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Unit 3 Based on the work of Gregor Mendel Genetics – › the study of patterns of inheritance as heredity characteristics (characters or traits) Genes – › particles of inheritance, segments of chromosomal DNA Began studying crosses involving only one trait at a time Crossed a pure-breeding strain of red flowered pea plants with a pure-breeding strain of white flowered pea plants Produced hybrids – › genetically mixed offspring, not purebred › All were red He then allowed all seeds to self-pollinate › Produced ¾ all red & ¼ all white Parental Generation Red x White (RR) (rr) First Filial Generation (F1) All Red (Rr) Second Filial Generation (F2) ¾ Red ¼ White II. Gene Pairs – A. Alleles – different forms of a genetic trait (ex. red, white) B. Homozygous – having the same alleles C. Heterozygous – having different alleles III. Dominant and Recessive Alleles – A. Dominant Allele – the allele that expresses itself B. Recessive Allele – the allele that is masked or not expressed IV. Genotype and Phenotype – A. Genotype – genetic make-up that cannot determined merely by inspection (Example – Rr) B. Phenotype – the expression of its genes, visually observed or chemically tested A genetic cross in which only one trait of the parents is of interest P1 Red Flower (RR) P1 Gametes F1 (one gene from X White Flower (rr) R r Rr (Red Flowers) each parent) F2 (self-pollinated) RR, Rr, Rr, rr A genetic cross in which two different pairs of contrasting alleles are of interest Smooth Yellow Seed P1 (SSYY) P1 Gametes Wrinkled Green Seeds x S,Y F1 (ssyy) s,y SsYy (Smooth Yellow Seeds) F2 Gametes SY, Sy, sY, sy (gene pair combinations for F2) F2 SY Sy sY sy SY SSYY SSYy SsYY SsYy Sy SSYy SSyy SsYy Ssyy SsYY SsYy ssYY ssYy SsYy Ssyy ssYy ssyy sY sy Genotype Ratios – SSYY – 1/16 SSYy – 2/16 SsYY – 2/16 SsYy – 4/16 SSyy – 1/16 Ssyy – 2/16 ssYY – 1/16 ssYy – 2/16 ssyy – 1/16 Phenotype Ratios – Smooth/YellowSmooth/GreenWrinkled/YellowWrinkled/Green - 9/16 3/16 3/16 1/16 Occurs when two alleles combine their effects – neither is recessive The heterozygote has a different phenotype, as well as a different genotype, from either homozygote Codominance – › Occurs when each allele is expressed separately – › Example: black and white feathers R= red flower R1 = White flower (not r because it is not recessive) P1 P1 Gametes RR x R R1 F1 Pink Flowered F2 Genotype Ratio Phenotype Ratio R1R1 RR1 RR 1 RR 1 red RR1 RR1 2 RR1 2 pink R1R1 1 R1R1 1 white Humans contain 23 pairs of chromosome Each single chromosome contains many to hundreds of genes A. Mutation – 1. A rare, unpredictable and inheritable change in a cell’s genetic material 2. Becomes a part of the genotype of the cell and of all its descendants 3. Most mutations produce recessive alleles B. Lethal Alleles: - Most will die prematurely because their DNA lacks a certain protein 1. Brachdactyly in humans (shortening of the middle fingers) Parents BB1 BB1 Offspring BB BB1 BB1 B1B1 **Homozygous babies lack fingers and show other skeletal defects that cause death in infancy. ** 2. 3. Manx cat (no tail) Sickle Cell Anemia - shows co-dominance with normal alleles - If heterozygous = HbAHbS (carrier) - If homozygous = HbSHbS (affected individual) - 50% die by age 20 4. Tay-Sachs - metabolic disorder resulting in deterioration of the brain - death by 5 - 1/30 East European Jewish persons are carriers 5. Cystic Fibrosis - 1/20 Caucasians is a carrier - death the age of 20 - high concentration of sodium, potassium & chloride in sweat - production of thick mucous, clogging the respiratory passages A. Mammals – Parents Female XX Gametes Offspring B. X Male XY X or Y XX XY Birds, moths, butterflies & amphibians are opposite – female(ZW) & male(ZZ) c. Sex Chromosomes One sex is heterozygous, one homozygous for the entire pair of chromosomes 2. The possession of a particular sex chromosome causes an embryo to develop into a male or a female = process of sex differentiation 3. Most genes involved in the production of male and female characteristics, sex hormones, are located on autosomes - not sex chromosomes! 