chapter 15

Bio1151 Chapter 15 The Chromosomal Basis of Inheritance

Mendel's laws can be explained by the theory of inheritance and production of gametes by .
- Chromosomes can be tagged to reveal location of a specific gene.
- Chromosomal basis of Mendel's laws.
  The arrangement of chromosomes at metaphase I and their separation in anaphase I account for the segregation and independent assortment of alleles.
- Independent assortment.
  F[1] plants produce equal numbers of four kinds of gametes because the alternative chromosome arrangements at metaphase I are equally likely.
  The result is a 9:3:3:1 phenotype ratio in the F[2].
Thomas Hunt Morgan's experiments with fruit flies (Drosophila) showed that the eye-color is sex-linked and located on the chromosome.
- A wild-type (most common phenotype) red-eyed female is mated with a mutant white-eyed male.
  The F[1] offspring all had red eyes.
  The F[2] showed a typical Mendelian 3:1 phenotype ratio of red:white eyes.
  However, all females had red eyes. The males exhibited a 1:1 phenotype ratio.
  conclusion
- Since all F[1] offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w^+). Since the recessive white-eye trait was expressed only in males in the F[2] generation, the eye-color gene must be located on the X chromosome: sex-linked.
In humans and other mammals, the sex chromosomes and determine an organism's gender.
- Human somatic cells have 22 pairs of homologous autosomes plus one pair of sex chromosomes. XX individuals are female, while XY are male.
Other forms of sex determination include a system used by many social insects.
- The haplo-diploid system.
  Many bees and ants have no sex chromosomes.
  Females develop from fertilized eggs and are diploid.
  Males develop from unfertilized eggs and are haploid. They have no fathers.
-linked disorders exhibit peculiar inheritance patterns.
- X-linked inheritance.
- Fathers usually pass a recessive allele to his daughter as a carrier.
- The carrier has a 50% chance of passing the disease to her sons.
- If the carrier marries an affected male, 50% of the offspring will have the disorder.
  These recessive disorders are more common in males than in females, since females have 2 X chromosomes and need to be homozygous to express the trait.
  Exercise:
- Father-daughter X-linkage.
  A father with the disorder will transmit the mutant allele to all daughters but to no sons.
  If the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers.
- Mother-son X-linkage.
  If a female carrier mates with a normal male:
- There is a 50% chance that a daughter will be a carrier like her mother.
- There is a 50% chance that a son will have the disorder.
- X-linkage.
  If a carrier mates with a male who has the disorder, there is a 50% chance that a child will have the disorder, regardless of sex.
  Daughters who do not have the disorder will be carriers.
occurs when homologous chromosomes or sister chromatids do not separate normally in meiosis.
- Nondisjunction during meiosis can produce gametes with an extra or missing chromosome, or aneuploidy. Homologous chromosomes may fail to separate during meiosis I. Sister chromatids may fail to separate during meiosis II.
- Down syndrome is caused by nondisjunction.
  This usually results in the aneuploidy called trisomy 21, an extra chromosome 21.
A female heterozygous for an X-resident gene may show phenotype, due to X inactivation.
- X inactivation.
  In mammalian females, one of the two X chromosomes in somatic cells is randomly inactivated during development as a compact Barr body.
  A "tortoiseshell" ("calico") cat has 2 different alleles for fur color: one for orange fur and one for black fur.
  In a heterozygous "mosaic" female, orange patches are formed by cells in which the orange allele is active; black patches have cells in which the black allele is active.
Chromosome alterations include , , , and .
- Chromosome alterations.
- A deletion removes a chromosomal segment.
- A duplication repeats a segment.
- An inversion reverses a segment within a chromosome.
- A translocation moves a segment from one chromosome to a non-homologous chromosome. A reciprocal translocation results from the exchange of fragments between non-homologous chromosomes.
- A reciprocal translocation produces a short chromosome 22, (Philadelphia chromosome), and a long chromosome 9, leading blood cells to escape control of the cell cycle, becoming cancerous and causing leukemia.