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A type of life cycle adopted by plants and some algae in which a multicellular diploid organism (the sporophyte) produces haploid spores that give rise to multicellular haploid organisms (the gametophytes). Gametophytes produce haploid gametes by mitotic cell division that then fuse at fertilization to produce a diploid sporophyte generation, thus completing the life cycle.
a copy of a gene residing on one chromosome. In diploid organisms, a homologous chromosome is present which will have another copy of the same gene. This copy can be identical (genetically identical alleles) in which case the cell is homozygous for that gene. Alternatively, the two alleles can have different nucleotide sequences, in which case the cell is heterozygous for that gene.
A stage of Meiotic cell division in which homologous chromosomes, each made up of two chromatids, are separated into two daughter cells. Note that at this stage, each homolog is made up of two joined chromatids which will only separate in Anaphase II. See Anaphase II for comparison.
A stage of Meiotic cell division in which sister chromatids are separated into two daughter cells. Note that at this stage, each only one homolog of a pair of homologous chromosomes found in the original mother cell is present in this daughter cell. Technically, this means that during Anaphase II, a 1n mother cell having chromosomes with two chromatids produces two new 1n daughter cells with chromosomes having only one chromatid. See Anaphase I for comparison.
A form of reproduction where only one parent is involved and the offspring are genetically identical to the parent. Genetic variation can arise over time through random mutation or rare incorporation of foreign DNA.
Chromosomes that contain genes that are not involved in the determination of an organisms’ gender. See sex chromosomes for comparison.
joined copies of a DNA strands produced by replication of a single chromosome template. Prior to anaphase, two joined chromatids are considered to be only one chromosome. Once the chromatids separate in anaphase of mitosis or anaphase II of meiosis, they are considered to be chromosomes, not chromatids.
a (usually highly compacted) complex of DNA and protein. Chromosomes can be made up of one or two chromatids.
an offspring produced asexually by a single parent, having the identical genetic makeup as its parent. Clones are produced naturally by some organisms. Clones can also be produced artificially be several methods. Note that genetic variation in clones can occur but only through random mutation or rare incorporation of foreign DNA.
The physical breaking and rejoining of DNA between non-sister chromatids during meiotic prophase I such that regions of DNA containing parental and maternal alleles are exchanged. Note that this results in chromosome homologs with sister chromatids that are not identical.
An organism that has two non-identical alleles of two different genes. It can also refer to a breeding experiment where two characteristics are considered. Note that Mendel performed dihybrid crosses to deduce the Law of Independent Assortment.
refers to a cell or the cells of an organism containing two copies of each gene, or two homologs of each of its chromosomes in its nuclei. This can also be referred to as having a 2n amount of DNA (see haploid for comparison). For example, human somatic cells are diploid and contain 23 pairs of chromosomes.
a group of organisms produced by parents. The first generation produced by a set of parents is known as the first filial, or F1 generation. Offspring produced by breeding F1 organisms result in the production of F2 generations, and so on.
A sexually reproducing eukaryotic life forms which displays a life cycle in which two haploid gametes fuse to produce a single celled zygote that then divides immediately by meiosis. The resulting haploid cells go on to develop into a haploid adult organism which produces gametes by mitotic cell division.
Haploid cells produced by organism displaying sexual reproduction. Two gametes fuse to form a diploid zygote which will divide by mitosis to give rise to a diploid adult (animals and the gametophyte of plants) or divide by meiosis to produce a haploid adult plant (fungi and some protists).
A haploid form of life characteristic of plant and some algae which produces haploid gametes by mitosis. See plants, algae for further discussion.
sequences of DNA that encode the information needed to produce a specific RNA molecule under appropriate circumstances. The term includes the DNA coding region itself as well as any regulatory regions such as promoters, operators, enhancers or suppressors. Can also be used to describe the DNA that encodes a specific trait, as for example “the gene for green eyes”.
Differences in the genetic makeup (genotype) of parents versus their offspring. In animals that exhibit sexual reproduction, major sources of variation include random combination of homologous chromosomes derived from maternal and paternal sources during the production of gametes, DNA crossover during meiosis, and the recombination of two gametes from pools of gametes containing thousands or even millions of possible genetic combinations during fertilization.
The genetic makeup of an organism, ie the designation of the actual DNA sequences that make up the genes of an organism (see phenotype for comparison).
refers to a cell or the cells of an organism containing only one copy of each gene or chromosome in its nuclei. This can also be referred to as having a 1n amount of DNA.
having two genetically different copies (alleles) of a specific gene. This means the DNA sequences for the gene on both homologous chromosomes are different. Note that this does not necessarily mean the organisms’ appearance is different from a homozygous organism, since appearance is determined by whether the genotypic differences in the two alleles actually results in a unique phenotype. The presence of an allele with a silent mutation or a recessive allele, for example, may have no effect on the resulting appearance (phenotype) of the organism.
Pairs of chromosomes in the cells of diploid cells that have the same shape, number of genes and order of genes along the chromosomes. During fertilization, a zygote obtains one of its pairs from the gamete of one parent and the other from the gamete of the other parent. All cells in a resulting multicellular diploid organism will have copies of these original maternally and paternally derived pairs.
having two genetically identical copies (alleles) of a gene encoding a specific gene product. This means the DNA sequences for the gene on both homologous chromosomes are identical.
