Inheritance and Monohybrid Crosses

Jamie Z 2024-11-04

Learning Goals

• Define and distinguish between the terms trait, gene, allele, genotype and phenotype, homozygous, heterozygous, dominant, recessive
• Represent an organism's phenotype & genotype using standard notation.
• Identify phenotypes of organisms with a given genotypes
• Determine the potential genotypes and phenotypes of offspring from a given monohybrid cross.
• Predict simple ratios of offspring genotypes and phenotypes in crosses involving dominant/recessive gene pairs

Traits

• Traits are determined by the genes on the chromosomes (segment of DNA which determines a trait).
• Traits are physical characterises of an organism
• Chromosomes come in homologous pairs, thus genes come in pairs
• The principle of independent assortment - Inheritance of one trait has no effect on the inheritance of another trait

💡

Dominant and Recessive Alleles

• Alleles that prevent another gene from showing are dominant
• Allele that is not expressed when a dominant allele is present, is recessive

Symbol

• Dominant Allele with a capital letter
• Recessive Allele with a lowercase letter

Name	Definition	Example
Homozygous dominant	Both genes of a pair are dominant for a particular trait	TT
Homozygous recessive	Both genes of a pair are recessive for a particular trait	tt
Heterozygous (or hybrid)	A dominant and recessive gene for a particular trait	Tt

Genotype and Phenotype

• Genotype - Combination of genes an organism has (actual gene makeup)
• Phenotype - Actual trait resulting from genetic make-up

Punnet Square

• Used to predict the possible genetic makeup of offspring

💡

Incomplete dominant and Codominance

• Incomplete dominance - When one allele is not completely dominant over another, they blend
• Example: In carnations the colour red (R) is incompletely dominant over white (W). The hybrid colour is pink. Give the genotypic and phenotypic ratio from a cross between 2 pink flowers.

• When both alleles are expressed, it is codominance

Multiple Alleles

• 3 or more alleles of the same gene that codes for a single trait
• Example: Blood type in humans is determined by 3 alleles - A, B and O
• But we can only inherit 2 of the traits