Sexual Reproduction and the Diversity of Life
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Sexual Reproduction and the Diversity of Life
BC Science Probe 9Section 3.2Pages 79-82
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Fertilization Produces Diploid Cells
• Fertilization: The joining or fusing of two gametes.– Gametes are produced by meiosis– They are haploid (only have ½ the regular number
of chromosomes of the parent cell)– When two gametes fuse during fertilization, they
make a diploid cell (a cell with a full set of chromosomes)
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Fertilization Produces Diploid Cells
• The first diploid (2n) cell of the offspring is called the zygote.
sperm(n) + egg (n) = zygote (2n)
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Fertilization Produces Diploid Cells
• Zygote:– Gets half of its chromosomes from each parent.– Human gametes each have 23 chromosomes, so a
human zygote will have 46, then divide by mitosis to produce more genetically identical cells with 46 chromosomes
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Diversity Among Offspring
• Diversity = variation or differences
• There is diversity among different species and also within a species.
• Sexual reproduction creates diversity
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Diversity Among Offspring
• It’s the homologous pairs of chromosomes that produce the variations among members of a species.– Each of the chromosomes in the homologous pair
have corresponding genes (genes that code for the same trait ex. hairline)
– But… these genes can be slightly different from one another!
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Diversity Among Offspring
• The different forms of the same gene are called alleles.– Back to the hairline example: there are two
different alleles.• One is for a straight hairline• One is for a widow’s peak
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Diversity Among Offspring
• The allele for a widow’s peak is dominant.– This means that it will express its trait (show up) if
it is present
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Diversity Among Offspring
• The allele for a straight hairline is recessive.– This means that it will only show up if both
chromosomes in the homologous pair have the recessive allele.
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Diversity Among Offspring
• If an individual has one of each allele, the dominant one gets expressed.
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Diversity Among Offspring
• Some alleles are not dominant or recessive.• There are such things as incomplete
dominance and codominance.
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Diversity Among Offspring
• Incomplete dominance:– Two different alleles produce a mixture of the
traits.– Red carnation + white carnation = pink carnation
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Diversity Among Offspring
• Codominance is when both traits get expressed.– This horse has one allele for white hair and one for
red. Both get expressed.
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Diversity Among Offspring
• There are even some traits that are controlled by more than one gene!
• Human eye colour is one of these traits. That’s why there are so many variations!
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