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Heredity AP Biology: Chapters 14- 15 3/10/16 Genetic Problems • Most of the material in this unit prepares you for solving genetic problems. • A genetics problem is an analysis of the characteristics (traits) of parents and offspring. • Given the traits of one of these generations, you are required to determine the traits of the other generation. Probability Rules • If a coin is tossed, there is a ½ (50%) chance, or probability that it will be heads. • If a coin is tossed again, there is again, a ½ (50%) chance that it will be heads. • The first toss does not affect the second toss, that it, the two tosses are independent. Probability Rules • To determine the probability of two or more independent event occurring together, you merely multiply the probabilities of each event happening separately. • This is the rule of multiplication of probability. • For 2 consecutive tosses of a coin, the probability of getting 2 heads is ½ x ½ = ¼ • For 3 tosses, the probability of 3 heads would be ½ x ½ x ½ = 1/8 Genetic Terms: • A gene represents the genetic material on a chromosome that contains instructions for creating a particular trait. (coding for a protein) • An allele is one of several varieties of a gene. In pea plants, for example, there are two alleles of the gene for flower color: purple allele and white allele. Genetic Terms: • A locus refers to the location on a chromosome where a gene is located. Every gene has a unique locus on a particular chromosome. • Every cell contains two copies of each chromosome, one inherited from each parent. This pair is called a homologous pair. 3/10 Homologues • This pair of chromosomes (homologues) each contains a gene for the same trait exactly at the same loci. • At any one particular locus, the two genes on a pair of homologues may represent the different alleles for that gene because they originated from different parents. Genetic Terms: • If the two alleles inherited for a gene are different, one allele may be dominant, while the other is recessive. • The trait coded by the dominant allele is the actual trait expressed. In pea plants, the purple flower is dominant, the white is recessive. Genetic Terms: • Homozygous dominant refers to the inheritance of two dominant alleles (PP). In this case, the dominant trait is expressed. • In the homozygous recessive condition, two recessive alleles are inherited (pp), and the recessive trait is expressed. • Heterozygous refers to the condition where the two inherited alleles are different (Pp). In this condition, only the dominant allele is expressed. Genetic Terms: • The phenotype is the actual expression of a gene. Purple flowers, blue eyes, and brown hair. • The genotype represents the actual alleles (genetic make up). For example, PP describes the genotype for the homozygous dominant condition. If P represents the allele for purple flowers, and p represents the allele for white flowers, the genotype Pp would express the phenotype of purple flowers. Law of Segregation • Remember, during Meiosis I, the two members of a homologous pair of chromosomes migrate to opposite poles. • As a result, each gamete will contain one allele for each gene. This is random! • Thus, the Law of Segregation refers to this random segregation of alleles and their chromosomes to separate gametes. Law of Independent Assortment • In addition, the migration of homologues within one pair of homologous chromosomes to opposite poles does not influence the migration of any OTHER homologous pair. • Each homologue will assort to one pole or the other of a cell separately and independently of each other. This is called independent assortment. Segregation & Assortment • Because both laws of segregation and independent assortment are random processes, the rules of probability can be used to describe how the different chromosomes (and their alleles) in parents assemble in gametes and offspring. Mendel • Mendel, a 19th century monk, is credited with the discovery the laws of segregation and independent assortment • In his experiments, he crossed two varieties of pea plants to form offspring, or hybrids. • A monohybrid cross involves only one trait (color or size, etc) Monohybrid Crosses: • In monohybrid crosses, a single trait is examined. • Using pea plants as an example, let P represent the allele for purple flowers and p represents the allele for white flowers. • Suppose a plant heterozygous for purple flowers (Pp) is crossed with a white flowered plant (pp). The cross can be represented by the genotypes Pp x pp Punnett Square: One parent along the top, one parent along the side. p p P Pp Pp P Pp Pp Punnett Square • In a Punnett square for a monohybrid cross, the gametes for one parent are represented in two spaces a the top of the diagram. • The gametes for the second parent are represented along the left side of the diagram. • Combine the