Genetics and Inheritance
Explore how traits are passed from parents to offspring through alleles, using Mendel's laws and Punnett squares to predict inheritance patterns.
Mendel's Laws of Inheritance
Gregor Mendel, an Austrian monk, conducted experiments with pea plants in the 1860s and established the foundational principles of genetics. His work revealed that traits are determined by discrete units we now call genes, and that these units come in different versions called alleles.
Law of Segregation
Each organism carries two alleles for each gene (one from each parent). During gamete formation (meiosis), these alleles separate so each gamete carries only one allele.
Law of Independent Assortment
Alleles for different genes are sorted independently of each other during gamete formation (provided the genes are on different chromosomes).
Key terms: An organism with two identical alleles (e.g. BB or bb) is homozygous. An organism with two different alleles (e.g. Bb) is heterozygous. The dominant allele (B) masks the effect of the recessive allele (b) in a heterozygote.
Genotype, Phenotype and Punnett Squares
The genotype is an organism's genetic makeup (e.g. Bb), while the phenotype is the observable characteristic (e.g. brown eyes). A Punnett square is a grid used to predict the probability of offspring genotypes from a genetic cross.
Monohybrid Cross: Bb × Bb
| B | b | |
| B | BB | Bb |
| b | Bb | bb |
Genotype ratio: 1 BB : 2 Bb : 1 bb
Phenotype ratio: 3 dominant : 1 recessive (3:1)
Remember: The 3:1 phenotype ratio only appears in crosses between two heterozygous parents. If one parent is homozygous dominant (BB) and the other heterozygous (Bb), all offspring show the dominant phenotype.
Beyond Simple Dominance
Not all inheritance follows the simple dominant/recessive pattern. Several other patterns exist in nature.
Incomplete Dominance
The heterozygote shows a blended phenotype. E.g. red flower × white flower = pink flower.
Co-dominance
Both alleles are fully expressed. E.g. ABO blood groups where alleles IA and IB are co-dominant, producing AB blood type.
Sex-linked Traits
Genes on the X chromosome show different inheritance in males and females. E.g. red-green colour blindness is more common in males.
Why Males Are More Affected by X-linked Recessive Conditions
Males have only one X chromosome (XY), so a single recessive allele on the X chromosome will be expressed. Females (XX) need two copies of the recessive allele to show the trait, because the dominant allele on the second X can mask it.
Key Vocabulary
Allele
An alternative form of a gene. For example, a gene for flower colour may have a purple allele (B) and a white allele (b).
Genotype
The combination of alleles an organism carries for a particular gene (e.g. BB, Bb or bb).
Phenotype
The observable physical or biochemical characteristic of an organism, determined by its genotype and environment.
Homozygous
Having two identical alleles for a gene (e.g. BB or bb). Heterozygous means having two different alleles (e.g. Bb).
Worked Examples
A heterozygous brown-eyed parent (Bb) crosses with a blue-eyed parent (bb). What are the expected offspring genotypes and phenotypes?
Step 1: Set up the Punnett square with gametes B and b from one parent, and b and b from the other.
Step 2: Fill in: Bb, Bb, bb, bb.
Step 3: Genotype ratio = 2 Bb : 2 bb = 1:1.
Answer: 50% heterozygous brown-eyed (Bb) and 50% homozygous blue-eyed (bb).
In snapdragons, red (R) and white (W) show incomplete dominance. What phenotypes result from RW × RW?
Step 1: Punnett square: RR, RW, RW, WW.
Step 2: RR = red, RW = pink (blended), WW = white.
Answer: Phenotype ratio = 1 red : 2 pink : 1 white (1:2:1).
A carrier mother (XCXc) and a normal father (XCY) have children. What is the probability of a colour-blind son?
Step 1: Mother's gametes: XC or Xc. Father's gametes: XC or Y.
Step 2: Sons: XCY (normal vision) or XcY (colour-blind). Daughters: XCXC (normal) or XCXc (carrier).
Answer: 25% of all children (or 50% of sons) will be colour-blind.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What genotype ratio is expected from a cross between two heterozygous parents (Bb × Bb)?
Question 2
Mendel's Law of Segregation states that:
Question 3
An organism with the genotype Bb is described as:
Question 4
In incomplete dominance, the heterozygote phenotype is:
Question 5
Why are males more likely to express X-linked recessive disorders?
Key Concepts Summary
- ●Mendel's Law of Segregation: allele pairs separate during gamete formation.
- ●A Punnett square predicts offspring genotype and phenotype ratios from a genetic cross.
- ●Dominant alleles mask recessive alleles in heterozygotes; both must be recessive for the recessive phenotype.
- ●Incomplete dominance produces blended phenotypes; co-dominance shows both alleles equally.
- ●Sex-linked traits on the X chromosome affect males more often because they have only one X.