Lecture 18 Outline of Last Lecture I. Intermediate InheritanceII. Multiple AllelesIII. EpistasisIV. Polygenic TraitsV. PleiotrophyVI. Environmental InfluenceOutline of Current Lecture I. ChromosomesII. Sex-Linked InheritanceIII. Genetic Problems with X-Linked GenesIV. How to Solve a Genetics ProblemV. Chromosomal AlterationsVI. Chromosomal Alterations Based on the Number of ChromosomesVII. Chromosomal Alterations Based on Chromosomal Structure ChangesVIII. HW - Genetics Problems with Solutions (due 4/15/14)Current LectureChapter 15 – ChromosomesI. Chromosomesa. Genes are located on chromosomesb. “Linked genes”  genes on the same chromosome tend to be inherited togetheri. Do not assort independently during meiosisii. Go to the same gametec. Sex chromosomes – determine sexi. One pair of sex chromosomes that act as homologs during meiosisii. Each gamete gets one sex chromosome to contribute to the zygoted. Homogametic sex – produces one kind of gamete with regard to sex chromosomes BIOL 1st Editione. Heterogametic sex – produces 2 kinds of gametei. This determines the sex of offspringf. “X-Y system” – system of sex determination in humans i. Male is heterogametic sex  half of the sperm carries “X” and the other carries “Y”ii. Female is homogametic  all eggs carry the “X”iii. XX = femaleiv. XY = maleII. Sex-Linked Inheritancea. One pair of sex chromosomes in humans called the “X” and “Y”b. The “X” chromosome i. Much largerii. Carries many genes unrelated to gender determination (color blindness, etc.)iii. Many more traits that the “Y” chromosome because it is so much biggerc. The “Y” chromosomei. Smallerii. Carries traits that are generally ONLY found in males (testes determining factor)d. Genes found on the “X” chromosome generally do not have counterparts on the “Y” and vice-versae. “Sex-linked” because they are inherited according to genderIII. Genetic Problems with X-Linked Genesa. Males receive their “X” chromosome from their mothersb. Males receive their “Y” chromosome from their fathersi. Fathers cannot pass on X-linked traits to their sonsc. If an X-linked trait is recessive:i. Females will express it ONLY if they are homozygous recessiveii. Males will express it even though it is recessive because they only have one “X” geneIV. How to Solve a Genetics Problema. The colorblindness allele (b) is sex-linked and recessive. The normal allele (B) is dominant. Thereis no allele for colorblindness on the “Y” chromosome. A colorblind woman marries a normal man.i. Set-up the problem:1. Mother’s genotype  She is XX (as all females are), and she has the colorblind phenotype, which is recessive. Both her “X” chromosomes are therefore the “b” recessive allele – “bb”.2. Father’s genotype  He is XY (as all males are), and the “Y” chromosome can’t carry the colorblindness allele. His phenotype, therefore, is entirely determined by the “X” chromosome, which is normal – “B”. His genotype is written as “BY”.3. Punnett Square – Set-up the cross that will occur when the father and mother mate by using a Punnett Square, crossing “bb” (mother) with “BY” (father).b bB Bb BbY bY bY4. The resulting offspring are 2Bb, 2bY.b. What is the chance that their daughter will be colorblind?i. 50% of the offspring are female. They are represented by the “Bb” genotype.ii. They all received the dominant “B” allele from their father, and the recessive “b” allele from their mother.iii. There is a 0% chance they will be colorblind.c. What is the chance that their sons will be colorblind?i. 50% of the offspring are male. They are represented by the “bY” genotype.ii. They all received the single recessive “b” allele from their mother, and the ”Y” chromosome from their father.iii. There is a 100% chance that they will be colorblind.d. What is the chance that their daughter will carry the colorblind allele?i. 50% of the offspring are female. They are represented by the “Bb” genotype.ii. They all received the dominant “B” allele from their father, and the recessive “b” allele from their mother.iii. There is a 100% chance they will be carriers of the recessive “b” allele. V. Chromosomal Alterationsa. These are due to errors in meiosis or to mutationsb. These 2 basic kinds:i. Change in the number of chromosomesii. Change in the structure of chromosomesVI. Chromosomal Alterations Based on the Number of Chromosomesa. “Aneuploidy” – abnormal number of chromosomes per cellb. “Nondisjunction” – a pair of homologs does not separate properly during Meiosis Ic. Example of nondisjunction:i. Pair #1 and Pair #2  Meiosis  Produces 2 cellsii. First daughter cell has 2 chromosomes from of pair #1 and 1 chromosome from pair #21. If this gamete is fertilized, zygote will be trisomic for chromosome #1 3 copies instead of 1iii. Second daughter cell has no copies of chromosomes from pair #1, and only one from pair #21. If this gamete is fertilized, zygote will be monosomic for chromosome #1 only 1copy instead of 2d. A number of serious human disorders are due to aneuploidye. The frequency of aneuploid disorders is high, but most aneuploidy zygotes are spontaneously abortedf. Some are less upsetting to genetic balance, and those surviveg. Examples:i. Down’s Syndrome – trisomy of chromosome of #21 (the smallest human chromosome) 1. Occurs in 1 in 700 in US.2. Incidence increases with age of mother3. Syndrome includes characteristic facial features, shortness, heart defects, and mental retardationii. Klinefelter’s Syndrome – “XXY”, extra male chromosome in males1. 1/2000 in US2. Male sex organs = always sterile3. Feminine body contours4. Normal intelligenceiii. Turner’s Syndrome – “XO”, only known human monosomy1. 1/10,000 in US2. Phenotypically female3. Do not mature sexually = sterile4. Normal intelligenceVII. Chromosomal Alterations Based on Chromosomal Structure Changesa. Deletions – fraction of the chromosome breaks off and is missingi. Homozygous deletions are usually lethalii. Example: Cru-de-Chat Syndrome1. Due to heterozygous deletion of chromosome #52. Small head; unusual cry3. Mentally retarded4. Usually die in infancy or childhoodb. Inversion – fragment of chromosome breaks off and the reattaches somewhere else, backwards.c. Duplication – fragment of chromosome breaks off one homolog and reattaches