Chromosomal Basis of Inheritance Ch. 15

Chromosome theory of inheritance: Genes have specific loci on chromosomes and the chromosomes go through segregation and independent assortment The behavior of chromosomes matches up with Mendel’s laws Wild type: the most common phenotype in a natural population

Sex-linked Genes genes carried on the X-chromosome (very few genes are carried on Y-chromosome) Most sex-linked traits are recessive If a female has two recessive mutated genes she will express the phenotype If a female has one recessive mutated gene, she will not express the phenotype, but be a carrier Males only inherit one X-chromosome, so if they have a mutated X-linked gene they will express the gene This makes males MUCH more likely to express

Other Important Facts Common examples of recessive sex-linked traits are color blindness and hemophelia All daughters of affected fathers are carriers Sons cannot inherit a sex-linked trait from the father because the son inherits the Y chromosome from father A son has a 50% chance of inheriting a sex-linked trait from a carrier mother Males cannot be carriers, if they have the gene they will express it It is uncommon for a female to have a sex-linked condition because she must have inherited a mutant gene from BOTH parents

Linked genes: genes on the same chromosome Genetic Recombination: production of offspring with new combinations of traits from two parents Parental Types: offspring that inherit phenotype of parents Recombinants: offspring that have new phenotype Linkage map: genetic map based on recombination frequencies The greater the distance between two genes, the more points there are between them where crossing over can take place

Genetic Defects Nondisjunction: error that happens during meiosis in which homologous chromosome don’t separate as they should Aneuploidy: abnormal condition Trisomy: three chromosomes Monosomy: only one chromosome

Down Syndrome: Trisomy 21; mental retardation, prone to alzheimer’s and leukemia Turner’s Syndrome: (XO) Only have 45 chromosomes due to missing sex chromosome. Small stature and mental retardation Klinefelter’s Syndrome: (XXY) Have 47 chromosomes due to an extra X chromosome. Male genitals, but testes are small and sterile *Genomic Imprinting: some traits expression varies depending on which parent they are inherited from *Extranuclear Inheritance: extranuclear genes found in mitochondria and chloroplasts. Mitochondria passed to zygote come from cytoplasm of egg, so these disease are always inherited from the mother