top and side genotypes and place them in the corresponding box. Punnett Square • Can you find the frequencies of the F2 generation for the cross of Pp x Pp? • ¼ are:_______ • ½ are: ______ • ¼ are: ______ P P p p Understanding check point • Yellow • Cross a homozygous yellow pages? pea with a green pea • Cross a heterozygous Tall pea with a short pea • Cross a Heterozygous Purple flower with a Homozygous Purple flower • Cross a Homozygous Round pea with a wrinkled pea Test Cross • Suppose you wanted to know the genotype of a white- flowered pea plant. – That’s easy, since white is recessive, and the only way to get white is with pp. • Suppose you wanted to know the genotype of a purple- flowered pea plant. Is it PP or Pp? – You must perform a Test Cross to determine the unknown genotype. Test Cross - p. 267 • To determine the unknown genotype, you cross the unknown with a genotype that is known. (pp). – Always cross your unknown with double recessive! • This is represented as: P__ x pp. – Since we do not know the other allele, we use an underscore. – To determine the unknown allele we use a Punnett Square and do a PP x pp cross and a Pp x pp cross. Dihybrid Crosses • In a dihybrid cross, genes for two different traits are observed. • For pea plants a dihybrid cross might observe flower color (white/purple) and plant height (short/tall) • Another example of a dihybrid cross might involve two characteristics of seeds: color and texture. Dihybrid Crosses • If Y and y are used to represent the two alleles yellow (Y) and green (y) and the letters (R) for round and (r) for wrinkled what would the cross look like? • A cross between one pea plant homozygous for both traits and second plat homozygous recessive for both traits would be: – YYRR x yyrr Dihybrid Cross w/ Punnett Square • Use the distributive property to distribute the alleles and write the pairs along the top and side of the Punnett Square: – YYRR x yyrr – YR, YR, YR, YR – yr, yr, yr, yr • Example ? • Blue packet questions? • Note the F2 generation phenotypic ratio •9:3:3:1 • Usual ratio with dihybrid hetero cross Problems - Sample • Cross a green, heterozygous round pea plant with a homozygous yellow and heterozygous round pea plant. – Determine parents genotype – Determine the gamete combination possibility – Use punnett square or math to determine the Genotypic RATIO and the Phenotypic RATIO (GR / PR) of the offspring What did you get? • Parents Genotypes – yyRr – YYRr • Determine gamete combinations – yR and yr – YR and Yr • (½ yR + ½ yr) X (½ YR + Yr) OR Punnett Square Punnett Square method YR yR YyRR yr YyRr Yr YyRr Yyrr Final Answer? Genotypic Ratio • ¼ YyRR • ¼ YyRr • ¼ YyRr • ¼ Yyrr Phenotypic Ratio • ¾ Yellow and Round • ¼ Yellow and wrinkled Mode of Inheritance • Complete Dominance • Monohybrid & Dihybrid Crosses • Incomplete Dominance • Codominance • Multiple Alleles • Epistasis • Polygenic – ABO Blood typing Activity!!! Incomplete Dominance • Sometimes, the alleles for a gene do not exhibit the dominant and recessive behaviors above. Instead, the combined expression of two different alleles in the heterozygous condition produces a blending of the individual expressions. Incomplete Dominance • In snapdragons, for example, the heterozygous condition of one allele for red flowers (R) and one allele for white flowers (r) is pink (Rr). • White board problem: – Cross a white snapdragon with a pink snapdragon. Determine the GR & PR for offspring Codominance • Another kind of inheritance pattern is codominance. • In this pattern, both inherited alleles are completely expressed. • For example, the M and N blood types produce two molecules that appear on the surface of human red blood cells. • The M (or LM) allele produces a certain blood molecule. • The N (or LN) allele produces a different kind of molecule Codominance • Individuals who are MM (LMLM) produce one kind of molecule. • Individuals who are NN (LNLN) produce a second kind of molecule. • Individuals who are MN (LMLN) produce BOTH kinds of molecule (Codominance) • White Board Problem – Cross a MM with a MN Multiple Alleles • In the blood group that produces A,B, and O blood types, there are three possible alleles, represented by IA, IB, or i. • Superscripts are used because the two alleles, A and B are codominant. • A lower case i is used for the 3rd allele because it is recessive when expressed with IA or IB . • There are 6 possible genotypes representing all 3 alleles. Multiple Alleles • • • • • • • • • IAIA and IAi represent A blood type. IBIB and IBi represent B blood type. IAIB represents AB blood type. ii represents O blood type. The four phenotypes (A, B, AB, and O) correspond to the presence or absence an A or B carbohydrate glycoprotein on the surface of an RBC. IAIA/ IAi = A carbohydrate IBIB/ IBi = B carbohydrate IAIB = both carbohydrates ii = no carbohydrates ABO Blood Typing Activity • White Board Problem: – Cross an AB with a O blood type • Using synthetic blood… you will type 4 patients’ blood samples… • Finish some notes first before activity • Complete today! Epistasis • Epistasis occurs when one gene affects the phenotypic expression of a second gene. • This frequently occurs in the expression of pigmentation. • One gene turns on (or off) the production of pigment, while a second gene controls either the amount of pigment produced or the color of the pigment. • Dog example page 273 Epistasis – mice example • Epistasis occurs in the pigmentation of fur in mice. One gene codes for the presence or absence of pigmentation. A second gene codes for the color (black or brown) of the fur. • Thus C and c represent the alleles for the presence or absence of color. • B and b represent the alleles for black or brown fur. • A mouse has to inherit at least one C to have colored fur, while the B or b determines what color. • What fur type is observed with a genotype of cc? Pleiotropy • Pleiotropy occurs when a single gene has more than one phenotypic expression. • For example, the gene in pea plants that expresses the round or wrinkled texture of seeds also influences the phenotypic expression of starch metabolism and water absorption. Pleiotropy • The allele for round seeds codes for a greater conversion of glucose to starch than does the allele for wrinkled seeds. • In wrinkled seeds, then, there is more unconverted glucose. • A higher concentration of glucose increases osmotic gradient, which increases the absorption of water. • Immature wrinkled seeds thus contain more water. When these seeds dehydrate, the water is lost, which results in a wrinkled pattern. Pleiotropy • Many disease-causing genes exhibit pleiotropy. • Sickle-cell anemia, a human blood disease, is caused by an allele that incorrectly codes for hemoglobin, valine instead of glutamic acid… leads to rigid protein structure of hgb • As a result, the abnormal hemoglobin molecule causes the RBC to become sickle shaped, especially in low Oxygen conc. • In response, the RBC’s do not flow through capillaries freely and oxygen is not adequately delivered throughout the body. Polygenic Inheritance –3/13/15 • Polygenic – more than one gene effecting a single phenotype such as skin color • Additive effect of two or more genes • AABBCC --- very dark skin color • Note graphic on p274 (simplified model of skin color inheritance) p274 Need extra practice?? • More genetic problem practice? • Check out Page 282 for extra questions and review… • Includes helpful suggestions as well. Nature vs Nurture • Effect of environment on phenotype occurs as well • Note fig 14.14 (p274) – hydrangea flower’ color varies due to pH of the soil • What about skin color variation due to environment? Blood typing Activity NOW! Pedigree 3/13/15 • Family Tree record that is used to determine the genotypes and phenotypes of related organisms • Essential to research of a genetic condition • P 276 Using Pedigrees • Check out Pedigree A and Pedigree B • Work on worksheet together Technology provides tools • Genetic counselors in many hospitals can provide information to prospective parents concerned about a family history for a genetic disorder • Fetal testing can be done to determine the presence of a disease on a fetus • Chorionic Villus Sampling (CVS) provides a small amount of fetal tissue to karyotype quickly • PKU testing determines the presence of phenylketonuria ???????not done 2015???? • Genetic Disorders ?????… • You will be doing a report of a genetic disorder or condition of your choice. – Present/poster walk to class next Friday – 3/30/12 The following describes a few interesting additional points regarding Genetic Inheritance Recessively inherited disorders • Recessive alleles that cause human disorders are usually defective versions of normal alleles • Defective alleles code for either a malfunctional or no protein • Heterozygotes can be phenotypically normal, if one copy of the normal allele is all that is needed to produce sufficient quantities of specific protein • (example – Sickle Cell Carrier) ?Recessively inherited disorders • Recessively inherited disorders range in severity from nonlethal traits (albinism) to lethal disease (cystic fibrosis). Since these disorders are caused by recessive alleles: – The phenotypes are expressed ONLY in the recessive homozygotes (aa) who inherit one recessive allele from each parent – Heterzygotes (Aa) can be phenotypically normal and act as carriers possibly transmitting the recessive allele to their offspring • Most people with recessive disorders are born to unaffected parents, both of whom are carriers ?Recessively inherited disorders • Human genetic disorders that are recessively inherited include: – Cystic Fibrosis – Tay-Sachs – Sickle-Cell Anemia • The probability of inheriting a recessively inherited disorder is greater if the parents are closely