A stage of the cell cycle in which cell growth occurs and DNA is replicated. For a diploid organism, a cell that completes interphase is still considered to have pairs of homologous chromosomes, but each chromosome is made up of two joined chromatids. The total amount of DNA is therefore twice as great as a normal somatic cell in G0, and four times as great as that found in a gamete. This is sometimes referred to as having a 4n amount of DNA, but not all sources use this terminology.
Mendel’s principle which states that genes for one factor are inherited independently of the genes for other factors. In fact, this is completely true only for genes that are located on different chromosomes, mostly true for genes located far apart on the same chromosome (due to DNA crossover during meiosis), and pretty much not true for genes that are located very close together on the same chromosome.
A term indicating that something is of or from a mother. In a diploid cell derived from an organism with sexually dimorphic parents (two types of parents) one chromosome of each pair of homologous chromosomes will be copies of a chromosome present in the maternal gamete at fertilization. Note that some organisms conduct sexual reproduction between parents with identical appearances, in which case the designation of which is the maternal and which is the paternal parent becomes kind of vague.
The first phase of meiotic cell division which results in the separation of homologous chromosomes from one another in Anaphase I. Note that this is also the stage where genetic recombination or crossover of DNA between non-sister chromatids occurs (in Prophase I).
The stage of the second meiotic cell division in which chromosomes containing two chromatids align at the metaphase plate before Anaphase II separates them. Note that the cell is 1n at this stage as only one homolog of each original chromosome pair is present in the cell at this point.
A form of cell division in which daughter cells end up with the same amount of DNA as the mother cell. For example, if a diploid cell divides by mitosis, its daughters will also be diploid. If a haploid cell divides by mitosis, its daughters will also be haploid. See meiosis for comparison.
A designation given to how many chromosome homologs or how many copies of a specific gene, is found in a cell or in the cells of an organism. For example, a gamete is considered 1n, because it contains only one copy of each chromosome and therefore only one copy of each gene found on them. A somatic cell of a diplod organism is designated 2n because it contains pairs of homologous chromosomes and therefore has two copies (which may or may not be identical at the DNA sequence level) of each gene on it. Note that somatic cells in haploid organisms are 1n.
The physical appearance of an organism, which is determined by its genes. Note that the phenotype is related to the genotype but several mechanisms determine the final appearance of an organism such as whether alleles show dominance, recessiveness, co-dominance, or incomplete dominance. In addition, the environment may also alter the appearance of an organism in a way that is not completely determined by its genetic makeup.
a life form which has a life cycle with alternation of generations, where a diploid form (known as the sporophyte) produces haploid spores by meiosis which then germinate directly to form a haploid life form known as the gametophyte. The haploid gametophyte produces gametes by mitosis, and the resulting haploid gametes fuse to form a diploid zygote which then develops into the diploid adult sporophyte to complete the life cycle.
a phase of meiotic cell division in which DNA crossover between nonsister chromatids of homologous chromosomes occurs.
a phase of meitic cell division in which chromosomes containing two joined sister chromatids condense. Note that homologs are not present at this point because they were separated into a different daughter cell in Meiosis I.
Single celled eukaryotic life forms, some of which display a life cycle in which two haploid gametes fuse to produce a single celled zygote that then divides immediately by meiosis. The resulting haploid cells go on to develop into a haploid adult organism which produces gametes by mitotic cell division.
Chromosomes that have genes encoding gender specific development or appearance. Note that not all organisms use specific sex chromosomes to determine the gender of their offspring.
A haploid reproductive cell produced by the diploid sporophyte of plants and some algae using meiotic cell division which develops directly into the haploid gametophyte organism.
A diploid form of life characteristic of plants and some algae which produces haploid spores using meiotic cell division (see plants, algae for full description).
The process of physically fusing or joining homologous chromosome pairs during the first stage of meiosis (Meiosis I). The mechanism of this joining is the formation of synaptonemal complexes that also create DNA crossovers between the non-sister chromatids of each pair of homologs.
a protein complex that holds two homologous chromosomes together during Meiosis I and creates physical exchange of DNA between non-sister chromatids.
the end of Meiosis I during meiotic cell division, resulting in two daughter cells each with on chromosome homolog, the other homolog of the pair of homologous chromosomes ends up in the other daughter cell. Note that at this stage each chromosome is made up of two joined sister chromatids. Because of DNA crossover that occurred in Prophase I, these chromatids are actually non-identical.
the end of Meiosis II during meiotic cell division, resulting in 1n (haploid) cells with a single chromosome (comprised of only one chromatid).
A term indicating that something is of or from a father. In a diploid cell derived from an organism with sexually dimorphic parents (two types of parents) one chromosome of each pair of homologous chromosomes will be copies of a chromosome present in the paternal gamete at fertilization. Note that some organisms conduct sexual reproduction between parents with identical appearances, in which case the designation of which is the maternal and which is the paternal parent becomes kind of vague.
a specific variant of an inherited character. For example, blond is a trait, while “hair color” is a character.
With respect to a specific heritable trait, a true breeding organism is a strain or variant that when bred with another member of the same group always produces offspring with the same trait.
Differences in characteristics between an offspring and its parents.
A single diploid cell resulting from the union of two haploid gametes during sexual reproduction. In some organisms, the zygote divides by mitosis to give rise to all the cells of a diploid organism. The resulting diploid organism produces gametes by meiosis to complete the life cycle. However, in fungi and some protists, the zygote divides by meiosis and the resulting haploid cells go on to develop into a haploid organism which will the produce gametes by mitotic cell division.
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