Sex Linkages

• Mendelian Hereditary Factors had no identified cellular structures that could house them, so most biologists were skeptical about it
• Chromosome Theory of Inheritance

  • Developed by Walter S. Sutton, Theodor Boveri, and others around 1902
  • Mendelian genes have specific loci along chromosomes
  • Chromosomes undergo segregation and independent assortment
  • Locus: specific location of a gene
• Thomas Hunt Morgan


  • His experiment proved that chromosomes are the location of Mendel's heritable factors
  • Studied the Drosophila melanogaster (fruit fly) which is a common insect that feeds on the fungi growing on fruit
  • "The fly room" was used to pertain to Morgan's laboratory
• Why Morgan used fruit flies

  • A large number of offspring can be produced within a single mating
  • Fast generation time (every two weeks, a new generation can be bred)
  • Fruit flies has only 4 pairs of chromosomes which are also easily distinguishable
• • Wild type - red eyes ; Mutant - white eyes / traits that are alternatives to the wild type
  • Wild type allele is dominant while the mutant allele is recessive
  • After breeding the F1 flies, he observed the 3:1 phenotypic ratio among F2 offspring however, the white-eye trait showed up only in males
  • A fly's eye color is linked to its sex; the gene involved in his white-eye mutant was located exclusively on the X chromosome
• Chromosomal Basis of Sex

  • XX - female (two X chromosomes)
  • XY - male (one X chromosome & one Y chromosome)
  • Y chromosome is much SMALLER than the X chromosome
  • Short segments at either end of the Y chromosome are the only regions homologous with regions on X; these allow X & Y chromosomes to pair and behave like homologs
  • The two sex chromosomes segregate during meiosis; each egg receives one X chromosome while for sperm, half of it receives an X chromosome and the other half receives a Y chromosome
• Sex-linked Gene


  • Gene located on either sex chromosome
  • X-linked genes: the genes (approx. 1,100) contained in the human X chromosome
  • Y-linked genes: genes (78) located on the human Y chromosome
• Very few disorders are passed on from father to son on the Y chromosome because there are only a few Y-linked genes
• WNT4


  A gene on chromosome 1 (autosome) which encodes a protein that promotes ovary development
• Intersex


  Individuals with intermediate sex characteristics
• Transgender


  Individuals with anatomical features that do not match their sense of their gender
• Inheritance of X-Linked Genes

  • Fathers pass X-linked alleles to all of their daughters (but NONE to their sons)
  • Mothers can pass X-linked alleles to both sons and daughters
  • Homozygous, Heterozygous: A female needs two copies of the allele (a female with a genotype of homozygous recessive for a trait exhibits the trait phenotypically)
  • Hemizygous: A male needs only one copy of the allele
• Far more males than females have X-linked recessive disorders
• Duchenne muscular dystrophy


  • A disease that is characterized by a progressive weakening of the muscles and loss of coordination which affects the individual's life expectancy (shortened)
  • The disorder is due to the absence of a key muscle protein called dystrophin
• Hemophilia


  • An X-linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting
  • A hemophiliac individual experiences prolonged bleeding when injured because a firm clot is slow to form
• X Inactivation in Female Mammals

  • Almost all of one X chromosome in each cell in female mammals becomes inactive during early embryonic development
  • Thus, cells of females and males have the same effective dose of most X-linked genes
  • Barr Body: a compact object made when the inactive X in each cell of a female condenses
• Linked Genes


  Genetically linked genes are two or more genes that are located near each other on the same chromosome which tend to be inherited together
• Genetic Recombination


  • Production of offspring with combinations of traits that differ from those found in either P generation parent
  • Parental types: offspring whose phenotype matches either of the phenotypes of the P generation
  • Recombinant types: (nonparental phenotypes found among offspring) offspring which have new combinations phenotypically
• Crossing Over


  Process which accounts for the recombination of linked genes. A set of proteins breaks the DNA molecules of one maternal and one paternal chromatid and rejoins each to the other.
• Nondisjunction


  • Members of a pair of homologous chromosomes do not move apart properly (during Meiosis I) or sister chromatids fail to separate (during Meiosis II)
  • Thus, one gamete receives two of the same type of chromosome while the other gamete receives NO copy
• Aneuploidy


  • A condition wherein the zygote has an abnormal number of a particular chromosome
  • Monosomic (2n-1 chromosomes): If there is no copy of a particular chromosome, it will lead to a missing chromosome in the zygote
  • Trisomic (2n+1 chromosomes): A chromosome is present in triplicate in the zygote
• Polyploidy


  • Having more than two complete chromosome sets in all somatic cells
  • Triploidy (3n) -3 chromosomal sets - happens by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes
  • Tetraploidy (4n) -4 chromosomal sets -Results from the failure of a 2n zygote to divide after replicating its chromosomes
• Deletion


  • Occurs when a chromosomal fragment is lost; thus, the affected chromosome is missing certain genes
  • Broken fragment may become reattached as an extra segment to a sister/nonsister chromatid
• Duplication


  A portion of a chromosome is present in two (an extra) copies (duplicated)
• Inversion


  Reattachment of a chromosomal fragment to the original chromosome but in the reverse orientation
• Translocation


  A fragment joins to a nonhomologous chromosome (rearrangement)
• Down Syndrome


  • Aneuploid condition
  • Result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes
  • Also called as trisomy 21 (since cells are trisomic for chromosome 21)
• Klinefelter Syndrome


  • Extra X chromosome in male (XXY)
  • Individuals that suffer under this syndrome have male sex organs but their testes are small and produce little/no sperm
• Males with extra Y chromosome (XYY) undergo typical sexual development and do not exhibit any well-defined syndrome, but tend to be taller than average
• Trisomy X


  • Females with 3 X chromosomes (XXX)
  • Generally healthy individuals and have no unusual physical features other than being slightly taller than average
• Monosomy X (Turner Syndrome)


  • XO
  • Individuals are phenotypically female but they are usually sterile (their sex organs do not mature)
• Cri du Chat


  • 'Cry of the cat'
  • Results from a specific deletion in chromosome 5
  • Individuals born with this condition has a small head with unusual facial features, severe intellectual disabilities, and a cry that sounds like the mewing of a distressed cat
• Chronic Myelogenous Leukemia (CML)

  • Occurs when a reciprocal translocation happens during mitosis of cells which are precursors of white blood cells
  • The exchange causes cancer by creating a new 'fused' gene that leads to uncontrolled cell cycle progression
• Pedigree Analysis


  Collected information about a family's history for a particular trait that is assembled into a family tree describing the trait across generations
• Recessively Inherited Disorders


  • Heterozygotes may transmit the recessive allele to their offspring (carrier)
  • Heterozygotes are phenotypically normal yet are carriers
• Recessively inherited disorders


  • Albinism
  • Cystic Fibrosis
  • Sickle-cell Disease
• Dominantly Inherited Disorders


  • Disorders due to dominant alleles
  • Traits for which the recessive allele is much more prevalent than the corresponding dominant allele
• Dominantly inherited disorders


  • Achondroplasia
  • Huntington's Disease
  • Polydactyly