Inheritance

Inheritance is the transmission of genetic information from parents to offspring. The PMDC MDCAT 2026 syllabus expects you to apply Mendel’s three laws, work Punnett squares for monohybrid and dihybrid crosses, recognise gene linkage and crossing over, and predict X-linked recessive disorders such as haemophilia and red-green colour blindness. This is one of the most heavily tested chapters — expect 4-6 MCQs.

PMC Table of Specifications. This chapter covers three PMDC subtopics — Mendel’s laws, gene linkage and crossing over, and X-linked recessive inheritance. Numerical Punnett-square questions are almost guaranteed.

Key Genetic Terms

Gene: A unit of inheritance — a length of DNA coding for a polypeptide or functional RNA.
Allele: An alternative form of a gene at a given locus (e.g. T for tall, t for short).
Genotype / Phenotype: Genotype is the genetic make-up (TT, Tt, tt). Phenotype is the observable trait (tall, short).
Homozygous / Heterozygous: Two identical alleles (TT or tt) vs two different alleles (Tt).
Dominant / Recessive: A dominant allele expresses its phenotype in the heterozygote; a recessive allele needs both copies to be expressed.
Test cross: Cross of an unknown phenotype with a homozygous recessive individual to reveal the unknown genotype.

Mendel’s Laws of Inheritance

Gregor Mendel worked with the garden pea (Pisum sativum) and seven contrasting traits. From thousands of crosses he deduced three statistical laws.

Law 1 — Law of Dominance

When two pure-breeding parents differing in one character are crossed, only one form (the dominant) appears in the F₁; the other (recessive) is hidden. Example: TT × tt → all Tt (tall).

Law 2 — Law of Segregation

The two alleles of a gene separate during gamete formation, so each gamete carries only one allele. They reunite at random in fertilisation. A monohybrid F₁ × F₁ cross gives a phenotypic ratio of 3 : 1 and a genotypic ratio of 1 : 2 : 1.

Law 3 — Law of Independent Assortment

Alleles of different genes located on different chromosomes assort independently in gamete formation. A dihybrid F₁ × F₁ cross (RrYy × RrYy) gives a phenotypic ratio of 9 : 3 : 3 : 1.

Worked monohybrid cross

Parental cross: TT × tt → F₁ = Tt (all tall). F₁ self-cross: Tt × Tt → gametes T and t from each side. Punnett square:

TT, Tt, Tt, tt → genotypic ratio 1 TT : 2 Tt : 1 tt
Tall : short = 3 : 1

Worked dihybrid cross

RrYy × RrYy (round/yellow vs wrinkled/green pea seeds). Each parent makes 4 gamete types: RY, Ry, rY, ry. The 16-cell Punnett square gives phenotypes:

9 round yellow
3 round green
3 wrinkled yellow
1 wrinkled green

Hence the famous 9 : 3 : 3 : 1 ratio.

Common Mendelian cross outcomes (F₂ generation)
Cross	Genotypic ratio	Phenotypic ratio	When to expect
Monohybrid (Aa × Aa)	1 AA : 2 Aa : 1 aa	3 : 1	One gene, complete dominance
Test cross (Aa × aa)	1 Aa : 1 aa	1 : 1	To determine an unknown genotype
Dihybrid (AaBb × AaBb)	9 : 3 : 3 : 1 phenotypes (16 genotypes)	9 : 3 : 3 : 1	Two genes, independent, complete dominance
Dihybrid test cross (AaBb × aabb)	1 : 1 : 1 : 1	1 : 1 : 1 : 1	Two genes, independent (no linkage)
Incomplete dominance (Aa × Aa)	1 : 2 : 1	1 : 2 : 1	Heterozygote shows intermediate phenotype
Codominance (e.g. ABO)	1 : 2 : 1	Both alleles fully expressed	I^AI^B = AB blood group

Common trap. The 9 : 3 : 3 : 1 ratio holds only when the two genes are on different chromosomes (truly independent). If they are linked on the same chromosome, ratios deviate predictably (more parental types, fewer recombinants).

Gene Linkage and Crossing Over

Genes located close together on the same chromosome tend to be inherited as a unit — they are linked. Linked genes violate Mendel’s law of independent assortment.

Linkage groups

All genes on a single chromosome form one linkage group. Humans have 23 linkage groups (one per pair). The closer two loci sit, the more tightly they are linked.

Crossing over

During prophase I of meiosis, homologous chromosomes pair up (synapsis) and exchange equivalent segments at chiasmata. This recombines maternal and paternal alleles, producing recombinant gametes. The frequency of recombination between two loci (recombination frequency, RF) is roughly proportional to the distance between them. 1% RF = 1 map unit (centimorgan).

