Study Guide for Genetics Test:
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.
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.
1950’s - Rosalind Franklin took an x-ray of DNA and discovered the spiral structure of DNA.
1950’s - Watson and Crick used Rosalind Franklins discoveries to develop the first model of DNA.
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.
Traditional Dominance: One allele is completely dominant over the other. Most of the genetics examples we
use in 7th grade science are like this. Example: Brown eyes are dominant to blue eyes.
Incomplete Dominance occurs or happens because one trait is not completely dominant over another. Each
allele for a trait has its own influence on the phenotype. Red and white flower is crossed results in a pink
flower (flower is neither red nor white, but a mix between the two).
Co-Dominance occurs or happens when 2 alleles are dominant and so you see both traits.
Zebras are both black and white so both alleles are dominant.
One gene can influences many traits. This happens when different traits are found together because are
coded for by the same gene. Example: white tigers always have blue eyes, because the gene for white fur is
the same for blue eyes.
Many genes can influence a single trait. Several genes act together to make one characteristic. Color of skin,
and eye color. These traits can be influenced by your environment. Healthy diet and exercise can influence
these traits.
Different types of alleles 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.
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: either dominant (BB) or recessive (bb).
Heterozygous / Hybrid– Genotypes that include 2 different alleles, (Dd).
*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%
Genetic Engineering is the transfer of genes from one organism to another. This transfer of genes from one
organism to another is usually intended to make new, improved plants and animals, or to fight diseases using
gene therapies. Examples: making plants that are resistant to parasites, corn that grows bigger to provide
more food, using harmless viruses to introduce helpful genes to patients suffering from genetic disorders.
Some scientists argue that genetic engineering may cause unknown health risks or could cause organisms to
become genetically weak and prone to disease or disorders.