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Transcript
Introduction to Genetics Mendel Was a monk in Austria in mid-1800’s, studied math and science As he was tending peas in the garden, he began to notice patterns in the pea characteristics ( tall or short, white or purple flowers, etc) Did a series of experiments over a 10 year period of time—mating, growing and counting pea plants with differing characteristics (30,000 pea plants total) and compared them to the previous and next generations Concluded that some “factor” was passed between generations that could be hidden but not lost Figure 14.2 Mendel tracked heritable characters for three generations Mendel’s Laws Law of segregation: homologous chromosomes separate and are packaged into different gametes Law of independent assortment: each pair of homologous chromosomes lines up independently of each other during meiosis Figure 14.4 Mendel’s law of segregation (Layer 2) Dihybrid Cross Dihybrid cross: crosses between individuals that differ in two traits In Mendel’s crosses round and yellow seeds are dominant Wrinkled and green seeds are recessive Try a cross between plants that are true- breeding for round and yellow seeds with plants that are true breeding for wrinkled and green seeds Can you predict the genotypes and phenotypes of the F1 generation? Figure 15.1 The chomosomal basis of Mendel’s laws Vocabulary Trait: characteristic of an organism Gene: sequence of nucleotides that code for a certain trait Allele: different form of a gene; symbolized by a letter –Dominant: allele that can hide another allele; always symbolized by a capital letter; ex: B –Recessive: allele that can be hidden by the dominant allele; always a lower case letter; ex: b Homozygous: 2 of the same allele –H. dominant: BB –H. recessive: bb Heterozygous: 2 different alleles: Bb Genotype: the alleles an organism has: BB, Bb Phenotype: how the trait looks; blue eyes, etc P: parental generation F1: their children F2: the children of the F1 Punnett square: tool for predicting the probability of producing a certain type of offspring Figure 14.3 Alleles, alternative versions of a gene Inheritance patterns Complete dominance: one allele can completely mask another; not always the most common phenotype 6 different crosses (yes, you have to memorize them) –TT x TT, TT x Tt, TT x tt, Tt x Tt, Tt x tt, tt x tt –Complete all the above crosses in your journal Figure 14.6 A testcross Incomplete dominance Heterozygote shows a blending of the parental traits CRCW , RR’, RW Co-dominance Heterozygote shows both parental traits MN blood type: MM has only M proteins, NN has only N proteins, MN has both M and N proteins; Roan cows, etc Multiple alleles More than 2 alleles for a given trait’ ABO blood type Pheno. Geno Antigen antibodies A A A –A I I or I i A B B B –B I or I i B A A B –AB I I A, B none –O ii none A,B Probability and Punnett Squares A. Genetics and Probability B. Punnett Squares C. Probability and Segregation D. Probabilities Predict Averages Probability Probability: branch of mathematics that predicts the chances that a certain event will occur What are chances that a tossed coin will turn up heads? -one out of 2, or ½ Your chance of drawing an ace from a deck of cards: 4 out of 52, or 1/13 The chance that all four tosses will land heads up: =1/2x1/2x1/2x1/2 =1/16 chance Sex chromosomes and linkage Sex chromosomes Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes Females are XX, males are XY (X and Y look different from each other); females give all their eggs one X chromosome plus cytoplasm and organelles; Males determine the sex of the child since ½ of their sperm get the X and ½ get the Y Any information on the X chromosome will appear in males, whether recessive or dominant; females require 2 recessive alleles to show a recessive trait Information solely on the Y chromosome are called holandric genes (porcupine quill body hair, hairy ear rims, SRY gene); only effect men Barr bodies In body cells of females one X chromosome at random is turned off early in development; inactivated X is called a Barr body All the cells descended from that cell have the same X turned off If female is heterozygous she becomes a mosaic—some areas have the dominant gene expressed, some have the recessive Ex: calico cats, patches of colorblindness X chromosome inactivation Cat’s fur color Males: XY Females: XX Extra X: inactivated, bar body Spots of 2 colors: female cat One color spots: male cat Non-disjunction Failure of chromosome pairs to separate during meiosis Results in gametes with too many or too few chromosomes Aneuploidy: abnormal # of a certain chromosome Polyploidy: more than 2 complete chromosome sets An embryo needs at least one X chromosome to survive Sex linkage problems It is now important if the offspring is male or female so the X and Y have to be used along with superscripts to show the alleles Examples of X-linked disorders: colorblindness, hemophilia, muscular dystrophy, eye color in fruit flies Hemophilia H-normal, h-hemophiliac Phenotype Genotype Normal female Carrier female Hemophiliac female Normal male Hemophiliac male XHXH XHXh XhXh XHY XhY Colorblindness Colorblindness- Sex-linked disorder which causes loss of color. