BIOL 1441 1st Edition Lecture 27 Outline of Last Lecture I. Chromosome Theory of InheritanceII. Morgan’s Experimental EvidenceIII. Sex-Linked GenesIV. Gene LinkageV. Linkage MappingOutline of Current Lecture I. Inheritance of sex-linked genesII. Color blindnessIII. Barr BodyIV. Genetic disordersV.Current LectureI. Inheritance of Sex-Linked Genesa. Most of the genes on Y chromosome are related to sex determinationb. Many genes on the X chromosome are not related to sex determinationi. Most sex-linked inherited traits are on Xc. Father gives an X- he passes his traits to his daughteri. Daughter has 2 X chromosomesd. Father gives a Y- his son will express only his mothers X linked traitsi. Son has 1 X chromosome from mome. Mom can pass her traits equally to sons and daughters- she can only give an Xf. Daughter has 2 X chromosomes- needs to 2 recessive alleles for the recessive trait to be expressedg. Son only has 1 X, therefore only one allele of a recessive trait will be expressedThese 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.i. Recessive trait behaves as a dominant traith. If a son receives a recessive trait from mom, it will be expressedII. Colorblindnessa. Sex-linked trait- seen mostly malesb. Dad is colorblind- will his daughter be? i. Only if mom also gives recessive colorblind alleleii. Rare allele- not likelyiii. If mom is a carrier it is possiblec. Dad is colorblind- will his son be?i. Has no effect on his son, gene is on Xii. If mom is a carrier, 50% chanced. Women pass this trait along, not mene. Disorders caused by recessive alleles on the X chromosome i. Duchenne muscular dystrophy- affects 1/3500 males born in US1. Progressive weakening of muscles and loss of coordination2. Rarely live past their early 20’sii. Hemophilia- absence of blood clotting proteins1. Bleed to deathIII. X inactivation – Barr Bodya. 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 chromosomeb. Inactivated chromosome- Barr bodyi. Compact object lies along inside of nuclear envelopec. 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 characteri. Some cells express one allele, other cells will express the othere. Barr Body Inactivationi. How is it turned “off”?1. Some DNA is modified- methylated (-CH3)a. Silences the gene, it cannot be transcribed2. XIST gene- X-inactive specific transcripta. Active only on Barr-body chromosomeIV. Genetic Disordersa. Base mutationi. Sickle cell anemiab. Nondisjunction- aneuploidyi. Down syndrome, Klinefelters syndrome, Turners syndromec. Alter chromosome structurei. 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 Numberi. Nondisjunction- pairs of homologous chromosomes do not separate normally during meiosisii. As a result- one gamete receives two of the same type of chromosome, and another gamete receives no copyiii. Can happen in meiosis I or IIe. Aneuploidyi. Aneuploidy- results from the fertilization of gametes in which nondisjunction occurredii. Offspring have an abnormal number of a particular chromosomeiii. Trisomic zygote- 3 copies of a particular chromosome1. Trisomy 21- down syndromeiv. Monosomic zygote- 1 copy of a particular chromosomev. Polyploidy- organism has more than two complete sets of chromosomesvi. Down Syndrome1. Aneuploid condition that results from 3 copies of chromosome 21 (trisomy 21)a. 1/700 children born in US2. Characteristic facial features, short stature, heart defects, mental retardationvii. Aneuploidy of Sex Chromosomes1. Klinefelter syndrome- extra X chromosome in a male, producing XXY individualsa. Sterile2. Monosomy X (Turner syndrome)- X0 femalesa.