Practice Quiz #3 PDF

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Practice quiz 3...

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Pedigree Analysis 1. “Males are more frequently affected than females and the phenotype often skips a generation.” Which inheritance pattern does this statement best describe? An X-linked recessive trait is more common in males because they are hemizygous for the genes on the X chromosome (they do not have another X-chromosome to mask the affect of a recessive allele). A generation is often skipped because affected males pass down their Y chromosome (which does not carry the trait) to sons and the recessive allele on the X chromosome is transmitted to their daughters. In the daughters, the recessive trait is often masked by the dominant allele from the mother, so is not seen in this generation. It could not be the other options for the following reasons: 

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Autosomal traits (either dominant or recessive) would not tend to appear in every-other generation, and are not usually associated with gender and often affect males and females in roughly equal numbers. Alleles passed down on the Y chromosome only affect males and the trait is observed in all sons of an affected male in every generation. In humans there are no known Y-linked traits. In X-linked dominant inheritance, both males and females would likely show the trait. (Affected males will pass the trait to all their daughters and none of their sons since the Y allele is passed to the sons).

Review and understand all characteristics that pertain to each inheritance pattern.

2. Which terms best fills in the blanks? When both parents are affected by a(n) _______________ trait, it is possible for them to have an unaffected child if both their genotypes are _______________ Ans: autosomal dominant; heterozygous When both parents are heterozygous for an autosomal dominant trait (eg. genotypes Aa x Aa ), they both express the trait. Because they both carry a recessive allele, it is possible for them to have a homozygous recessive child that does not express the trait. If the inheritance was autosomal recessive the affected parents could only be homozygous. Similarly in X-linked recessive inheritance, all parental X chromosomes carry the recessive allele and it would not be possible to have an unaffected child. It would be possible to observe the scenario with X-linked dominant inheritance to produce unaffected sons; however, the mother's genotype would be heterozygous and not hemizygous dominant.

3. If the phenotype presented in the pedigree is autosomal recessive, how many individuals definitely have a heterozygous genotype?

All four individuals from II are heterozygous. Offspring II-1, II-2 and II-3 must be heterozygous because they are unaffected by the trait but carry a recessive allele from their mother (I-2). Individual II-4 from outside the family, must also be heterozygous because he passes a recessive allele to produce a homozygous recessive offspring (III-1). As for individuals I-1, III-2, III-3 and III-4, we cannot say with 100% certainty that they are heterozygous, as there is a possibility they are homozygous dominant. 4. Could the characteristics presented in this pedigree be caused by an X-linked recessive allele? Why or not?

If this pedigree were to follow an X-linked recessive inheritance pattern, the genotype of the father is XY and the mother's genotype is xx . Both sons (individuals II-2 and II-3) would inherit their X chromosome from their mother and their genotypes should be xY ; thus, they should be affected by this trait. Individual II-1 also violates the suggested inheritance pattern because the X chromosome inherited from the father should mask the recessive allele from the mother; she should be a carrier, but not affected by the trait. The expected genotypes of these three individuals contradict an X-linked recessive inheritance pattern.

5. What is the most likely mode of inheritance for this pedigree?

There are a few common patterns that arise that you should know in order to begin to narrow your search. If the trait is present frequently in every generation (and it is here), this indicates dominant. If the trait appears infrequently and not in every generation, then it indicates recessive. If it affects males more than females and skips a generation of females, then it indicates X-linked recessive. If it appears frequently and is being passed from affected fathers to all daughters, then it indicates X-linked dominant. Now test for each one. Start with autosomal dominant; this fits with female I-2 being heterozygous. There may be more than one inheritance pattern that is acceptable, so test the others to be sure. The mode of inheritance cannot be X-linked recessive, because individual II-3 should be affected and individuals II-8 and III-11 should be unaffected. Its likely not autosomal recessive because mates 1, 4, and 6 coming into generation 2 would have to be heterozygous. This would be highly unlikely if the trait was a detrimental disease (however the question does not specify this). The most likely inheritance pattern for this pedigree is X-linked dominant because there is a relationship between the gender of the affected parent and the gender and number of affected offspring. Look at hemizygous affected fathers II-2 and II-5. They pass the trait to ALL daughters.

6. Examine the pedigree. What is the most likely mode of inheritance and what is the chance that III-1 is a carrier?

The trait appears infrequently. The likely mode of inheritance is autosomal recessive. This trait must be recessive because the affected individuals have unaffected parents. The mode of inheritance cannot be X-linked recessive because individual II-7 contradicts this inheritance pattern. The chance that III-1 is a carrier is 1/2 or 50%. To produce affected III-3,

we know the mother is a carrier and so is the father. When we cross two heterozygotes, the probability of obtaining a heterozygous offspring is 50%....