Genes and alleles, monohybrid and dihybrid crosses, codominance, multiple alleles and sex linkage, with genetic diagrams.
Inheritance is the passing of alleles from parents to offspring. Key terms: a gene is a length of DNA coding for a protein; an allele is a version of a gene; genotype is the alleles present; phenotype is the observable characteristic; homozygous = two same alleles; heterozygous = two different; dominant alleles are expressed in the heterozygote, recessive only when homozygous.
Monohybrid crosses
A cross involving one gene. Show parental genotypes, gametes, a Punnett square, and offspring ratios.
A cross of two heterozygotes (Bb × Bb) gives a 3 : 1 phenotypic ratio. A test cross (with a homozygous recessive) reveals an unknown genotype: offspring all dominant → homozygous; a 1:1 ratio → heterozygous.
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