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Genetics & Heredity

Unit 8: Genetics & Heredity Unit 9: Human Genetic Disorders Ch. 26: Inheritance of Traits & Ch. 27: Human Genetics. Genetics & Heredity. What is genetics? The study of heredity passing of traits from parents to offspring. Chromosomes in Cells. Remember… Body cells are diploid

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Genetics & Heredity

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  1. Unit 8: Genetics & HeredityUnit 9: Human Genetic DisordersCh. 26: Inheritance of Traits& Ch. 27: Human Genetics

  2. Genetics & Heredity • What is genetics? • The study of heredity • passing of traits from parents to offspring

  3. Chromosomes in Cells • Remember… • Body cells are diploid • 2 of each chromosome • 1 from mom & 1 from dad • Gametes (sperm & eggs) are haploid • 1 of each chromosome • Why?

  4. Genes • Why is your combination of genes unique? • Chance… don’t know which sperm will fertilize which egg • get ½ of your chromosomes from mom & ½ from dad • Meiosis • crossing over during prophase 1 • “independent assortment” of chromosomes based on alignment during metaphase 1

  5. Genes & Alleles • What is a gene? • section of chromosome that determines a specific trait (ex. hair color, eye color, ear shape, etc.) • genes are paired on homologous chromosomes • different forms of genes for the same trait are called “alleles”

  6. Dominant & Recessive Alleles • Each parent contributes 1 allele (form of gene) for trait • Can be: • dominant • prevents expression of (“masks”/“hides”) recessive trait • recessive • seen only when pure (homozygous) for trait • Represented with letters • usually first letter of dominant trait • same letter used for dominant & recessive • CAPITAL = dominant • lowercase = recessive

  7. Allele Combinations • If both alleles are: • the same • homozygous (pure) dominant (ex. AA) • homozygous (pure) recessive (ex. aa) • different • heterozygous (hybrid) (ex. Aa)

  8. Genotype vs. Phenotype • genotype = actual genetic make-up of individual (alleles) • codes for phenotype (trait) • represented by 2 letters • represent alleles from mom & dad • ex. PP, Pp, pp • phenotype = outward (physical) expression of the genotype • trait we “see” • (due to) the protein that is produced • usually represented by an adjective • ex. purple, white, etc.

  9. Genotype is Expressed as a Phenotype • Ex. Let P = purple & p = white • homozygous (pure) dominant • genotype PP • phenotype = purple • heterozygous (hybrid) • genotype Pp • phenotype = purple • dominant trait “masks/hides” recessive trait • homozygous (pure) recessive • genotype pp • phenotype = white

  10. Predicting Traits in Offspring • Punnett Squares • Help predict the results of crosses (mating) • Letters along top & side represent possible alleles in gametes of each parent • Boxes represent possible allele combinations (genotypes & resulting phenotypes) in offspring • Can be used to determine probability and ratios

  11. Making a Punnett Square • Parents are Tt & tt genotypes… • So… Tt x tt is our cross

  12. Passing Traits to Offspring & Probability • Probability • the chance an event will occur • What is the chance of getting heads? Tails? • If you flip two coins, of getting 2 heads? 2 tails? • What is the chance of a couple having a boy? A girl? Of having four boys? Five girls?

  13. Passing Traits to Offspring & Ratios • genotypic ratio = probable ratio of genotypes in offspring of a cross • Ex. If cross Pp & Pp • 1PP : 2Pp : 1 pp • phenotypic ratio= probable ratio of phenotypes resulting from the genotypic ratio • Ex. If cross Pp & Pp • 3 purple : 1 white

  14. Passing Traits to Offspring & Ratios • expected ratio= ratio expected based on probability (Punnett Square) • observed ratio= what actually occurs • Why would these be different?

  15. Passing Traits to Offspring • If one parent is homozygous dominant & other is homozygous recessive • each parent can only produce gametes with 1 type of allele • All offspring will always have: • heterozygous (hybrid) genotype • ex. Ss or Pp • dominant phenotype • ex. smooth or purple

  16. Passing Traits to Offspring • If both parents are heterozygous • each parent can produce gametes with 2 types of alleles • Offspring will always have: • 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive genotype ratio • ex. 1 SS : 2 Ss : 1 ss • 3 dominant phenotype : 1 recessive phenotype ratio • ex. 3 smooth : 1 wrinkled

  17. Phenotype genotype P p p P Pp Pp Pp Phenotype genotype Pp

  18. Gregor Mendel – the Father of Genetics1822-1884

  19. Mendel’s Experiments • Studied garden pea plants • 7 different traits with clearly different forms • Tried to determine how these traits were passed from parent to offspring

  20. Mendel’s Experiments • Mated pure purple parent (PP) & pure white parent (pp) • All offspring had: • purple phenotype • heterozygous (hybrid) genotype • Pp

  21. Mendel’s Experiments • Heterozygous (hybrid) offspring allowed to self- pollinate • So… Pp x Pp • New offspring weren’t all purple

  22. Mendel’s Principle of Dominance • Mendel noted that one form dominates over the other • dominant trait prevents the expression of the recessive trait • Ex. In peas, purple x white gives all purple offspring • PUPRLE = dominant • white = recessive

  23. Dominant/Recessive is Not Always the Mode of Inheritance • Traits are not always as clearly defined as the 7 pea plant traits Mendel studied • Examples of non-dominant/recessive inheritance • Sex determination • Sex-linked traits • Codominance • Multiple alleles

  24. Sex Determination • humans have 46 chromosomes (in body cells) • 23 pairs • Pairs 1 – 22 = autosomes (body chromosomes) • 23rd pair determines gender = sex chromosomes • XX = female • XY = male • Which parent’s chromosomes determines if the offspring will be a boy or girl???? • Why? What is the probability of having a son? A daughter?

