Mendelian Genetics and Inheritance Patterns
Predicting Inheritance
Alleles and dominance
Genes can exist in multiple allelic forms. Dominant alleles affect phenotype in heterozygotes, while recessive alleles are expressed only when both copies are recessive.
Probability in genetics
Genetic crosses are solved with probability. For a monohybrid cross of two heterozygotes, the genotype ratio is and the phenotype ratio is often if complete dominance applies.
If two heterozygous parents () have a child, what is the probability the child is homozygous recessive?
The Punnett square gives , , , and , so occurs with probability .
Correct answer:
What is a test cross used for?
Crossing the unknown individual with a homozygous recessive reveals whether the unknown is heterozygous or homozygous dominant.
Correct answer: To determine the genotype of an individual with a dominant phenotype.
Inheritance patterns
Complete dominance
- One allele masks another in heterozygotes
- Common phenotypic ratio in monohybrid crosses
Incomplete dominance
- Heterozygote has intermediate phenotype
- Phenotypic ratio often matches genotype ratio
A test cross for an unknown dominant phenotype typically uses a partner that is:
A homozygous recessive partner reveals the unknown genotype through offspring phenotypes.
Correct answer: Homozygous recessive