Chapter 15 Chromosomal Basis of Inheritance

Chromosome Theory Chromosome theory of inheritance – states that genes have specific loci on the chromosome and they are what go through segregation and independent assortment Thomas Hunt Morgan is credited with providing support to theory with work on fruit flies (Drosophila Melanogaster) - showed early framework for sex linked genes

Sex-linked Genes Chromosomal basis of sex determination is simply X or Y. - one allele from each parent - XX is female -XY is male - ovaries produce only X chromosomes , whereas sperm produce X and Y that segregate during meiosis

(d) The haplo-diploid system Figure 15.6 44 + XY Parents 44 + XX or Sperm 22 + X 22 + X 22 + Y Egg 76 + ZW 76 + ZZ 44 + XX 44 + XY or Zygotes (offspring) (a) The X-Y system (c) The Z-W system Figure 15.6 Some chromosomal systems of sex determination 32 (Diploid) 16 (Haploid) 22 + XX 22 + X (b) The X-0 system (d) The haplo-diploid system

Sex-linked Genes Sex linked gene- gene located on the sex chromosome - historically referred to gene on X chromosome - fathers may pass sex linked allele to daughters but not sons - mothers could pass alleles to both - males have only one locus meaning if they receive recessive allele from the mother, they express the trait *leads to male more prominently expressing sex-linked recessive disorders Ex: Muscular dystrophy – lack of muscle protein dystrophin located on the X chromosome Hemophilia- absence of one or more proteins for clotting blood

X Inactivation in Females Barr body – inactive X chromosome in a cell of a female -genes will not be expressed -occurs randomly and independently in each embryonic cell -creates 2 cell types: active X from father and active X from mother -once inactivated all mitotic descendants have inactive X possible cause comes from multiple copies of RNA transcript gene on Barr body called X-inactive specific transcript (XIST)

Gene Linkage Linked genes – genes located on the same chromosome that are inherited together in genetic crosses Genetic recombination – production of offspring with combinations of traits different from either parent *For unlinked genes recombination is caused by random orientation of homologous chromosomes – gives 50% frequency *Crossing over of homologous chromosomes accounts for the break in connection between genes on the same chromosome and allows for the recombination of linked genes

Gene Maps Genetic map – ordered list of genetic loci along a chromosome Linkage map – genetic map based on recombination frequencies map unit – distance between genes; 1 unit = 1% recombination frequency Alfred Sturtevant contributed the idea that the farther apart two genes are, the higher the probability of crossover and thus higher frequency of recombination -possible for genes on the same chromosome to be physically linked but not genetically linked

Abnormal Chromosome Number Sex linked traits and mutations are not the only ways of changing phenotypes Physical and chemical disturbances, errors in meiosis, or damage to chromosome can alter genes and lead to disorders Nondisjunction – condition in which members of a pair of homologous chromosomes do not move apart in meiosis I, or sister chromatids fail to separate in meiosis II -leads to abnormal chromosome numbers

Abnormal Chromosome Number Aneuploidy – condition in which offspring have an abnormal number of a particular chromosome -trisomic – 3 chromosomes present or 1 extra -monosomic – 1 chromosome present or 1 missing If nondisjunction occurs during mitosis and early in embryonic development then the condition can be passed to a large a number of cells and have a greater effect on organism

Abnormal Sets Polyploidy- term for alteration of chromosome sets or multiple chromosome sets - refers to somatic cells after mitosis -triploidy – 3 sets of chromosomes -tetraploidy – 4 sets -condition more common in plants Ex: wheat – hexaploid strawberries - octoploid

Alterations of Structure

Human Disorders Aneuploid gametes can result from nondisjunction in meiosis creating aneuploid zygote -some conditions can cause embryos to spontaneously abort -some survive birth and beyond but have set of symptoms labeled a syndrome Ex: Down syndrome or trisomy 21

Aneuploidy of Sex Chromosomes Klinefelter syndrome- males born with extra X chromosome (XXY) -occurs 1 in 2,000 births (2004) - occurs 1 in 500 – 1,000 male births (2014) -presence of sex organs but sterile individual -some breast enlargement or female characterstics may occur -normal intelligence -Males with extra Y (XYY) exhibit taller stature than average but not known syndrome ( 1 in 1,000)

Aneuploidy of Sex Chromosomes Females with extra X chromosome (XXX) are normal and healthy Turner’s syndrome – (X0) only known monosomy in humans - 1 in 2,500 female births -individuals appear female but sex organs do not mature and become sterile - normal intelligence

Altered Chromosome Disorders Cri du chat – deletion in chromosome 5 that results in mental retardation, small head, facial abnormalities – cat sound cries -most die as infants or early childhood Chronic myelogenous leukemia – results from translocation of chromosomes - results from long portion of chromosome 22 and tip of chromosome 9 exchange(forms Philadelphia chromosome) - gene activated causes uncontrolled cell cycle progression

Inheritance patterns Genomic imprinting - patterns of methylation passes from parent to offspring keeping record of what occurred during development -permanently regulates expression of either maternal or paternal allele of certain genes Extranuclear genes – found in organelles such as mitochondria and chloroplasts contain small circular DNA that can code for proteins and RNA