Monohybrid and dihybrid crosses are key concepts in genetics. They help us understand how traits are inherited from parents to offspring. By using these crosses, we can predict the likelihood of specific genetic combinations appearing in future generations.

Punnett squares are handy tools for visualizing these crosses. They show us the possible genotypes and phenotypes of offspring, helping us grasp the different types of genetic dominance. This knowledge is crucial for understanding inheritance patterns in various organisms.

Monohybrid Crosses

Monohybrid cross predictions

• Involves inheritance of single gene with two alleles
  • Dominant allele represented by capital letter (A)
  • Recessive allele represented by lowercase letter (a)
• Genotypic ratios: proportion of different genotypes in offspring
  • Homozygous dominant (AA)
  • Heterozygous (Aa)
  • Homozygous recessive (aa)
• Phenotypic ratios: proportion of different phenotypes (observable traits) in offspring
  • Determined by genotype and dominance relationship between alleles
• Predict ratios by considering parent genotypes and possible allele combinations in offspring

Punnett squares for genetic probabilities

• Visual tool to predict genotypic and phenotypic ratios of offspring
• Monohybrid crosses use 2x2 Punnett square
  • Possible alleles from each parent placed on top and left side
  • Each cell represents possible offspring genotype
• Genotypic ratio determined by counting number of each genotype in square
• Phenotypic ratio determined by considering allele dominance and counting phenotypes in square

Types of genetic dominance

• Complete dominance: dominant allele completely masks recessive allele expression
  • Heterozygous (Aa) have same phenotype as homozygous dominant (AA)
  • Phenotypic ratio typically 3:1 (dominant:recessive)
• Codominance: both alleles expressed equally in heterozygous state
  • Heterozygous (Aa) have distinct phenotype from homozygous dominant (AA) and recessive (aa)
  • Phenotypic ratio typically 1:2:1 (AA:Aa:aa)
• Incomplete dominance: dominant allele doesn't completely mask recessive allele
  • Heterozygous (Aa) have intermediate phenotype between homozygous dominant (AA) and recessive (aa)
  • Phenotypic ratio typically 1:2:1 (AA:Aa:aa)

Dihybrid Crosses

Dihybrid cross predictions

• Involves inheritance of two genes, each with two alleles
  • Genes assumed to be on different chromosomes or far apart, allowing independent assortment
• Predict genotypic and phenotypic ratios by considering parent genotypes for both genes and possible allele combinations in offspring
• Genotypic ratio for dihybrid cross with two heterozygous parents (AaBb x AaBb) is 9:3:3:1 (AABB:AAbb:aaBB:aabb)
• Phenotypic ratio depends on dominance relationships between alleles for each gene

Punnett squares for genetic probabilities

• Dihybrid crosses use 4x4 Punnett square
  • Possible allele combinations from each parent placed on top and left side
  • Each cell represents possible offspring genotype for both genes
• Genotypic ratio determined by counting number of each genotype combination in square
• Phenotypic ratio determined by considering allele dominance for each gene and counting phenotype combinations in square