UPCAT Biology — Genetics & HeredityFlash Cards

What is a gene and how does it differ from an allele?

A gene is a section of DNA that carries information for constructing a protein or part of a protein. It determines a specific trait (like eye color). An allele is a variant form of a gene - a specific version of that gene (like brown eyes vs blue eyes). Genes don't occur in pairs, but alleles do. For example, the gene for eye color has different alleles like brown, blue, or green.

Explain Mendel's Law of Segregation with an example.

The Law of Segregation states that: 1) Every trait is defined by a gene pair, 2) Allelic pairs randomly segregate during sex cell formation, 3) Each gamete contains only one allele per trait, 4) Offspring inherit one allele from each parent. Example: If a parent has Tt genotype for height, their gametes will carry either T or t, not both. During fertilization, the offspring receives one allele from each parent, forming combinations like TT, Tt, or tt.

What are the three components of a nucleotide?

A nucleotide consists of three components: 1) Nitrogenous bases (purines: adenine and guanine; pyrimidines: cytosine, thymine, and uracil), 2) Sugar backbone (deoxyribose in DNA, ribose in RNA), 3) Phosphate group. These components link together to form the building blocks of DNA and RNA molecules.

Compare incomplete dominance and codominance with examples.

Incomplete Dominance: Both alleles are expressed but blend together, like mixing paint colors. Example: Red flower (RR) × White flower (WW) = Pink flower (RW). Codominance: Both alleles are fully expressed simultaneously without blending. Example: AB blood type shows both A and B antigens. In incomplete dominance, you see a new phenotype; in codominance, you see both original phenotypes together.

What is the phenotype ratio in a dihybrid cross (F2 generation)?

In a dihybrid cross F2 generation, the phenotype ratio is 9:3:3:1. This means: 9 individuals show both dominant traits, 3 show first dominant and second recessive, 3 show first recessive and second dominant, and 1 shows both recessive traits. Example: Yellow Round : Yellow Wrinkled : Green Round : Green Wrinkled = 9:3:3:1

Describe the central dogma of molecular biology.

The central dogma describes the flow of genetic information: DNA → RNA → Protein. The process involves: 1) Replication: DNA makes copies of itself, 2) Transcription: DNA information is copied to RNA (mRNA), 3) Translation: mRNA is used to make proteins at ribosomes. This explains how genetic information stored in DNA ultimately produces proteins that determine traits.

What is a sex-linked trait and why are males more affected?

Sex-linked traits are determined by genes located on sex chromosomes, usually the X chromosome. Males are more affected because they have only one X chromosome (XY), so if they inherit a recessive allele on the X chromosome, they will express the trait. Females have two X chromosomes (XX), so they need two copies of the recessive allele to express the trait. Examples include color blindness and hemophilia.

Define genotype and phenotype with examples.

Genotype is the genetic composition (allelic makeup) of an organism for a trait, while phenotype is the observable expression of that trait. Example: For plant height, TT, Tt, or tt are genotypes (genetic codes), while 'tall' or 'short' are phenotypes (what you actually see). The genotype determines the phenotype, but environmental factors can also influence phenotype expression.

What are the four types of mutations and their effects?

1) Substitution: One base is replaced by another (can be silent, missense, or nonsense), 2) Insertion: Extra base pairs are added, 3) Deletion: Base pairs are removed, 4) Frameshift: Insertions/deletions that change the reading frame. Effects range from no change (silent) to severe protein dysfunction. Example: Sickle cell anemia results from a substitution mutation in the beta-hemoglobin gene.

Explain Mendel's Law of Independent Assortment.

This law states that genes for different traits are inherited independently of each other during gamete formation. The inheritance of one trait doesn't affect the inheritance of another trait. This happens because genes on different chromosomes assort independently during meiosis. Example: Inheriting the gene for seed color doesn't influence which gene you inherit for seed shape.

What is multiple allelism? Provide an example.

