gamete

Examples of gamete in the following topics:

• Disorders in Chromosome Number

  • If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome.
  • If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome .
  • If a gamete with two copies of the chromosome combines with a normal gamete during fertilization, the result is trisomy; if a gamete with no copies of the chromosomes combines with a normal gamete during fertilization, the result is monosomy.

• Sympatric Speciation

  • For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have.
  • These new gametes will be incompatible with the normal gametes produced by this plant species.
  • Therefore, an allopolyploid occurs when gametes from two different species combine .
  • In the example shown, a normal gamete from one species fuses with a polyploidy gamete from another.
  • Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis.

• Life Cycles of Sexually Reproducing Organisms

  • Once the haploid gametes are formed, they lose the ability to divide again.
  • Meiosis is not directly involved in the production of gametes because the organism that produces the gametes is already a haploid.
  • Fertilization between the gametes forms a diploid zygote.
  • In animals, sexually-reproducing adults form haploid gametes from diploid germ cells.
  • Fusion of the gametes gives rise to a fertilized egg cell, or zygote.

• Mendel's Law of Segregation

  • In essence, the law states that copies of genes separate or segregate so that each gamete receives only one allele.
  • The behavior of homologous chromosomes during meiosis can account for the segregation of the alleles at each genetic locus to different gametes.
  • As chromosomes separate into different gametes during meiosis, the two different alleles for a particular gene also segregate so that each gamete acquires one of the two alleles.
  • The Law of Segregation states that alleles segregate randomly into gametes
  • When gametes are formed, each allele of one parent segregates randomly into the gametes, such that half of the parent's gametes carry each allele.

• Fertilization

  • If pregnancy is considered to begin at the point of implantation, the process leading to pregnancy occurs earlier as the result of the female gamete, or oocyte, merging with the male gamete, or spermatozoon.
  • After the point of fertilization, the fused product of the female and male gamete is referred to as a zygote or fertilized egg.
  • For species that undergo internal fertilization, such as humans, the fusion of male and female gametes usually occurs following the act of sexual intercourse.
  • Through fertilization, the egg is activated to begin its developmental process (progressing through Meiosis II), and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.

• Genetic Linkage and Violation of the Law of Independent Assortment

  • If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and the short and yellow alleles will go into other gametes.
  • These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes.
  • But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles.
  • If you create the Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply.
  • Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome.

• Down Syndrome

  • With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes.
  • When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21.
  • Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.
  • Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected.
  • During reproduction, normal disjunctions leading to gametes have a significant chance of creating a gamete with an extra chromosome 21, producing a child with Down syndrome.

• Testes

  • The testis is homologous to the ovary in that it produces the male gamete (sperm) while the ovary produces the female gamete (eggs).
  • The tubules are lined with a layer of cells (germ cells) that develop into sperm cells (also known as spermatozoa or male gametes) from puberty into old age.

• The Evolution of Seed Plants and Adaptations for Land

  • The evolution of seeds allowed plants to reproduce independently of water; pollen allows them to disperse their gametes great distances.
  • Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain.
  • The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain.

• Mendel's Law of Independent Assortment

  • Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are yr, while the gametes for the yellow/round plant are all YR.
  • For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele.
  • The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele.
  • Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed as follows: YR, Yr, yR, and yr.
  • Arranging these gametes along the top and left of a 4 × 4 Punnett square gives us 16 equally likely genotypic combinations.