenotypes of the two parental varieties Pink F1 Generation CRCW 1⁄ 1⁄ 2 2 W Gametes C R C Eggs ⁄2C ⁄2C Sperm 1 R 1 W F2 Generation 1⁄ R 2C CR CR CR CW 1⁄ W 2C CR CW CW CW The Relation Between Dominance and Phenotype Dominant and recessive alleles – Do not really “interact” – Lead to synthesis of different proteins that produce a phenotype Dominant Alleles Frequency of Dominant Alleles Dominant alleles – Are not necessarily more common in populations than recessive alleles Frequency of Dominant Alleles Dominant alleles – Are not necessarily more common in populations than recessive alleles Hemmingway’s home CFI: Multiple Alleles Most genes exist in populations – In more than two allelic forms CFI: Multiple Alleles The ABO blood group in humans – Is determined by multiple alleles Carbohydrates on the surface of the cell Multiple alleles for a single trait showing an order of dominance Four different alleles exist for the rabbit coat color (C) gene. C is dominant to cch cch is dominant to ch ch is dominant to c CFI: Pleiotropy In pleiotropy – An allele has multiple phenotypic effects Ex. In sickle cell disease : 1. Deformed red blood cells 2. Physical weakness 3. Pain and organ damage CFI: Extending Mendelian Genetics for Two or More Genes Some traits – May be determined by two or more genes CFI: Epistasis In epistasis – A gene at one locus alters the phenotypic expression of a gene at a second locus An example of epistasis: --coat color in Labrador retrievers B locus determines the color of the pigment E locus determines if it is deposited in the hairs CFI: Polygenic Inheritance Many human characters – Vary in the population along a continuum and are called quantitative characters  Quantitative variation usually indicates polygenic inheritance – An additive effect of two or more genes on a single phenotype  AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20⁄ 64 15⁄ 64 6⁄ 64 1⁄ 64 Quantitative Traits… CFI: Nature and Nurture The Environmental Impact on Phenotype Another departure from simple Mendelian genetics arises – When the phenotype for a character depends on environment as well as on genotype Multifactorial characters – Are those that are influenced by both genetic and environmental factors  The norm of reaction – Is the phenotypic range of a particular genotype that is influenced by the environment (Alkaline soil) (Acidic soil) The effect of pH on Hydrangea (same genotype!) Integrating a Mendelian View of Heredity and Variation An organism’s phenotype – Includes its physical appearance, internal anatomy, physiology, and behavior – Reflects its overall genotype and unique environmental history  Even in more complex inheritance patterns – Mendel’s fundamental laws of segregation and independent assortment still apply!!!  Mendelian inheritance has its physical basis in the behavior of chromosomes Several researchers proposed in the early 1900s that genes are located on chromosomes The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment The chromosome theory of inheritance states that: Mendelian genes have specific loci on chromosomes Chromosomes undergo segregation and independent assortment The chromosomal basis of Mendel’s laws  Thomas Hunt Morgan – Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors – See biographical sketch on Canvas, if interested Morgan’s Choice of Experimental Organism Morgan worked with fruit flies – Because they breed at a high rate – A new generation can be bred every two weeks – They have only four pairs of chromosomes  Morgan first observed and noted – Wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type – Are called mutant phenotypes Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – The F1 generation all had red eyes – The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes  Morgan determined – That the white-eye mutant allele must be located on the X chromosome EXPERIMENT Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes. P X Generation F1 Generation Morgan then bred an F1 red-eyed female to an F1 red-eyed male to produce the F2 generation. RESULTS The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. F2 Generation  W+ W X X CONCLUSION Since all F1 offspring had P X X Y red eyes, the mutant Generation W+ white-eye trait (w) must be recessive to the wild-type red-eye trait (w+). Since the recessive trait—white eyes—was expressed only in males in the F2 generation, Morgan hypothesized that the eye-color gene is W Ova located on the X chromosome and that there (eggs) Sperm is no corresponding locus on the Y chromosome, as diagrammed here. F1 W+ W+ W+ Generation W W+ Ova (eggs) Sperm F2 W+ W+ Generation W+ W+ W W W W+ What’s the point? Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait – Was the first solid evidence indicating that a specific gene is associated with a specific chromosome – Earned him the Nobel Prize in Physiology or Medicine in 1933 The Chromosomal Basis of Sex An organism’s sex – Is an inherited phenotypic character determined in large part by the presence or absence of certain chromosomes In humans and other mammals – There are two varieties of sex chromosomes, X and Y SRY (Sex determining Region Y) gene is required for testicular development 44 + Parents 44 + XY XX 22 + Sperm 22 + Ova 22 + X Y X 44 + Zygotes 44 + XX (offspring) XY (a) The X-Y system  Different systems of sex determination – Are found in other organisms 22 + 22 + XX X (b) The X–0 system 76 + 76 + ZW ZZ (c) The Z–W system 16 16 (Diploid) (Haploid) (d) The haplo-diploid system Inheritance of Sex-Linked Genes The sex chromosomes – Have genes for many characters unrelated to sex A gene located on either sex chromosome – Is called a sex-linked gene Sex-linked genes exhibit unique patterns of inheritance  Sex-linked genes (a) A father with the disorder will transmit the XAXA XaY mutant allele to all daughters but to no sons. – Follow specific When the mother is a dominant homozygote, the daughters will have the normal phenotype Xa Y Sperm patterns of but will be carriers of the mutation. Ova XA XAXa XAY inheritance XA X X X Y A a A XAXa  XAY (b) If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a Sperm XA Y carrier like her mother, and a 50% chance that each son will have the Ova XA XAXA XAY disorder. Xa XaXA XaY (c) If a carrier mates with a male who has XAXa  XaY the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Sperm Daughters who do not have the Xa Y disorder will be carriers, where as males without the disorder will be Ova XA XAXa XAY completely free of the recessive allele. Xa XaXa XaY  Sex-linked genes – Follow specific patterns of inheritance Some recessive alleles found on the X chromosome in humans cause certain types of disorders – Color blindness – Duchenne muscular dystrophy – Hemophilia  Many human traits follow Mendelian patterns of inheritance Humans are not convenient subjects for genetic research – However, the study of human genetics continues to advance Pedigree Analysis A pedigree – Is a family tree that describes the interrelationships of parents and children across generations Tips for working with pedigrees: If parents without the trait have offspring with the trait, the trait must be recessive, and the parents carriers. If the trait is seen in every generation, it is likely dominant. If both parents have the trait, and the trait is recessive, all offspring will have the trait. Start by labeling the likely genotype of all family members that you can—even if it is just P_  Inheritance patterns of particular traits – Can be traced and described using pedigrees First generation Ww ww ww Ww (grandparents) Ff Ff ff Ff Second generation (parents plus aunts Ww ww ww Ww Ww ww and uncles) FF or Ff ff ff Ff Ff ff Third WW ww generation ff FF (two sisters) or or Ww Ff Widow’s peak No Widow’s peak Attached earlobe Free earlobe = male affected (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe) = female affected Hemophilia: An Example In this pedigree, only males are affected, and sons do not share the phenotypes of their fathers. – Thus, hemophilia is linked to a sex chromosome–the X. Expression of hemophilia skips generations. – Thus, it is recessive.  Pedigrees – Can also be used to make predictions about future offspring Recessively Inherited Disorders Many genetic disorders – Are inherited in a recessive manner 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  Cystic Fibrosis—a recessively inherited disease Symptoms of cystic fibrosis include: – Mucus buildup in some internal organs – Abnormal absorption of nutrients in the small intestine Sickle-Cell Disease Sickle-cell disease – Affects one out of 400 African- Americans – Is caused by the substitution of a single amino acid in the β-hemoglobin protein in red blood cells Symptoms include – Physical weakness, pain, organ damage, and even paralysis Sickle-Cell Disease Mating of Close Relatives Matings between relatives – Can increase the probability of the appearance of a genetic disease – Are called consanguineous matings Dominantly Inherited Disorders Some human disorders – Are due to dominant alleles Achondroplasia—a dominantly inherited trait One example is achondroplasia – A form of dwarfism that is lethal when homozygous for the dominant allele Huntington’s disease – Is a degenerative disease of the nervous system – Has no obvious phenotypic effects until about 35 to 40 years of age – Is caused by