Download Genetic Disorders as Models for Evolution

Genetic Disorders as Models for Evolution: Case Study (Page 567 to 569) [21]
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a) Genotypes of the offspring are Hb Hb , 2 Hb Hb , and Hb Hb [2]
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i)
probability ¼ (Hb Hb ) normal phenotype or ¾
ii)
probability ¼
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b) Normal parent (Hb Hb ) x carrier parent (Hb Hb ) = all offspring are normal [1]
None would be sickle-celled.
c) All offspring will be homozygous for the dominant condition. There is also little chance
that they would have offspring as most would die prior to reproductive age. [1]
d) If demonstrated that the heterozygous phenotype was related to the sickling condition
which was expressed by the homozygous recessive genotype. [1]
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e) They are resistant to malaria, a parasitic infection. Selective advantage ~ (Hb Hb ) [1]
f) The sickle-cell gene would eventually disappear, since there would no longer be any
advantage in being a carrier. However, this would take a number of generations. [1]
g) The mosquito that carriers the disorder is being controlled. Also, many North American
Blacks may have migrated to areas where malaria doesn’t exist, breed out with other
races, increased their gene pool. [1]
h) ¼. The next generation should yield the same results unless the tt condition which
causes Tay-Sachs provides some environmental disadvantage or some advantage. [1]
i) Otherwise the parent would have the disorder. The phenotype is expressed as the
homozygous genotype. [1]
j) Both are caused by an abnormal protein (enzyme). In both cases the DNA sequence has
been altered (sickle cell anemia ~ substitution and Tay-Sacks ~ deletion) [1]
k) Individuals with tt have motor coordination problems and often convulsions. Die early. [1]
l) The tt genotype often caused death or severe deterioration of the nervous system, while
Tt condition may have provided resistance against tuberculosis. Isolation, genetic drift,
resistance to disease, death, interbreeding etc. [2]
m) Individuals with the heterozygous genotype have an increased survival rate. [1]
n) It may indicate that migration from this population was restricted or that some other
environmental factor may have occurred in this area. This group’s genotype may have
been susceptible to these environmental influences. [1]
o) The environment likely induces gene mutations, mutations are not universal. [1]
Case Study Application Questions
1. The environment selects certain genotypes. Genotypes that are deleterious in the
homozygous condition continue to remain part of the gene pool because their
heterozygous condition provides some selective advantage. [1]
2. The couple should be informed about the chances that a child would have the
homozygous recessive condition. Treatment for the homozygous recessive condition
could also be explored. [1]
3. The recessive disorders provide a selective advantage in the heterozygous condition.
The heterozygous condition requires that some genes remain in the gene pool. The
mating of heterozygous individuals, by chance, will produce some homozygous recessive
individuals. You cannot produce a 100% heterozygous population. [2]