Meiosis And Sexual Life Cycles

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allele

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Any of the alternative versions of a gene that produce distinguishable phenotypic effects.

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alternation of generations

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A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants and some algae.

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asexual reproduction

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The generation of offspring from a single parent that occurs without the fusion of gametes (by budding, division of a single cell, or division of the entire organism into two or more parts). In most cases, the offspring are genetically identical to the parent.

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autosome

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A chromosome that is not directly involved in determining sex; not a sex chromosome.

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chiasma (pl., chiasmata)

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The X-shaped, microscopically visible region where homologous nonsister chromatids have exchanged genetic material through crossing over during meiosis, the two homologs remaining associated due to sister chromatid cohesion.

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clone

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(1) A lineage of genetically identical individuals or cells. (2) In popular usage, a single individual organism that is genetically identical to another individual. (3) As a verb, to make one or more genetic replicas of an individual or cell. See also gene cloning.

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crossing over

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The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.

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diploid cell

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A cell containing two sets of chromosomes (2n), one set inherited from each parent.

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gametophyte

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In organisms (plants and some algae) that have alternation of generations, the multicellular haploid form that produces haploid gametes by mitosis. The haploid gametes unite and develop into sporophytes.

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gene

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A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).

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genetics

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The scientific study of heredity and hereditary variation.

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haploid cell

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A cell containing only one set of chromosomes (n).

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heredity

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The transmission of traits from one generation to the next.

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homologous chromosomes

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A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One is inherited from the organism’s father, the other from the mother. Also called homologs, or a homologous pair.

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karyotype

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A display of the chromosome pairs of a cell arranged by size and shape.

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life cycle

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The generation-to-generation sequence of stages in the reproductive history of an organism.

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locus (pl., loci)

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A specific place along the length of a chromosome where a given gene is located.

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meiosis

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A modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell.

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meiosis I

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The first division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell.

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meiosis II

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The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell.

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recombinant chromosome

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A chromosome created when crossing over combines the DNA from two parents into a single chromosome.

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sex chromosome

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A chromosome responsible for determining the sex of an individual.

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sexual reproduction

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A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from the gametes of the parents.

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somatic cell

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Any cell in a multicellular organism except a sperm or egg.

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spore

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(1) In the life cycle of a plant or alga undergoing alternation of generations, a haploid cell produced in the sporophyte by meiosis. It can divide by mitosis to develop into a multicellular haploid individual, the gametophyte, without fusing with another cell. (2) In fungi, a haploid cell, produced either sexually or asexually, that produces a mycelium after germination.

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sporophyte

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In organisms (plants and some algae) that have alternation of generations, the multicellular diploid form that results from the union of gametes. It produces haploid spores by meiosis that develop into gametophytes.

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synapsis

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The pairing and physical connection of replicated homologous chromosomes during prophase I of meiosis.

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variation

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Differences between members of the same species.

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zygote

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The diploid product of the union of haploid gametes during fertilization; a fertilized egg.