4. All individuals have the genes needed to develop into a member of either sex – hormonal treatment- birds, fish 1. 5. Sometimes, sex chromosomes fail to segregate normally during meiosis, resulting in individuals with abnormal numbers of sex chromosomes. a. XXX = female, fertile or sterile, usually normal b. X = Turner’s syndrome; female, sterile, ovaries are rudimentary or absent c. XXY = Klinefelter’s syndrome; male sterile, possible mental retardation d. XXXY = male e. XYY = male, tall, acne-prone, impaired fertility A. B. C. D. E. F. Sex chromosome carry genes Mammalian X chromosomes have large nonhomologous portions (no mates of the Y) = sex linked In males, any recessive allele on a nonhomologous parts of the X chromosome will be expressed in the phenotype Females must have two copies of the recessive allele for a phenotype Since many recessive alleles are bad more males have these disorders Examples: Red-green colorblindness, hemophilia, one type of muscular dystrophy, SCID A. B. Genes that are expressed to a greater or lesser degree as a result of the level of sex hormones Examples: baldness, gout (Affects males mostly, high uric acid in the blood which allows crystals to form which are deposited into the connective tissues which causes pain. This disorder can lead to arthritis). A. B. C. D. Long, Twisted, double stranded helix consisting of a chain of nucleotides. Nucleotides consist of a sugar, a phosphate & a nitrogen base The four base pairs include – adenine, cytosine, guanine & thymine Strands are “complimentary” – they can only pair one other base pair – Adenine – Thymine & Cytosine - Guanine Copyright Pearson Prentice Hall A. DNA directs the synthesis of proteins using RNA (ribonucleic acid) Three types of RNA – 1. rRNA – makes up the ribosome of the cell 2. mRNA – carries the DNA message from the nucleus to the ribosome for protein synthesis 3. tRNA – carries amino acids to the ribosome B. Transcription – › › › When DNA instructions are copied into RNA RNA is synthesized on one of the two strands of DNA Not an exact copy due to the replacement of Uracil for Thymine (A,U,C,G) Translation – C. › › Process by which the cells convert the genetic message from the mRNA into proteins mRNA is translated by the ribosome resulting in a chain of amino acids called proteins. As tRNA moves the amino acids together, long chains are formed (proteins) There are 20 different amino acids The structure and function of the protein depends upon the amino acids present and the order in which they are attached The base pairs (A, U, C, G) are arranged in codons or words of 3 Each codon stands for one amino acid XV. Factors that affect gene expression 1. Age 2. Diet 3. Light 4. Temperature XVI. Genetic Counseling – 1. Blood tests 2. Amniocentesis – - detects abnormal chromosomal numbers in the fetus during the 16th week of pregnancy A. Recombinant DNA – › › › › Produced from recombining DNA from different organisms Allowed because the genetic code is mostly universal among all organisms Using enzymes, scientists cut DNA sections and paste them into bacterial cell plasmids, which then replicate them quickly. Examples – Insulin Production Cancer and AIDS research Cattle Production XVIII. Scientists Important to Genetics Gregor Mendel Charles Darwin – › Father of Evolution/Natural Selection Watson & Crick › Discovered the shape of DNA Beadle & Tatum – › Worked with Gene Mutations Hershey & Chase – › Determined that genes are on chromosomes, not proteins Thomas Morgan – worked with mutant Fruit Flies Mary Lyon – Studied Barr bodies (inactive X chromosome) Tsui & Riordan – cloned Cystic Fibrosis Gene