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Chapter 12

Chapter 12 . Inheritance Patterns and Human Genetics. MORGAN. In early 1900’s, geneticist Thomas Hunt Morgan begin breeding Drosophila, the fruit fly Drosophila has four pairs of chromosomes Morgan observed that one chromosome was different in males than in females

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Chapter 12

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  1. Chapter 12 Inheritance Patterns and Human Genetics

  2. MORGAN In early 1900’s, geneticist Thomas Hunt Morgan begin breeding Drosophila, the fruit fly • Drosophila has four pairs of chromosomes • Morgan observed that one chromosome was different in males than in females • He called this smaller chromosome the X chromosome, and the larger chromosome the Y chromosome

  3. Morgan hypothesized that the X and Y chromosomes are sex chromosomes • All other chromosomes are autosomes Like other chromosomes, sex chromosomes form pairs and segregate into separate cells during meiosis I - Gametes that form in meiosis II have either an X chromosome or a Y chromosome

  4. - Gametes produced by males can contain either an X chromosome or a Y chromosome • Gametes produced by females contain only X chromosomes MALES DETERMINE SEX OF AN OFFSPRING

  5. In humans and fruit flies, an egg that is fertilized by a sperm with an X chromosome will be a female zygote (XX) and an egg that is fertilized with a Y chromosome will be a male zygote (XY) - This system means that 50% of offspring of any mating will be male, and 50% will be female

  6. Sex Linkage • X-linked genes- genes found on the X chromosome • Y-linked genes- genes found on the Y-chromosome • The presence of a gene on a sex chromosome is called Sex Linkage

  7. Morgan experimented with eye color in Drosophila

  8. Although most fruit flies have red eyes, a few males have white eyes • When Morgan crossed a white eyed male with a red-eyed female: F1= all red eyed flies F2= 3 red :1 white BUT ALL WHITE-EYED FLIES WERE MALE! WHY NO FEMALES?

  9. Morgan hypothesized that the gene for eye color must be carried on the X chromosome • Eye color in Drosophila is an X-linked trait • Another cross:

  10. Linkage Groups • There are thousands of genes, so there must be more than one per chromosome • Linkage Group- genes located on one chromosome • Another experiment!

  11. MORE FLIES! - In Drosophila, the allele G for gray body is dominant over the allele g for black body • The allele L for long wings is dominant over l for short wings Morgan crossed homozygous gray, long-winged flies (GGLL) with homozygous black, short-winged flies (ggll) to produce a heterozygous F1 generation: GgLl

  12. He then crossed members of this F1 generation (GgLl x GgLl) to produce an F2 generation • Morgan knew that if alleles for body color and wing length were on different chromosomes, they should assort independently and produce an F2 generation with a phenotypic ratio of 9:3:3:1

  13. Morgan predicted that if alleles were on the same chromosome, he would get a 3:1 ratio in the F2 generation • His results were a 3:1 phenotypic ratio (3 gray long : 1 black short) • Morgan hypothesized that the genes for body color and wing length are LINKED

  14. Crossing-Over Though the 3:1 ratio was produced, there were still a few offspring that had the genotype Ggll and ggLl HOW??? - Crossing over caused the linked traits to be inherited independently and not together Do you remember what happens in crossing-over?

  15. CROSSING OVER- the exchange of pieces of DNA between homologous chromosomes - This accounts for the unexpected phenotypes that Morgan obtained

  16. Chromosome Mapping The farther apart 2 genes are on a chromosome, the greater the chance of crossing-over occurring • Chromosome Map- diagram that shows the linear sequence of genes on a chromosome

  17. Mutation • Remember, a mutation is the change in the DNA of an organism • Can be helpful or harmful - Some mutations help an organism adapt to new surroundings - Some mutations cause diseases (cancer)

  18. Different types of Mutations 1. Germ-cell: occur in gametes, do not affect organism 2. Somatic: occur in body cells, can affect organism Ex: Skin cancer and Leukemia 3. Lethal Mutations- cause death, usually before birth

  19. Chromosome Mutations 1. Deletion- loss of a piece of chromosome due to chromosomal breakage 2. Inversion- segment breaks off and then reattaches in reverse to the same chromosome

  20. Chromosome Mutations cont. 3. Translocation- chromosome breaks off and then reattaches to another homologous chromosome 4. Nondisjunction- failure of a chromosome to separate from its homologue during meiosis - one gamete gets an extra chromosome, and one is missing one

