Genetics & Inheritance — Part 1: 40 MCQs with Explanations

The gene for hemophilia is located on the X chromosome, making it sex-linked.

Helicase breaks hydrogen bonds between complementary base pairs to unwind the DNA double helix.

In DNA, adenine (A) forms two hydrogen bonds with thymine (T).

Aneuploidy is the presence of an abnormal chromosome number (e.g., Down syndrome, trisomy 21).

DNA (deoxyribonucleic acid) is a nucleic acid that carries genetic information.

Huntington's disease is caused by a dominant allele on an autosome (chromosome 4).

In RNA, adenine pairs with uracil (A-U) instead of thymine.

The central dogma, first proposed by Francis Crick, states that genetic information flows from DNA to RNA (transcription) and then from RNA to Protein (translation).

According to Chargaff's rules, A=T and G=C. If A is 30%, then T is also 30%, totaling 60%. The remaining 40% is split equally between G and C, so G is 20%.

Hemophilia is a classic example of an X-linked recessive disorder, specifically mentioned in the syllabus under sex linkage in humans.

Gregor Mendel's experiments on pea plants led to the principles of inheritance, including the law of independent assortment for two traits.

Lamarck's theory of evolution, specifically the inheritance of acquired characteristics, is a key concept included in the MDCAT evolution syllabus.

Sex-linked traits are associated with genes located on sex chromosomes (X or Y), with X-linked disorders being more common.

Huntington's disease is caused by a dominant allele on an autosome. An affected individual has a 50% chance of passing it to each child.

Pyrimidine bases have a single-ring structure and include cytosine, thymine (in DNA), and uracil (in RNA). Adenine is a purine base with a double-ring structure.

Hemophilia is an X-linked recessive disorder caused by mutations in genes encoding clotting factors VIII or IX, primarily affecting males.

DNA replication is semi-conservative, as demonstrated by Meselson and Stahl; each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

DNA ligase catalyzes the formation of phosphodiester bonds between adjacent Okazaki fragments on the lagging strand during DNA replication.

Down syndrome (Trisomy 21) results from an extra copy of chromosome 21, causing characteristic facial features, intellectual disability, and other health issues.

Helicase breaks hydrogen bonds between complementary base pairs, unwinding the double helix to create a replication fork.

RNA polymerase catalyzes transcription—the synthesis of RNA using a DNA template strand during gene expression.

DNA polymerase adds nucleotides to the growing DNA strand in the 5'→3' direction during replication, using the template strand.

Most bacteria have a single circular chromosome located in the nucleoid region, along with possible plasmids (small circular DNA molecules).

Males (XY) express X-linked recessive disorders with a single mutant allele since they lack a second X chromosome to mask the effect.

Meselson-Stahl experiment demonstrated that each daughter DNA molecule consists of one parental strand and one newly synthesized strand.

DNA ligase catalyzes phosphodiester bond formation between adjacent Okazaki fragments on the lagging strand.

Turner syndrome (45,X) is caused by complete or partial absence of one X chromosome in females, leading to short stature and infertility.

Down syndrome results from trisomy 21 (three copies of chromosome 21), causing intellectual disability and characteristic physical features.

Helicase breaks hydrogen bonds between complementary bases, separating DNA strands to form replication forks.

DNA polymerase adds nucleotides to the 3' end of growing DNA strand in 5'→3' direction during replication, using template strand.

Degeneracy: most amino acids are specified by multiple codons (e.g., leucine has 6 codons), providing mutation resistance.

Mutation introduces new genetic variants; sexual reproduction (crossing over, independent assortment, random fertilization) reshuffles existing variation.

Hardy-Weinberg equilibrium (p² + 2pq + q² = 1) describes allele/genotype frequencies in non-evolving populations (no selection, mutation, migration, drift, random mating).

Mendel's first law: during gamete formation, the two alleles for a gene segregate so each gamete receives only one allele.

Leading strand is synthesized continuously in 5'→3' direction toward replication fork. Lagging strand is synthesized discontinuously as Okazaki fragments.

Mendel's second law: alleles of different genes assort independently during gamete formation if genes are on different chromosomes (or far apart on same chromosome).

RNA contains uracil (U) instead of thymine (T) found in DNA. Both nucleic acids contain adenine, guanine, and cytosine.

Males (XY) express X-linked recessive disorders with a single mutant allele since they lack a second X chromosome to mask the effect. Females are typically carriers.

Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers, each containing a sugar, phosphate group, and nitrogenous base.

During gamete formation, allele pairs separate so each gamete receives only one allele for each gene.