Q: How does a sex-linked gene affect a human?
A: It is a gene located on either sex chromosome and in humans, it specifically refers to a gene on the X chromosome. Because males have only one locus, the terms homozygous and heterozygous lack meaning for describing their sex-linked genes in this case.
Q: What causes Down syndrome?
A: An extra chromosome 21.
Q: What is the production of offspring with new combinations of traits inherited from two parents called?
A: Genetic recombination.
1. Mendelian inheritance has its physical basis in the behavior of chromosomes
2. Sex-linked genes exhibit unique patterns of inheritance.
3. Linked genes tend to be inherited together because they are located near each other on the same chromosome.
4. Alterations of chromosome number or structure cause some genetic disorders.
5. Loci found on the same chromosome can be genetically recombined only via molecular recombination
Figure 15.15 (d)
A translocation moves a segment from one chromosome to a nonhomologous chromosome. In a reciprocal translocation, the most common type, nonhomologous chromosomes exchange fragments. In a nonreciprocal translocation, which is less common, a chromosome transfers a fragment without receiving a fragment in return.
Mendel proposed the idea that the factors responsible for traits of each pair are independent of every other pair in the process of their distribution into the gametes (law of independent assortment). It is now known that during meiosis, the chromosomes of various homologous pairs assort at random so that the chromosomes of each pair segregate independently of the chromosomes of every other pair.
Because females have twice as many copies of X-linked genes as males, one copy of each must be turned off. This occurs by inactivating one X chromosome in every cell of a female.
