AP Biology PowerPoint Notes Chapter 11 & 12 Patterns of Heredity and Human Genetics

Transcription

1 AP Biology PowerPoint Notes Chapter 11 & 12 Patterns of Heredity and Human Genetics Mendelism and Genotype Genotype must be considered an integrated whole of all the genes because genes often work together to control the phenotype. There are forms of inheritance that involve degrees of dominance, interactions, multiple alleles, and polygenes. Environmental conditions can influence gene expression. Human Genetic Disorders Recessively inherited if a disorder is lethal before reproductive age, the allele frequency decreases because homozygotes don t reproduce to pass on the gene. Usually, heterozygotes can survive because enough of the normal protein is produced. Human Genetic Disorders Dominantly inherited the lethal dominant allele is more rare because even heterozygotes die. If the disorder only affects individuals after reproductive age, the allele will be maintained in the population because they can pass it on before the disorder is fatal. Genetic Counseling For many genetic disorders it is wise for parents who are carriers to be counseled so they can be prepared to cope with the disorder or terminate the pregnancy. 1. Carrier recognition screening prospective parents to determine if they are carriers of a genetic disorder. 2. Fetal testing methods for testing a fetus in utero to determine if it carries genetic disorders a. Amniocentesis a small sample of amniotic fluid is withdrawn and the fetal cells it contains are cultured for a few weeks. The cells can then be tested for genetic disorders. This procedure can be done by the 14 th to 16 th week. b. CVS a sample of the chorionic villi is obtained and the cells are tested for genetic disorders. The technique can be done by the 8 th to 10 th week and results are available in 24 h. c. Fetoscopy a small camera is inserted into the uterus to visually examine the fetus for physical defects. New born screening

2 A neonatal technique whereby the baby is screened for genetic disorders. Ethical and Moral implications for many, these techniques are controversial as they could be used as grounds for terminating a pregnancy. Parents who are not prepared for the challenge of having a child with a particular disorder can choose abortion. Dominance is not always complete: In Mendel s alleles dominance was complete when an individual was heterozygous (Ss expresses S phenotype) Many genes have alleles that are not dominant or recessive to one another. I.e.. Heterozygous shows an intermediate phenotype. Unusual Inheritance Patterns Incomplete Dominance Codominance Multliple Alleles Epistasis Pleiotropic Polygenic Incomplete Dominance Results: Before Mendel, it was believed that parental traits were blended in offspring and could not be separated in later generations. The traits that Mendel studied showed this was not the case, but this is only true of traits which show so called simple dominance. With incomplete dominance, two phenotypes are blended rather than showing two different possibilities. E.g. flower color in snapdragons Snap Dragons Incomplete Dominance F1 = 1 all pink

3 F2 = 1:2:1 Codominance Both alleles are expressed at the same time. For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches. When the F1 roan individuals self fertilize, the F2 progeny have a phenotypic ratio of 1 red:2 roan:1 white. Multiple Alleles For traits we ve discussed so far, there have always been only two alleles in a population. For some traits, there are more than two alleles. Remember, no matter how many alleles exist, each individual has only two one from each parent. E.g., AB blood groups, eye color Multiple Alleles and Codominance In humans, there are four blood types (phenotypes): A, B, AB, and O Blood type is controlled by three alleles. A, B, O O is recessive, two O alleles must be present for the person to have type O blood A and B are codominant. If a person receives an A allele and a B allele, their blood type is type AB Blood Types Crosses involving blood type often use an I to denote the alleles Possible blood types: A, B, AB, O Blood Types The blood type determines what antibodies are located within the blood. Type A blood has type B antibodies. If type B blood is put into their bodies, their immune system reacts as if it were a foreign invader, the antibodies clump the blood can cause death.

4 Type AB blood has no antibodies, any blood can be donated to them they are called the "universal acceptors" Type O blood has no surface markers on it, antibodies in the blood do not react to type O blood, they are called the "universal donors" Epistasis A gene at one location affects or alters the expression of a gene at another location. Ex. coat color in mammals In horses, brown coat color (B) is dominant over tan (b). Gene expression is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene (C) codes for the presence of pigment in hair, whereas the recessive gene (c) codes for the absence of pigment. If a horse is homozygous recessive for the second gene (cc), it will have a white coat regardless of the genetically programmed coat color (B gene) because pigment is not deposited in the hair. Pleiotropy The ability of a single gene to exert an influence on several characteristics (have multiple phenotypic effects). Many pleiotropic conditions arise from genes whose products are involved in signaling and regulation pathways. Because these proteins coordinate daily life in numerous tissues, defects in them have numerous consequences, as one breakdown leads to another. Example: Sickle cell anemia One gene can also influence a combination of seemingly unrelated characteristics. Pleiotrophy Albinism Example: Siamese Cats pigment that controls fur pigmentation also influences the connections between a cat s eyes and the brain. A defective brain causes both abnormal pigmentation and cross eyed condition. Albino individuals lack pigment in their skin and hair, and also have crossed eyes at a higher frequency than pigmented individuals. This occurs because the gene that causes albinism can also cause defects in the nerve connections between the eyes and the brain.

