Principles of Inheritance and Variation

PRINCIPLES OF INHERITANCE AND VARIATION

**Inheritance** is the process by which characters are passed on from parents to offspring; it forms the basis of heredity. **Variation** is the degree by which offspring differ from their parents. Sexual reproduction is the primary cause of variation in diploid organisms. Human knowledge of selective breeding for desirable traits dates back to 8000-1000 B.C., seen in domestication of cattle breeds like Sahiwal cows in Punjab, though the scientific basis remained unknown until Mendel's work.

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MENDEL'S LAWS OF INHERITANCE

**Gregor Mendel** (1822-1884) conducted hybridisation experiments on garden peas (*Pisum sativum*) from 1856-1863, laying the foundation of modern genetics. His work was revolutionary because:

**First application of statistical analysis and mathematical logic** to biological inheritance problems

Used a large sample size, lending credibility to collected data

Conducted experiments across successive generations (F₁, F₂, F₃) to verify general rules rather than unsubstantiated ideas

Selected contrasting traits that appeared as only two opposing phenotypes

**Mendel's Selected Contrasting Traits in Pea Plants:**

Stem height: Tall (6-7 feet) vs Dwarf (1-1.5 feet)

Seed shape: Round (smooth) vs Wrinkled

Seed colour: Yellow vs Green

Pod shape: Inflated vs Constricted

Pod colour: Green vs Yellow

Flower colour: Violet vs White

Flower position: Axial (along stem) vs Terminal (at tip)

**True-breeding lines** are pure-breeding varieties that have undergone continuous self-pollination for several generations, showing stable trait inheritance and expression. Mendel selected 14 true-breeding pea plant varieties as contrasting pairs, differing in one character with opposing traits.

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INHERITANCE OF ONE GENE (MONOHYBRID CROSS)

**Monohybrid cross** involves breeding two organisms that differ in one character controlled by one gene. A classic example is the cross between tall (TT) and dwarf (tt) pea plants.

**Mendel's Observations in Monohybrid Cross (Tall × Dwarf):**

**P Generation (Parents):**

Tall plant: TT (homozygous dominant)

Dwarf plant: tt (homozygous recessive)

**F₁ Generation (First filial):**

**All offspring are tall (Tt)**

No segregation of traits; no blending

The dwarf trait completely disappears

**F₂ Generation (Self-pollination of F₁):**

3/4 tall plants (both TT and Tt genotypes)

1/4 dwarf plants (tt genotype)

**Phenotypic ratio: 3 tall : 1 dwarf**

**Genotypic ratio: 1 TT : 2 Tt : 1 tt**

Dwarf trait reappears unchanged, showing no blending

**Key Terminologies:**

**Gene** is the unit of inheritance containing information to express a particular trait.

**Alleles** are slightly different forms of the same gene coding for contrasting traits. For example, T (for tallness) and t (for dwarfness) are alleles of the height gene.

**Genotype** is the genetic composition of an organism (TT, Tt, or tt). It is not visible and represents the actual allelic combination.

**Phenotype** is the observable physical appearance of an organism (tall or dwarf). It is what we see and is determined by both genotype and environment.

**Homozygous** organisms have identical alleles for a gene (TT or tt). They produce only one type of gamete.

**Heterozygous** organisms have different alleles for a gene (Tt). They produce two types of gametes in equal proportions.

**Dominant allele** (capital letter, e.g., T) masks the expression of the recessive allele in heterozygotes. The dominant trait appears in F₁ and 3/4 of F₂.

**Recessive allele** (lowercase letter, e.g., t) is expressed only when present in homozygous condition (tt). It reappears in F₂ in 1/4 proportion.

