Genetics and Heredity
Grade 7 · Science · Study Guide
Understand how traits are inherited from parents to offspring through genes, alleles, and Punnett squares.
Why do you look like your parents? The answer is genetics — the study of how traits are passed from one generation to the next. Your traits are determined by genes, which are segments of DNA located on chromosomes. Most human body cells contain 46 chromosomes — 23 pairs — with one chromosome in each pair coming from your mother and one from your father.
Each gene can exist in different versions called alleles. If both alleles are the same (like BB or bb), the individual is homozygous for that trait. If the alleles are different (like Bb), they are heterozygous. Alleles also have a relationship: dominant alleles mask recessive alleles when both are present. For example, brown eye color (B) is dominant over blue (b). A person with Bb has brown eyes, because the B allele dominates.
Gregor Mendel, a 19th-century monk who experimented on pea plants, figured out the basic rules of inheritance before anyone even knew what DNA was. His work led to two major laws: the Law of Segregation (each parent passes only one allele per gene to offspring) and the Law of Independent Assortment (genes for different traits are inherited independently of each other).
A Punnett square is a simple grid tool that lets you predict the probability of offspring inheriting certain traits. You put one parent's alleles across the top and the other's down the side, then fill in the boxes. Each box represents a 25% chance. Some traits (like blood type) are more complicated — they can involve multiple alleles or codominance, where both alleles are fully expressed at the same time.
| R | r | |
|---|---|---|
| R | RR (Red) | Rr (Red) |
| r | Rr (Red) | rr (White) |
Key terms
- Gene
- A segment of DNA on a chromosome that codes for a specific trait or protein.
- Allele
- One of two or more versions of a gene.
- Dominant
- An allele that masks or overrides the expression of the recessive allele.
- Recessive
- An allele whose effect is hidden when a dominant allele is present.
- Homozygous
- Having two identical alleles for a trait (e.g., BB or bb).
- Heterozygous
- Having two different alleles for a trait (e.g., Bb).
- Punnett square
- A grid used to predict the probability of offspring genotypes and phenotypes.
- Phenotype
- The observable physical traits of an organism (what you can see).
- Genotype
- The actual allele combination an organism carries (the genetic code).
Flashcards
- Genetics
- The branch of biology that studies heredity and variation in organisms.
- Dominant allele
- An allele that is expressed whenever it is present, even if only one copy exists.
- Recessive allele
- An allele that is only expressed when two copies are present (homozygous recessive).
- Punnett square
- A diagram used to predict ratios of offspring genotypes in a genetic cross.
- Law of Segregation
- Mendel's law stating each parent passes only one allele per gene to offspring.
- Codominance
- When both alleles are fully expressed simultaneously in the phenotype (e.g., AB blood type).
Practice questions
What is the term for an allele that is hidden when a dominant allele is present?
- Dominant
- Heterozygous
- Recessive
- Codominant
Show answer
Answer: Recessive
Recessive alleles are only expressed (visible) when an organism inherits two copies — one from each parent.
A person with genotype Bb is homozygous.
Show answer
Answer: False
Bb has two DIFFERENT alleles, making it heterozygous. Homozygous means both alleles are the same (BB or bb).
The physical appearance of an organism based on its genes is called its ___.
Show answer
Answer: phenotype
Phenotype refers to observable traits like eye color, height, or blood type — the physical expression of the genotype.
In a Punnett square cross of Rr × Rr, what fraction of offspring will be homozygous recessive (rr)?
- 1/4
- 1/2
- 3/4
- All of them
Show answer
Answer: 1/4
In an Rr × Rr cross, the four possible outcomes are RR, Rr, Rr, and rr. Only one of four (25%) is homozygous recessive.
Gregor Mendel discovered the basic rules of inheritance by studying pea plants.
Show answer
Answer: True
Mendel crossed thousands of pea plants and counted ratios of traits, discovering the mathematical laws of inheritance decades before DNA was understood.
Fun facts
- You share about 50% of your DNA with a banana — because many basic cell functions use the same ancient genes.
- Gregor Mendel published his findings on genetics in 1866, but they were largely ignored for 35 years until other scientists rediscovered them.
Get a personalized course on Genetics and Heredity built around your child.