Genetics and Heredity
Grade 8 · Science · Study Guide
Understand how traits are passed from parents to offspring through genes and chromosomes.
Heredity is the passing of traits from parents to offspring through genes. Genes are segments of DNA found on chromosomes, and each gene may have different versions called alleles. Most organisms inherit two alleles for each trait — one from each parent.
Dominant alleles mask recessive alleles when both are present. An organism's genotype (allele combination) determines its phenotype (observable trait). Homozygous means both alleles are the same (AA or aa); heterozygous means they differ (Aa).
Punnett squares are tools used to predict the probability of offspring inheriting specific traits. By crossing parent genotypes in a grid, you can calculate the likelihood of each possible genotype and phenotype in the offspring.
| Term | Definition | Example |
|---|---|---|
| Genotype | The allele combination an organism has | Tt, TT, tt |
| Phenotype | The observable trait expressed | Tall plant, short plant |
| Homozygous | Both alleles are the same | TT or tt |
| Heterozygous | Alleles are different | Tt |
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Key terms
- Allele
- A specific version of a gene that codes for a particular trait.
- Dominant
- An allele that is expressed whenever it is present, masking the recessive allele.
- Recessive
- An allele that is only expressed when two copies are present (homozygous recessive).
- Punnett Square
- A grid used to predict the probability of offspring inheriting specific genotypes and phenotypes.
Practice questions
In genetics, what is a phenotype?
- The allele combination of an organism
- The observable physical trait of an organism
- The location of a gene on a chromosome
- The number of chromosomes in a cell
Show answer
Phenotype is what you can see or measure — like height, eye color, or flower color — determined by the genotype.
If T = tall (dominant) and t = short (recessive), what is the phenotype of a plant with genotype Tt?
- Short
- Tall
- Medium height
- Cannot be determined
Show answer
Since T is dominant, it masks the recessive t allele. A plant with Tt has the dominant phenotype — tall.
A recessive allele is always expressed in the phenotype.
Show answer
A recessive allele is only expressed when the organism is homozygous recessive (two copies of the recessive allele).
An organism with two different alleles for a trait (like Tt) is called ___.
Show answer
Heterozygous means two different alleles; homozygous means two identical alleles.
Two heterozygous tall plants (Tt × Tt) are crossed. What is the probability of producing a short (tt) offspring?
Show answer
The Tt × Tt cross yields a 3:1 phenotypic ratio — 3 tall : 1 short.
Fun facts
- Humans share approximately 99.9% of their DNA with every other human — our differences come from just 0.1% of our genetic code.
- Gregor Mendel, the 'father of genetics,' discovered the principles of heredity by studying pea plants in the 1860s before DNA was even discovered.
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