Chapter 15

Chromosome Theory of Inheritance Chromosomes actually segregate (homologs) and assort (nonhomologous) in meiosis Compare with dihybrid cross from chapter 14 Influenced by Mendel’s laws

Determining Sex Anatomical and physiological differences are numerous Chromosomal basis for determining sex is presence of the Y chromosome – Significantly smaller Only short regions on either end of Y are homologous with X

Gametic Sex Males produce sperm either X or Y Females produce eggs all X Offspring’s sex determined the moment of conception – X sperm fertilizes egg = female – Y sperm fertilizes egg = male Gonads, reproductive organs, are generic until 2 months – Development depends on SRY gene, located on Y chromosomes – Actually codes for proteins that regulate other genes – Without, gonads become ovaries; default sex is female

Alternate Chromosomal Systems X-Y system – Mammals – Determined by sperm, X or Y X-0 system – Insects, only one sex chromosome, X – Females XX, males X0 Z-W system – Birds and fishes – Determined by egg, Z – Female ZW, male ZZ Haploid-diploid system – Bees and ants – Fertilization determines – Fertilized = female = diploid = 2 parents – Unfertilized = male = haploid = no father

Sex (X)-linked Genes Genes located on either sex chromosome, especially the X (in humans) Fathers pass X to all daughters but no sons Mothers pass X to all offspring Recessive alleles Female express phenotype only if homozygous Males express only if egg bore the recessive allele Males are hemizygous because other terms can’t apply More males have sex-linked recessive disorders

Color Blindness N represents color-blind gene Loci carried on sex (X) chromosomes Muscular dystrophy and hemophilia are similar examples F: normal; M: affectedF: carrier; M: carrierF: carrier; M: normal

X Inactivation Females have 2 X chromosomes, but don’t make double the proteins – Barr body is the inactivated and condensed X chromosome Reactivated in ovaries so both X’s are transferable – Males and females each with same dose (1 copy) of genes Decision is random and independent, can be mom or dad’s allele, in each embryonic cell – Inactivity is conserved in all mitotic descendents – Females are mosaics Heterozygous for a sex-linked = ½ cells express 1 allele and ½ express other E.g coat color in cats

Tortiseshell/Calico Cats X-linked gene (O) determines orange fur Males have one X so if O is orange, o is black Females have 2 X’s – Heterozygotes (Oo) have 1 X inactivated – Some cells express O others o = patches of orange and black Rare, but possible to have male’s this color

Phenotype Alterations Small-scale – Random mutations create all new alleles = new phenotypic traits Large-scale – Physical and chemical disturbances can damage chromosomes – Errors in meiosis can alter chromosomal number – Often lead to miscarriages or genetic disorders Plants often tolerate better than animals

Nondisjunction Separation during anaphase doesn’t occur properly Fertilization of gamates from nondisjunction results in aneuploidy – Abnormal chromosome number extra = trisomy, fewer = monosomy Polyploidy 2+ sets of chromosomes Common in plants

Human Disorders Fetal testing can diagnose most, often times affected individuals are miscarried Down Syndrome Trisomy 21; 47 chromosomes; risk increases with age Sex chromosome relationships Less disruptive Y chromosome carries few genes X chromosome extra copies are inactivated

Sex Chromosome Disorders Sex Chromosome Abnormalities GenotypeGenderSyndromePhysical Traits XXY, XXYY, XXXYMaleKlinefelter syndrome Sterile, small testicles, breast enlargement XYYMaleJacob’s syndrome Normal male traits, previously thought ‘super- males’, karyotype ID XOFemaleTurner syndrome sex organs don't mature, sterile, short stature XXXfemaleTrisomy X tall stature, learning disabilities, limited fertility, karyotype ID

Altering Chromosomal Structure Deletion: a chromosomal fragment is lost Duplication: segments repeat – Pieces from deletions can attach to another chromosome Inversion: a segment breaks and flips 180° Translocation: chromosome segment moves from 1 to another non-homolog

Altered Chromosome Disorders Cri du chat (cry of the cat) Deletion in chromsome 5 Small head, unusual facial features, cry of a cat Huntington’s disease Duplication of CAG sequence in DNA on chromosome 4 Effected has copies, normal is copies Codes for glutamine, produces a polyglutamine <36 glutamines produces huntingtin protein, more is mutant