Inheritance

Vocabulary.

• Gene — DNA section coding for a trait.
• Allele — version of a gene; often denoted by upper-case for dominant (T), lower-case for recessive (t).
• Genotype — the alleles an organism has (e.g. TT, Tt, tt).
• Phenotype — the trait expressed (tall or short).
• Homozygous — two identical alleles (TT or tt).
• Heterozygous — two different alleles (Tt).
• Dominant — allele expressed in heterozygote; masks recessive.

Mendel's laws:

• Law of Segregation — pair of alleles separate during gamete formation; each gamete gets one.
• Law of Independent Assortment — alleles of different genes (on different chromosomes) sort independently.

Monohybrid cross — Punnett square. Tt × Tt:

	T	t
T	TT	Tt
t	Tt	tt

Offspring: 1 TT : 2 Tt : 1 tt. Phenotype ratio: 3 tall : 1 short.

Test cross with a homozygous recessive (Tt × tt) reveals the unknown's genotype: if any short offspring appear, the unknown was Tt.

Codominance. Both alleles fully expressed in heterozygotes. ABO blood groups:

Genotype	Phenotype
I^A I^A or I^A i	Blood group A
I^B I^B or I^B i	Blood group B
I^A I^B	Blood group AB (codominance)
i i	Blood group O

Worked example. Mother I^A i × Father I^B i.

Mother gametes: I^A, i. Father gametes: I^B, i.

	I^B	i
I^A	I^A I^B (AB)	I^A i (A)
i	I^B i (B)	i i (O)

Probability ratio: 1 AB : 1 A : 1 B : 1 O. All four blood groups possible.

Sex linkage. Genes carried on sex chromosomes. Most are X-linked because the Y is small.

X-linked recessive (e.g. haemophilia, red-green colour blindness). Notation: X^H = normal allele, X^h = recessive allele.

Genotype	Phenotype
X^H X^H	Female, unaffected
X^H X^h	Female, carrier (unaffected but can pass on)
X^h X^h	Female, affected (rare)
X^H Y	Male, unaffected
X^h Y	Male, affected

Why more common in males. Males have only ONE X — a recessive allele is automatically expressed. Females need TWO copies.

Worked example. Carrier mother (X^H X^h) × normal father (X^H Y).

	X^H	Y
X^H	X^H X^H (unaffected ♀)	X^H Y (unaffected ♂)
X^h	X^H X^h (carrier ♀)	X^h Y (affected ♂)

Daughters: 50% unaffected, 50% carriers. Sons: 50% unaffected, 50% affected.

Pedigree analysis. Family tree showing trait inheritance.

• Squares = males; circles = females.
• Filled = affected; half-filled (sometimes) = carrier.
• Horizontal lines between symbols = mating.

Reading rules.

• Two unaffected parents have an affected child → trait is RECESSIVE; parents are carriers.
• Affected child is FEMALE → cannot be Y-linked.
• Father affected, all daughters affected → likely AUTOSOMAL dominant or X-linked dominant.
• Skips generations → recessive (carriers in between).