Mendelian Inheritance: Learning Objectives
1. Define and distinguish between true-breeding organisms, hybrids, the P
generation, the F1 generation, and the F2 generation.
- True-breeding is homozygous. When self-fertilization produces offspring
identical to the parent
- Hybrids are heterozygous. Is the offspring of two true-breeding varieties
- P generation: True breeding parents
-F1 generation: the first generation of the P generation offspring
-F2 generation: The offspring of the F1 generation
2. Define and distinguish between the following pairs of terms: homozygous
and heterozygous; dominant allele and recessive allele; genotype and
Two alleles are inherited from each parent:
Homozygous: An organism that has two identical alleles for a gene (PP or
pp). All gametes carry that allele. They are true breeding
Heterozygous: An organism that has two different alleles for a gene (Pp).
Half of the gametes carries one allele (P) and the remaining half carries
the other (p). They are NOT true breeding.
There are different versions of genes that cause change in the offspring. Different
versions of genes are called alleles
Dominant allele: when two alleles differ, and the one that determines the
Recessive allele: when two alleles differ, the one that has not noticeable
effect on the organism's appearance
Example: Pp, where P is dominant and p is recessive
• Phenotype: physical trait, ex. white or purple colour. The organisms expressed
trait. In Mendel's experiment the F2generation had a 3:1 phenotypic ratio of
plants with purple flowers to plants with white flowers.
Genotype: genetic makeup, ex. PP, pp, Pp. A gene with two allelic expressions,
one dominant and one recessive, possesses three genotypes but only two
o 3 genotypes:
PP - homozygous dominant (purple)
Pp - heterozygous (purple)
pp - homozygous recessive (white)
* The genotypic ratio of the F 2eneration was 1:2:1*
3. Also, define a monohybrid cross and a Punnett square.
Punnet Square: used to show four possible combinations of alleles that could
occur when these gametes combine. Is used to keep track of gametes.
Monohybrid cross: Cross-breeding in which there is a difference between the
parents of only one character For example: Mendel cross-bred between true breeding purple pea plant
and true breeding white pea plant
The punnet square after this monohybrid cross was:
- F1 generation resulted in all purple coloured plants but their offsprings
were 3/4 purple and 1/4 white
4. Explain how Mendel’s law of segregation describes the inheritance of a single
Law of segregation: A sperm or egg carries only one allele for each inherited
character because the two members of an allele pair segregate (separate) from
each other during gamete formation; when sperm and egg unite at fertilization,
each contributes its alleles, restoring the paired genes in the offspring.
Each gamete contains a single copy of each gene and is based on the events in
meiosis (anaphase I and II)
Mendel deduced that an organism has two genes (alleles) for each inherited
characteristic. The law of segregation Predicts that allele pairs separate from each
other during the production of gametes
5. Describe the genetic relationships between homologous chromosomes.
Homologous: are chromosomes that carry alleles of the same genes.
Alleles of a gene reside at the same locus on homologous chromosomes. Pair of
alleles is found in the same chromosome.
6. Explain how Mendel’s law of independent assortment applies to a dihybrid
Law of independent assortment: The inheritance of one character has no effect
on the inheritance of another. Each pair of alleles segregates independently during
Dihybrid cross: The crossing of parental varieties differing in two characters.
In a dihybrid cross - two parents will each have two characteristic that are passed
to their offspring. Each parent will therefore present 4 sets of possibilities in the
genotype. Mother (4) x Father (4) = 16 possible combinations of the genotype.
Mendel used peas to explain a dihybrid cross where two traits/characteristics were
crossed. A genetic trait for a round seed versus a wrinkled seed will be
independent of the combinations for a yellow versus green colour of the seeds.
This means that seeds can be round and yellow, round and green, wrinkled and
yellow or wrinkled and green – so, there will be 16 combinations.
7. Explain how and when the rule of multiplication and the rule of addition can
be used to determine the probability of an event.
Rule of multiplication: A rule stating that the probability of a compound event is
the product of the separate probabilities of the independent events. So the
probability of two events occurring together.
Rule of addition: If there is more then one way an outcome can occur
(determining chances of heterozygous mixtures)
8. Explain how family pedigrees can help determine the inheritance of many
Family pedigrees: a family tree representing the occurrence of heritable traits in
parents and offspring across a number of generations. 9. Explain how recessive and dominant disorders are inherited. Provide
examples of each.
Recessive disorder: Recessive alleles that cause human disorders are usually defective
versions of normal alleles. Defective alleles code for either a malfunctional protein or no
protein at all. Recessively inherited disorders range in severity from nonlethal traits (e.g.
albinism) to lethal diseases (e.g. cystic fibrosis). Since these disorders are caused by
recessive alleles: The phenotypes are expressed only in homozygotes (aa) who inherit one
recessive allele from each parent. Heterozygotes (Aa) can be phenotypically normal and
act as carriers, possibly transmitting the recessive allele to their offspring. Carrier:
Carrier of the recessive allele for a genetic disorder but the parent appears normal
themselves. Three examples are cystic fibrosis, Tay-Sachs disease and sickle-cell disease.
• Cystic fibrosis
o The most common lethal genetic disease
o The dominant allele codes for a membrane protein that controls
chloride traffic across the cell membrane. Chloride channels are
defective or absent in individuals that are homozygous recessive
for the cystic fibrosis allele.
o Disease symptoms result from the accumulation of thickened
mucus in the pancreas and lungs.
• Tay-Sachs disease
o Brain cells of babies with this disease are unable to metabolize
gangliosides (a type of lipid), because a crucial enzyme does not
o As lipids accumulate in the brain, the infant begins to suffer
seizures, blindness and degeneration of motor and mental
performance. The child usually dies after a few years.
• Sickle-cell disease
o The disease is caused by a single amino acid substitution in