related. ?Dominantly inherited disorders • Some human disorders are dominantly inherited – For example, achondroplasia (a type of dwarfism) affects 1 in 10,000 people who are heterozygous for this gene – Homozygous dominant condition results in spontaneous abortion of the fetus • Lethal dominant alleles are much rarer than lethal recessives • Late-acting lethal dominants can escape elimination of the disorder does not appear until an advanced age, after the afflicted individuals may have transmitted the lethal gene to their children. (Huntington’s disease) ?Genetic condition research • POSTER ACTIVITY WILL BE DONE WHEN MRS. SHAW RETURNS • Poster presentation – Poster Walk • Modeling college-level practice of showing others what you know and what you have researched • Note handout for detail info • You will evaluate 3 posters Chapter 15 begin 3/20/15 …start Figure 15.0 Chromosomes Relating Mendelism to Chromosomes: • Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles • A convergence of Genetics and Cytology (the study of cells and cell history) in the early 1900’s to describe chromosomal behaviors Relating Mendelism to Chromosomes: • Based on these observations, biologists developed the CHROMOSOME THEORY OF INHERITENCE. According to this theory: – Mendelian factors or genes are located on chromosomes – It is the chromosomes that segregate and independently assort Figure 15.2 The chromosomal basis of Mendel’s laws p287 Linked Genes • If two genes are on different chromosomes, such as seed color and seed texture of pea plants, the genes segregate independently of one another. • Linked genes are genes that reside on the SAME chromosome and thus cannot segregate because they are physically connected. • Genes that are linked are usually inherited together. Linked Genes • In the fruit fly, Drosophila melanogaster, flies reared in the laboratory have occasionally been found to exhibit abnormal traits. These traits originate from gene mutations, or molecular changes in the DNA. • Two such mutations, affecting body color and wing structure, are linked. Evidence for linked genes in Drosophila (p293) Figure 15.5a Recombination due to crossing over (p295) Linkage Map/Gene Distance • The greater the distance between two genes on a chromosome, the more places between the genes than the chromosome can break and thus the more likely the two genes will cross over during synapsis. • As a result, recombination frequencies are used to give a picture of the arrangement of genes on a chromosome. (The % of crossover is the recombination frequency/gene distance). Page 296 Morgan’s choice of an experimental organism • Wild-type: normal or most frequently observed phenotype • Mutant Phenotypes: phenotypes which are alternatives to the wild type due to mutations in the wildtype gene Sex Determination • Note that sex determination varies with organisms (p290) • Chromosomal Numbers vary • SRY Gene in humans – Required for development of testes – Found ONLY on Y chromosome *p290 *Sex-Linked Inheritance • There is one pair of homologous chromosomes in animals that does not have exactly the same genes. • These two chromosomes, the X and Y chromosomes are called the sex chromosomes , or autosomes. • Sex-linked genes (or X-linked) are genes that reside on the X chromosome. • Y-linked genes are possible, but few genes reside on the Y chromosome, Y-linkage is rare *p291 *Discovery of a sex linkage • After a year of breeding Drosophila to find variant phenotypes, Morgan discovered a single male fly with white eyes instead of the wild-type red. • Morgan mated this mutant white-eyed with a red-eyed female. *Figure 15.3 Morgan’s first mutant p288 *Sex-Linked • There are additional considerations when working with sex-linked genes because when females (XX) inherit a sex-linked gene, they receive two copies of the gene, one on each X chromosome. • In contrast, a male (XY) will inherit only one copy of the gene because only the X chromosome delivers the gene. • Whichever allele is on the X chromosome (dominant or recessive) is expressed. INQUIRY Figure 15.4 p289 3/15 • With a partner review the problem presented. Note the Genotypic and Phenotypic ratio of F2 generation! • Solve and record on white board the answer to the following cross. – White eyed female fruit fly crossed with a wild eyed male… what will be their Genotypic and Phenotypic ratio for their F1generation? Sex-Linked • Hemophilia is caused by a sex-linked recessive gene (h) in humans. • Hemophiliacs cannot properly form blood clots and in the worst cases can die from minor injuries by bleeding to death. • Females and males who inherit the normal allele (H) and XHXH and XHY. • In order for a female to be affected she must have 2 copies of the defective allele: XhXh. • A male, however need only inherit one copy of the affected allele XhY • Therefore, sex-linked traits are much more common in males. • Females who carry the