AP Bio Pedigree Practice Quiz

In pedigree charts, a circle is conventionally used to represent a female individual. This standard symbol helps in quickly identifying genders in family history analyses.

A square is the standard symbol for a male in pedigree charts. Recognizing these symbols is fundamental to analyzing genetic inheritance patterns.

Filled symbols typically represent individuals who express or are affected by a trait. This convention is used to quickly visualize the distribution of a trait within a family.

For an autosomal recessive trait to appear in a child, both unaffected parents must carry one copy of the recessive allele. Carriers are phenotypically normal but can pass on the allele to their offspring.

A Punnett square is used to predict possible genetic outcomes by mapping the segregation of parental alleles. It is a fundamental tool in understanding inheritance patterns in pedigree analysis.

When one parent is heterozygous for an autosomal dominant trait, there is a 50% chance of passing on the dominant allele. Since the other parent does not contribute an affected allele, the probability remains 50% for each child.

In X-linked dominant inheritance, an affected father passes his X chromosome to all his daughters, causing them to be affected, while his sons receive the Y chromosome and remain unaffected. This distinct pattern points to an X-linked dominant mode of inheritance.

An unaffected individual with an affected parent in an autosomal recessive disorder must have inherited one recessive allele from the affected parent and one dominant allele from the other parent, making them a heterozygous carrier. Carriers do not show the trait but can pass the allele to offspring.

Males are more susceptible to X-linked recessive disorders because they possess only one X chromosome; a single recessive mutation will result in the trait being expressed. Females have two X chromosomes, so a normal allele on one can mask the effect of a recessive mutation on the other.

Autosomal recessive conditions can manifest in a child born to two unaffected parents because both parents may be carriers of the recessive allele. When both contribute the recessive allele, the child becomes affected despite each parent showing no symptoms.

Consanguineous marriages increase the odds that both parents share the same recessive allele because of their common ancestry. This shared genotype significantly raises the risk of autosomal recessive disorders in their offspring.

In autosomal dominant inheritance with complete penetrance, the presence of a single dominant allele guarantees expression of the trait. Therefore, an affected individual must inherit the allele from one of their affected parents.

A trait that is observed in every generation typically indicates autosomal dominant inheritance. The dominant allele, when present, tends to be expressed, preventing the trait from skipping generations.

Incomplete penetrance occurs when not all individuals with the mutant allele exhibit the phenotype. This result can cause an autosomal dominant trait to appear as if it skips generations, despite the genetic predisposition being present.

Mitochondrial inheritance is exclusively maternal, meaning that only females pass on their mitochondria to their children. This results in all children of an affected mother having the potential to express the trait, while fathers do not transmit it.

When both parents are heterozygous carriers for an autosomal recessive trait, the Mendelian inheritance ratio dictates that 25% of their children will inherit both recessive alleles and thus be affected. This is a fundamental outcome predicted by classic genetics.

The unaffected sibling has a 2/3 probability of being a carrier and the partner has a 1/25 chance. If both are carriers, there is a 1/4 chance the child will be affected. Multiplying these together (2/3 Ă - 1/25 Ă - 1/4) yields approximately 1/150.

X-linked recessive traits are more frequently expressed in males because they have only one X chromosome, and these traits do not show male-to-male transmission since fathers pass their Y chromosome to sons. This pattern makes X-linked recessive inheritance the most likely explanation.

Given that an unaffected female with an affected brother has about a 50% chance of being a carrier for an X-linked recessive trait, and if she is a carrier her son has a 50% chance of inheriting the mutant allele, the overall probability for an affected son is 0.5 Ă - 0.5 = 25%.

A dihybrid cross examines the inheritance of two traits that assort independently, allowing prediction of the combined phenotype in offspring. This method is best applied when analyzing two separate, unlinked traits within a pedigree.