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Unit 8: Genetics & Heredity

Unit 8: Genetics & Heredity Unit 9: Human Genetic Disorders Ch. 26: Inheritance of Traits & Ch. 27: Human Genetics. Unit 8: 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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Unit 8: Genetics & Heredity

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

  2. Unit 8: 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? • So zygote gets right # of chromosomes…

  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 (formation of gametes) • crossing-over during prophase 1 • alignment of chromosomes during metaphase 1

  5. Genes & Alleles • What is a “gene”? • section of chromosome that codes for a specific protein • & 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 or recessive • What is a dominant allele? • allele that prevents expression of (“masks”/“hides”) recessive trait • What is a recessive allele? • allele whose trait can be seen only when the organism is pure (homozygous) for that trait

  7. Dominant & Recessive Alleles • How are alleles represented? • with letters • usually the first letter of the dominant trait • If the same letter is used for dominant & recessive, how do we know which allele is which? • CAPITAL = DOMINANT • lowercase = recessive

  8. Allele Combinations • What does “homozygous” mean? • both alleles are the same • homozygous (pure) dominant (ex. AA) • homozygous (pure) recessive (ex. aa) • What does “heterozygous” mean? • both alleles are different • heterozygous (hybrid) (ex. Aa)

  9. Genotype vs. Phenotype • What is “genotype”? • organism’s actual genetic “code”/make-up (alleles) • What does the genotype do? • codes for protein that causes trait (phenotype) • How do we represent an organism’s genotype? • 2 letters (one for each allele) • one from mom & one from dad • ex. PP, Pp, pp

  10. Genotype vs. Phenotype • What is “phenotype”? • the outward (physical) expression of the genotype (trait we “see”) • What actually causes the “phenotype” (trait) we see? • the protein that is produced (due to the organism’s genotype “code”/alleles) • How do we represent an organism’s phenotype? • usually an adjective • ex. purple, white, tall, short, etc.

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

  12. PP Pp pp

  13. Predicting Traits in Offspring • What are Punnett Squares? • a way to predict the results of crosses (mating) • letters outside represent possible alleles in gametes of each parent • top = one parent & side = other parent • letters inside boxes represent possible allele combinations (genotypes) in offspring (& phenotypes) • can be used to determine probability and ratios Bb BB

  14. Making a Punnett Square • Parents are Tt & tt genotypes… • So… Tt x tt is our cross (mating)

  15. Passing Traits to Offspring & Probability • What is probability? • chance an event will occur • What is the chance of getting heads? tails? • ½ • If you flip two coins, of getting 2 heads? 2 tails? • ½ x ½ = 1/4 • What is the chance of a couple having a boy? a girl? • 1/2 • of having five girls? • ½ x ½ x ½ x ½ x ½ = 1/32 • or ( ½ )5 = 1/32

  16. Passing Traits to Offspring & Ratios • What is a “genotypic ratio”? • probable ratio of genotypes (alleles) in offspring of a given cross • Ex. If cross Pp & Pp • 1PP : 2Pp : 1 pp

  17. Passing Traits to Offspring & Ratios • What is a “phenotypic ratio”? • probable ratio of phenotypes (traits) in offspring of a given cross • resulting from the genotypes of the offspring • Ex. If cross Pp & Pp • 3 purple : 1 white

  18. Passing Traits to Offspring & Ratios • What is an “expected ratio”? • ratio we expect to get based on probability (Punnett Square) • What is an “observed ratio”? • ratio we actually get • Why would these be different? • fertilization is random • some embryos die during early stages

  19. Gregor Mendel • Father of Genetics • 1822-1884 • studied garden pea plants • 7 different traits with clearly different forms • tried to determine how they were passed from parent to offspring

  20. Mendel’s Experiments • What happened when Mendel mated a pure purple parent (PP) & a pure white parent (pp)? • all offspring had: • purple phenotype • heterozygous (hybrid) genotype • Pp

  21. Mendel’s Experiments • What happened when Mendel let the heterozygous (hybrid) offspring from his first experiment self-pollinate? • So… Pp x Pp • new offspring weren’t all purple…

  22. Mendel’s Principle of Dominance • What did Mendel notice from his experiments? • that one form dominates over the other • …dominant trait prevents the expression of the recessive trait • What trait was dominant in these plants? • PUPRLE = dominant • What trait was recessive? • white = recessive

  23. Dominant/Recessive is Not Always the Method 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 • How many chromosomes do humans have (in body cells)? • 46… 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? • Dad’s b/c he is the only one that can give a Y; mom always gives X. What is the probability of having a son? A daughter?

