5 pts - Why are most genetics flaws in humans inherited as recessive traits?
The recessive disorders are more common because they are less prone to be eliminated than the dominant traits by natural selection. Recessive mutations in single copy are less harmful than dominant mutations and can be inherited. Commonly, the recessive mutations involve enzymes; 50% level activity of the enzyme is sufficient for normal function and survival of the affected individual. Heterozygous people with a recessive mutation may also be clinically normal; an example is the sickle cell disease. The autosomal recessive allele can be transmitted to the progeny because the parents have the chance to survive. In many cases, people do not know that they have a mutated recessive allele (they are carriers) until they have children that have both recessive alleles and show the disease phenotype.

4 pts - What do the terms penetrance and expressivity refer to?
Penetrance is the percentage of individuals with a specific genotype that shows the corresponding phenotype. People with a genetic mutation that exhibit the correspondent phenotype can be less of 100%, and lack of penetrance means that there are individuals whose phenotype does not reflect the genotype.
Expressivity is the degree of phenotype expression, which can be different among individuals. The trait is expressed differently, and the causes usually are environmentally or variation of the gene loci (mutations in the same gene). For instance, egg pigmentation of Oo’Aa bird, the heterozygous Bb organisms present different shade of blue.
Penetrance refers to the expression or not expression of a phenotype among different individuals. Expressivity is how much the correspondent phenotype is displayed.

Mutant alleles can cause a dominant pedigree inheritance pattern in various ways. Discuss the following examples. Your discussion should include enough molecular detail to prove you understand how each example fits the category. How does the term “penetrance” fit into each story?

3pts - Shh gain of function – Polydactyl
In this case, the mutation leads to a new function of the protein or abnormal. Polydactyl is caused by a mutation in the Sonic Hedgehog gene (SHH) gene, which decides the pattern and digit of hands and feet. There is ectopic expression of this gene that causes polydactyly (more fingers are developed).

3pts - loss of heterozygosity – retinoblastoma
The RB gene encodes for a cell cycle control protein, the gene

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