Mendel's Laws of Inheritance: MDCAT Biology notes

Key terms

• Gene: unit of heredity at a fixed position (locus) on a chromosome. Alleles are alternative forms of a gene (e.g. R and r).
• Homozygous: two identical alleles (RR or rr). Heterozygous: two different alleles (Rr). A dominant allele is written as a capital letter, the recessive as a small letter.
• Genotype: the alleles an organism carries (its genetic makeup). Phenotype: the physical appearance of the trait. A trait is any physical characteristic.
• True breeding (pure line): homozygous; on self-pollination it gives only its own type. Repeated self-fertilisation produces true-breeding plants.

Mendel worked on the garden pea because it normally self-pollinates (pollen reaches the stigma of the same flower), yet can be cross-pollinated by hand, and it has clear contrasting traits.

Mendel's laws

Law of dominance

In a heterozygote one allele (dominant) completely masks the other (recessive). Rr looks the same as RR. Mendel's traits all showed complete dominance. So a child with one brown-eye allele has brown eyes even if one parent is blue-eyed.

Law of segregation (first law)

The two alleles of a pair separate during gamete formation (meiosis), so each gamete receives only one. In a monohybrid cross, the recessive phenotype disappears in $F_1$ and reappears in one quarter of $F_2$.

Law of independent assortment (second law)

Each pair of alleles assorts into gametes independently of other pairs. This holds only for genes on non-homologous (different) chromosomes; genes on the same chromosome are linked. The 9:3:3:1 ratio of a dihybrid cross is the evidence.

Standard crosses

Cross	Phenotype ratio	Genotype ratio
Rr $\times$ Rr (monohybrid $F_2$)	3 : 1	1 RR : 2 Rr : 1 rr
Rr $\times$ rr (test cross)	1 : 1	1 Rr : 1 rr
RrYy $\times$ RrYy (dihybrid $F_2$)	9 : 3 : 3 : 1	9 genotypes

In the monohybrid $F_2$, homozygous dominant : homozygous recessive = 1 : 1, and all homozygotes (RR + rr) : heterozygotes = 2 : 2 = 1 : 1. RRYY $\times$ rryy gives an $F_1$ that is all RrYy, round yellow. For a single class in the dihybrid $F_2$, multiply: dominant for one trait and recessive for the other = $\frac{3}{4}\times\frac{1}{4}=\frac{3}{16}$.

Test cross

An organism showing the dominant phenotype is crossed with a homozygous recessive (tt). If all offspring show the dominant trait, the unknown is homozygous dominant; if half show the recessive trait, it is heterozygous.

Beyond simple dominance

Pattern	Meaning	Example
Incomplete dominance	Heterozygote is intermediate	Pink four o'clock flowers
Codominance	Both alleles fully expressed in heterozygote, not blended	AB blood group
Multiple alleles	More than two alleles in the population	ABO: $I^A$, $I^B$, $i$
Epistasis	Gene at one locus masks a gene at another locus	Bombay phenotype
Polygenic	Many genes add up; continuous variation	Human skin colour, height, wheat kernel colour

Human inheritance

Humans cannot be crossed experimentally, so modes of inheritance are traced by pedigree analysis. Autosomal traits appear equally in males and females. Two normal parents with an affected child must both be carriers of a recessive allele. Cousin marriages raise the risk of recessive disorders because relatives share alleles.

Common MDCAT traps

• A test cross is always with the homozygous recessive, not with Tt or TT.
• Carrier $\times$ affected (Aa $\times$ aa) gives 50% affected, e.g. a woman with an albino father marrying an albino man.
• Carrier $\times$ carrier gives 50% homozygotes (25% AA + 25% aa), not 25%.
• Codominance (both expressed) differs from incomplete dominance (intermediate).
• Tongue rolling is discontinuous; skin colour and height are continuous.

Quick revision

• Alleles = alternative forms of a gene.
• Phenotype = appearance; genotype = allele makeup.
• Segregation: alleles separate into gametes.
• Independent assortment only for genes on different chromosomes.
• ABO: multiple alleles, with $I^A$ and $I^B$ codominant.