**Punnett Square:**

The **Punnett Square**, developed by Reginald C. Punnett, is a graphical representation to calculate the probability of all possible genotypes in offspring. Steps:

1. Write possible gametes of one parent in the top row

2. Write possible gametes of other parent in left column

3. Fill boxes by combining gamete types from row and column

4. Read all possible genotypes and count ratios

**Example (F₁ self-pollination):**

F₁ plant: Tt produces gametes T (50%) and t (50%)

Punnett square results: 1 TT : 2 Tt : 1 tt (genotypic ratio)

Phenotype: 3 tall : 1 dwarf (because Tt is tall like TT)

**Mathematical Expression of Monohybrid Cross:**

The 1:2:1 genotypic ratio follows the binomial expansion:

**(1/2 T + 1/2 t)² = 1/4 TT + 1/2 Tt + 1/4 tt**

This mathematical relationship proves Mendel's law is based on probability of gamete union.

**Test Cross:**

A **test cross** determines the genotype of an organism displaying the dominant phenotype. The organism is crossed with a homozygous recessive individual (tt).

**Example:** To determine if a tall F₂ plant is TT or Tt:

Cross tall plant (unknown genotype) with dwarf plant (tt)

If tall plant is TT: All offspring are Tt (tall) — 100% tall

If tall plant is Tt: Offspring are 1/2 Tt (tall) and 1/2 tt (dwarf) — 1:1 ratio

This ratio immediately reveals the genotype of the tested parent.

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LAW OF DOMINANCE

The **Law of Dominance** (Mendel's First Law) states:

1. Characters are controlled by **discrete units called factors** (now called genes)

2. Factors occur in **pairs** (alleles in diploid organisms)

3. In a **dissimilar pair, one dominates the other**: the dominant factor masks the recessive factor

**Explanation:** In the heterozygote Tt, the T allele (tall) completely masks the t allele (dwarf), resulting in a tall phenotype. The dominance of T over t explains why:

All F₁ (Tt) plants are tall, resembling the TT parent

In F₂, 3/4 plants are tall (both TT and Tt) and 1/4 are dwarf (tt only)

The phenotypic ratio is 3:1 (dominant:recessive)

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LAW OF SEGREGATION

The **Law of Segregation** (Mendel's Second Law) states:

**Alleles do not blend** in offspring; both parental traits appear unchanged in F₂

During **gamete formation (meiosis), alleles of a pair segregate** such that each gamete receives only **one allele**

**Segregation is random**, giving each allele 50% chance of inclusion in any gamete

**Homozygous parents** (TT or tt) produce all similar gametes

**Heterozygous parents** (Tt) produce two types of gametes (T and t) in 1:1 ratio

**Evidence supporting segregation:**

Dwarf plants (tt) produce only t gametes; self-pollinated offspring are all dwarf (tt), never tall

This proves the alleles separated during meiosis and did not blend

The reappearance of the recessive trait in F₂ without modification proves segregation occurred

**Genetic basis of segregation:** During meiosis I, homologous chromosomes (carrying different alleles) separate, distributing one allele to each daughter cell.

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INCOMPLETE DOMINANCE

In cases where the F₁ phenotype is **intermediate between the two parents** and does not resemble either parent, the trait shows **incomplete dominance**.

**Example - Snapdragon Flower Color:**

P generation: Red flowers (RR) × White flowers (rr)

F₁ generation: Pink flowers (Rr) — intermediate phenotype

F₂ generation: 1 Red (RR) : 2 Pink (Rr) : 1 White (rr)

**Key differences from complete dominance:**

**F₁ phenotype is intermediate**, not matching either parent

**Phenotypic ratio equals genotypic ratio** (1:2:1), because all three genotypes show distinct phenotypes

The heterozygote Rr is visibly different from both homozygotes

**Explanation:** One functional allele (R) produces 50% of the enzyme/protein needed for red color. This insufficient amount results in pink (intermediate) phenotype. Two alleles (RR) produce 100% (red), while zero functional alleles (rr) produce no color product (white).

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CO-DOMINANCE

**Co-dominance** occurs when both alleles of a pair are completely expressed in the heterozygous condition, producing a **phenotype that displays both traits simultaneously** rather than a blend.

The heterozygote shows **both parental phenotypes together**, not an intermediate phenotype.

