Biology Chapter 12

“Chromosomes”______________Biology

12th Chapter

A. What is a chromosome? - strand of DNA, plus proteins, and RNA Chromosome pairs are distinguished by size, banding pattern, and centromere position B. Linked Genes – genes carried on the same chromosome * linked genes violate Mendel’s Law of Assortment because they won’t necessarily separate C. Sex Determination * mechanism by which an individual develops as a male or a female 1. Total Chromosome Number a. female is diploid b. male is haploid 2. X-O System (number of X chromosomes determines sex) Female is XX Male is XO *** Insects *** 3. X-Y System (presence of Y chromosome determines sex) *** Mammals *** D. Inheritance of Genes on Sex Chromosomes * X-Linked Traits are carried on the X Chromosome and most are recessive * More common in females, affected males are hemizygous, cannot be carriers * Male inherits X-linked condition from carrier or affected mother * Y-Linked traits are very rare because Y has few genes E. X Inactivation Female mammals have 2 alleles for every gene on the X Chromosome, while males have only 1 This inequality is balanced by turning off one of the X Chromosomes in each cell of a 3 wk old embryo • some cells turn off paternal X • some cells turn off maternal X F. Chromosomal Abnormalities 1. Polyploidy – extra full sets of chromosomes * animal polyploids spontaneously abort of die shortly after birth * plant polyploids are relatively common (wheat, lilies) 2. Aneudoploidy – an extra (trisomy) or missing (monosomy) chromosome * Aneuploidy is usually due to a meiotic error call non disjunction

3. Deletion – part of a chromosome is missing 4. Duplication – part of a chromosome is present twice 5. Inversion – part of a chromosome is reversed 6. Translocation – nonhomogolous chromosomes exchange parts (reciprocal translocation) or combine (Robertsonian translocation)