Topic 10: Genetics and evolution (HL)
10.1 Meiosis
Crossing-over:
- Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis.
- Chromosomes intertwine and break at the exact same positions in non-sister chromatids.
- The two chromosomes are now attached at the same corresponding position on the non-sister chromatid.
- Many chiasmata can form between the chromatids.
- Once attached the non-attached portions of the chromatids actually repel each other.
- Chiasmata refer to the actual break of the phosphodiester bond during crossing over.
- The chiasmata are separated during anaphase 1 which can result in an exchange of alleles between the non-sister chromatids from the maternal and paternal chromosomes
Independent assortment:
- Homologous chromosomes are separated in meiosis I
- Sister chromatids are separated in meiosis II
- When Mendel first did his experiments on pea plants, he looked at the traits that were passed on from generation to generation. He did not know how the traits were inherited in terms of meiosis.
- We now know that independent assortment is an essential component in explaining how chromosomes align themselves during meiosis.
- It also explains how unlinked genes are passed on from generation to generation.
- As explained above, when homologues line up along the equatorial plate in metaphase I, the orientation of each pair of is random;
meaning the maternal or paternal homologue can orient towards either pole. - Also the orientation of how one set of homologues line up has no effect on how any of the other homologues line up.
- For example, if chromosome pair one is heterozygous for a certain trait, there is a 50% chance that the gamete will receive the dominant trait and a 50% chance that the gamete will receive the recessive trait.
- Also if chromosome pair five is heterozygous for a particular trait, again there is a 50% chance that the gamete will receive the dominant allele and a 50% chance that it will receive the recessive allele.
- Both of these homologues line up independently during meiosis and have no effect on which gamete the other alleles will end up in