Heredity: Inheritance and Variation of Traits
Core concepts
DNA stores genetic information in a sequence of nucleotides; genes are segments that code for proteins or RNA. Replication copies DNA before cell division. Transcription makes mRNA from DNA in the nucleus, and translation builds a polypeptide at the ribosome using codons and tRNA. Traits are influenced by genotype and environment. In Mendelian inheritance, alleles separate during meiosis (law of segregation) and assort independently when genes are on different chromosomes. Punnett squares predict offspring ratios; a monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio. Meiosis produces haploid gametes with genetic variation through crossing over and independent assortment; mitosis produces diploid body cells. Mutations are changes in DNA sequence and are the original source of new alleles. Pedigrees and karyotypes reveal inheritance patterns and chromosome abnormalities.
Traps to watch
- Dominant alleles are not necessarily more common in a population; dominance only describes expression in heterozygotes.
- Genotype is the allele combination; phenotype is the observable trait.
- Transcription occurs in the nucleus (in eukaryotes); translation occurs at the ribosome.
- Meiosis halves chromosome number; mitosis keeps it constant.
- Codominance shows both traits at once; incomplete dominance produces an intermediate blend.
Worked example
Cross Aa × Aa. Each parent gives A or a with equal chance. Offspring genotypes are 1 AA : 2 Aa : 1 aa. If A is completely dominant, phenotypes are 3 dominant : 1 recessive. Rule of thumb: count gamete combinations, then group by phenotype.