XHXh are carriers. Sex Chromosomes • Problem A woman whose father was a hemophiliac marries a man without hemophilia. Determine the Genotypic and Phenotypic ratio for their possible offspring. • Chi-Square…Were you able to finish all crosses??? – Practice problem… • Df = ? • Did you calculate the chi-square? • Volunteer to calculate on board! • Solve following problem (board) – A white-eyed female Drosophila is mated with a red-eyed (wild type) male. What are the genotypic and phenotypic ratio of their offspring? X-Inactivation (p292) • During embryonic development in female mammals, one of the two X chromosomes in each does not uncoil into chromatin. • Instead, X-inactivation occurs, and one chromosome remains coiled, as a dark compact body, called a Barr body. • Barr bodies are mostly inactive X chromosomes, and the alleles on them do not get expressed. • Thus, only the genes on one active X chromosome are expressed by the cell. X -Inactivation • Calico cats have yellow, black, and white hair, randomly arranged in patches over their bodies. • The yellow and black colors are coded by a gene on the X chromosome. (white is controlled by a different gene) • When the gene for yellow is inactivated, black patches of fur appear. The same for yellow fur. X- Inactivation • Subsequent daughter cells will have the same X chromosome inactivated as did the embryonic parent cell from which they originated. • The inactivated X chromosome is reactivated during meiosis. Nondisjunction p297 • In the normal process of meiosis, chromosomes pair at the metaphase plate and subsequently separate and migrate to opposite poles. • When nondisjunction occurs, the chromosome pair migrate to the same pole (do not separate). Nondisjunction Video Clip - Non-disjunction 3/25 15_13Nondisjunction.mpg Nondisjunction • As a result, half the gametes will have an extra chromosome, and the other half will be missing a chromosome. Nondisjunction • Down syndrome occurs when an egg or sperm with an extra number 21 chromosome fuses with a normal gamete. (3 - #21 chromosome) • The result is trisomy of chromosome 21. • Down syndrome results in mental retardation, heart defects, respiratory problems, and abnormalities of external features. Nondisjunction • Turner syndrome results when there is nondisjunction of the sex chromosomes. • Sperm or an egg will have no chromosome (O) • Zygote will result with an XO • Turner syndrome zygotes (X0) is female with no second sex chromosome. • Turner syndrome individuals are physically abnormal and sterile. Errors and Exceptions in Chromosomal Inheritance • Aneuploidy: A condition of having an abnormal number of certain chromosome – An aneuploid cell that has a chromosome triplet is said to be trisomic – An aneuploid with a missing chromosome is said to be monosomic – Abnormal gene dosage in aneuploids causes characteristic symptoms in survivors. Trisomy of chromosome 21 is…? Errors and Exceptions in Chromosomal Inheritance • Polyploidy: A chromosome number that is more than two complete chromosome sets – Triploidy: a polyploid chromosome number of 3N – Tetraploidy: a polyploid chromosome number of 4N Alterations of chromosome structure • Chromosome breakage can alter chromosome structure in four ways: – Deletion (fragment lacking) – Duplication (fragment joined to another homologue) – Translocation (fragments joins to nonhomologue) – Inversion (fragment attaches in reverse order) Figure 15.x1 Translocation Karyotype Changes • Human disorders due to chromosomal alterations – Down Syndrome: affects 1 out of 700 children in the U.S. – Trisomy of chromosome 21 – Phenotypic facial features, short stature, heart defects, mental retardation, respiratory infections, proneness to leukemia and alzheimers Figure 15.14 Down syndrome p273 Karyotype Changes • Klinefelter’s Syndrome: Usually an XXY male (can be multiple X’s) affected individual may have female phenotypes • Triple X Syndrome: Usually XXX, fertile and can show normal phenotype • Turner’s Syndrome: Genotype of XO (human monosomy) short stature, some sexual characteristic fail to develop, internal sex organs do not develop • Cri du chat Syndrome: caused by a deletion on chromosome 5, symptoms include mental retardation, small head, with unusual facial features and a cry that sounds like a mewing cat Figure 15.x2 Klinefelter syndrome Figure 15.x3 XYY karyotype Translocation (ex: chronic Myelongenous leukemia) Translocation • Chronic Myelongenous Leukemia: portion of chromosome 22 switches places with chromosome 9, symptoms include those seen in other types of leukemia • Fragile X Syndrome: most common genetic cause of mental retardation. It is an abnormal X chromosome, the tip of the X chromosome hangs on the rest of the chromosome by a thin DNA strand • Prader-Wili Syndrome: caused by a deletion at chromosome 15, characterized by mental retardation, obesity, short stature, small hands and