GENETICS SUPPLEMENT
I.
Introduction
GENETICS is the study of heredity and variation. Heredity is the transmission of characters from parents to
offspring, while VARIATION is the tendency of offspring to differ from their parents and each other.
Inherited variation may be due to genetic recombination or mutations. Noninherited variation may be due to
environmental influences such as nutrition, weather factors, crowding and pollution.
II.
Gamete Formation
Gamete formation involves independent assortment. Every diploid cell contains pairs of HOMOLOGOUS
CHROMOSOMES on which are found GENES. The genes are in pairs, and govern hereditary traits such as
flower color or height in plants. The two members of a given pair of genes are known as ALLELES.
During meiosis, at which time the gametes (sperm & egg) are formed, the pairs of homologous chromosomes are
separated along with the genes they bear. Only one of each pair is eventually sorted into each egg or sperm.
In studying genetics, it is helpful to consider one pair at a time. The following problems should help fix in your
mind the concept of INDEPENDENT ASSORTMENT of gene pairs in the formation of gametes.
In each of the following examples, determine how many DIFFERENT KINDS of gametes may be formed from
the diploid parent cells. The LETTERS within the parent cells represent GENES for a particular trait. The
capital letters A, B, D represent genes for DOMINANT TRAITS. While lower case letters a, b, d represent
genes for RECESSIVE TRAITS.
In examples #1 and #2 below, fill in the gamete cell with the letter or letters which represent all the
hereditary combinations. Assume that each pair of genes is located on a different pair of homologous
chromosomes. NOTE: each gamete is HAPLOID and contains only ONE gene of a pair. This means ONLY ONE
letter (either dominant or recessive) of each pair must be represented.
You can use the “FOIL”, demonstrated in class, for determination of gamete genotype for dihybrid crosses.
Example: AaBb x AABb
First 
AB
AB
Outside 
Ab
Ab
Inside 
aB
AB
Last

ab
Ab
This in-class activity has been adapted from a laboratory exercise written by Bette Doezema, Professor at
Grand Rapids Community College.
Determining Genetic Makeup of Gametes
III.
Example #1
Example #2
Father’s Cells (2n)
Aa
Father’s Cells (2n)
AaBb
Possible Male Gametes (n)
Possible Male Gametes (n)
Monohybrid Cross (The recombination of gametes)
When both genes of a pair of a particular trait are the same, they are said to be HOMOZYGOUS. When the
gene pair consists of genes that produce contrasting effects for the trait, they are called HETEROZYGOUS.
A DOMINANT gene (A, for example) of a contrasting pair (alleles Aa) is the one which expresses its effect,
while the RECESSIVE gene (a) of the pair does not express its effect. The genetic combination of the gene pair
is called a GENOTYPE, and is symbolized by letters (AA, Aa, or aa). The PHENOTYPE is the expressed physical
characteristic described in words such as “red”, “white”, “tall”, “smooth”, etc.
A MONOHYBRID CROSS involves the transmission and expression of a SINGLE PAIR of genes.
The problems and demonstrations involve MONOHYBRID CROSSES, the concept of COMPLETE and
INCOMPLETE DOMINANCE, and Mendel’s LAW OF SEGREGATION.
This in-class activity has been adapted from a laboratory exercise written by Bette Doezema, Professor at
Grand Rapids Community College.
1.
Mendel’s Law of Segregation
Mendel concluded from his breeding experiments that an individual plant has two genes for each hereditary
characteristic. These genes behave as independent units, passing intact from one generation to the next
where they may or may not visibly affect phenotype depending upon their dominant or recessive
characteristics.
Mendel further concluded that gene pairs SEGREGATE during the formation of the gametes. Consequently,
a gamete contains only one allele of each gene. He reasoned that when genes recombine at fertilization, they
produce the predictable phenotypic ratios that he had observed in the offspring.
2. Complete Dominance
Mendel determined that, in pea plants, tall height (T) is dominant to dwarf (t). This means that
heterozygous plants (Tt) are tall. The allele for tall (T) completely masks the allele for dwarf (t) in
producing the phenotype. This allele relationship is called COMPLETE DOMINANCE.
You can use a PUNNETT SQUARE to predict the outcome of crossing a homozygous tall plant (TT) with a
dwarf plant (tt). The resulting offspring are called the F1 generation.
Then, you can use another Punnett square to cross two of the F1 offspring and obtain the F2 generation.
Cross the two parents, homozygous dominant tall plant (TT) and recessive dwarf plant (tt).
