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Meiosis and Introduction to Inheritance
Prelab Reading
All of us can think of traits that have been passed down in our own families. You might have hair color similar to
your father, or eye shape similar to your mother; you also might worry that you have inherited the trait for an illness
that is common among your relatives, such as heart disease or breast cancer. The study of the inheritance of traits is
called genetics.
Traits are passed from one generation to the next when genes are passed from parents to offspring. Traits are passed
on structures called chromosomes. Chromosomes are composed of a molecule called deoxyribonucleic acid (DNA)
and a variety of proteins. Sections of DNA that encode traits are called genes. Each chromosome carries hundreds of
genes. Figure 1 shows all of the chromosomes found in a human body cell. This picture is called a karyotype.
Figure 1. A normal human karyotype.
You can see from the karyotype that it is possible to arrange chromosomes into pairs. These pairs are called
homologous pairs. Humans have 22 homologous pairs of chromosomes and one pair of sex chromosomes. Females
have two X sex chromosomes, and males have one X and one Y sex chromosome.
Homologous pairs of chromosomes carry the same genes, but can carry different versions of each gene. Different
versions of a gene are said to be alleles of a particular gene. Two individuals with different alleles of a gene will have
different appearances or phenotypes. This is because an individual’s phenotype is determined by the complement of
alleles present, called the genotype. Check out Table 1 on the next page to get an idea of the relationship between
genes and alleles.
Table 1. Relationship between genes and alleles.
Examples of Genes
Examples of Alleles of a Particular Gene
eye color
brown eyes
blue eyes
plant height in peas
tall
dwarf
number of fingers
six fingers per hand
five fingers per hand
Each of these genes has two alleles—different versions of the same gene—that result in different traits. The three
examples here are easy ones because we only have two alleles for each gene. Some genes have many more than two
alleles. We’ll explore an example of this in a later lab.
Organisms pass their genes to their offspring via special cells that are produced by a process of cell division called
meiosis. In humans, meiosis occurs in the cells of the ovaries and testes to produce eggs and sperm. Cells produced
by meiosis are called gametes and they contain ½ of an individual’s genes and chromosomes. Gametes are united at
fertilization. When a gamete from a female fuses with a gamete from a male, genetic information from each parent
will be present in the offspring.
Some genes that are passed on chromosomes are dominant, or expressed when there is one allele present, and some
genes are recessive, or only expressed when two of the same alleles are present. Dominant alleles are usually written
as capital letters, while recessive alleles are written as lowercase letters. See Table 2:
Table 2. Dominant vs. recessive alleles
Examples of Genes
Examples of Alleles of a Particular Gene
eye color
Dominant allele: brown eyes (B)
Recessive allele: blue eyes (b)
plant height in peas
Dominant allele: tall (T)
Recessive allele: dwarf (t)
number of fingers
Dominant allele: six fingers per hand (F)
Recessive allele: five fingers per hand (f)
Look at the table above, and pay attention to the following:
•
For each gene listed, there are two naturally-occurring alleles.
•
One allele is the dominant allele and is abbreviated with a capital letter, while the other is the recessive
allele and is abbreviated with a lowercase letter.
•
Also notice that what we think of as “normal” is not always the dominant trait. Polydactlyly, the condition
of having six fingers on each hand, is dominant to the allele for “normal” five-fingered hands.
An understanding of dominant and recessive interactions allows one to predict the outcomes of various crosses. In
the lab that we’ll do this week, you will use imaginary organisms to help you understand the basic principles of
genetics.