BIO 101 1st Edition Lecture 19 Outline of Last Lecture I 17th Century genetics II Mendel s Experiments a Law of Segregation b Law of Independent Assortment III Human Genetics a Recessive Disorders b Dominant Disorders IV Complications of Mendelian Genetics Outline of Current Lecture I Chromosomal basis of sex II Genetics Problems a X linked diseases b Chromosomal alterations i Change in Chromosome number ii Changes in the structure of the chromosome Chapter 15 Chromosomal Alterations Genes are located on chromosomes Genes on the same chromosome tend to be inherited together called linked genes 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 offspring Several systems of Sex Determination Humans have X Y system Male is the heterogametic sex sperm carry X sperm carry Y Female is homogametic all eggs carry the X XX female XY male BIO 101 1st Edition Sex 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 bigger Genetics problems involving X linked genes Males receive their X chromosome ONLY from their mother from their father they get the Y fathers cannot pass X linked trait to their sons If an X linked trait is recessive then o Females will express it ONLY if they are homozygous recessive o Males will express it even though it is recessive because they have only one X their other copy is a Y and does not have X linked genes Sample X linked Problem The 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 Problem Mother 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 and her 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 BY Cross between bb x BY Gametes Mother s eggs all carry X with b allele Father s sperm have X with B allele have Y with no colorblind gene at all BIO 101 1st Edition Girls 50 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 Boys 50 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 chromosomes b change in the structure of chromosomes A Change in Chromosome Number Aneuploidy abnormal number of chromosomes per cell Nondisjunction 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 survive Examples 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 retardation BIO 101 1st Edition 2 Klinefelter s syndrome XXY extra X in male 1 2000 in US male sex organs always sterile feminine body contours normal intelligence 3 Turner s Syndrome XO only known human monosomy 1 40 000 in US phenotypically female do not mature sexually sterile short normal intelligence B 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 childhood 2 Inversion fragment of chromosome breaks off and then re attaches to some place but backwards 3 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 4 Translation fragment of chromosome breaks off and then re attaches to a different chromosome NOT the same homologous pair