University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

122

Pages:

3
Author:

Mariam A. A.

Chromosomal Crossing Over ✓ Crossing over is process involving the exchange of corresponding gene segments of non-sister chromatids between homologous chromosome pairs (double-stranded chromosome pairs for the most organisms). ✓ This effectively creates new allele combinations, known as recombination. ✓ The crossing over occurs between non-sister chromatids, one from each double-stranded chromosome of the homologous pair. Each parent of the organism contributes one doublestranded chromosome. ✓ Chiasma is the point where crossing over occur. ✓ Crossing over does not occur in mitosis because the homologous chromosomes are NOT aligned side-by-side along the equator of the cell to allow overlapping segments of non-sister chromatids along the equator in the nuclear membrane. ✓ Sister chromatids are exact copies of each other, this means that the alleles on sister chromatids (coding for the any of the genes) are identical. ✓ Prior to crossing over, each locus (position) on each sister chromatid has the same allele. ✓ Capital letter represents a dominant allele and lowercase represents a recessive allele. We will talk about dominant and recessive alleles. Importance of crossing over. The process of crossing over creates new allele combinations adds diversity of the gene pool of the population as the four gametes formed in meiosis will not be identical but rather can have different allele for each gene. Also, upon fertilisation, two random gametes with some variation in their alleles for certain genes will combine and produce an offspring with an unique allele combination to their parents. This therefore increases genetic variation in a population and can introduce new phenotypes that potentially could be expressed. Genetic variation in a population provides a pathway for evolution to occur. Without genetic variation, there will be no mechanisms for evolution. Genetic variation can be introduced into a population’s gene pool via: • Sexual reproduction (crossing over, independent assortment, random segregation and fertilisation) or • Mutation