Genetics and Heredity
Grade 9 · Science · Study Guide
Explore how traits are passed from parents to offspring through genes, alleles, and Mendel's laws of inheritance.
Genetics is the study of how traits are inherited from parent to offspring. Gregor Mendel, a 19th-century Austrian monk, conducted careful experiments with pea plants and established the foundational laws of heredity. His work was rediscovered in 1900 and forms the basis of classical genetics.
Each trait is controlled by genes, which are specific segments of DNA located on chromosomes. Most organisms have two copies of each gene (one from each parent), called alleles. If the two alleles are the same, the organism is homozygous for that trait; if they differ, it is heterozygous. A dominant allele (represented by a capital letter) masks the expression of a recessive allele (lowercase). An organism's genetic makeup is its genotype; its observable characteristics are its phenotype.
Mendel's Law of Segregation states that the two alleles for a trait separate during gamete formation so each gamete carries only one allele. His Law of Independent Assortment states that alleles for different traits are distributed to gametes independently of each other (assuming genes are on different chromosomes). Punnett squares are used to predict the probability of offspring genotypes and phenotypes. Some traits show incomplete dominance (blending) or codominance (both alleles expressed equally), adding complexity beyond simple dominant/recessive patterns. Human blood type (ABO system) is a classic example of codominance and multiple alleles.
| T (dominant) | t (recessive) | |
|---|---|---|
| T (dominant) | TT (tall) | Tt (tall) |
| t (recessive) | Tt (tall) | tt (short) |
| Pattern | Description | Example |
|---|---|---|
| Complete Dominance | Dominant allele fully masks recessive | Brown eyes over blue |
| Incomplete Dominance | Blend of two alleles | Red + white flower = pink |
| Codominance | Both alleles expressed equally | AB blood type |
Key terms
- Gene
- A segment of DNA that codes for a specific protein or trait; located at a specific locus on a chromosome.
- Allele
- One of two or more versions of a gene; inherited one from each parent.
- Dominant
- An allele that is expressed in the phenotype whenever present, masking recessive alleles.
- Recessive
- An allele that is only expressed in the phenotype when two copies are present (homozygous recessive).
- Genotype
- The genetic makeup of an organism; described using allele symbols (e.g., Bb, TT, tt).
- Phenotype
- The observable physical expression of the genotype (e.g., tall, blue eyes, blood type A).
- Punnett Square
- A grid used to predict the probability of offspring genotypes and phenotypes from a cross.
- Law of Segregation
- Mendel's first law: allele pairs separate during meiosis so each gamete carries one allele per gene.
Flashcards
- Allele
- A version of a gene; you have two alleles per gene, one from each parent.
- Dominant
- Allele expressed whenever present; written as capital letter.
- Recessive
- Allele only expressed when two copies present (homozygous); written as lowercase.
- Genotype
- Actual genetic makeup (e.g., Tt, BB, bb).
- Phenotype
- Observable trait resulting from genotype + environment.
- Punnett Square
- Grid tool for predicting offspring genotype and phenotype ratios.
Practice questions
An organism with two different alleles for the same trait is described as:
- Homozygous dominant
- Homozygous recessive
- Heterozygous
- Codominant
Show answer
Answer: Heterozygous
Heterozygous means having two DIFFERENT alleles for a trait (e.g., Bb, Tt). Homozygous means both alleles are the same (BB or bb).
A recessive trait can be expressed in an organism that has one dominant and one recessive allele.
Show answer
Answer: False
When one dominant allele is present, it masks the recessive allele. Only organisms with two recessive alleles (homozygous recessive, e.g., tt) express the recessive trait.
An organism's observable traits make up its ___.
Show answer
Answer: phenotype
Phenotype is what you can observe — height, eye color, blood type. Genotype is the underlying genetic code. Two organisms can share a phenotype but have different genotypes (e.g., TT and Tt both look tall).
In a cross between two Tt (heterozygous tall) plants, what fraction of offspring will be short (tt)?
- 1/4
- 1/2
- 3/4
- 0
Show answer
Answer: 1/4
Tt × Tt Punnett square gives: TT, Tt, Tt, tt. That is 1 TT : 2 Tt : 1 tt. Only tt is short, so 1 out of 4 offspring (25%) will be short.
Which of Mendel's laws states that allele pairs separate during gamete formation?
- Law of Independent Assortment
- Law of Dominance
- Law of Segregation
- Law of Codominance
Show answer
Answer: Law of Segregation
Mendel's First Law states allele pairs segregate (separate) during meiosis, so each gamete receives only one allele for each gene — the basis of how traits are inherited.
Fun facts
- Mendel studied over 29,000 pea plants over 8 years — making him one of history's most patient scientists.
- You share about 99.9% of your DNA with every other human on Earth — all human diversity comes from that 0.1%.
- Identical twins have the same genotype but can have different phenotypes due to environmental influences — one twin might develop a disease while the other does not.
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