**Classic Example - ABO Blood Grouping System:**

Blood type is controlled by three alleles: I^A, I^B, and i

**Genotypes and Phenotypes:**

I^A I^A or I^A i → Blood Type A (N-acetylgalactosamine antigen on RBC surface)

I^B I^B or I^B i → Blood Type B (D-galactosamine antigen on RBC surface)

**I^A I^B → Blood Type AB** (both A and B antigens; CO-DOMINANT)

ii → Blood Type O (no antigen; recessive)

**In AB blood group:**

Both I^A and I^B alleles are equally dominant

The heterozygote I^A I^B displays both antigen A and antigen B on red blood cells

This is co-dominance, not incomplete dominance, because both traits are fully expressed (antigens are present in full amount), not blended

**Key distinction from incomplete dominance:**

**Incomplete dominance:** F₁ shows intermediate phenotype (pink from red + white)

**Co-dominance:** F₁ shows both parental phenotypes simultaneously (AB has both A and B antigens)

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INHERITANCE OF TWO GENES (DIHYBRID CROSS)

A **dihybrid cross** involves breeding organisms differing in two characters, each controlled by different genes.

**Mendel's Dihybrid Cross - Seed Shape and Color:**

**P Generation:**

Round yellow seeds: RRYY (homozygous for both traits)

Wrinkled green seeds: rryy (homozygous for both traits)

**F₁ Generation:**

All offspring: RrYy

Phenotype: All round yellow seeds

Both dominant traits (R for round, Y for yellow) are expressed

**F₂ Generation (Self-pollination of F₁):**

When RrYy plants self-pollinate:

**Phenotypic ratio: 9:3:3:1**

9/16 Round yellow (R_Y_)

3/16 Round green (R_yy)

3/16 Wrinkled yellow (rrY_)

1/16 Wrinkled green (rryy)

**Genotypic ratio: 1:2:1:2:4:2:1:2:1** (nine different genotypes among 16 individuals)

**Mendel's Second Law: Law of Independent Assortment**

**Law of Independent Assortment** states:

**Alleles of different genes segregate independently** during gamete formation

The segregation of one pair of alleles does not influence the segregation of another pair

This applies to genes on different (non-homologous) chromosomes

**Explanation:** During meiosis II, the orientation of one bivalent (pair of homologous chromosomes) is random and independent of other bivalents. Therefore:

An RrYy parent produces four types of gametes: RY, Ry, rY, ry (in 1:1:1:1 ratio)

Assortment of R/r is independent of assortment of Y/y

**Evidence supporting independent assortment:**

The 9:3:3:1 ratio in F₂ is exactly the product of two independent monohybrid crosses (3:1 for R_/rr × 3:1 for Y_/yy)

Traits assort independently into new combinations (round green and wrinkled yellow appear, which were not in parents)

Genetic basis: genes on different chromosomes segregate randomly during meiosis

**Punnett Square for Dihybrid Cross:**

A 4×4 Punnett square shows:

4 types of female gametes: RY, Ry, rY, ry (top row)

4 types of male gametes: RY, Ry, rY, ry (left column)

16 boxes showing all possible offspring genotypes

Reading phenotypes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

**Note:** Mendel studied 7 characters in peas, and each pair assorted independently, supporting his law across multiple traits.

**Test Cross for Dihybrid:**

An F₁ individual (RrYy) crossed with homozygous recessive (rryy) produces offspring:

1/4 RrYy (round yellow)

1/4 Rryyy (round green)

1/4 rrYy (wrinkled yellow)

1/4 rryy (wrinkled green)

**Ratio: 1:1:1:1**

This 1:1:1:1 ratio directly reflects the four gamete types of the dihybrid and confirms independent assortment.

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MULTIPLE ALLELES

While Mendel's work focused on traits controlled by two alleles, **some genes exist in more than two allelic forms within a population**. These are called **multiple alleles**.

