BIOL 1441 1st Edition Lecture 27 Outline of Last Lecture I Chromosome Theory of Inheritance II Morgan s Experimental Evidence III Sex Linked Genes IV Gene Linkage V Linkage Mapping Outline of Current Lecture I Inheritance of sex linked genes II Color blindness III Barr Body IV Genetic disorders V Current Lecture I Inheritance of Sex Linked Genes a Most of the genes on Y chromosome are related to sex determination b Many genes on the X chromosome are not related to sex determination i Most sex linked inherited traits are on X c Father gives an X he passes his traits to his daughter i Daughter has 2 X chromosomes d Father gives a Y his son will express only his mothers X linked traits i Son has 1 X chromosome from mom e Mom can pass her traits equally to sons and daughters she can only give an X f Daughter has 2 X chromosomes needs to 2 recessive alleles for the recessive trait to be expressed g Son only has 1 X therefore only one allele of a recessive trait will be expressed These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute II III IV i Recessive trait behaves as a dominant trait h If a son receives a recessive trait from mom it will be expressed Colorblindness a Sex linked trait seen mostly males b Dad is colorblind will his daughter be i Only if mom also gives recessive colorblind allele ii Rare allele not likely iii If mom is a carrier it is possible c Dad is colorblind will his son be i Has no effect on his son gene is on X ii If mom is a carrier 50 chance d Women pass this trait along not men e Disorders caused by recessive alleles on the X chromosome i Duchenne muscular dystrophy affects 1 3500 males born in US 1 Progressive weakening of muscles and loss of coordination 2 Rarely live past their early 20 s ii Hemophilia absence of blood clotting proteins 1 Bleed to death X inactivation Barr Body a Mammalian females one of the two X chromosomes in each cell is randomly inactivated during embryonic development i Only express mom or dad s X chromosome b Inactivated chromosome Barr body i Compact object lies along inside of nuclear envelope c In EACH embryonic cell present at the time of X inactivation one of the X chromosome is inactivated RANDOMLY d If a female is heterozygous for a particular gene located on the X chromosome she will be a mosaic for that character i Some cells express one allele other cells will express the other e Barr Body Inactivation i How is it turned off 1 Some DNA is modified methylated CH3 a Silences the gene it cannot be transcribed 2 XIST gene X inactive specific transcript a Active only on Barr body chromosome Genetic Disorders a Base mutation i Sickle cell anemia b Nondisjunction aneuploidy i Down syndrome Klinefelters syndrome Turners syndrome c Alter chromosome structure i Deletion Cri du chat lose portion of chromosome ii Duplication repeats a segment iii Inversion breaks in 2 places flips and reinserts iv Translocation CML part of chromosomes breaks off and attaches to another d Abnormal Chromosome Number i Nondisjunction pairs of homologous chromosomes do not separate normally during meiosis ii As a result one gamete receives two of the same type of chromosome and another gamete receives no copy iii Can happen in meiosis I or II e Aneuploidy i Aneuploidy results from the fertilization of gametes in which nondisjunction occurred ii Offspring have an abnormal number of a particular chromosome iii Trisomic zygote 3 copies of a particular chromosome 1 Trisomy 21 down syndrome iv Monosomic zygote 1 copy of a particular chromosome v Polyploidy organism has more than two complete sets of chromosomes vi Down Syndrome 1 Aneuploid condition that results from 3 copies of chromosome 21 trisomy 21 a 1 700 children born in US 2 Characteristic facial features short stature heart defects mental retardation vii Aneuploidy of Sex Chromosomes 1 Klinefelter syndrome extra X chromosome in a male producing XXY individuals a Sterile 2 Monosomy X Turner syndrome X0 females a Sterile