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Day 82:
Gregor Mendel and the Gene Idea
Key Concept Questions:
1. What is the relation between phenotype and complete dominance?...incomplete
dominance?...co-dominance?
2. If both parents are “carriers” for Tay-Sachs disease what is the probability that their
child will develop the disease? Why?
3. How is sickle cell anemia different from Tay-Sachs disease?
4. What type of plant did Mendel use when experimenting with heredity?
5. What are the basic principles of heredity? Explain them.
 Mendel discovered the basic principles of heredity by breeding garden peas in carefully
planned experiments.
 These basic principles are:
o Complete Dominance(ex.: Purple flower (PP) + White flower (pp) = all purple(Pp)
o Incomplete Dominance(ex.: Red flower + White flower = pink flower)
o Co-dominance(ex.: Orange flower + Yellow flower = Orange & Yellow flower)
 Tay-Sachs disease occurs when the body
lacks a protein that helps break down a
chemical found in nerve tissue. Without this
protein, this chemical builds up in cells,
especially nerve cells in the brain.
o Most people with Tay-Sachs have
the infantile form. Symptoms usually
appear when the child is 3 to 6
months old. The disease tends to
get worse very quickly, and the child
usually dies by age 4 or 5.
o Tay-Sachs disease is from a
defective recessive gene. Because
of this the disease is not extremely
common but there are people who
are carriers, which means they have
only one of the recessive genes
instead of the two needed for the disease to show.
 Sickle cell anemia is caused by an abnormal type of hemoglobin, called hemoglobin S.
Hemoglobin is a protein inside red blood cells that carries oxygen. Hemoglobin S
changes the shape of red blood cells, especially when the cells are exposed to low
oxygen levels. The red blood cells become shaped like crescents or sickles.
o The fragile, sickle-shaped cells deliver less oxygen to the body's tissues. They
can also get stuck more easily in small blood vessels, and break into pieces that
interupt healthy blood flow.
o Sickle cell anemia is another disease that is caused by a recessive gene, just like
Tay-Sachs, but there is a difference. Sickle cell is a disease most commonly
found in of tropical and sub-tropical regions where malaria is or was common.
One-third of all indigenous inhabitants of Sub-Saharan Africa carry the gene,
because in areas where malaria is common, there is a benefit in carrying only
one sickle-cell gene.
 Those with only one of the two alleles of the sickle-cell disease, while not
totally resistant, are more tolerant to the infection of malaria and thus
show less severe symptoms when infected.
Answers:
1. With complete dominance the phenotype is only from the dominant gene with the
recessive gene hidden. With incomplete dominance the phenotype can be a blending of
the two genes with no one gene overpowering the other. With co-dominance the
phenotype is a mixture of the two different genes showing both but not blending.
2. The probability that a child will inherit Tay-Sachs disease from their parents if they are
both carriers is 1/4.
3. Sickle cell disease is different from Tay-Sachs because when one has only one of the
genes for sickle cell the person would become more likely to be more resistant to malaria
which is a disease that is common in the places in the world in which the people of the
area are more likely to have a natural slight advantage to malaria which turns into having
one of the genes for sickle cell anemia.
4. Mendel used simple pea plants to explore the concepts of heredity.
5. Complete dominance – one gene has complete dominance over another so only the
dominant genes traits will show through.
Incomplete dominance – the dominance between the genes is not complete and both
genes have a visual impact on the organism; even though, it can be hard to distinguish
between two incomplete dominant genes and just a different complete dominance gene.
Co-dominance – both genes show visually their characteristics but they are not blended
together they still have a separation from each other but both show-up.