  25. Sex-linked Inheritance • X & Y chromosomes not fully homologous • X is bigger & carries more genes • Males will have only 1 allele for traits carried only on X • called X-linked or sex-linked • Ex.: • In Drosophila (fruit flies) eye color • In humans  hemophilia & colorblindness • X-linked traits & disorders are more common in males • Why???

  26. Sex-linked Inheritance • Predictions made using Punnett square • Consider the sex chromosomes (X or Y) & genes they carry (shown as superscript/exponent) together as a unit… • ex. XG (= dominant gene), Xg (= recessive gene), Y (= no gene) • If a female is heterozygous, she does not show the trait/have the disorder, but is a carrier • can pass gene to offspring XG female Xg XG XG XG Xg XG Y Xg Y XG Male Y

  27. Sex-linked Inheritance • Ex. In Drosophila (fruit flies) eye color • What are the sex, genotype, & phenotype of each offspring? • Are there any female carriers for the white eye gene?

  28. heterozygote (hybrid) shows both traits shown by 2 different capital letters Ex. Roan cow phenotype = mix of both red & white hairs genotype = RW Codominance

  29. Multiple Alleles • How many possible genotypes are there? • How many phenotypes? • Can you spot the blood type that is the result of codominance? • more than 2 different forms of an allele exist • but individual still has just 2 • Ex. human blood types • exhibits multiple alleles (3) • IA (A) • IB (B) • i (o) • also exhibits codominance • IA = IB (A & B are codominant) • i (o is recessive) • So… (IA = IB) > i

  30. Human Genetic Disorders

  31. Human Genetic Disorders • Due to DNA mutation (usually recessive) or chromosome abnormalities (in # or structure) • Causes production of abnormal proteins • Examples: • Autosomal recessive disorders (***most genetic disorders) • Cystic Fibrosis • Sickle-cell Anemia • Tay-Sachs Disease • Autosomal dominant disorders • Huntington’s Disease • Sex-linked disorders • Hemophilia • Color Blindness • Chromosomal abnormality disorders • Down Syndrome (trisomy 21) • Klinefelter’s Syndrome (XXY)

  32. Autosomal Recessive Disorders • To be affected, must be homozygous b/c allele is recessive • Cystic Fibrosis • Sickle-cell Anemia • Tay-Sachs Disease

  33. Autosomal Dominant Disorders • To be affected, can be homozygous or heterozygous b/c allele is dominant • Huntington’s Disease

  34. Sex-linked Disorders • Hemophilia • Colorblindness

  35. Sex-linked Disorders • Hemophilia is X-linked recessive • If mother is carrier & father has hemophilia: • genotypic ratio? • phenotypic ratio? • If mother is carrier & father is normal: • Make a Punnett square • genotypic ratio? • phenotypic ratio?

  36. Sex-linked Disorders • Colorblindness is X-linked recessive • In this Punnett square, what are the genotypes & phenotypes of the parents? Ishihara test for red- green color blindness

  37. Chromosomal Abnormalities in Number • abnormal number of chromosomes: • Caused by non-disjunction • failure of paired chromosomes to separate during meiosis 1 or meiosis 2

  38. Chromosomal Abnormality Disorders • Down Syndrome (trisomy 21) • person has 3 copies of chromosome # 21 • Caused by non-disjunction

  39. Chromosomal Abnormality Disorders • Klinefelter’s Syndrome • Sex chromosome disorder • Males have extra copy of X chromosome • XXY (or 47, XXY b/c 47 total chromosomes) • caused by non-disjunction

  40. Chromosomal Abnormalities in Structure • abnormal structure of chromosomes: • added, deleted, inverted, or translocated pieces

  41. Detecting Abnormalities • karyotyping • “picture of human chromosomes” • From blood sample • Can detect extra chromosomes or chromosomal abnormalities

  42. Detecting Abnormalities • Amniocentesis • sample of fluid surrounding fetus • can detect Down Syndrome • Chorionic villus biopsy • sample of cells from chorion

  43. Review & Animations • Vocab interactive • http://nortonbooks.com/college/biology/animations/ch10a02.htm • Crosses • http://www.sonefe.org/online-biyoloji-dersleri/grade-12/monohybrid-cross/ • Drag & drop genetics • http://www.zerobio.com/drag_gr11/mono.htm • Various • http://www.abpischools.org.uk/page/modules/genome/dna4.cfm?coSiteNavigation_allTopic=1 • Genetic disorders • http://www.humanillnesses.com/original/Gas-Hep/Genetic-Diseases.html

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