**Key points:**

An individual diploid organism still carries **only two alleles** (one from each parent)

But the **population may have three or more different allelic forms**

Multiple alleles occupy the same locus on homologous chromosomes

**ABO Blood Group System - Multiple Alleles Example:**

The ABO blood group is controlled by **three alleles**: I^A, I^B, and i

**Allelic relationship:**

I^A is co-dominant with I^B

Both I^A and I^B are dominant over i

I^A and I^B are not dominant over each other

**Possible genotypes and phenotypes:**

1. **I^A I^A** → Type A blood (homozygous)

2. **I^A i** → Type A blood (heterozygous)

3. **I^B I^B** → Type B blood (homozygous)

4. **I^B i** → Type B blood (heterozygous)

5. **I^A I^B** → Type AB blood (co-dominant; both antigens present)

6. **ii** → Type O blood (recessive; no antigen)

**Genetic basis:**

I^A allele codes for enzyme adding N-acetylgalactosamine (antigen A) to red blood cell surface

I^B allele codes for enzyme adding D-galactosamine (antigen B)

i allele (recessive) codes for non-functional enzyme; no antigen added

**Clinical significance:**

Blood transfusions must match ABO type to prevent hemolysis

Type AB individuals are universal recipients (have both antigens; accept all types)

Type O individuals are universal donors (no antigens; not recognized as foreign)

**Example cross:**

Parent 1: I^A I^B (Type AB) × Parent 2: I^B i (Type B)

Possible offspring: I^A I^B (Type AB), I^B I^B (Type B), I^A i (Type A), I^B i (Type B)

Offspring phenotypes: Type AB, Type B, or Type A

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PLEIOTROPY

**Pleiotropy** is a condition where **a single gene controls multiple, seemingly unrelated traits**. The allele affects the phenotype in multiple ways.

**Explanation:** A gene codes for a protein (enzyme or structural protein) used in multiple metabolic pathways or tissues. A mutation in that gene therefore affects multiple traits.

**Example - Phenylketonuria (PKU) in Humans:**

The gene coding for the enzyme **phenylalanine hydroxylase** exhibits pleiotropy.

**Normal allele (P):**

Enzyme converts amino acid phenylalanine → tyrosine

Tyrosine is used for melanin production (skin pigment), thyroid hormone synthesis, and normal brain development

**Mutant recessive allele (p):**

Produces non-functional enzyme or no enzyme

Phenylalanine accumulates to toxic levels

**Multiple phenotypic effects:**

Severe intellectual disability (brain damage from phenylalanine toxicity)

Light skin color (reduced melanin; cannot convert phenylalanine to tyrosine for melanin synthesis)

Light-colored hair

Musty/mousy body odor (from phenylalanine metabolites)

Eczema

**Individuals (pp) show all these traits together** because they all stem from a single enzyme deficiency.

**Genetic basis of pleiotropy:**

One gene produces one polypeptide/enzyme

That protein functions in multiple metabolic pathways or tissues

Mutation affects all dependent pathways simultaneously

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POLYGENIC INHERITANCE (QUANTITATIVE INHERITANCE)

**Polygenic inheritance** involves **multiple genes**, each contributing a small additive effect to a single trait, producing a **continuous range of phenotypes** rather than discrete categories.

**Characteristics:**

**Controlled by many genes** (polygenes), each with small individual effect

Traits show **continuous variation** (bell-curve distribution)

Phenotype heavily influenced by **environmental factors**

**Intermediate phenotypes are most common**; extreme phenotypes are rare

**Examples:**

1. **Human Height:**

Controlled by 700+ genetic variants (multiple genes)

Environmental factors (nutrition, climate) significantly affect final height

Population shows bell-curve distribution from short to tall

No clear Mendelian ratios (not 3:1 or 9:3:3:1)

2. **Skin Color in Humans:**

Multiple genes controlling melanin production

Shows spectrum from very light to very dark, not discrete categories

Environmental exposure (sun) also influences pigmentation

3. **Eye Color:**

Though once thought to be simple Mendelian, actually involves multiple genes

Results in continuous variation

**Quantitative Inheritance Model - Three Gene Example:**

Suppose skin color is controlled by three genes (A, B, C), each with two alleles contributing additively:

**Genotype → Phenotype:**

AABBCC → Very dark (6 dominant alleles)

AaBbCc → Medium (3 dominant alleles)

aabbcc → Very light (0 dominant alleles)

