MCAT - Bio/Biochem Foundations:: Genetics Inheritance

Mendel's law of segregation states that:

- **A)** Genes for different traits are always inherited together as a single unit
- **B)** The two alleles for a given gene separate (segregate) from each other during gamete formation, so each gamete receives only one of the two alleles
- **C)** Only dominant alleles are passed to offspring, with recessive alleles being destroyed
- **D)** All offspring will always show the identical phenotype as one parent

Mendel's law of independent assortment states that:

- **A)** Alleles of genes located on different (nonhomologous) chromosomes segregate independently of one another during gamete formation, so the inheritance of one gene's alleles doesn't influence the inheritance of another (unlinked) gene's alleles
- **B)** All genes are always inherited together, regardless of chromosome location
- **C)** Only recessive alleles assort independently, while dominant alleles do not
- **D)** Independent assortment applies only to genes located on the same chromosome

In a monohybrid cross between two heterozygous individuals (Aa x Aa) for a trait with simple complete dominance, what phenotypic ratio is expected in the offspring?

- **A)** 1:1
- **B)** 1:2:1 phenotypic ratio
- **C)** 3:1 (three-quarters showing the dominant phenotype, one-quarter showing the recessive phenotype)
- **D)** 1:1:1:1

A dihybrid cross between two individuals heterozygous for two independently assorting genes (AaBb x AaBb) produces offspring in what classic phenotypic ratio (assuming complete dominance at both loci)?

- **A)** 3:1
- **B)** 1:1:1:1
- **C)** 1:2:1
- **D)** 9:3:3:1 (9 showing both dominant traits, 3 showing one dominant/one recessive, 3 showing the other dominant/recessive combination, and 1 showing both recessive traits)

Incomplete dominance, a deviation from simple Mendelian dominance, occurs when:

- **A)** The recessive allele is completely masked, identical to complete dominance
- **B)** The heterozygous phenotype is an intermediate blend between the two homozygous phenotypes (e.g., red x white flowers producing pink offspring), reflecting neither allele being fully dominant over the other
- **C)** Both alleles are fully and separately expressed in the heterozygote with no blending, producing a combination phenotype
- **D)** Only males can display the heterozygous phenotype

Codominance, another deviation from simple dominance, differs from incomplete dominance in that codominance produces a heterozygous phenotype in which:

- **A)** Neither allele's trait is expressed at all
- **B)** The two traits blend into an intermediate phenotype, identical to incomplete dominance
- **C)** Both alleles are fully and separately/distinctly expressed simultaneously, without blending (e.g., the AB blood type, in which both the A and B antigens are fully expressed together on red blood cells, rather than an intermediate blend)
- **D)** Only the recessive allele's trait appears

The ABO blood group system illustrates multiple allelism, meaning:

- **A)** More than two alleles (IA, IB, and i) exist for this single gene within the population, even though any individual can carry only two of these alleles at a time
- **B)** Only two alleles exist for this gene, identical to simple Mendelian traits
- **C)** The gene is located on the Y chromosome
- **D)** Each individual carries all three alleles simultaneously

A gene exhibiting pleiotropy is one in which:

- **A)** The gene has no observable phenotypic effect on the organism at all
- **B)** The gene's expression depends entirely on the sex of the individual
- **C)** The gene is located on multiple different chromosomes simultaneously
- **D)** A single gene influences multiple, seemingly unrelated phenotypic traits - a classic human example is the gene responsible for phenylketonuria (PKU), whose mutation affects not just one but several distinct physiologic systems