Somatic Cells, Gametes and Fertilization

• Organisms that produce sexually contain two types of cells: somatic and gametes.

Somatic Cells

• Somatic cells are body cells with a normal number of chromosomes, known as the diploid number (2n).
• Skin cells and brain cells are examples of somatic cells.

Gametes

• Gametes are sex cells containing half the normal number of chromosomes, known as the haploid number (n).
• Sperm cells and ova are gametes.
• The male gamete, sperm, is produced in the testes.
• The female gamete, ovum, is produced in the ovaries.
• During ovulation, the ovum is released from the ovary for fertilization.
• Fertilization, the joining of sperm and ovum, occurs in the fallopian tube in humans.
• Fertilization results in the formation of a zygote (fertilized egg).
• Sperm + Ovum (egg) → Zygote.

Fertilization

• Fertilization is the fusion of sperm and egg to form a zygote.
• A zygote is a fertilized egg.

Homologous Chromosomes

• These are pairs of chromosomes (maternal and paternal) similar in shape and size.
• Homologous pairs carry genes controlling the same inherited traits.
• Each gene locus is in the same position on homologous chromosomes.
• Humans have 23 pairs of homologous chromosomes.
• 22 pairs are autosomes.
• 1 pair are sex chromosomes.

Tetrads

• Before cell division homologous chromosomes replicate to create a tetrad (4 sister chromatids).

Sex Chromosomes

• Sex chromosomes determine the offspring's sex.
• Offspring with "XX" chromosomes will be female.
• Offspring with "XY" chromosomes will be male.

Meiosis

• Meiosis is the process that produces gametes (sex cells) with half the number of chromosomes.
• During meiosis, diploid cells are reduced to haploid cells, Diploid (2n) → Haploid (n).
• Meiosis involves two cell divisions (meiosis I and meiosis II) with one duplication of chromosomes.

Interphase I

• This phase prepares the cell for meiosis.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres.
• Centriole pairs also replicate, and the nucleus and nucleolus are visible.

Meiosis I

• A cell division that reduces the chromosome number by one-half.
• It has four phases: Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

• The longest and most complex phase, taking up 90% of the meiotic process.
• Chromosomes condense.
• Synapsis occurs where homologous chromosomes come together to form a tetrad.
• A tetrad consists of two chromosomes or four chromatids (sister and non-sister chromatids).
• Crossing over occurs during Prophase I.
• During crossing over segments of non-sister chromatids break and reattach to other chromatids.
• Chiasmata are the sites of crossing over.
• Crossing over creates variation (diversity) in offspring traits.

Metaphase I

• The shortest phase.
• Tetrads align on the metaphase plate.
• Independent assortment takes place.
• The orientation of homologous pairs to poles is random.
• Variation is increased via the formula 2n, for example: 2^23 = 8,388,608 possible combinations if 2N = 46.

Anaphase I

• Homologous chromosomes separate and move towards the poles.
• Sister chromatids remain attached at their centromeres.

Telophase I

• Each pole now has a diploid set of chromosomes.
• Cytokinesis occurs, forming two diploid daughter cells.

Meiosis II

• No interphase II occurs.
• Prophase II is the same as prophase in mitosis.
• Spindle fibers connect to centromeres.
• Metaphase II is the same as metaphase in mitosis.
• Anaphase II is the same as anaphase in mitosis.
• Sister chromatids separate.
• Telophase II is the same as telophase in mitosis.
• Nuclei form, and cytokinesis occurs, resulting in four haploid daughter cells (gametes).

Gene Mapping

• Linked genes are located on the same chromosome and are more likely to be inherited together.
• Having an allele from one grandparent for a gene on chromosome 3 increases the likelihood of inheriting alleles from the same grandparent for other genes on chromosome 3.
• On a particular chromosome, some genes are separated more often by crossing over than others are.

Mapping and % Recombination

• Recombination is the mixing of alleles of a gene due to crossing over.
• The frequency of recombination between two genes indicates how far apart those genes are on a chromosome.
• The higher the frequency of recombination, the further apart the genes are.

Genetic Maps

• Genes are shown in relative order and distance from each other based on pedigree studies.