Parent (TT)
Parent (tt)
F1 Generation
Note: Always put the dominant allele (T) first when writing a genotype (Example - Tt)
What is the expected genotype of the F1 generation? ______________
Describe the expected phenotype(s) of the F1 generation: ___________
This in-class activity has been adapted from a laboratory exercise written by Bette Doezema, Professor at
Grand Rapids Community College.
Cross two members of the F1 generation:
Parent (Tt)
Parent (Tt)
F2 Generation
What is the expected genotype ratio of the F2 generation? ________
What is the expected phenotype ratio of the F2 generation: ________
3. Incomplete Dominance
In some instances, a cross between two individuals, each differently homozygous for a particular
characteristic, results in partial expression of both traits in the F1 phenotype. One gene incompletely masks
the other in the heterozygote.
Complete the following Punnett squares using the information. Since dominance is not operating, a slight
modification of gene symbols is used. We will use the letter C to represent color in snapdragons. C R will
represent the allele for red color and CW will represent the allele for white color. The hybrid of a red and a
white snapdragon is pink, so would be represented as C R CW.
Use this Punnett square to reveal the genotype of the F1 generation, where the parent plants consist of a
red and a white snapdragon plant.
Parent (CR CR)
Parent (CW CW)
F1 Generation
This in-class activity has been adapted from a laboratory exercise written by Bette Doezema, Professor at
Grand Rapids Community College.
What is the expected genotype of the F1 generation? ______________
Describe the expected phenotype(s) of the F1 generation: ___________
Next, cross two of these F1 snapdragon plants to obtain the theoretical or expected phenotypic and
genotypic rations of the F2 generation.
Parent (CR CW)
Parent (CR CW)
F2 Generation
What is the expected genotype ratio of the F2 generation? ________
What is the expected phenotype ratio of the F2 generation: ________
IV.
Probability: The likelihood of occurrence of a given event.
The laws of probability may be applied to any event which involves chance or random happenings. For
example, if one tosses a penny, the chance of it falling heads is one in two, or ½, and the chance of it falling
tails is ½. Another way to express ½ is to divide 1 by 2 to obtain the decimal 0.5, which is the same as ½ or
50%.
In this case the EXPECTED RATIO is 1:1. This is determined by dividing heads (0.5) and tails (0.5) by the
lowest value (in this case 0.5). Ratios are always expressed as 1: “something”.
The PROBABILITY OF AN EVENT can be predicted by dividing the number of favorable outcomes for that
even by the number of possible outcomes. The answer, expressed numerically, will always be some value
between ZERO (totally impossible) or ONE (completely certain).
Because this is a mathematical measure of what can be expected, the values are theoretical. In practice,
you will not always achieve the expected. Many causes, including SMALL SAMPLE SIZE, can contribute to
the actual outcome not being equivalent to the expected outcome.
This in-class activity has been adapted from a laboratory exercise written by Bette Doezema, Professor at
Grand Rapids Community College.
1.
Expected ratio for sex of offspring.
In humans, the fusion of gametes at fertilization results in 46 chromosomes for the developing fetus.
Twenty two of those are homologous autosomes plus the sex chromosomes that determine the sex of the
offspring. The mother can only contribute an X chromosome. The father can contribute an X or a Y,
depending on which was independently assorted to the particular sperm that fertilized the egg.
Father (XY)
Mother (XX)
Sex Chromosomes of Offspring
What’s the probability of any sperm cell containing a Y-chromosome? _______ An X-chromosome? ______
What is the probability of fertilization resulting in a female child? _____________
Since there are only two possibilities for the sex of a baby (male or female), theoretically, any fetus has a
50% chance of being male and a 50% chance of being female. No matter how many children of one sex or
the other there already are in a family, the probability of having a boy or the probability of having a girl is
50% for each fertilization.
V. Mendelian Inheritance in Humans
Human possess several single-gene traits that are easy to observe and which are controlled by genes that have
only two alleles. Based on the specific combination of alleles (AA, Aa = dominant phenotype; aa = recessive
phenotype) the trait is either present or absent. There is no “in between”. Many other human traits, such as skin
color and height are due to several pairs of genes, and the phenotypes show a continuous range of variation.
See the genetic traits worksheet for examples.
For the traits described in the following list, determine your phenotype and possible genotype.
This in-class activity has been adapted from a laboratory exercise written by Bette Doezema, Professor at
Grand Rapids Community College.