Hobbies And Interests
Meiosis
The process of meiosis creates what are called "daughter cells," cells that contain half the number of chromosomes found in the parent cell. In the case of humans, during fertilization an egg with 23 chromosomes fuses with a sperm carrying 23 chromosomes.
Crossing Over
During Prophase 1 of meiosis, chromosomes line up in homologous pairs. They then "cross over," meaning they switch parts of their DNA sequences with each other. In later stages of meiosis, when the chromosomes are separated into daughter cells, each cell contains a different accumulation of genetic information.
Think of this in terms of fraternal twins. The process begins with separate eggs and separate sperm. Before they became eggs and sperm, each underwent crossing over to various extents, providing quite an array of possible combinations. These combinations often affect physical traits, or the phenotypes of individuals.
Punnett Squares
A simple way to introduce genetic inheritance is with Punnett squares. These squares are ways of determining the gene combinations that are possible between a cross of parents. If a person inherits two forms of a gene, one from each parent, called alleles, then how the gene affects a person's phenotype is determined by whether each allele is dominant or recessive. The trait most likely to have an effect is called the dominant trait; however, if a recessive trait is homozygous, meaning both parents pass it down, then it can be expressed (see Resources for several links to information on Punnett Squares and example crosses).
Polygenic Traits and Pleiotropy
Although dominant and recessive genes often determine the physical traits a person inherits, many traits are the result of the combined effect of multiple genes. Hair and eye color are examples. Pleiotropy, a single gene affecting multiple traits, is essentially the opposite of this.
Intermediate Inheritance
Intermediate inheritance essentially means intermediate expression of a trait. This occurs when a person is heterozygous, meaning that there is a mixture between dominant and recessive alleles. In flowers, a cross between a purple and white flower may produce a flower with light purple petals (reference 3, resource 2). However, not all heterozygous traits are intermediate. Often, heterozygous genes express a dominant trait.
Identical Twins and Multiples
Identical twins and multiples almost always carry the exact same genetic code, making them ideal blood and organ donors for each other. This is also why identical twins and multiples look almost exactly the same.