feet, uncontrollable laughter and jerky movement, often associated with uncontrollable desire to eat, never feeling satiated Some inheritance patterns are exceptions to standard Mendelian inheritance • There are two normal exceptions to Mendelian genetics: • One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus • In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance Genomic Imprinting (p301) • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits • Such variation in phenotype is called genomic imprinting • Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production • It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides • Genomic imprinting is thought to affect only a small fraction of mammalian genes • Most imprinted genes are critical for embryonic development © 2011 Pearson Education, Inc. Additional situations • Methylation --- (CH3) • Adding methyl groups to certain nucleotides at a specific loci on chromosomes • This causes the gene to be inactivated (silenced) • At this point the animal probably uses the allele that is not imprinted Page 302 (mutations in plastids – inherited from maternal parent) Figure 15.UN04 Extranuclear genes • Small circles of DNA found in mitochondria & plastids (chloroplasts) • They reproduce themselves • Passed to daughter organelles • Mitochondrial DNA – Mother’s family genes ONLY (Mitochondrial DNA originates in the OVUM) Used in many geneology work -”Journey of Man” • Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems – For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy © 2011 Pearson Education, Inc. Technology provides tools • Genetic counselors in many hospitals can provide information to prospective parents concerned about a family history for a genetic disorder • Fetal testing can be done to determine the presence of a disease on a fetus • Chorionic Villus Sampling (CVS) provides a small amount of fetal tissue to karyotype quickly • PKU testing determines the presence of phenylketonuria Research Project • Genetic Disorders • You will be doing a report of a genetic disorder or condition of your choice. – Poster Walk will occur Friday, 3/17/15 The following describes a few interesting additional points regarding Genetic Inheritance Human Genetic Defects • Genetic defects can be caused by the inheritance of an allele, or it can be caused by chromosomal abnormalities. – – – – A missing chromosome (nondisjunction) An extra chromosome (nondisjunction) Chromosome portions deleted (deletion) Chromosome portions duplicated (duplication) – Chromosome portion moved to another chromosome (translocation) – Chromosome portions rearranged in reverse orientation (inversion) Recessively inherited disorders • Recessive alleles that cause human disorders are usually defective versions of normal alleles • Defective alleles code for either a malfunctional or no protein • Heterozygotes can be phenotypically normal, if one copy of the normal allele is all that is needed to produce sufficient quantities of specific protein • (example – Sickle Cell Carrier) Recessively inherited disorders • Recessively inherited disorders range in severity from nonlethal traits (albinism) to lethal disease (cystic fibrosis). Since these disorders are caused by recessive alleles: – The phenotypes are expressed ONLY in the recessive homozygotes (aa) who inherit one recessive allele from each parent – Heterzygotes (Aa) can be phenotypically normal and act as carriers possibly transmitting the recessive allele to their offspring • Most people with recessive disorders are born to unaffected parents, both of whom are carriers Recessively inherited disorders • Human genetic disorders that are recessively inherited include: – Cystic Fibrosis – Tay-Sachs – Sickle-Cell Anemia • The probability of inheriting a recessively inherited disorder is greater if the parents are closely related. Dominantly inherited disorders • Some human disorders are dominantly inherited – For example, achondroplasia (a type of dwarfism) affects 1 in 10,000 people who are heterozygous for this gene – Homozygous dominant condition results in spontaneous abortion of the fetus • Lethal dominant alleles are much rarer than lethal recessives • Late-acting lethal dominants can escape elimination of the disorder does not appear until an advanced age, after the afflicted individuals may have transmitted the lethal gene to their children. (Huntington’s disease) Human Genetic Defects: • The AP Exam expects you to know the more common genetic defects in humans and their underlying causes. • Research Project… begin today! • Poster presentation due Friday! • Take Home test for Genetics Unit & Begin Evolution Unit on Monday! Genetic condition research • Poster presentation – Poster Walk • Modeling college-level practice of showing others what you know and what you have researched • Note handout for detail info • You will evaluate at least 2 other posters