**Population distribution:**

Extremes (AABBCC or aabbcc) are rare

Intermediates (3-4 dominant alleles) are most frequent

Forms bell-shaped curve (normal distribution)

**Key difference from Mendelian inheritance:**

Mendelian: Discrete phenotypic classes, exact mathematical ratios (3:1, 9:3:3:1)

Polygenic: Continuous phenotypic variation, bell-curve distribution, Mendelian ratios not apparent

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CHROMOSOME THEORY OF INHERITANCE

**Chromosome Theory of Inheritance** (Sutton and Boveri, 1902-1915) states:

**Genes are located on chromosomes; the behavior of chromosomes during meiosis and fertilisation exactly parallels the inheritance of traits as described by Mendel's laws.**

**Parallels between Chromosome Behavior and Mendelian Inheritance:**

| **Mendelian Law** | **Chromosome Behavior** |

|---|---|

| Factors occur in pairs | Chromosomes occur in pairs (homologous pairs) |

| Dominance of one factor | Dominance of one allele on chromosome |

| Segregation of alleles during gamete formation | Separation of homologous chromosomes during meiosis I |

| Each gamete receives one allele | Each gamete receives one chromosome from each pair |

| Alleles of two genes assort independently | Chromosomes from different pairs assort randomly during meiosis II |

| Recombination of alleles in offspring | Recombination of chromosomes during fertilisation |

**Evidence Supporting Chromosome Theory:**

1. **Correlation of trait segregation with chromosome segregation:**

Genes segregate in same proportions as chromosomes (1:1 in test cross)

Both occur during meiosis

2. **Sex-linked inheritance patterns:**

Traits following the X chromosome show X-linked inheritance pattern

Male (XY) shows trait if single recessive allele; female needs two copies

Proves genes are on chromosomes

3. **Crossing over:**

Produces new combinations of alleles

Corresponds to recombination of chromosome segments during prophase I

**Significance:**

The chromosome theory unified Mendelian genetics with cytology, explaining the physical basis of inheritance and validating Mendel's laws at the chromosomal level.

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SEX DETERMINATION

**Sex determination** is the mechanism by which an organism's sexual phenotype (male or female) is genetically established.

Different organisms use different systems:

**XY Sex Determination System (Humans, Most Mammals, Some Insects):**

**Sex chromosomes:**

**X chromosome:** Larger; carries many genes unrelated to sex determination

**Y chromosome:** Smaller; carries male-determining genes, including SRY gene (sex-determining region Y)

**Genotypes:**

**XX → Female** (receives X from mother and X from father)

**XY → Male** (receives X from mother and Y from father)

**Inheritance pattern:**

Father (XY) produces two types of sperm: X (50%) and Y (50%)

Mother (XX) produces only X eggs

**Cross:** XX (mother) × XY (father)

50% XX (daughters)

50% XY (sons)

**Sex ratio:** 1:1 (male:female) at conception; slight variations due to differential survival

**Genetic basis:**

SRY gene on Y chromosome triggers testis development

Without Y chromosome (or SRY), default developmental pathway is female

Testosterone and anti-Müllerian hormone from testis cause male development

**ZW Sex Determination System (Birds, Some Butterflies, Some Reptiles):**

**Reversal of XY system:**

**ZZ → Male** (homogametic; produces one type of gamete)

**ZW → Female** (heterogametic; produces two types of gametes)

**Inheritance:**

Female (ZW) is heterogametic; produces gametes with Z or W

Male (ZZ) is homogametic; produces gametes all with Z

**Cross:** ZZ (male) × ZW (female)

50% ZZ (males)

50% ZW (females)

**Note:** Sex ratio also 1:1, but female is the heterogametic sex

**Haplodiploidy System (Bees, Wasps, Some Ants):**

**Unique system based on ploidy:**

**Diploid (2n) → Female** (develops from fertilised egg)

**Haploid (n) → Male** (develops from unfertilised egg; parthenogenesis)

**Mechanism:**

Female lays both fertilised eggs (develop into females) and unfertilised eggs (develop into males)

Sex determined by whether egg is fertilised, not by sex chromosomes

Males are haploid; females are diploid

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SEX-LINKED INHERITANCE

**Sex-linked inheritance** refers to traits controlled by genes located on sex chromosomes (X chromosome in XY system), showing inheritance patterns different from autosomal genes.