5 Pleiotropy Sickle Cell Anemia This disease develops in persons carrying two defective alleles for a blood protein, betahemoglobin. Mutant beta hemoglobin's are misaligned inside a blood cell and cause sickle shaped red blood cells at low oxygen concentrations. Deformed blood cells impair circulation. Impaired circulation damages kidneys and bone. Polygenic Inheritance Some traits show additive effects of 2 or more genes (quantitative) E.g., skin color in humans Skin color in humans shows that three genes interact to determine the level of pigment in an individual's skin. The dominant alleles (A, B, and C) each contribute one "unit" of pigment to the individual (additive effect), so that individuals with more of these alleles will be darker than those with fewer alleles. The recessive alleles (a, b, and c) do not contribute any units of pigment. Therefore, skin color is related to the number of dominant alleles present in each individual's genotype. Environmental Impact on Phenotype: A single genotype can produce a range of phenotypes Norm of reaction = range of phenotypic variability produced by a single genotype under various environmental conditions. Limited genotype can only produce a specific genotype APO blood type Wide Range blood cell count can vary with environment Broad for polygenetic & behavioral traits; skin and fur color. Comprehensive theory of Mendelian genetics: An organism s entire phenotype reflects its overall genotype and unique environmental history Extending the principles of segregation and independent assortment helps explain more complex hereditary patterns (Epistasis & polygenetics)

6 Chromosomal Theory Main Points: A. Chromosomes carry genes, the units of hereditary B. Paired chromosomes separate during meiosis. Each gamete has half the number of chromosomes found in a somatic cell C. Chromosomes assort independently during meiosis. In other words, each gamete receives one chromosome from each pair and the chromosomes it receives have no influence on the inheritance of any other pair. D. Fertilization restores diploid chromosome number and paired alleles in zygote. Chromosome Gene Expression Autosomes nonsex chromosomes Sex chromosomes XX = female XY = male Morgan s Work Sex Linked Traits X chromosome contains genes that are not related to sex Y chromosome contains little genetic material American geneticist, Thomas Morgan noticed a white eye fly among many red eye flies. He crossed a white eyed male with a red eyed female. The F1 was all red eyed as expected. The F2 was 3 red to 1 white. However, only males had white eyes; also, among the males there was a ratio of 1 red:1 white. A karyotype showed that the males had a different chromosome from the females the sex chromosomes. Morgan, therefore, reasoned that the gene for eye color must be located on the sex chromosomes and called these traits sex linked. Remember that a male always inherits a sex linked trait from the female parent because the father always supplies the y chromosome. E.g., in humans, red green color blindness is X linked.

7 Karyotype X Linked Chromosomes Recessive mutant associated with the X chromosomes Females are carriers but males develop the condition Example: color blindness, hemophilia Barr Body One X chromosome in each cell of a female becomes inactive during embryonic development. E.g., Calico cats. One gene involved has two alleles: Orange allele O, which is the dominant form, (XO), and produces orange fur "Black" allele, "o", which is the recessive form, (Xo), and produces black fur. For a cat to be a calico, it must simultaneously express both of the alleles, O and o, which are two versions of the same gene, located at the same locus on the X chromosome Y Linked Chromosomes Dominant mutant associated with the Y chromosome Males are carriers and only males develop the condition Rare Linked genes Example: baldness Note: Baldness is normally x linked and passed on by the females. Morgan also investigated traits that seem to be inherited together. He called these traits linked. In certain dihybrid crosses, he found that most offspring had phenotypes similar to the parents, but other phenotypes were also displayed. How could these phenotypes arise? Autosomal Recessive Disorders Tay Sachs Disease Cystic Fibrosis

