The Chromosomal Basis of Inheritance 21 and 24 October, 2005 Text Chapter 15

Genes and Chromosomes The behavior of chromosomes in meiosis and fertilization explains Mendel’s rules of inheritance. Genes on different chromosomes assort independently.

Morgan’s Mutant

Morgan’s experiments showed that some genes are inherited along with the X chromosome.

Genes that are on the same chromosome are linked Genes that are on the same chromosome are linked. Those on the X chromosome are X-linked.

Crossing over allows linked genes to appear to assort independently. Remember, the purpose of crossing over is to generate diversity in offspring arising from sexual reproduction.

Crossing over results in recombinant gametes.

Recombinant gametes lead to offspring.

Recombination Distance Genes that are far apart are more likely to have a crossing-over event occur between them than genes that are closer together on the chromosome.

Mapping Knowing the recombination frequencies between several linked genes allows the researcher to construct a genetic map. Note that the maximum genetic distance discernable in a cross is 50% (indistinguishable from independent assortment.) Larger genetic distances are determined by adding.

Sex Determination

Sex-linked genes (carried on the X chromosome) show unique patterns of inheritance. Since the Y chromosome has few genes, recessive sex-linked alleles are not masked in males. A father with an x-linked trait will transmit the allele to all daughters, but no sons. A carrier mother will pass the allele to 1/2 of her offspring. Sons will be affected, daughters will be carriers. Carrier female x affected male produces affected females

X Inactivation

Nondisjunction

Meiotic nondisjunction occurs when homologues or sister chromatids fail to separate in anaphase. This error leads to gametes that have too many or too few chromosomes. If these gametes are involved in fertilization, the resulting offspring will have an abnormal number of chromosomes (aneuploidy). They may have one (monosomy) or three (triploidy) copies of a chromosome. This changes the gene dosage of the genes on the affected chromosome

Alterations of Chromosome Structure Gene dosage can also be changed by structural alterations within a chromosome, including deletion, duplication, translocation, and inversion.

Down Syndrome