Bio 113 - Genetics
Genetics examines (and predicts) the results of sexual reproduction. These results are governed by three simple rules.
Genetic Rules for Inheritance
  • Hereditary traits are determined by specific genes on specific chromosomes.
  • When an individual produces sex cells, the two homologues split up (segregate) and end up in separate gametes.
  • When the gametes fuse (fertilization) the new individual carries two genes for each trait, one from the mother’s egg and one from the father’s sperm.

The new individuals are formed by a combining of two haploid sex cells (gametes - a.k.a. sperm and egg).sperm fusing with egg Meiosis is the cell division process that converts a diploid cell into a haploid gamete. Meiosis also allows changes in the genetic information in order to increase diversity in the offspring.

Fertilization then combines the genetic information of gametes, each having one half the original genetic information of the parents, to form a new genetically unique individual.

Two other words that are used often in genetics are phenotype and genotype. The phenotype of an organism is the result of the genes it carries, sometimes thought of as its observable traits, and these traits are produced by the organism's genotype. Genotype is literally the genes an organism has, for any particular trait. Using the movie below, determine the genotype of each parent (for simplicity we only show one set of homologous chromosomes).

Using meiosis, the Female produces eggs and Male produces sperm. These gametes are haploid, each with only one of the homologous pair of chromosomes. Which of the homologous pair a gamete receives is completely random. The gametes can combine to form a new individual. The Punet square in the movie shows the gametes being combined (by fertilization) in all possible combinations to see all the possible phenotypes for the children of this couple.

The new individuals (single-celled) are called zygotes. A zygote has two of each type of chromosome (homologous chromosomes) and is diploid.

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As you examine the following pages, keep in mind the differences between Chromatids and homologues. Think about the relationship between Genotype and Phenotype. Keeping the terms straight will improve your understanding (and grade). Also, examine the below guidelines for understanding inheritance of dominant and recessive traits (actually genes).

Every individual has two genes for each trait, they received one of these from their mom, and one from their dad. The two genes interact to produce the final physical characteristics of the individual. These interactions can vary between gene pairs. Sometimes, certain genes are "dominant" to others. With a Dominant gene you only need have one copy to give you its trait. If you don't have a dominant gene, then you are recessive for that trait (two copies are necessary for you to have the corresponding trait). Dominant genes are usually represented as capital letters and recessive genes as lower-case letters.

Pedigree patterns for dominant traits.
1. Affected children always have an affected parent.
2. An affected parent has on average 50% affected children (assuming that the affected parent is heterozygous and the other parent is unaffected).
Pedigree patterns for recessive traits.
1. Affected children typically do not have affected parents (but see 3 below).
2. An affected family has on average 1/4 affected children.
3. The vast majority of recessive traits are very rare. However if a recessive trait is common, you may often see families that arise from one homozygous and one heterozygous parent,/ where one parent and half of the children are affected. This can be confused with a dominant trait).

Inheritance of traits next