Meiosis + Outcrossing = Extreme diversity
Meiosis produces seemingly innumerable combinations of genes from a single individual. The randomness of meiosis is why biological siblings can be so different—they have unique combinations of genes, even though they came from the same parents. While the creation of sex cells is a key part of what makes sex such a great promoter of diversity, a significant amount of diversity comes from sex itself—specifically outcrossing, or mating involving two individuals.
Meiosis produces sex cells, each of which contains only one chromosome of each homologous chromosome pair. To make a genetically complete cell (with two copies of each chromosome pair), fertilization needs to occur. Fertilization is the merging of a sperm and an egg cell to make a cell with genetic material from both parents. This new cell, the zygote, contains a set of chromosomes, with a unique combination of genes, from both its mother and father.
Recap: Genotype, phenotype, and mutation
You have over 3 billion genetic “letters” (the nucleotides) that collectively represent your genotype. The genotype is the raw genetic data that you inherited and could potentially pass down to offspring. Your genotype, along with environmental cues and other factors that affect gene expression (see chapter 5) make up your phenotype. Simply, your phenotype is the expression of your genotype.
We can use an understanding of genetics to predict inheritance patterns of certain traits. You may be interested in the inheritance of traits that are advantageous (such as resistance to certain diseases), neutral (such as hair color), and harmful, or deleterious (such as disease traits). In the following section we discuss patterns of inheritance for different types of expressed traits, or phenotypes. Keep in mind that many traits are neutral, some traits are advantageous in some environments and deleterious in others, and some traits might be advantageous when an individual has one copy and deleterious when an individual has two.