Genetics Review
Gregor Mendel “The Father of Genetics”: A 19th century monk who is credited with being the
founder of the branch of Biology dealing with heredity—Genetics. Mendel did a number of
groundbreaking studies on heredity using pea plants.
Why Pea plants? Pea plants are capable of 1) self-pollination and cross-pollination 2) pea plant
seeds were readily available 3) pea plants have traits that are easy to identify, categorize, and
analyze
Mendel’s Experiment:

Two “True-Breeding” populations
(purple and white) are cross-pollinated

Individual plants from the F1
generation are self-pollinated

Individual plants in the F2
generation are self-pollinated and it is
seen that ¼ are true-breeding purple, ¼
are true-breeding white, and ½ are nontrue-breeding and produce purple and
white flowers in a 3:1 ratio
From his experiments, Mendel deduced 3 laws describing patterns of inheritance from
parents to offspring:
1. The Law of Dominance: If an individual has one or more copies of the dominant allele,
then the dominant phenotype will be expressed (Complete Dominance). A recessive
phenotype is only expressed in the absence of a dominant allele (i.e. 2 copies of the
recessive allele).
2. The Law of Segregation: During gamete production, each of an individual’s 2 alleles split
into separate gametes (Ex. A male who is Aa would produce half of his sperm with “A”
and half of his sperm with “a”).
3. The Law of Independent Assortment: During gamete production, any combination of
genetic alleles is possible (Ex. A female who is AaBb may produce eggs that are “AB” or
“Ab” or “aB” or “ab”).
Trait vs. Gene. vs. Allele
Trait: essentially the same as “characteristic”, a trait describes how a
gene is expressed
EX. Eye color, hair color, blood type, etc.
Gene: a segment of DNA coding for a protein that determines a single
trait
EX. Gene that codes for the protein Immunoglobulin (plays a
part in determining blood type)
Allele: a variation of a gene, can be one or more, new alleles may enter
the gene pool due to mutation
EX. The gene for hair color: allele for dark hair and allele for light
hair; Blood type: Immunoglobulin A (type A), Immunoglobulin B (type
B), no Immunoglobulin (type O)
Genotype vs. Phenotype
Genotype: Think “Genes”, and from “genes” think genetic code. The
genotype tells us the set of alleles (gene versions) that an individual
has. We use letters (alleles from mom and alleles from dad) to describe
a genotype.
1. Homozygous (Homo=SAME): two of the same alleles for a gene
a. Homozygous Dominant: DD
b. Homozygous Recessive: dd
2. Heterozygous (Hetero=DIFFERENT): two different alleles for a
gene
a. Only one way to get this: Dd and dD are the same thing!
Phenotype: Think “Physical”, and from “physical” think what you
physically see. Multiple genotypes can produce the same phenotype.
We use words (usually adjectives) to describe a phenotype.
1. Both Homozygous Dominant (DD) and Heterozygous (Dd)
GENOTYPES will produce the dominant phenotype (see Law of
Dominance)
2. ONLY the Homozygous Recessive (dd) GENOTYPE will produce the
Recessive phenotype
Inheritance Patterns
1. Complete Dominance: Follows the Law of Dominance, the
presence of a dominant allele will always produce the dominant
phenotype
2. Incomplete Dominance: Only homozygous dominant genotypes
show the full dominant phenotype heterozygous genotypes
show a blending of the recessive and dominant phenotypes
3.
4.
5.
6.
7.
8.
9.
a. EX. Red Flower (RR) X White Flower (rr) produces Pink
flowers (Rr) [red and white mixes to form pink]
Multiple Alleles: Some genes have more than just a dominant
and recessive allele, often seen in CODOMINANCE
Codominance: Two or more dominant alleles are produced in the
phenotype if both are present
a. EX. Blood Type: IA (Type A) and IB (Type B) are dominant to i
(Type O) but codominant to each other giving A, B, AB, or O
as possible blood types
Polygenic Traits: Traits that are influenced/determined by
multiple genes
a. EX. Height, skin color, eye color
Pleiotropy: When one gene has effects on seemingly unrelated
traits
a. EX. Albinism (albino mutation): the gene affects the ability of
an individual to produce pigment in the skin, hair, eyes, and
other tissuesoften can result in visual difficulties in
humans and increased incidence of skin cancer
Autosomal Inheritance: any pattern of inheritance attributed to
chromosomes 1-22 (in humans) NOT Sex-linked…Most
inheritance is this type and is equally displayed in BOTH sexes
Sex-Linked Inheritance: any pattern of inheritance attributed to
the X or Y chromosomes because males only have one copy of
the X chromosomes, any heritable traits on the X chromosome
have a HIGHER INCIDENCE IN MEN THAN WOMEN
Polyploidy: the presence of more than two sets of chromosomes
in a cellassociated with fruiting plants
a. EX. Strawberries may be 8n…meaning that they have 8
copies of each chromosome in their genome
Useful Genetics Tools
1. Punnett Square: Used to visualize all possible combinations of the
gametes of two parents (one parent in self-pollination). Useful for
determining the probability of producing a given genotype (allele
combination) or phenotype (observable description) from two
individuals with known genotypes OR determining the
genotypes/phenotypes of parents based on the
genotypes/phenotypes of their offspring.
2. Karyotype: A picture of an individual’s chromosomes. Useful for
determining the sex of an individual and any defective polyploidies
a. EX. The presence of XX or XY indicates a female or male
b. EX. The presence of an extra 21st chromosome indicates Down
Syndrome in humans
3. Pedigree: Used to trace a trait through multiple generations of a
family and determine the pattern of inheritance
a. EX. A sex-linked trait will be seen mostly (if not only) in male
members of the family
b. EX. A recessive trait may appear in the offspring of parents who
do not show the recessive phenotype
TWINS: When two offspring are born in the same
human birthing event (born at the same time)
 Fraternal Twins: Two offspring resulting from two
separately fertilized eggs
o Can be different sexes
o Share the same relatedness as any other two
siblings (NOT genetically identical)
 Identical Twins: Two offspring resulting from the
splitting of a single fertilized egg
o Genetically IDENTICAL
o So…must be the same sex