Genetics Practice Questions

Correct answer: Phenotype

The phenotype refers to the observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences. The genotype, on the other hand, is the actual genetic code. Alleles are different variants of a gene that contribute to the phenotype.

Correct answer: Delivering specific amino acids to the ribosome based on the mRNA codon

Transfer RNA (tRNA) acts as an adapter molecule that matches its anticodon with the complementary codon on the messenger RNA (mRNA) strand. By doing so, it delivers the correct amino acid to the growing polypeptide chain. Messenger RNA carries the code from DNA, while ribosomal RNA forms the ribosome structure.

Correct answer: 1:2:1

Crossing two heterozygous individuals (Tt x Tt) results in one homozygous dominant (TT), two heterozygous (Tt), and one homozygous recessive (tt) offspring. This gives a genotypic ratio of 1:2:1. The 3:1 ratio represents the phenotypic ratio (tall to short).

Correct answer: Nonsense mutation

A nonsense mutation changes an amino acid codon into a premature stop codon, leading to an abnormally shortened and usually nonfunctional protein. Missense mutations alter a single amino acid, while silent mutations do not change the amino acid sequence at all.

Correct answer: 50%

A son inherits his Y chromosome from his father and his X chromosome from his mother. Since the mother is a carrier (XNXcb), there is a 50% chance she will pass on the X chromosome carrying the color-blindness allele (Xcb) to her son, making him color-blind.

Correct answer: Law of Independent Assortment

Gregor Mendel's Law of Independent Assortment states that the alleles of two or more different genes get sorted into gametes independently of one another. This law holds true for genes located on different chromosomes or far apart on the same chromosome. The Law of Segregation applies specifically to the separation of alleles for a single gene.

Correct answer: Codominance

Codominance occurs when both alleles in a heterozygous organism contribute equally and distinctly to the phenotype, such as in AB blood types or roan cattle fur. In contrast, incomplete dominance results in a blended, intermediate phenotype. Epistasis involves one gene masking the expression of another gene entirely.

Correct answer: Helicase

Helicase breaks the hydrogen bonds holding the complementary base pairs together, effectively unwinding and separating the double-stranded DNA. DNA polymerase then synthesizes the new strands, while DNA ligase seals any gaps in the sugar-phosphate backbone.

Correct answer: Incomplete dominance

Incomplete dominance occurs when the heterozygous phenotype is an intermediate blend between the two homozygous phenotypes, as seen here with pink flowers. Codominance would display both distinct red and white patches. Multiple alleles refers to a gene having more than two allelic variants in a population.

Correct answer: Polygenic inheritance

Polygenic inheritance occurs when one characteristic is controlled by two or more genes, typically resulting in continuous variation across a population. Pleiotropy is the opposite situation, where a single gene influences multiple unrelated phenotypic traits.

Correct answer: Hydrogen bonds

Hydrogen bonds form between complementary nitrogenous bases (adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three). Phosphodiester bonds form the strong covalent backbone of each individual strand, connecting the sugars and phosphates.

Correct answer: Uracil

RNA contains the pyrimidine base uracil instead of thymine, which is found only in DNA. Cytosine is a nitrogenous base present in both DNA and RNA, and ribose is a sugar rather than a nitrogenous base, so neither fits the question.

Correct answer: Crossing over

Crossing over occurs during prophase I of meiosis when homologous chromosomes pair up and exchange segments of their genetic material. This creates new combinations of alleles in the resulting gametes. Nondisjunction is an error where chromosomes fail to separate properly.

Correct answer: Down syndrome

Down syndrome occurs when an individual has a third copy of chromosome 21 instead of the usual pair, a condition known as trisomy 21. Turner syndrome involves a missing X chromosome in females, Klinefelter syndrome involves an extra X chromosome in males (XXY), and XYY syndrome involves an extra Y chromosome in males — none of which involve chromosome 21.

Correct answer: Epistatic gene

Epistasis is a genetic phenomenon where the effect of one gene is dependent on the presence of one or more 'modifier' genes, effectively masking the other gene's phenotype. A classic example is coat color in Labrador retrievers, where one gene determines pigment color and another determines whether the pigment is deposited in the fur.

Correct answer: 20%

Chargaff's rules state that DNA has equal amounts of Adenine (A) and Thymine (T), and equal amounts of Cytosine (C) and Guanine (G). If A is 30%, then T is also 30%, accounting for 60% of the total DNA. The remaining 40% must be split equally between C and G, resulting in 20% Cytosine.

Correct answer: Ribosome

Translation, the process of decoding mRNA to build a protein, takes place on ribosomes located in the cytoplasm or attached to the rough endoplasmic reticulum. Transcription, the process of copying DNA into mRNA, occurs inside the nucleus.

Correct answer: Huntington's disease

Huntington's disease is an autosomal dominant disorder, meaning an individual needs only one copy of the defective gene from either parent to develop the condition. Cystic fibrosis and sickle cell anemia are autosomal recessive, requiring two copies, while hemophilia is an X-linked recessive disorder.

Correct answer: Aneuploidy

Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell, such as 45 or 47 chromosomes instead of the normal 46 in humans. Polyploidy refers to having one or more complete extra sets of chromosomes (e.g., 3n or 4n), which is common in plants but lethal in humans.

Correct answer: Gregor Mendel

Gregor Mendel was an Austrian monk who discovered the fundamental principles of inheritance through his systematic breeding of pea plants in the 1850s and 1860s. His work laid the foundation for modern genetics, though its significance was not fully realized until decades later.