Alleles segregate randomly in gametes introducing genetic variation in a variety of ways. This contributes to variation in populations and offspring. This can be done by an independent assortment of chromosomes in meiosis or by the crossing over of chromosomes.

Independent assortment generates genetic variation. For instance, a cell with two copies (one from mom, one from dad) of each chromosome, undergoes independent assortment that shuffles these chromosomes, and then only one copy of each into the gamete. This way, the gamete may have chromosome one from mom, chromosome 2 from dad, chromosome 3 from dad, and so on. The mechanism of independent assortment during meiosis is the following:

1. During metaphase I, homologous chromosome pair up along the metaphase line in random orientation – sometimes the mom’s chromosome is on the left, sometimes it’s on the right.
2. During anaphase I, the homologous chromosomes are pulled apart. Those on the left will be put into one daughter cell; those on the right will be put into another.

Because of independent assortment, genes on different chromosomes are randomized. However, genes on the same chromosome can not be randomized by this mechanism. The segregation of alleles into gametes can be influenced by linkage. The physically closer the genes are on the chromosome, the more linked they are. However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. Although crossing over is a mechanism that reduces linkage, crossing over is only efficient when the genes are physically apart from each other on the chromosome. When the genes are further apart on the chromosome, crossing over makes them less linked.

Nondisjunction occurs when chromosomes fail to segregate during meiosis; when this happens, gametes with an abnormal number of chromosomes are produced. This can occur in Anaphase 1 and Anaphase 2 and can lead to gametes with too many or too few chromosomes.

Genetic recombination is the process that introduces genetic diversity into the gametes during meiosis. The two processes that makeup recombination are independent assortment and crossing over.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis, occur during prophase I, and may consist of a single, double, or multiple crossovers.

• A single crossover is when homologous chromosomes are aligned, and chromatids from two different chromosomes can exchange segments resulting in genetic recombination.
• In double crossovers, chromatids from two homologous chromosomes come in contact at two points.

The synaptonemal complex, the protein complex that glues the tetrad together, first forms at specific locations and then spreads to cover the entire length of the chromosomes. It supports the crossing over. At the end of prophase I, the pairs are held together only at the chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Tetrads form for crossing over to occur. It breaks apart when the homologous chromosomes separate in meiosis I.

Genes that are located on the sex chromosomes are called sex-linked genes. Sex-linked genes generally refer to genes on the X chromosome in humans, because the Y chromosome has very few genes. Because males only have on X chromosome, they only inherit one allele of sex-linked genes, making them more susceptible to diseases caused by sex-linked genetic mutations. This is different from imprinted genes, in which genes are expressed in a parent-specific manner based on inherited epigenetic modifications.

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Key Points

• Independent assortment generates genetic variation.

• Nondisjunction occurs when chromosomes fail to segregate during meiosis.

• Crossing over is a mechanism that reduces linkage. However, crossing over is only efficient when the genes are physically apart from each other on the chromosome.

• Genetic recombination is the process that introduces genetic diversity into the gametes during meiosis.

• The crossover events occur during prophase I and may consist of a single, double, or multiple crossovers.

• The synaptonemal complex is the protein complex that glues the tetrad together; tetrads form for crossing over to occur. It breaks apart when the homologous chromosomes separate in meiosis I.

• Sex-linked genes are genes located on sex chromosomes, this mainly refers to genes located on the X chromosome

Key Terms

Gene: a unit of heredity; the functional groups of chromosomes that determine specific characteristics by coding for particular proteins

Chromosome: a structure in the cell nucleus that contains DNA, histone protein, and other structural proteins

Allele: one of several alternative forms of the same gene occupying a given position on a chromosome

Linkage: the property of genes of being inherited together

Recombination: the formation of genetic combinations in offspring that are not present in the parents

Crossing over: the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes

Tetrad: two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process

Chromatid: either of the two strands of a chromosome that separate during meiosis

Independent assortment: when alleles of two (or more) different genes get sorted into gametes independently of one another

Meiosis: cell division in sexually-reproducing organisms used to produce haploid gametes

Metaphase: the second stage of cell division, between prophase and anaphase, during which the chromosomes become attached to the spindle fibers

Anaphase: the stage of meiotic or mitotic cell division in which the chromosomes move away from one another to opposite poles of the spindle

Prophase: the first stage of cell division, before metaphase, during which the chromosomes become visible as paired chromatids and the nuclear envelope disappears

Chiasmata: a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.

Synaptonemal complex: is a protein structure that forms between homologous chromosomes to mediate synapsis and recombination during meiosis

Sex-linked genes: genes located on sex chromosomes, mainly refers to genes located on the X chromosome in humans

Imprinted genes: genes that are expressed in a parent-specific manner, based on epigenetic mechanisms