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Genetics

Genetics. Ms. Napolitano & Mrs. Haas CP Biology. Introduction to Genetics. Every living thing inherits characteristics from its parent(s) Genetics – the scientific study of heredity. Gregor Mendel & his pea plants. Studied genetics of garden peas Started with true-breeding plants

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Genetics

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  1. Genetics Ms. Napolitano & Mrs. Haas CP Biology

  2. Introduction to Genetics • Every living thing inherits characteristics from its parent(s) • Genetics – the scientific study of heredity

  3. Gregor Mendel & his pea plants • Studied genetics of garden peas • Started with true-breeding plants • If allowed to self-pollinate, would create identical offspring • One only tall, one only short, one green, one yellow, etc. (called traits) • Self-pollinated the plants by hand • Controlled breeding • Called “cross-pollination”

  4. Genes & Dominance • Traits– specific characteristics that vary from one individual to another • Freckles, eye color, right vs. left handed, etc. • Original cross – P generation • Mendel = true breeding plants • Offspring = F1 generation • Offspring of F1 = F2, etc. • Hybrids – parents have different traits

  5. Mendel’s Pea Plants

  6. Mendel’s Conclusions • Inheritance determined by factors that are passed down from one generation to the next • Genes – chemical factors that determine traits • Alleles – different forms of a gene • Ex: brown vs. blue eyes, curly vs. straight hair, etc. • Principle of dominance – some alleles are dominant and others are recessive • Dominant (T) will always be expressed • Recessive (t) only expressed if dominant allele is absent

  7. Mendel’s second cross • Mendel not satisfied – what happened to recessive alleles? • Crossed F1 generations with themselves • Made F2 generations • Results – traits from recessive alleles came back in ~¼ of plants

  8. Segregation • Segregation – separation of alleles • Happens during meiosis during formation of gametes Mendel’s conclusions: • When F1 plants produce gametes, alleles separate so that each gamete carries only a single copy of the gene • F1 produces two types of gametes – one tall allele & one short allele

  9. Example of Segregation

  10. Genetics & Probability • Probability – the likelihood that a particular event will occur • Probability is used to predict the outcomes of genetic crosses • Punnett square – used to predict genetic variations • Homozygous – 2 identical alleles for a particular trait • TT or tt • Aka “true breeding” • Heterozygous – 2 different alleles for a particular trait • Tt • Aka “hybrid”

  11. Genotype & Phenotype • Phenotype – physical characteristics • Ex: Plant phenotype is tall or short • Genotype – genetic makeup • Ex: Plant genotype is TT, Tt, or tt

  12. Punnett Square – Pea seed color

  13. Punnett Square – Pea seed color What is the genotype of both parents? What is the phenotype of both parents? Are the parents homozygous or heterozygous?

  14. Probability & segregation • In previous Punnett square, ¼ seeds are green and ¾ seeds are yellow • 3 dominant, 1 recessive • Ratio = 3:1 • In previous Punnett square, ¼ seeds are YY, ¼ seeds are yy, and 2/4 (1/2) seeds are Yy • Ratio = 1:2:1

  15. Probabilities & Averages • Need a large sample size! • Probability cannot predict outcome of a single event • Ex: In theory, if you flipped a coin twice, you would get 1 heads and 1 tails – but won’t always happen • The larger the number of offspring, the closer the resulting numbers will get to expected values

  16. Independent Assortment • Does segregation of one pair of alleles affect the segregation of another pair of alleles? • Mendel – crossed true-breeding, round, yellow peas (RRYY) with true-breeding, wrinkled, green peas (rryy) • Offspring: all round, yellow peas • Therefore: Yellow & round dominant to wrinkled & green

  17. Independent Assortment • Mendel – now crossed new plants (RRYY x rryy) • 556 total seeds produced • 315 seeds round, yellow • 32 seeds wrinkled, green • 209 seeds had combinations not found in either parent • Independent assortment – alleles segregate independently • In this example, color segregates independent of shape • In other words – the inheritance of one gene does not affect the inheritance of the other • Mendels results close to 9:3:3:1 ratio

  18. Mendel’s Second Cross

  19. Genetic Diseases • Tay Sachs • Recessive • Progressively destroys nerve cells • Sickle Cell Anemia • Recessive • Affects hemoglobin shape in red blood cells • Cannot get enough oxygen to cells • Huntington’s Disease • Dominant (ah!) • Neurodegerative • Cognitive decline • Symptoms do not arise until ~35-44 years old

  20. exceptions • Genetics is very complicated in many organisms! • Many genes have more than 2 alleles • Some alleles are neither dominant nor recessive • Examples: • Incomplete dominance • Codominance

  21. Incomplete dominance • Example: A white & red flower cross, making pink • One allele is not completely dominant over another • Traits “blend” Parents: • 1 WW (white) • 1 RR (red) Offspring • 4 RW (pink)

  22. Codominance • Both alleles contribute to phenotype • Both traits fully expressed • Ex: In chickens, black feathers is codominant with white feathers Parents • 1 CBCB • 1 CWCW Offspring • 4 CBCW

  23. Multiple Alleles • Genes with more than 2 alleles • More than 2 alleles possible in a population (Not per organism! Remember – 1 from mom, 1 from dad) • Ex: Rabbit coat color

  24. Sex-Linked Genes • Sex-linked genes - genes located on sex chromosomes • Mostly found on the X chromosome • Much bigger in size • Over 100 sex-linked diseases on X chromosome • Males will express X-linked alleles because they have only 1 X chromosome! • Doesn’t matter if dominant or recessive • In Punnett square, only the X chromosome gets the gene! • Ex: Parents will be XBXband XbY

  25. Sex-Linked Punnett Square

  26. Sex-Linked Diseases • Colorblindness • Inability to distinguish certain colors • Red-green colorblindness = 1 in 10 males in US • Only 1 in 100 females are colorblind (why?) • Hemophilia • Blood-clotting protein missing, can bleed to death easily • 1 in 10,000 males have hemophilia • Duchenne Muscular Dystrophy • Progressive weakening & loss of skeletal muscle • 1 in 3,000 males

  27. Blood Typing • Use ABO and Rh blood groups • Rh=either + or –, + is dominant • 3 alleles: IA, IB, & i • IA and IB are codominant, and both dominant to recessive i • Possible blood types: • IA IA – Type A • IA i – Type A • IB IB – Type B • IBi – Type B • IA IB – Type AB • ii – Type O

  28. Blood Type Punnett Squares

  29. Polygenic Traits • Polygenic traits - traits controlled by 2+ genes • Ex: Human skin color (4 genes)

  30. Human Chromosomes • Recall: Humans have 46 chromosomes • Karyotype – picture of human chromosomes lined up in order • 2 chromosomes = sex chromosomes • Determine gender • Females = XX • Males = XY • Other 44 chromosomes = autosomes

  31. Human Karyotype

  32. The “Magic” moment • Males & females born in ~50:50 ratio • All human egg cells carry a single X chromosome, but only half of all human sperm cells carry a single X chromosome. The others carry a single Y chromosome. • Females = (23,X) • Males = (23,Y) = Female + X X Male = + X Y

  33. Pedigree Charts • Show relationships within a family

  34. Pedigree Example

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