BIO 101 1st EditionLecture 19Outline of Last LectureI. 17th Century geneticsII. Mendel’s Experimentsa. Law of Segregationb. Law of Independent AssortmentIII. Human Geneticsa. Recessive Disordersb. Dominant DisordersIV. Complications of Mendelian GeneticsOutline of Current LectureI. Chromosomal basis of sexII. Genetics Problemsa. X-linked diseasesb. Chromosomal alterationsi. Change in Chromosome numberii. Changes in the structure of the chromosomeChapter 15- Chromosomal Alterations- Genes are located on chromosomes- Genes on the same chromosome tend to be inherited together called linkedgenes- Linked genes do NOT assort independently during meiosis (like genes that are located on different chromosomes do)- Linked genes go to the same gamete (unless crossing over occurred)Chromosomal basis of SEX (gender)- Sex is determined by presence or absence of special chromosomes called SEX CHROMOSOMES- There is ONE pair of sex chromosomes and these behave as homologs during meiosis- therefore each gamete gets ONE of the sex chromosomes to contribute to the zygote- Homogametic sex- produces only one kind of gamete with regard to the sex chromosomes- Heterogametic sex- produces 2 kinds of gamete determines the sex of the offspringSeveral systems of Sex DeterminationHumans have X-Y systemMale is the heterogametic sex- ½ sperm carry X, ½ sperm carry YFemale is homogametic- all eggs carry the XXX= femaleXY- maleBIO 101 1st EditionSex-Linked Inheritance- One pair of sex chromosomes in humans, called X and Y- They determine gender XX=female XY= male- The X chromosomes is much larger than the Y and carries many genes that don’t have to do with gender determination (ex: color blindness, hemophilia)- The small Y chromosome carries mostly genes that encode traits ONLY found in males (ex: testes determining factor)- Genes found on the X chromosome usually do NOT have counterparts on the Y and vice versa- Genes on the X and Y chromosomes are said to be SEX-LINKED because they tend to be inherited according to gender- There are many more X-linked traits than Y-linked traits because the X chromosome is biggerGenetics problems involving X-linked genes- Males receive their X-chromosome ONLY from their mother (from their fatherthey get the Y): fathers cannot pass X-linked trait to their sons- If an X-linked trait is recessive theno Females will express it ONLY if they are homozygous recessiveo Males will express it even though it is recessive because they have onlyone X (their other copy is a Y and does not have X-linked genes)Sample X-linked ProblemThe color blindness allele (b) is X-linked and recessive. The normal allele (B) is dominant. There is no allele for colorblindness on the Y chromosome. If a colorblind woman marries a normal man, what is the chance that their daughters will be color blind? What about their sons? What is the chance the daughters will carry the colorblind allele?Set Up the ProblemMother’s genotype: she is XX (as all females are) and she has the colorblind phenotype, which is RECESSIVE: both her X chromosomes must have the b allele andher genotype is bb/Father’s genotype: he has only one X (as all males do) and the Y doesn’t have a gene for color blindness: his phenotype is entirely determined by the one X and he is normal: his genotype must be BYCross between bb x BYGametes- Mother’s eggs all carry X with b allele- Father’s sperm ½ have X with B allele, ½ have Y with no colorblind gene at allBIO 101 1st EditionGirls50% chance offspring will be girls (XX)- they all receive dominant B allele from father and recessive b allele from mother. O% chance they will be colorblind, 100% chance they will be carriers.Boys50% chance offspring will be boys (XY)- they all receive single X (from mother) with b allele. 100% chance they will be colorblind. Chromosomal Alterations- Due to errors in meiosis or to mutations- 2 basic kinds:a. change in the number of chromosomesb. change in the structure of chromosomesA. Change in Chromosome NumberAneuploidy- abnormal number of chromosomes per cellNondisjunction- a pair of homologs does not separate properly during Meiosis I- A number of serious human disorders are due to aneuploidy- The frequency of aneuploid disorders may be high, but most aneuploid zygotes are spontaneously aborted- Some are less upsetting to genetic balance and therefore surviveExamples:1. Down’s Syndrome- trisomy of chromosome #21 (the smallest human chromosome) occurs 1 in 700 in US.- Incidence increases with age of the mother 0.04% less than 30, 1.25% early 30’s= 30x increase- Syndrome includes characteristic facial features, shortness, heart defects, mental retardationBIO 101 1st Edition2. Klinefelter’s syndrome- XXY, extra X in male, 1/2000 in US, male sex organs, always sterile, feminine body contours, normal intelligence3. Turner’s Syndrome- XO, only known human monosomy, 1/40,000 in US, phenotypically female, do not mature sexually, sterile, short, normal intelligenceB. Changes in the Structure of Chromosomes 1. Deletions- fragment of a chromosome breaks off and is missing- Homozygous deletions are usually lethal (many genes are required)- Example- Cri-Du-Chat Syndrome: due to heterozygous deletion on chromosome #5, small head, unusual cry, mentally retarded, usually die in infancy or early childhood2. Inversion- fragment of chromosome breaks off and then re-attaches to some place, but backwards3. Duplication- fragment of chromosome breaks off one homolog and re-attaches to the OTHER homolog (this sequence is then present 2 times in the other homolog) 4. Translation- fragment of chromosome breaks off and then re-attaches to adifferent chromosome (NOT the same homologous