  21. Gene Mutations Point Mutations- substitution, addition, or removal of a single nucleotide 1. Substitutions- one nucleotide in a codon is replaced with a different nucleotide, resulting in a new codon Sickle-cell anemia- adenine is substituted for thymine, causes misshaped red blood cells

  22. 2. Frame-shift mutations- addition or deletion of a nucleotide can cause other codons to shift

  23. 12-2 Human Genetics • Much of what has been discovered about the inheritance of traits has been learned in the study of human genetics

  24. Studying Human Inheritance • Humans have up to 20 times as many genes as the fruit fly, so inheritance is more complicated • Pedigrees • Family records that show how a trait is inherited over several generations • Certain phenotypes are usually repeated in predicable patterns- called Patterns of Inheritance

  25. Carriers- individuals who have one copy of a recessive autosomal allele • Do not express the allele, but can pass it to their offspring

  26. Genetic Traits and Disorders Genetic Disorders- diseases or debilitating conditions that have a genetic basis 1. Single-allele traits 2. Multiple-allele traits 3. Polygenic traits 4. X-linked traits 5. Sex-influenced traits

  27. Single-allele traits Traits controlled by a single allele of a gene Huntington’s Disease-caused by a dominant allele located on an autosome - mild forgetfulness, irritability, in 30’s or 40’s - in time causes loss of muscle control, spasms, severe mental illness and death - most people don’t know they have the disease until they have passed it onto their children

  28. Huntington's pedigree

  29. Genetic Marker- a short section of DNA that is known to have a close association with a particular gene located nearby - can give clues that genes such as the one that causes HD is present Other Diseases: - Sickle-cell anemia and Cystic Fybrosis

  30. Multiple-allele traits Controlled by 3 or more alleles of the same gene that code for a single trait • In humans, the ABO blood groups are controlled by 3 alleles, IA, IB, and i • Each individual’s blood type consists of two of these alleles

  31. IAIA genotype- type A blood IAi genotype- type A blood IBIB genotype- type B blood IBi genotype- type B blood ii genotype- type O blood IAIB genotype- type AB blood

  32. Polygenic Traits A trait controlled by 2 or more genes Ex: Skin color is controlled by 3 to 6 genes - Each gene results in a certain amount of melanin- pigment found in skin Ex: Eye color Many genes are also controlled by environment - Height is influenced by nutrition and disease

  33. X-Linked Traits Colorblindness- recessive X-linked disorder in which an individual cannot distinguish between certain colors Hemophilia- disease in which the ability of the blood to clot is impaired Duchene Muscular Dystrophy- weakens and destroys muscle tissue NOT ALL X-LINKED TRAITS ARE DISEASES

  34. Sex-influenced traits Needs to be a presence of male or female sex hormones in order for the trait to be expressed • Ex: Baldness- allele that causes baldness in men does not in women

  35. Disorders due to Nondisjunction Causes gamete to either lack a chromosome or have an extra one Normal chromosome # 46 1. Monosomy- zygote has only one copy of a particular chromosome (45 chromosomes) 2. Trisomy- zygote has 3 copies of a particular chromosome (47 chromosomes)

  36. Some abnormalities in chromosome number can be lethal, others are not -Down Syndrome- extra chromosome, causes mild to severe mental retardation Nondisjunction can also affect the sex chromosomes -Klinefelter’s Syndrome- males with an extra X chromosome (XXY) * these individuals have some feminine characteristics and are sometimes mentally retarded and infertile

  37. -Turner’s Syndrome- individuals with a single X chromosome (XO) * These individuals have a female appearance but do not mature sexually and remain infertile - Individuals with a single Y chromosome do not survive

  38. Detecting Genetic Disorders Genetic Screening- an examination of a person’s genetic makeup - May involve constructing a karyotype Genetic Counseling- a form of medical guidance that informs people about problems that could affect their offspring

  39. Diagnosing • Amniocentesis- a small amount of fluid from the sac that surrounds the fetus is removed and analyzed - between 14th and 16th week of pregnancy - fetal cells and proteins from fluid can be analyzed and a karyotype can be prepared 2. Chorionic villi sampling- a sample of the tissue chorionic villi is taken and analyzed - 8th and 10th week of pregnancy

  40. Sometimes tests are performed immediately after birth - Phenylketonuria- genetic disorder in which the body cannot metabolize the amino acid phenylalanine - the accumulation of this amino acid can cause severe brain damage - can be easily controlled by diet if known

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