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Genetics and Heredity
Mendel's Laws, Punnett Squares & Inheritance
Related Labs
Mendelian genetics explains how traits are passed from parents to offspring through discrete units called genes. Gregor Mendel discovered patterns of inheritance by studying pea plants and tracking visible traits across generations. His work showed that inheritance is not a blending process but follows predictable rules. These ideas are the foundation of classical genetics and help explain family resemblance, inherited disorders, and selective breeding.
Genes are carried on chromosomes, and different versions of a gene are called alleles. During meiosis, allele pairs separate so each gamete receives one allele, and fertilization restores the pair in the offspring. Dominant and recessive relationships affect which traits appear in the phenotype, while the genotype describes the actual allele combination. Tools such as Punnett squares and probability rules let students predict trait ratios and understand how heredity works from one generation to the next.
Key Facts
- Gene = a segment of DNA that influences a trait; allele = different version of a gene.
- Genotype is the allele combination, such as TT, Tt, or tt; phenotype is the observable trait.
- Law of Segregation: the two alleles for a gene separate during gamete formation.
- Law of Independent Assortment: alleles of different genes assort independently if the genes are on different chromosomes or far apart.
- Monohybrid cross of two heterozygotes: Tt x Tt gives genotype ratio 1 TT : 2 Tt : 1 tt and phenotype ratio 3 dominant : 1 recessive.
- Dihybrid cross of two heterozygotes: RrYy x RrYy gives phenotype ratio 9:3:3:1 under simple Mendelian inheritance.
Vocabulary
- Gene
- A gene is a section of DNA that contains information affecting a trait.
- Allele
- An allele is one of the alternative forms of a gene found at the same locus on homologous chromosomes.
- Genotype
- Genotype is the specific allele combination an organism has for a gene.
- Phenotype
- Phenotype is the observable characteristic produced by the genotype and the environment.
- Heterozygous
- Heterozygous means having two different alleles for a gene, such as Tt.
Common Mistakes to Avoid
- Confusing genotype with phenotype, which is wrong because genotype refers to allele combinations while phenotype refers to the expressed trait you can observe.
- Assuming dominant means more common or stronger, which is wrong because dominance only describes which allele is expressed in a heterozygote.
- Forgetting that each parent contributes only one allele for each gene, which is wrong because meiosis separates allele pairs before fertilization.
- Using phenotype ratios when the question asks for genotype ratios, which is wrong because different genotypes can produce the same phenotype under complete dominance.
Practice Questions
- 1 In pea plants, tall T is dominant over short t. What are the genotype ratio and phenotype ratio from a cross Tt x Tt?
- 2 In a dihybrid cross RrYy x RrYy, where R is dominant to r and Y is dominant to y, what is the probability of an offspring with genotype rryy?
- 3 A child shows a recessive trait, but both parents show the dominant phenotype. Explain what this suggests about the parents' genotypes.