Genetics and DNA
Grade 8 · Science · Study Guide
Learn how DNA stores genetic information, how traits are inherited, and how genes determine characteristics.
DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms. It is shaped like a double helix — two strands twisted together. DNA is made of units called nucleotides, each containing a sugar, a phosphate group, and one of four nitrogen bases: adenine (A), thymine (T), cytosine (C), and guanine (G). A always pairs with T, and C always pairs with G (complementary base pairing).
DNA is organized into segments called genes. Each gene contains instructions for making a specific protein, which carries out functions in the cell. Genes are found on chromosomes — humans have 46 chromosomes (23 pairs) in nearly every cell.
Genetics is the study of how traits are inherited from parents to offspring. Gregor Mendel's experiments with pea plants established the foundation of genetics. He identified dominant and recessive alleles — different versions of the same gene. A dominant allele (written with a capital letter, e.g., B) masks the effect of a recessive allele (lowercase, e.g., b). An organism with two identical alleles (BB or bb) is homozygous; with two different alleles (Bb) it is heterozygous.
The physical appearance of an organism is its phenotype; the genetic makeup is its genotype. A Punnett square is a tool used to predict the probability of offspring having certain genotypes.
Key terms
- DNA
- Deoxyribonucleic acid; the double-helix molecule carrying genetic instructions in living organisms
- Gene
- A segment of DNA that codes for a specific protein and determines a particular trait
- Allele
- One of two or more versions of a gene; inherited one from each parent
- Dominant allele
- An allele that is expressed in the phenotype even when only one copy is present
- Recessive allele
- An allele whose effect is hidden when a dominant allele is also present; expressed only when two copies are inherited
- Punnett square
- A diagram used to predict the possible genotype combinations of offspring from a genetic cross
Flashcards
- Chromosome
- A structure made of tightly coiled DNA and proteins; humans have 46 (23 pairs) in body cells
- Genotype
- The genetic makeup of an organism, represented by letters (e.g., BB, Bb, bb)
- Phenotype
- The observable physical characteristics of an organism determined by genotype and environment
- Homozygous
- Having two identical alleles for a gene (e.g., BB or bb)
- Heterozygous
- Having two different alleles for a gene (e.g., Bb)
- Mutation
- A change in the DNA sequence of a gene, which may alter the protein produced
- Double helix
- The twisted ladder shape of the DNA molecule, formed by two complementary strands
Questions
What is the complementary base pair for adenine (A) in DNA?
- Cytosine (C)
- Guanine (G)
- Thymine (T)
- Uracil (U)
Show answer
Answer: Thymine (T)
In DNA, adenine (A) always pairs with thymine (T) through two hydrogen bonds — this is called complementary base pairing.
An organism with the genotype Bb is considered homozygous.
Show answer
Answer: False
Bb is heterozygous — it has two different alleles. Homozygous means both alleles are the same (BB or bb).
The physical appearance of an organism as a result of its genes is called its ______.
Show answer
Answer: phenotype
Phenotype refers to observable traits (like eye color or height) that result from the interaction of genotype and environment.
How many chromosomes are found in a typical human body cell?
- 23
- 46
- 92
- 64
Show answer
Answer: 46
Human body cells (somatic cells) contain 46 chromosomes arranged in 23 pairs — one chromosome in each pair from each parent.
Gregor Mendel's work with pea plants laid the foundation for the science of genetics.
Show answer
Answer: True
Mendel's 19th-century pea plant experiments revealed the principles of inheritance, including dominant and recessive traits, making him the 'father of genetics'.
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