BIOL 1st Edition Lecture 18 Outline of Last Lecture I Intermediate Inheritance II Multiple Alleles III Epistasis IV Polygenic Traits V Pleiotrophy VI Environmental Influence Outline of Current Lecture I Chromosomes II Sex Linked Inheritance III Genetic Problems with X Linked Genes IV V VI How to Solve a Genetics Problem Chromosomal Alterations Chromosomal Alterations Based on the Number of Chromosomes VII Chromosomal Alterations Based on Chromosomal Structure Changes VIII HW Genetics Problems with Solutions due 4 15 14 Current Lecture Chapter 15 Chromosomes I Chromosomes a Genes are located on chromosomes b Linked genes genes on the same chromosome tend to be inherited together i Do not assort independently during meiosis ii Go to the same gamete c Sex chromosomes determine sex i One pair of sex chromosomes that act as homologs during meiosis ii Each gamete gets one sex chromosome to contribute to the zygote d Homogametic sex produces one kind of gamete with regard to sex chromosomes e Heterogametic sex produces 2 kinds of gamete i This determines the sex of offspring f 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 female iv XY male II Sex Linked Inheritance a One pair of sex chromosomes in humans called the X and Y b The X chromosome i Much larger ii Carries many genes unrelated to gender determination color blindness etc iii Many more traits that the Y chromosome because it is so much bigger c The Y chromosome i Smaller ii 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 viceversa e Sex linked because they are inherited according to gender III Genetic Problems with X Linked Genes a Males receive their X chromosome from their mothers b Males receive their Y chromosome from their fathers i Fathers cannot pass on X linked traits to their sons c If an X linked trait is recessive i Females will express it ONLY if they are homozygous recessive ii Males will express it even though it is recessive because they only have one X gene IV How to Solve a Genetics Problem a The colorblindness allele b is sex linked and recessive The normal allele B is dominant There is 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 b B Bb Bb Y bY bY 4 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 Alterations a These are due to errors in meiosis or to mutations b These 2 basic kinds i Change in the number of chromosomes ii Change in the structure of chromosomes VI Chromosomal Alterations Based on the Number of Chromosomes a Aneuploidy abnormal number of chromosomes per cell b Nondisjunction a pair of homologs does not separate properly during Meiosis I c Example of nondisjunction i Pair 1 and Pair 2 Meiosis Produces 2 cells ii First daughter cell has 2 chromosomes from of pair 1 and 1 chromosome from pair 2 1 If this gamete is fertilized zygote will be trisomic for chromosome 1 3 copies instead of 1 iii Second daughter cell has no copies of chromosomes from pair 1 and only one from pair 2 1 If this gamete is fertilized zygote will be monosomic for chromosome 1 only 1 copy instead of 2 d A number of serious human disorders are due to aneuploidy e The frequency of aneuploid disorders is high but most aneuploidy zygotes are spontaneously aborted f Some are less upsetting to genetic balance and those survive g 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 mother 3 Syndrome includes characteristic facial features shortness heart defects and mental retardation ii Klinefelter s Syndrome XXY extra male chromosome in males 1 1 2000 in US 2 Male sex organs always sterile 3 Feminine body contours 4 Normal intelligence iii Turner s Syndrome XO only known human monosomy 1 1 10 000 in US 2 Phenotypically female 3 Do not mature sexually sterile 4 Normal intelligence VII Chromosomal Alterations Based on Chromosomal Structure Changes a Deletions fraction of the chromosome breaks off and is missing i Homozygous deletions are usually lethal ii Example Cru de Chat Syndrome 1 Due to heterozygous deletion of chromosome 5 2 Small head unusual cry 3 Mentally retarded 4 Usually die in infancy or childhood b Inversion fragment of chromosome breaks off and the reattaches somewhere else backwards c Duplication fragment of chromosome breaks off one homolog and reattaches to the other homolog this sequence is then present 2 times in the other homolog d Translation fragment of chromosome breaks off and then reattaches to a different chromosome not the same homologous pair VIII HW Genetics Problems a The sickle cell allele h is recessive and is NOT sex linked The normal allele H is dominant Two people plan to have children and both are heterozygous at this locus Show a Punnett Square analysis i Set up 1 Mother s genotype Hh