Multiple allelism occurs when a trait is controlled by more than two alleles in a population, though an individual can only have two alleles. Example: ABO blood type system has three alleles (IA, IB, i). Possible genotypes are IAIA, IAIB, IAi, IBIB, IBi, and ii, resulting in four phenotypes: Type A, Type B, Type AB, and Type O blood.

Compare DNA and RNA structures and functions.

DNA: Double-stranded, contains deoxyribose sugar, has thymine, stores genetic information, found in nucleus. RNA: Single-stranded, contains ribose sugar, has uracil instead of thymine, involved in protein synthesis, found in nucleus and cytoplasm. DNA is the permanent storage of genetic information, while RNA is the temporary messenger that helps make proteins.

What happens during transcription and where does it occur?

Transcription is the process of RNA synthesis from DNA template. Steps: 1) Initiation - RNA polymerase binds to promoter, DNA unwinds, 2) Elongation - RNA polymerase moves along DNA, synthesizing mRNA, 3) Termination - RNA polymerase releases completed mRNA. It occurs in the nucleus of eukaryotic cells. The result is mRNA that carries genetic information from DNA to ribosomes.

Describe the translation process step by step.

Translation converts mRNA to proteins: 1) Ribosome binds to mRNA at start codon, 2) tRNA with matching anticodon brings amino acid to ribosome, 3) Amino acids are linked together forming polypeptide chain, 4) Process continues until stop codon is reached, 5) Completed protein is released and folds into functional shape. This occurs at ribosomes in the cytoplasm.

What is a Punnett square and how is it used?

A Punnett square is a diagram used to predict the probability of offspring genotypes and phenotypes from a genetic cross. It shows all possible gamete combinations from two parents. Steps: 1) Write parent genotypes, 2) List possible gametes for each parent, 3) Create grid with male gametes on one axis, female on another, 4) Fill squares with offspring genotypes, 5) Calculate ratios.

What are the key features of DNA replication?

DNA replication creates identical copies of DNA. Process: 1) DNA unwinds at origin of replication, 2) New bases are added complementarily (A with T, G with C), 3) One strand is made continuously (leading), other in pieces (lagging), 4) Primers are removed and gaps filled by DNA ligase. Result is two identical DNA molecules, each with one original and one new strand (semi-conservative).

What is genetic engineering and give examples of its applications?

Genetic engineering is the manipulation of genetic material using recombinant DNA technology. Applications include: 1) Producing human proteins in bacteria (insulin), 2) Creating genetically modified crops (pest-resistant corn), 3) Gene therapy for genetic diseases, 4) Transgenic animals for research, 5) Production of medicines and vaccines. It combines DNA from different sources to create beneficial products.

Explain the difference between dominant and recessive alleles.

Dominant alleles are expressed in the phenotype even when only one copy is present (heterozygous condition). They mask the expression of recessive alleles. Recessive alleles are only expressed when two copies are present (homozygous condition). Example: In Tt, T (tall) is dominant and expressed, while t (short) is recessive and masked. Only tt individuals show the recessive phenotype.

What is a monohybrid cross and what is its typical F2 ratio?

A monohybrid cross examines the inheritance of one trait controlled by one gene with two alleles. When crossing two heterozygotes (Tt × Tt), the F1 generation shows 100% dominant phenotype, and the F2 generation shows a 3:1 phenotype ratio (dominant:recessive) and 1:2:1 genotype ratio (TT:Tt:tt). This demonstrates Mendel's Law of Segregation.

What are chromosomes and what is their role in heredity?

Chromosomes are structures containing DNA wrapped around proteins (histones). They carry genes that determine inherited traits. Humans have 46 chromosomes (23 pairs) - 22 pairs of autosomes and 1 pair of sex chromosomes. During reproduction, chromosomes ensure that genetic information is passed from parents to offspring through the separation and recombination of genetic material during meiosis.