  25. Sex-linked Inheritance • X & Y chromosomes not fully homologous. Why? • X is bigger & carries more genes

  26. Sex-linked Inheritance • How many alleles will a male have for traits carried only on the X chromosome? • 1 b/c only have 1 X chromosome (Y doesn’t have allele) • What is this called? • X-linked or sex-linked • Ex. eye color in fruit flies, hemophilia in humans, colorblindness in humans • X-linked traits & disorders are more common in males. Why??? • b/c female has XX, more likely she will have a copy of dominant allele… males = XY… can only get dominant allele on X (& only have 1 X)

  27. Sex-linked Inheritance • How do we make predictions made using Punnett squares for sex-linked traits? • Consider the sex chromosome (X/Y) & allele for the trait it carries (“exponent”) TOGETHER as a unit… • ex. XG (= X w/ dominant allele), Xg (= X w/ recessive allele), Y (= Y w/ NO allele) • What if a female is heterozygous (XGXg)? • she does not show the trait/have the disorder, but is a carrier • & can pass gene to offspring • Can a male be a carrier? • No, b/c only has one X chromosome w/ allele… so either has it or doesn’t XG Xg XG XG XG Xg XG Y Xg Y XG Y

  28. Sex-linked Inheritance • Drosophila (fruit fly) eye color is sex-linked • What are the sex, genotype, & phenotype of each offspring? Are there any carriers for the white eye gene? • Left picture: 2 females with red eyes = XRXr (carrier white eye gene) & 2 males with white eyes = XrY • Right picture: female w/ red eyes = XRXR, female w/ red eyes = XRXr (carrier white eye gene), male w/ red eyes = XRY, & male w/ white eyes = XrY XR Y XR XR XR Y XR Xr Xr Y XR Xr

  29. Multiple Alleles & Codominance • How many possible genotypes are there? • How many phenotypes? • Can you spot the blood type that is the result of codominance? • What is meant by multiple alleles? • more than 2 different forms of an allele exist • but individual still has just 2 • Ex. human blood types • (3) multiple alleles • A (IB) • B(IB) • o (i)

  30. Multiple Alleles & Codominance • How many possible genotypes are there? • How many phenotypes? • What is meant by codominance? • both alleles are “expressed” equally • Ex. human blood types also exhibit codominance (as well as multiple alleles) • A & B are codominant and are “expressed” equally • A = B (codominant) • o (recessive) • So… (A = B) > o

  31. Unit 9: Human Genetic Disorders • What causes genetic disorders? • DNA mutation (usually recessive) or chromosome abnormalities (in # or structure) that cause the production of abnormal proteins

  32. Human Genetic Disorders • How can we group genetic disorders? • autosomal recessive disorders(*most genetic disorders) • allele is recessive & found on a chromosome from pairs 1 – 22 (autosomes or body chromosomes) • cystic fibrosis, sickle-cell anemia, Tay-Sachs disease • autosomal dominant disorders • allele is dominant & found on a chromosome from pairs 1 – 22 (autosomes or body chromosomes) • Huntington’s Disease • sex-linked disorders • allele (which is usually recessive) is found on the 23rd pair of chromosomes (sex chromosomes)… Usually on the X chromosome • hemophilia, color blindness • chromosomal abnormality disorders • result from errors in chromosome # or structure • Down Syndrome (trisomy 21), Klinefelter’s Syndrome (XXY)

  33. Autosomal Recessive Disorders • What genotype(s) must a person have to be affected? • homozygous recessive (gg) • cystic fibrosis • sickle-cell anemia • Tay-Sachs Disease • Can someone be a carrier? Why/why not? • yes • b/c if heterozygous (Gg), person carries the gene, but isn’t affected • due to having the “normal” dominant gene

  34. Autosomal Dominant Disorders • What genotype(s) must a person have to be affected? • can be homozygous (GG) or heterozygous (Gg) b/c allele is dominant • Huntington’s Disease • Can someone be a carrier? Why/why not? • No • b/c even if person is heterozygous (Gg), person will have disorder • due to dominant “disease” gene blocking “normal” recessive gene

  35. Sex-linked Disorders • Remember from earlier… hemophilia is X-linked & recessive • What are the possible genotypes & phenotypes? Can someone be a carrier? • XHXH = normal female • XHXh = carrier female (but not affected) • XhXh = female w/ hemophilia • XHY = normal male • XhY = male w/ hemophilia • Why can’t a male be a carrier? • b/c only has one X chromosome w/ allele… so either has it or doesn’t • Ex. mom = carrier & dad = normal: • Make a Punnett square. • genotypic ratio? • phenotypic ratio? 1 XHXH : 1 XHXh : 1 XHY : 1 XhY 1 normal female: 1 carrier female : 1 normal male: 1 hemophiliac male

  36. Sex-linked Disorders • Remember from earlier…colorblindness is X-linked recessive • What are the possible genotypes & phenotypes? Can someone be a carrier? • XCXC = normal female • XCXc = carrier female (but not affected) • XcXc = colorblind female • XCY = normal male • XcY = colorblind male • In this Punnett square, what are the genotypes & phenotypes of the parents? • father: • genotype = XCY & phenotype = colorblind • mother: • genotype = XCXc & phenotype = carrier Ishihara test for red- green color- blindness

  37. Chromosomal Abnormalities in Number • What causes an abnormal number of chromosomes? • non-disjunction • failure of paired chromosomes to separate during meiosis 1 or meiosis 2

  38. Disorders Due to Abnormal Chromosome # • What is Down Syndrome (trisomy 21)? • when person has 3 copies of chromosome # 21 • What is Klinefelter’s Syndrome? • a sex-chromosome disorder in which males have extra copy of X chromosome • XXY (or 47, XXY b/c 47 total chromosomes) • What causes Down Syndrome (trisomy 21) & Klinefelter’s Syndrome? • non-disjunction • failure of paired chromosomes to separate during meiosis 1 or meiosis 2

  39. Chromosomal Abnormalities in Structure • What is causes structural abnormalities in chromosomes? • pieces are added, deleted, inverted, or translocated

  40. 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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