Linked genes therefore produce more parental-type gametes than recombinant ones. If the two loci are far apart, RF approaches 50%, mimicking independent assortment.

X-linked Recessive Inheritance

Some genes are carried on the X chromosome. Because males have only one X (XY), a single recessive allele is enough to express the trait. Females (XX) need two copies. As a result, X-linked recessive disorders are far more common in males.

Classical examples

Haemophilia A — deficiency of clotting factor VIII; affected individuals bleed excessively. Famous in the European royal families descended from Queen Victoria.
Red-green colour blindness — defective opsin gene; cannot distinguish reds and greens.
Duchenne muscular dystrophy — defective dystrophin gene; progressive muscle wasting.

Pedigree pattern

Use X^H for the normal allele and X^h for the recessive disease allele.

X^HX^H — normal female
X^HX^h — carrier female (clinically normal)
X^hX^h — affected female (rare)
X^HY — normal male
X^hY — affected male

Carrier mother × normal father (X^HX^h × X^HY) → offspring:

1/4 normal daughter (X^HX^H)
1/4 carrier daughter (X^HX^h)
1/4 normal son (X^HY)
1/4 affected son (X^hY) — 50% of sons are affected

Pedigree shortcut. If a trait skips a generation, affects mostly males, and there is no father-to-son transmission, suspect X-linked recessive. (Sons inherit Y, not X, from their fathers, so the dad cannot pass an X-linked allele to a son.)

Other notable inheritance patterns

Autosomal recessive: sickle cell anaemia, cystic fibrosis, phenylketonuria, albinism.
Autosomal dominant: Huntington’s disease, achondroplasia.
Codominance: ABO blood groups (I^A and I^B together produce AB).
Incomplete dominance: snapdragon flower colour (red × white → pink).

Worked MCQs

Five MCQs covering numerical and conceptual patterns most often tested. Practise the cross on paper before reading the explanation.

Q1. A heterozygous tall pea plant (Tt) is self-pollinated. What is the expected phenotypic ratio in the offspring?

1 : 1
3 : 1 (tall : short)
9 : 3 : 3 : 1
1 : 2 : 1

Tt × Tt gives genotypes 1 TT : 2 Tt : 1 tt. Both TT and Tt are tall (3 tall) while tt is short (1 short), giving a 3 : 1 phenotypic ratio — the classic monohybrid result and a direct demonstration of the law of segregation.

Q2. A 9 : 3 : 3 : 1 phenotypic ratio in the F₂ generation supports which of Mendel’s laws?

Law of dominance
Law of segregation
Law of independent assortment
Law of linkage

A 9 : 3 : 3 : 1 dihybrid ratio appears only when the two gene loci segregate independently — that is, when they sit on different chromosomes (or far apart on the same chromosome). This is Mendel’s third law.

Q3. A carrier mother for haemophilia (X^HX^h) marries a normal man (X^HY). What is the probability that any one son is haemophilic?

0%
25%
50%
100%

Sons receive Y from the father and either X^H or X^h from the carrier mother — each with 50% probability. Half of the sons are X^HY (normal) and half are X^hY (haemophilic). Note: among all children the affected fraction is 25%.

Q4. Crossing over during meiosis occurs at structures called:

Centromeres
Chiasmata
Kinetochores
Telomeres

Chiasmata are the visible X-shaped points where non-sister chromatids of homologous chromosomes have exchanged equivalent segments during prophase I (specifically the pachytene stage). They are the cytological evidence of crossing over.

Q5. Why are X-linked recessive disorders such as red-green colour blindness more common in males than females?

The disease allele is dominant in males
Males produce more recessive gametes
Males have only one X chromosome, so a single recessive allele is fully expressed
The Y chromosome carries the same disease alleles

A male is XY — hemizygous for X-linked genes. There is no second X to mask a recessive allele, so the trait is expressed even from a single copy. Females need to be homozygous (X^hX^h) to show the disorder, which is far rarer.

Quick Recap

Mendel’s laws: dominance, segregation, independent assortment.
Monohybrid F₂ = 3 : 1 phenotypic; 1 : 2 : 1 genotypic.
Dihybrid F₂ = 9 : 3 : 3 : 1 (only if loci unlinked).
Linked genes → more parental than recombinant gametes; RF measures map distance.
Crossing over occurs at chiasmata in prophase I of meiosis.
X-linked recessive: more common in males; no father-to-son transmission; carrier mothers transmit to half of sons.
Examples: haemophilia, red-green colour blindness, Duchenne muscular dystrophy.

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.