Unit 2: Extensions of Mendelian Inheritance
What happens once alleles stop behaving in the simple dominant-recessive way Mendel's peas suggested.
8 topics · 31 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Incomplete Dominance
- How it worksHow does a heterozygote produce an intermediate phenotype when neither allele is fully dominant?
- Worked exampleFind the phenotype ratio from crossing two pink snapdragons under incomplete dominance.
- Common mistakeWhy isn't incomplete dominance the same thing as the two alleles blending together permanently?
- Going furtherWhat happens to a cross's phenotype ratio when incomplete dominance at one gene combines with ordinary dominance at a second gene?
Your turn1 of 2
Two pink snapdragons (Rr x Rr, incomplete dominance) are crossed, producing 151 offspring. How many are expected to be pink (the heterozygous, intermediate phenotype)?Codominance
- How it worksHow do two codominant alleles both get expressed in the same heterozygous organism?
- Worked exampleFind the genotype of a person with blood type AB given the codominant IA and IB alleles.
- Common mistakeWhy isn't codominance just another name for incomplete dominance?
- Going furtherWhat happens to a Punnett square's phenotype categories when a trait is codominant instead of dominant-recessive?
Your turn1 of 2
Two roan cattle (RW x RW, codominant red and white alleles) are crossed, producing 151 calves. How many are expected to be roan (heterozygous, patches of both colors) rather than solid red or solid white?Multiple Alleles and ABO Blood Type
- How it worksHow do three alleles at one gene combine to produce six possible genotypes for blood type?
- Worked exampleFind the possible blood types of children from a mother who is type A, genotype IAi, and a father who is type B, genotype IBi.
- Common mistakeWhy doesn't having three alleles in a population mean one person can carry all three at once?
- Worked exampleFind the possible genotypes and blood types of children from a type AB mother and a type O father.
Your turn1 of 2
A type A mother (genotype IAi) and a type B father (genotype IBi) have 124 children. Based on the expected 1:1:1:1 genotype ratio (IAIB, IAi, IBi, ii), how many are expected to be type O (ii)?Lethal Alleles
- How it worksHow does a homozygous lethal allele remove one genotype class from the offspring before birth?
- Worked exampleFind the surviving offspring ratio from crossing two yellow mice given the homozygous yellow genotype is lethal.
- Going furtherWhat happens to the expected 3 to 1 ratio when a recessive allele is lethal only in homozygotes?
Your turn1 of 2
Two yellow mice (Yy x Yy) are crossed, where the homozygous YY genotype is lethal in utero. If 124 embryos are conceived, how many are expected to survive to birth?Epistasis
- How it worksHow does one gene's product block or mask the phenotype controlled by a second gene in epistasis?
- Worked exampleFind the modified 9:3:4 phenotype ratio from a dihybrid cross where a recessive genotype at one gene masks the other gene.
- Common mistakeWhy isn't epistasis the same thing as one allele being dominant over another at the very same gene?
- Going furtherWhat happens to a 9:3:3:1 ratio when the two genes involved show duplicate recessive epistasis instead of single-gene recessive epistasis?
- Worked exampleFind the phenotype ratio from a dihybrid cross showing dominant epistasis, where one dominant allele masks the second gene entirely.
Your turn1 of 2
A dihybrid cross (AaBb x AaBb) shows recessive epistasis, where bb masks gene A's phenotype, giving a 9:3:4 ratio. Out of 494 offspring, how many are expected to show the epistatic (masked) phenotype, the 4/16 share?Pleiotropy
- How it worksHow can one gene's protein product ripple out to affect several unrelated organ systems?
- Worked exampleTrace how a single mutation in the beta-globin gene produces both misshapen red blood cells and blocked capillaries in sickle cell disease.
- Common mistakeWhy doesn't pleiotropy mean several different genes are secretly involved in one disorder?
Your turn1 of 2
A patient with sickle cell disease has misshapen red blood cells, chronic anemia, joint pain, and organ damage, all traced back to a single mutated beta-globin gene. What best explains why one gene mutation produces so many seemingly unrelated symptoms?Penetrance and Expressivity
- How it worksHow does incomplete penetrance let a carrier of a dominant allele show no trait at all?
- Worked exampleFind the expected versus observed number of affected individuals in a family of 20 carriers given 80 percent penetrance.
- Common mistakeWhy doesn't an unaffected parent passing a dominant disorder to their child mean the gene skipped a generation?
- Going furtherWhat changes about a pedigree's inheritance pattern when a trait has variable expressivity instead of constant severity?
Your turn1 of 2
A family has 45 people who carry a dominant disease allele. If the allele has 0.757 penetrance, how many of the 45 carriers are expected to actually show the disease phenotype?Gene Interaction and Modified Dihybrid Ratios
- How it worksHow do two genes acting in the same biochemical pathway create a 9 to 7 ratio instead of 9:3:3:1?
- Worked exampleFind the 9 to 7 phenotype ratio from a cross where two genes must each contribute a dominant allele to make pigment.
- Ties togetherHow does complementary gene interaction connect a shared biochemical pathway to the modified ratios seen in epistasis?
- Common mistakeWhy isn't a 9 to 7 ratio evidence that the two genes involved are linked instead of simply interacting in one pathway?
Your turn1 of 2
In a cross (AaBb x AaBb) where a dominant allele at BOTH genes is required to make pigment, giving a 9 to 7 ratio, 494 offspring are produced. How many are expected to be colored (the 9/16 share)?