11.1 What Is a Chromosome?
1. A chromosome is a continuous double-stranded molecule of DNA, with RNA and associated proteins that provide scaffolding or help carry out replication or transcription.
2. A karyotype is a size-ordered chart of chromosome pairs, which are distinguished by size, centromere position, and banding patterns of dark-staining heterochromatin and light-staining euchromatin.
3. The minimal requirements for chromosomal integrity are a centromere, telomeres, and origins of replication.
11.2 "Linking" Mendel's Laws to Chromosomes
4. Each species has a characteristic number of chromosomes. Cytogenetics associates abnormal chromosomes to phenotypes.
5. Genes on the same chromosome are linked; rather than demonstrating independent assortment, they produce a large number of parental genotypes and a small number of recombinant genotypes.
6. Genotype predictions and linkage maps are derived from knowing allele configurations (coupling or repulsion) and crossover frequencies, which are directly proportional to distances between genes.
11.3 How Do Chromosomes Determine Sex?
7. Sex determination mechanisms set development on a course toward malenss or femaleness.
8. Sex determination mechanisms are diverse. In chromosomal strategies, the sex with two different sex chromosomes is heterogametic, and the sex with two of the same type of sex chromosome is homogametic.
9. In humans, the male is heterogametic, and the female is homogametic. The SRY gene on the Y chromosome controls other genes that stimulate development of male structures and suppress development of female structures.
11.4 Inheritance of Genes on Sex Chromosomes
10. An X-linked trait passes from mother to hemizygous son because the male inherits his X chromosome from his mother and his Y chromosome from his father. Y-linked traits are very rare.
11. X inactivation shuts off one X chromosome in the cells of female mammals, equalizing the number of active X-linked genes in each sex. Early in development, each female cell inactivates one X chromosome. A female is mosaic for X-linked heterozygous genes on the X chromosome because the X chromosomes are inactivated at random with respect to their parental origin.
11.5 When Chromosomes Are Abnormal
12. Polyploid cells have extra full chromosome sets, and aneuploids have extra or missing individual chromosomes. A trisomy (one extra chromosome) is less harmful than a monosomy (one absent chromosome). Sex chromosome aneuploidy is less severe than autosomal aneuploidy. Nondisjunction, an uneven division of chromosomes in meiosis, causes aneuploidy.
13. Chromosomal rearrangements disrupt meiotic pairing, which can delete or duplicate genes. An inversion flips gene order, affecting the phenotype if it disrupts a vital gene. A Robertsonian translocation fuses the long arms of two nonhomologs. In a reciprocal translocation two nonhomologs exchange parts.