8 Phenylketonuria Sickle Cell Anemia Tay Sachs Disease caused by a genetic defect in a single gene with one defective copy of that gene inherited from each parent There is currently no cure or treatment. Tay Sachs disease is rare Causes deterioration of mental and physical abilities and usually results in death. Cystic Fibrosis Mutation of one gene is required to regulate the components of sweat, digestive juices, and mucus. Although most people without CF have two working copies of the CFTR gene, only one is needed to prevent cystic fibrosis. CF develops when neither gene works normally. Affects the entire body, causing progressive disability and often early death. Cyst form within the pancreas A multitude of symptoms, including difficulty in breathing, sinus infections, poor growth, diarrhea, and infertility result from the effects of CF on other parts of the body. Is most common among Caucasians. Most common life shortening inherited disease. Phenylketonuria Metabolic genetic disorder characterized by a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH). This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine. When PAH deficient, phenylalanine is accumulated and is converted into a phenylketone, which is detected in the urine. The condition is left untreated; it can cause problems with brain development, leading to progressive mental retardation, brain damage, and seizures.

9 Autosomal Dominant Disorders Neurofibromatosis Huntington s Disease Achondroplasia Neurofibromatosis (NF) Nerve tissues grow tumors. The tumors may cause bumps under the skin, colored spots, skeletal problems, pressure on spinal nerve roots, and other neurological problems. Huntington s Disease affects muscle coordination and leads to cognitive decline and dementia. cause of abnormal involuntary writhing movements called chorea. Much more common in people of Western European descent than in those from Asia or Africa. Achondroplasia Dwarfism: Achondroplastic dwarfs have short stature. Negative regulatory effect on bone growth. Leads to severely shortened bones. Chromosome Mapping The distance between two genes on a chromosome can be measured as the frequency of crossing over between those two genes. Genes far apart on a chromosome cross over more frequently, so we can assume that cross over frequency is directly proportional to the distance apart. One map unit is defined as 1% cross over frequency. Chromosome Mapping Mutations E.g., imagine three genes: A, B, C. Assume cross over frequencies between them are A and B = 12%; B and C = 7%, A and C = 5%. They must map as ACB.

10 New alleles arise by mutations Rare, stable, and inherited changes in the genetic material Increase the amount of variation among offspring (also; crossing over, recombination during meiosis, gamete fusion) Random process; different copies of the same alleles can be changed in different ways Wild type and mutant alleles may produce a different phenotype. Monosomy & Trisomy An individual has only one of a particular type of chromosome Monosomy Turner s Syndrome Only females Has one X chromosome in some or all cells or has two X chromosomes but one is damaged. Turner syndrome affects approximately 1 out of every 2,500 female live births worldwide Signs include: short stature delayed growth of the skeleton shortened fourth and fifth fingers broad chest low hair line on the back of neck sometimes heart abnormalities usually infertile due to ovarian failure Trisomy Down s Syndrome Non disjunction of chromosome # 21 during meiosis Lower than average cognitive ability Ranging from mild to moderate learning disabilities 1 per 800 to 1,000 births Congenital heart problems

11 Other Down Syndromes: Polyploidy Mosaics Robertson s Translocation The presence of more than two homologous sets of chromosomes in some biological cells and organisms Named according to the number of chromosome sets in the nucleus: Polyploidy triploid (three sets; 3x) tetraploid (four sets; 4x) pentaploid (five sets; 5x) hexaploid (six sets; 6x) Results of hybrids offspring often sterile and reproduce by parthenogenesis. Occurs in some animals, such as goldfish, salmon, and salamanders; fatal in higher organisms. Especially common among ferns and flowering plants, including both wild and cultivated species. Examples of Polyploid Crops Triploid crops: banana, apple, ginger, watermelon, and citrus Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek, tobacco, peanut Hexaploid crops: chrysanthemum, bread, wheat, oat. Octaploid crops: strawberry, dahlia, pansies, sugar cane. Chromosomal Mutations Environmental agents can cause chromosomes to break Mutagens: Radiation UV and X ray Organic chemicals (i.e. pesticides)

12 Viruses Ethnicity Duplication of Genes Mutants genes are displayed twice on the same chromosome due to duplication of these genes. This can prove to be an advantageous mutation as no genetic information is lost or altered and new genes are gained Deletion of a Gene Genes of a chromosome are permanently lost as they become unattached to the centromere and are lost forever Inversion of Genes The order of a particular order of genes are reversed Translocation of Genes Information from one of two homologous chromosomes breaks and binds to the other. Usually this sort of mutation is lethal