Techniques: The Patterns of Trait
Inheritance in Genetic Transmission
Background
All sexually reproducing organisms have two types of cell division: somatic and gametic.
Somatic division, or mitosis, is necessary for the growth, development and replacement
of damaged tissues. It forms two new somatic cells that are genetically identical to the
parent cell. Gametic division (gametogenesis), or meiosis, is necessary for gamete
formation and forms four cells that are genetically different, with half as many
chromosomes (n – haploid) as the parent cell (2n – diploid).
In animals, meiosis occurs in the ovaries or testes and forms haploid eggs or sperm. In
plants, meiosis occurs in the carpel or stamen of a flower and produces haploid eggs or
pollen. The reduction of chromosome number is important during sexual reproduction.
When the haploid male and female gametes fuse, the original diploid number of
chromosomes is restored to the diploid state in the zygote. Thus, a diploid organism
receives two sets of genes: one set through the haploid egg and another set through the
haploid sperm or pollen, contributing to novel genetic diversity.
The genes (genotype) for specific traits (phenotype) are located on the chromosomes.
Gregor Mendel used garden peas to propose the basic principles of heredity. According
to the Mendelian principles of classical dominance, genes exist in two alternative forms
or alleles. One form is the dominant allele which can be represented by the lower case of
the same letter. For the recessive trait to be expressed, two recessive alleles must be
present. It is masked by the presence of a dominant allele. The expression of the alleles
or the appearance of the trait is the phenotype, while the actual genetic composition is the
genotype.
If the inheritance of alleles for one trait is tracked, it is called a monohybrid cross,
whereas if the inheritance of alleles for two traits is tracked, it is called a dihybrid cross.
Mendel proposed the law of segregation to state that the two alleles for each trait separate
during gametogenesis. Therefore, each gamete gets only one of the two alleles for each
trait. Additionally, for a trait, if the genes are located on different chromosomes, they are
unlinked and inherited independently. This is referred to as the law of independent
assortment.
While working on genetics problems, it is important to establish the parental genotype,
apply the laws of segregation and independent assortment to determine the alleles passed
to the gametes, place the gamete types on the sides of a Punnett square and predict the
allele combination in the offspring.
Punnet squares are only one way to determine the probability of an outcome in a cross.
Another way involves using statistical rules called the rule of multiplication and the rule
of addition. The rule of multiplication states that the probability of both of two
independent events happening can be found by multiplying the odds of either event alone.
Alternatively, the rule of addition can be used to calculate the chances of either of two
events happening. For example, if the probability of being struck by lightning is 1 in a
million (10-6) and the probability of winning the lottery is one in ten million (10-7), the
probability of both happening is 10-6 x 10-7 = 10-13. The probability of either event
happening is 10-6 + 10-7 = 1.1 x 10-6. The figure below illustrates the applications of a
Punnett square and the rule of multiplication in a dihybrid cross.
Exceptions to Mendel’s basic principles of heredity: Chromosomal-Linkage
The specific traits in garden peas that Mendel studied had alleles located on separate
chromosomes which are therefore inherited independently of each other. These are
considered “un-linked” genes. Scientists working on other, far more technical species
discovered patterns of inheritance more complex than what Mendel had predicted.
Thomas Hunt Morgan found that the genes for wing shape and body color were not
independently assorted in the fruit fly. Instead, they are inherited together in a larger
percentage than expected for that genotype. These genes are located on the same
chromosome and are said to be “linked” genes. An exchange of alleles between
homologous chromosomes during prophase I (crossing-over or recombination) segregates
these genes. In addition to the parental types, recombinants (a new combination of
parental genes) are formed. The frequency of recombinants depends on the distance
between the two genes on the chromosome. To determine the percent crossover, the total
number of recombinant offspring is divided by the total number of offspring and then
multiplied by 100. This also gives the number of map units between the two genes and
helps in mapping of the genes on a chromosome. These concepts are disseminated and
conveyed in the figure below.
