4.3 Mendelian Genetics and Inheritance
Key Takeaways
- Mendel's three laws - dominance, segregation, and independent assortment - explain monohybrid and dihybrid ratios such as 3:1 and 9:3:3:1.
- A Punnett square is just a graphical use of the multiplication rule: the probability of inheriting any particular allele combination is the product of the probabilities of each independent event.
- Pedigrees are decoded by checking whether the trait skips generations (recessive), whether unaffected parents have affected children (recessive), and whether affected fathers pass it to all daughters but no sons (X-linked dominant).
- ABO blood type is the textbook example of codominance plus multiple alleles, while pink snapdragon flowers from a red-by-white cross illustrate incomplete dominance.
- Polygenic traits show continuous bell-curve variation, pleiotropy means one gene affects many traits, and epistasis means one gene's expression masks another (as in coat color in Labrador retrievers).
Why Mendelian Genetics Anchors Chapter 4
Mendelian problems are predictable points on the Praxis: you set up a cross, compute the ratio, and interpret the outcome. Mastering the framework also unlocks pedigree interpretation, which is a standard test question type.
Mendel's Three Laws
| Law | What it Says | Example |
|---|---|---|
| Law of Dominance | When two true-breeding parents differ in one trait, the F1 offspring all show the dominant phenotype. | Tall (TT) x short (tt) -> all Tt tall |
| Law of Segregation | The two alleles for each gene separate during gamete formation; each gamete receives only one. | Tt parent produces 1/2 T and 1/2 t gametes |
| Law of Independent Assortment | Alleles of different genes assort independently into gametes, provided the genes are on different chromosomes (or far apart on the same chromosome). | A RrYy plant produces RY, Ry, rY, and ry gametes in equal frequencies |
Working a Monohybrid Cross
For a single gene, write the parental genotypes, identify their possible gametes, and set up a 2-by-2 Punnett square. Example: Tt x Tt.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotype ratio: 1 TT : 2 Tt : 1 tt. Phenotype ratio (T dominant): 3 tall : 1 short.
Test Cross
A test cross breeds an individual showing the dominant phenotype with a homozygous recessive partner to determine the unknown's genotype. If the unknown is TT, all offspring are tall; if the unknown is Tt, you expect a 1:1 tall:short ratio.
Dihybrid Cross
For two genes on different chromosomes, the offspring phenotype ratio of a dihybrid cross (RrYy x RrYy) is 9 : 3 : 3 : 1 (9 dominant for both : 3 dominant for one : 3 dominant for the other : 1 recessive for both). This is the multiplication rule in action - the 3:1 ratio for one gene multiplied by the 3:1 ratio for the other.
Probability Shortcuts
Most Praxis problems can be solved without drawing a Punnett square.
- Multiplication rule: probability of two independent events both occurring = product of their probabilities. The probability that two Aa x Aa parents produce a child who is aa AND a girl = 1/4 x 1/2 = 1/8.
- Addition rule: probability of either of two mutually exclusive events = sum of their probabilities. The probability of an Aa x Aa child being either AA or aa = 1/4 + 1/4 = 1/2.
Pedigree Patterns
A pedigree diagrams a family. Squares = males, circles = females, filled symbols = affected.
| Pattern | Tell-tale Signs |
|---|---|
| Autosomal dominant | Trait appears in every generation. Affected parents typically have affected children. Roughly equal in males and females. |
| Autosomal recessive | Trait can skip generations. Unaffected (carrier) parents can have affected children. Equal in males and females. |
| X-linked recessive | Far more affected males than females. Affected fathers cannot pass it to sons but pass the carrier allele to all daughters. Carrier mothers transmit to 1/2 of sons. |
| X-linked dominant | Affected fathers pass it to all daughters but no sons. Both sexes affected; can appear in every generation. |
| Y-linked | Strictly father-to-son transmission. |
| Mitochondrial | Strictly mother-to-all-offspring transmission. |
Carrier Identification
A carrier is heterozygous for a recessive allele but phenotypically unaffected. In autosomal recessive disorders, when both parents are carriers (Aa x Aa), each child has 1/4 chance of being affected, 1/2 chance of being a carrier, and 1/4 chance of being homozygous unaffected.
Non-Mendelian Inheritance
Incomplete Dominance
Heterozygotes show an intermediate phenotype. Classic example: snapdragons. Red (RR) x white (rr) yields all pink (Rr) F1. A pink x pink cross gives 1 red : 2 pink : 1 white - the genotype ratio equals the phenotype ratio.
Codominance
Both alleles are fully expressed in heterozygotes. The textbook example is ABO blood groups, which also involves multiple alleles (I^A, I^B, i).
| Genotype | Blood Type |
|---|---|
| I^A I^A or I^A i | A |
| I^B I^B or I^B i | B |
| I^A I^B | AB (codominant) |
| ii | O |
Polygenic Inheritance
Many genes contribute additively to one trait, producing a continuous, bell-curve distribution of phenotypes. Examples: human height, skin color, and most quantitative traits.
Pleiotropy
One gene affects multiple, seemingly unrelated traits. Example: the CFTR mutation causes cystic fibrosis with effects on the lungs, pancreas, sweat glands, and male fertility.
Epistasis
One gene's product masks or modifies the expression of another gene. In Labrador retrievers, the E gene controls whether pigment is deposited in the fur: ee dogs are yellow regardless of their B (black/brown) genotype, so E is epistatic to B.
Two heterozygous parents (RrYy x RrYy) cross. Genes R and Y are on different chromosomes. What is the expected phenotype ratio in the F2 generation, assuming R and Y are completely dominant?
A man with blood type AB marries a woman with blood type O. Which blood types are possible in their biological children, and which are NOT possible?