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Duchenne Muscular Dystrophy Sex-linked disorder that results in progressive weakening and loss of skeletal muscle. In the U.S. 1/3000 males Defective muscle protein Chromosome 46, XX 46, XY 45, XO 47, XXY Name of Syndrome Normal Normal Turner's Syndrome Klinefelter's Syndrome Sex Female Male Female Male Frequency in Population 0.511* 0.489* 1/5,000 1/700 The Family Pedigree A Family Tree To understand how traits are passed on from generation to generation, a pedigree, or a diagram that shows the relationships within a family, is used. In a pedigree, a circle represents a female, and a square represents a male. A filled-in circle or square shows that the individual has the trait being studied. The horizontal line that connects a circle and a square represents a marriage. The vertical line(s) and brackets below that line show the child(ren) of that couple. 1. This pedigree shows the inheritance of attached ear lobes. Which parent has attached ear lobes? 2. How many children do the parents have? Which child has attached ear lobes? Which child is married? Does this child’s spouse have attached ear lobes? Do any of this child’s children have attached ear lobes? 3. Section 14-1 A circle represents a female. A square represents a male. A horizontal line connecting a male and female represents a marriage. A half-shaded circle or square indicates that a person is a carrier of the trait. A completely shaded circle or square indicates that a person expresses the trait. A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait. Genetics Human Genetic Disorders Chromosome Abnormalities Normal development requires 46 chromosomes; any other number results in an abnormality Chromosome abnormalities are seen in less than 1% of live births Most fetuses lost through spontaneous abortion have major chromosomal abnormalities These disorders are a result of non-disjunction—the failure of chromosomes to separate in meiosis 1 Types of disorders Turner’s syndrome: XO; 1/5000 births; results in short, sterile females with some degree of retardation Kleinfelter’s syndrome: XXY; 1/2000 births; results in tall, slender, sterile males with some mental retardation, have some female characteristics More disorders Down’s syndrome: trisomy 21; 3/2000 births more common in births to older women 1/50 at age 45 results in short stature, round heads, protruding tongues, prone to respiratory and heart disease, varying degrees of retardation And lastly.. Supermale: XYY; 1/2000 births; results in tall stature, subnormal intelligence, many have personality disorders and a tendency towards violence Allele disorders Recessive alleles cause these diseases Cystic Fibrosis: most common disorder in caucasians –Codes for a malfunctioning cell membrane protein which results in thick sticky mucous lining the lungs and digestive tract –Treatment: gene therapy—inhalation of good gene, but must be repeated More disorders Tay-sachs: most common in eastern european Jews; can be tested for the allele –Enzyme deficiency causes build-up of harmful substances in the brain –Babies begin showing symptoms by 6 months old—lose their eyesight, hearing, become paralyzed –Most die by age 4 And more.. Phenylketonuria: also more common in caucasians –Lack of enzyme that breaks down phenylalanine (found in nutrasweet) –Can cause mental retardation and convulsions –In US babies are tested at birth and treated with a low protein diet until their brains are fully developed And one more… Sickle cell anemia: most common disorder in African-americans –Only 1 base different causes the abnormal protein in red blood cells (hemoglobin) to crystallize under low oxygen conditions –Causes severe pain, strokes, tissue damage, etc –Heterozygous individuals are resistant to malaria Dominant allele disorders Only 1 dominant allele can cause these disorder Neurofibromatosis: causes tumors on the nerves, skin, bones, vital organs –Can vary in degree from small brown bumps to large disfiguring tumors And finally.. Huntington’s disease –Causes degeneration of the nerve cells in the brain, causes loss of motor control and eventually paralysis –Symptoms are not normally seen until after age 40