The arrangement of homologous chromosome pairs at metaphase I affects resulting gametes. A total of...
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Transcript of The arrangement of homologous chromosome pairs at metaphase I affects resulting gametes. A total of...
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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.
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The total number of combinations of chromosomes that meiosis can package into gametes is 2n, where n is the haploid number. EX. Organism has 4 chromosome. Haploid number is 2. 22=4. For a human 223 = about 8 million possible chromosome combinations.
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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).
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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.
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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.
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Non disjunction – the members of a chromosome pair fail to separate.
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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.
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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.
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Translocation – the attachment of a chromosomal fragment to a nonhomologous chromosome.
Reciprocal translocation – two nonhomologous chromosomes exchange segments.
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