Human Development: Cell Formation

• Human development starts with the fusion of sperm and egg cells.
• Sperm carries the father's genetic information, the egg the mother's.
• Offspring still have the same number of chromosomes as each parent.

Sex Cell Formation

• Human body cells contain 46 chromosomes, 23 from each parent.
• Chromosomes exist in pairs called homologous chromosome pairs, with 23 non-homologous chromosomes.
• The number of these chromosomes is specific to each species, referred to as the haploid chromosome set (n).
• Body cells contain the double, diploid set of chromosomes (2n).
• In humans, n = 23 and 2n = 46.
• Egg and sperm cells are sex cells, produced from diploid primordial germ cells.
• DNA is duplicated to create chromosomes with two identical sister chromatids.
• The primordial germ cell undergoes meiosis consisting of two maturation divisions.
• Meiosis I separates homologous 2-chromatid chromosomes, forming two haploid cells with 2-chromatid chromosomes.
• Meiosis II separates the 2-chromatid chromosomes to create four haploid sex cells with 1-chromatid chromosomes.
• Chromosome count is halved (reduction division).
• Fertilization fuses two haploid gametes into a diploid zygote, so offspring chromosome count stays constant.

Interchromosomal Recombination

• Meiosis involves homologous chromosomes aligning in pairs.
• Arrangement is random, leading to different distribution possibilities in daughter cells during Meiosis I.
• With a haploid chromosome set of n = 2, sex cells can have either two paternal or two maternal chromosomes.
• Sex cells can also possess a chromosome from each parent.
• There are four ways chromosomes distribute to sex cells.
• The larger the haploid chromosome set, the more genetically diverse gametes can be formed.
• With n = 3, there are 23 = 8 possibilities.
• In humans with n = 23, there are 223 = 8.4 million possibilities.
• Random recombination of homologous chromosomes allows genetic variability in sex cells and offspring.

Intrachromosomal Recombination

• Two homologous chromosome pairs join to form tetrads of four chromatids.
• These chromatids cross over at points called chiasmata, exchanging segments.
• Crossing over between non-sister chromatids leads to genetic information recombination.
• The resulting recombinant chromosome becomes a genetic mosaic of paternal and maternal DNA.
• Human meiosis involves one to three crossovers per chromosome pair.
• Intrachromosomal recombination greatly increases the number of genetically distinct sex cells.