Chapter Fifteen: the chromosomal basis of inheritance

Q1: What's that structure of chromosome?

A1: Each chromosome consists two sister chromotids, connecting by centremere.

Q2: Where are Mendel's hereditary factors located in the cell?

A2: DNA

Q3: What lead to genetic variation?

A3: Law of segregation and law of independent assortment

Five Facts:

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 locaed near each other on the same chromosome.

4. Alterations of chromosome number or structure cause some genetic disorders

5. some inheritance patterns are exceptions to the standard chromosome theroy.

Figure:

nondisjunction in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II

Summary:

Chromosomes and genes are both pressent in pairs in diploid cells; homologous chromosomes separate during the process of meiosis; and fetilization restores the paired condition for both chromosomes and genes. The behavior of chromosomes during meiosis accounts for Mendel's laws of segregation and independent assortment. Sex is an inherited phenotypic character usually determined by which sex chromosomes are present. A gene located on either sex chromosome is called a sex-linked gene. X chromosome has more genes than Y chromosome. In females, one of two X chromosome is radonly inactivate, which is called barr body.

Genes located on the same chromosome that tend to be inherited together in genetic crosses are said to be linked genes. Genetic recombination is the production of offspring with combinations of traits that differ from those found in either parent. A genetic map based on recombination frequencies is called a linkage map. The father apart genes are, the more likely their allele combinations will be recombined during crossing over. If either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a chromosome, a condition known as aneuploidy. missing one is monosomic; triplicate is trisomic. Polyploidy have more than two complete chromosome sets. Chromosome breakage can result in deletions, inversions. duplications, and translocations.

video:

http://www.youtube.com/watch?v=-ROhfKyxgCo