the presence of an allele on chromosome 4—we can test for this now Nancy Wexler Multifactorial Disorders Many human diseases – Have both genetic and environment components Examples include – Heart disease and cancer Genetic Testing and Counseling Genetic counselors – Can provide information to prospective parents concerned about a family history for a specific disease Counseling Based on Mendelian Genetics and Probability Rules Using family histories – Genetic counselors help couples determine the odds that their children will have genetic disorders Tests for Identifying Carriers For a growing number of diseases – Over 4,500 tests are available that identify carriers and help define the odds more accurately Alpha-1-antitrypsin deficiency (AAT; emphysema and liver disease) Amyotrophic lateral sclerosis (ALS; Lou Gehrig's Disease; progressive motor function loss leading to paralysis and death) Alzheimer's disease* (APOE; late-onset variety of senile dementia) Ataxia telangiectasia (AT; progressive brain disorder resulting in loss of muscle control and cancers) Gaucher disease (GD; enlarged liver and spleen, bone degeneration) Inherited breast and ovarian cancer* (BRCA 1 and 2; early-onset tumors of breasts and ovaries) Hereditary nonpolyposis colon cancer* (CA; early-onset tumors of colon and sometimes other organs) Charcot-Marie-Tooth (CMT; loss of feeling in ends of limbs) Congenital adrenal hyperplasia (CAH; hormone deficiency; ambiguous genitalia and male pseudohermaphroditism) Cystic fibrosis (CF; disease of lung and pancreas resulting in thick mucous accumulations and chronic infections) Duchenne muscular dystrophy/Becker muscular dystrophy (DMD; severe to mild muscle wasting, deterioration, weakness) Dystonia (DYT; muscle rigidity, repetitive twisting movements) Fanconi anemia, group C (FA; anemia, leukemia, skeletal deformities) Factor V-Leiden (FVL; blood-clotting disorder) Fragile X syndrome (FRAX; leading cause of inherited mental retardation) Hemophilia A and B (HEMA and HEMB; bleeding disorders) Hereditary Hemochromatosis (HFE; excess iron storage disorder) Huntington's disease (HD; usually midlife onset; progressive, lethal, degenerative neurological disease) Myotonic dystrophy (MD; progressive muscle weakness; most common form of adult muscular dystrophy) Neurofibromatosis type 1 (NF1; multiple benign nervous system tumors that can be disfiguring; cancers) Phenylketonuria (PKU; progressive mental retardation due to missing enzyme; correctable by diet) Adult Polycystic Kidney Disease (APKD; kidney failure and liver disease) Prader Willi/Angelman syndromes (PW/A; decreased motor skills, cognitive impairment, early death) Sickle cell disease (SS; blood cell disorder; chronic pain and infections) Spinocerebellar ataxia, type 1 (SCA1; involuntary muscle movements, reflex disorders, explosive speech) Spinal muscular atrophy (SMA; severe, usually lethal progressive muscle-wasting disorder in children) Thalassemias (THAL; anemias - reduced red blood cell levels) Tay-Sachs Disease (TS; fatal neurological disease of early childhood; seizures, paralysis) [3/99] Fetal Testing In amniocentesis – The liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS) – A sample of the placenta is removed and tested Cell-free, maternal blood screening – Based on fetal DNA in the mother’s bloodstream. Is a screen, not diagnostic. Fetal testing 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 Ex. PKU (phenylketonuria) U.S. Air Force photo/Staff Sgt Eric T. Sheler - Phenylalanine USAF Photographic Archives Tyrosine  GINA Genetic Information Nondiscrimination Act Be able to: Describe Mendel’s experiments showing that different alleles of a genetic trait can segregate into different gametes. Explain how Mendel’s results demonstrated that alleles of a genetic trait behaved as discrete particles and were not blended together. Distinguish between the genotype and phenotype of an organism. Use a test cross to determine the genotype of an organism. Explain how a dihybrid cross demonstrates that alleles of one gene can sort independently of alleles for another, unlinked gene. Use probability rules to predict the outcomes of monohybrid and dihybrid crosses. Explain how the concept of dominance, codominance, incomplete dominance and multiple alleles influences phenotype. Use pedigree analysis to determine inheritance patterns of genes. Be able to: Explain how chromosome behavior in meiosis reflects inheritance patterns seen by Mendel (segregation of maternal and paternal alleles and independent assortment of different genes into gametes). Describe how genes on sex chromosomes have a different pattern of inheritance than genes on autosomes.