The arrangement of homologous chromosome pairs at metaphase I affects resulting gametes. A total of four chromosome combinations is possible in the gametes, and in fact the organism will produce gametes of all four types in roughly equal quantities. For a species such as human, all the chromosome pairs orient independently at metaphase I.

The total number of combinations of chromosomes that meiosis can package into gametes is 2 n, where n is the haploid number. EX. Organism has 4 chromosome. Haploid number is =4. For a human 2 23 = about 8 million possible chromosome combinations.

How many possibilities are there when a gamete from on individual unites with a gamete from another individual in fertilization. The random fusion of a single sperm with a single egg (ovum) will produce a zygote with any of about 64 trillion combinations of chromosomes (8 million x 8 million).

Crossing over – an exchange of corresponding segments between two homologous chromosomes. Chiasma – is the place where two homologous (nonsister) chromatids are attached to each other.

Crossing over begins very early in prophase. Chromosomes with these combinations would not exist if not for crossing over – they are called recombinant. Genetic recombination – the production of gene combination different from those carried by the original chromosomes.

Non disjunction – the members of a chromosome pair fail to separate.

Deletion – fragment of a chromosome is lost. Duplication – if a fragment from one chromosome joins to the sister chromatid or homolgous chromosome. Inversion – if the fragment reattaches to the original chromosome but in the reverse direction.

Inversions are less likely to cause serious harm because all the genes are still present. Deletions cause serious physical and mental problems. A deletion in a gene on chromosome 5 causes cri-du-chat. A child born with this is mentally retarded, has a small head with unusual facial features, and has a cry that sounds like a the meowing of a distressed cat. Usually die in infancy.

Translocation – the attachment of a chromosomal fragment to a nonhomologous chromosome. Reciprocal translocation – two nonhomologous chromosomes exchange segments.