Exceptions to Mendel’s basic principles of heredity: Sex-Linkage Circumstances
Traits that are expressed in a similar proportion in males and females are controlled by
the genes located on the autosomes (in humans, these are chromosomes 1 through 22; in
the fruit fly, these are chromosomes 1 through 3). Morgan observed that in the fruit fly,
certain traits such as the red and white eye colors are unequally expressed in the two
sexes. The genes for the inheritance of these traits are called the sex-linked genes (in
humans, chromosome 23; in the fruit fly, chromosome 4). They are mostly recessive and
are carried on the X-chromosome with no corresponding gene on the Y chromosome.
Females have two X chromosomes and therefore inherit two alleles for each trait, one
from each parent. Thus, females are genotypically homozygous (two dominant or
recessive alleles) or heterozygous (one dominant and one recessive allele). Males have
only one X chromosome that is inherited from the mother and carry only one allele for
the trait; males are genotypically hemizygous (recessive or dominant allele). Therefore,
males exhibit these traits more commonly than females. Examples of sex-linked
recessive traits in humans include hemophilia, red-green color-blindness and Duchenne
muscular dystrophy.
Exceptions to Mendel’s basic principles of heredity: Non-Classical Dominance
Some alleles of genes display neither dominant or recessive patterns of expression. If the
phenotype of a heterozygote is a blended mix of both alleles, this is called incomplete
dominance, and the alleles for that trait are given different, upper-case letters. For
example, if a gene for flower color has two incompletely dominant alleles, where R could
be used to indicate the allele for red color and W to indicate the allele for white color, the
a heterozygote would have a “blended” phenotype of pink color.
Codominance is a slightly different situation, in which two alleles are both expressed but
are not blended. For example, the alleles of the gene for ABO blood group antigens that
are found on the surface of red blood cells display codominance. Each of the alleles is
expressed on red blood cells, regardless of the second allele in the cell. There are three
alleles for the ABO blood group antigens: IA, IB and i. The alleles IA and IB are
codominant and will be expressed regardless of the second allele, while i is recessive to
both IA and IB. The alleles IA and IB cause type A or type B antigens to be expressed,
while i does not cause antigen expression.
The alteration of a gene is said to have pleiotropic effects if the result alters many
different, seemingly unrelated aspects of the organism’s total phenotype. As an example,
the mutation of hypothetical gene X alters the development of the heart, bone and inner
ears.
Complex traits that are influenced by many different genes are called polygenic. An
example includes height, since it will be influenced by genes for growth factors,
receptors, hormones, bone deposition, muscle development, energy utilization, etc…
Epistasis refers to a situation where expression of alleles for one gene is dependent on a
different gene. For example, a gene for curly hair cannot be expressed if a different gene
causes baldness.
Penetrance describes the likelihood that a person with a given genotype will express the
expected phenotype. While many traits are completely penetrant, there is a spectrum of
options which is dependent on the allele. Some have age-related penetrance, others have
environmental and lifestyle modifiers. Finally, many alleles have genetic modifiers that
affect penetrance; since several human traits are polygenic, allels at different loci can
affect penetrance.
In short, genetics is rather complicated!
Purpose
The purpose of this exercise is to familiarize you with the Mendelian laws of heredity and
their exceptions, and to solve genetic crosses.
Materials per team
Your brain, a pencil and a piece of paper
Procedures
There are no experimental procedures for this exercise; a whiteboard workshop is
included.
WORKSHEET
Punnet, Probability and Pedigree Analysis Workshop
(bust out more paper, ‘cause you’re gonna’ need it!)
1. Classical Dominance:
a. Monohybrid Crosses
1. Homozygous dominant c. homozygous recessive
2. Homozygous dominant c. homozygous dominant
-or- Homozygous recessive c. homozygous recessive
3. Heterozygous c. homozygous dominant
-or- Heterozygous c. homozygous recessive
4. Heterozygous c. heterozygous
b. Dihybrid Cross
1. Heterozygote c. heterozygote
c. Trihybrid Cross (case study; applied rules of probability)
2. Chromosomal Linkage:
a. Unlinked vs. linked genes; gametes produced
b. Case study in recombination frequency
3. Sex-Linkage (example: hemophilia)
4. Non-Classical Dominance:
a. Incomplete dominance (example: flower petal color)
b. Codominance (example: human blood typing)
5. Pedigree Analysis