Study Guide for Genetics Quiz:
Structure of DNA:
DNA molecules are made up of nucleotides. Nucleotides are composed of sugars, phosphates and bases.
The 4 different bases that are part of nucleotides are Adenine, Guanine, Thymine and Cytosine. Chargaff’s
bas-pairing rules state that bases pair like this; Adenine always pairs with Thymine. Cytosine always pairs with
Guanine. A DNA strand such as; TACTCA would pair with strand ATGAGT. Genes
are small pieces of DNA that code for individual traits. Genes are located on
chromosomes. Human body cells have 46 chromosomes and over 20,000 genes.
Historical Contributions to the study of Genetics:
1800’s - Gregor Mendel discovered that traits are passed down from parents to offspring when he studied pea
plants. He also discovered dominant and recessive traits.
Early 1900’s – Reginald Punnett developed a square that is used to visualize all the possible combinations of
alleles in offspring resulting from a genetic cross.
Inheritance of Traits:
Heredity is the passing of traits from parents to offspring. We get 23 chromosomes from each of our parents.
Genes are located on chromosomes and are a “blueprint” or set of instructions for each trait. Each parent
donates one allele for each trait to its offspring. The two alleles (versions of a gene) donated, one by each
parent, together make up an offspring’s genotype for that trait (Bb). The physical expression of the trait (what
you see), that is inherited such as brown eyes or freckled skin, is an organisms phenotype. Organisms inherit
many different traits. Inherited traits are those coded in our DNA. Some examples of inherited traits include;
hair color, eye color, skin color, nose shape, or size of ears. Non- inherited traits (such as personality,
hairstyle, and level of physical fitness) are products of one’s environment or habits.
Alleles (different versions of a gene) can be either dominant or recessive. In genetics, dominant genes are
represented by capital letters (B), and recessive genes are represented by lowercase letters (b). A dominant
gene will always show through if it is present (BB, Bb). A recessive trait cannot appear unless two recessive
alleles (bb) are inherited. See picture below. Most of the genetics examples we use in 7th grade science are
like this. Example: Brown eyes are dominant to blue eyes.
Probability is the mathematical chance that an event will occur. Punnett Squares are used to show the
probability that offspring (babies) will inherit different traits, based on their parents’ genotypes. Each box
represents a 25% chance of offspring inheriting that combination of alleles.
Gene: Eye color
Alleles: Brown (B) and blue (b)
Homozygous / Purebred - Genotypes that have two of the same alleles: dominant (BB) or recessive (bb).
Heterozygous / Hybrid– Genotypes that include 2 different alleles, (Dd). The dominant allele shows.
*Example Punnett Square Problem: Two brown eyed parents are both heterozygous for that trait. What are
the chances that they will have a child with blue eyes?
B
B
b
Genotypic Ratio is:
b
BB
Bb
Bb
bb
1 : 2: 1
BB Bb bb
Phenotypic Ratio is: ___3__ : ___1__
Dominant : Recessive
Chance of child having blue eyes: 25%
Mitosis vs. Meiosis:
Mitosis
Mitosis – Division of the nucleus and DNA in eukaryotic body cells.
In humans, body cells each have 46 chromosomes. Before mitosis, the DNA is
copied (92 chromosomes) and then the DNA is divided through mitosis, resulting
in two identical daughter cells, each with 46 chromosomes.
Meiosis – Production of eukaryotic sex cells. In humans, meiosis results in the
creation of egg and sperms. Egg and sperm cells each have 23 chromosomes
(a random combination of the individual’s genes). This ensures that the
offspring will have 46 chromosomes, a random combination of ½ of mother and
½ of father’s genes. This is why two siblings (brothers and sisters) can look very
similar or very different.
Meiosis