**Key principle:**

Males (XY) are **hemizygous** for X-linked genes (only one copy; express both dominant and recessive alleles)

Females (XX) are **diploid** for X-linked genes (two copies; follow normal dominance rules)

**X-Linked Recessive Traits - Hemophilia (Bleeding Disorder):**

**Controlled by recessive allele (h) on X chromosome; normal allele is H**

**Genotypes and phenotypes:**

**X^H X^H** → Normal female (homozygous)

**X^H X^h** → Carrier female (heterozygous; phenotypically normal but carries one recessive allele)

**X^h X^h** → Affected female (homozygous recessive; rare, requires affected father and carrier mother)

**X^H Y** → Normal male (hemizygous; has normal allele)

**X^h Y** → Affected male (hemizygous; expresses recessive trait; more common than affected females)

**Key observations:**

**Males more frequently affected** than females because they need only one recessive allele

**Females rarely affected** unless father is affected and mother is homozygous or carrier

**Affected males cannot pass condition to sons** (sons receive Y from father)

**All daughters of affected males are at least carriers** (inherit father's X^h)

**Example cross - Affected male (X^h Y) × Normal female (X^H X^H):**

All daughters: X^H X^h (carriers; phenotypically normal)

All sons: X^H Y (normal)

Phenotypic ratio: All normal (but daughters are carriers)

**Example cross - Carrier female (X^H X^h) × Normal male (X^H Y):**

Daughters: 1/2 X^H X^H (normal), 1/2 X^H X^h (carrier)

Sons: 1/2 X^H Y (normal), 1/2 X^h Y (affected)

Sons show 1:1 ratio of normal:affected; daughters are all phenotypically normal

**Medical significance:** Hemophilia affects blood clotting factor VIII or IX. Historical occurrence in European royal families (Queen Victoria's family).

**X-Linked Recessive - Color Blindness (Red-Green):**

**Similar inheritance pattern to hemophilia**

**Genotypes:**

**X^N X^N** → Normal female (can distinguish red and green)

**X^N X^c** → Carrier/Color-blind-carrier female (normal vision but carries allele)

**X^c X^c** → Color-blind female (rare; cannot distinguish red and green)

**X^N Y** → Normal male

**X^c Y** → Color-blind male (common; ~8% of human males)

**Pedigree pattern:**

More males affected than females

Carrier mothers have 50% affected sons

Affected fathers have all carrier daughters (but no affected sons)

**Incidence:** Color blindness affects ~1/12 males (8-10%) and ~1/200 females in human populations

**Difference between Carrier and Affected Female:**

**Carrier female (X^H X^h):** Heterozygous; phenotypically normal (one functional allele produces enough protein); can pass recessive allele to offspring

**Affected female (X^h X^h):** Homozygous recessive; phenotypically affected; both alleles non-functional

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CHROMOSOMAL ABNORMALITIES (CHROMOSOMAL DISORDERS)

**Chromosomal abnormalities** (also called chromosomal disorders or aneuploidies) result from abnormal number or structure of chromosomes.

**Types:**

**1. Aneuploidy** - Abnormal number of individual chromosomes (loss or gain of one/few chromosomes)

**2. Euploidy** - Change in complete sets of chromosomes (triploidy 3n, tetraploidy 4n, etc.)

**ANEUPLOIDY - COMMON TYPES IN HUMANS:**

#### **Down Syndrome (Trisomy 21):**

**Cause:** **Trisomy of chromosome 21** (three copies instead of two)

**Karyotype:** 47 chromosomes (instead of normal 46); specifically 47, XX, +21 or 47, XY, +21

**Occurrence:** Most common autosomal trisomy compatible with life; ~1 in 700 births

**Genetic basis:**

Results from **non-disjunction during meiosis II** in either parent

Non-disjunction: Failure of sister chromatids to separate during meiosis II

Results in one gamete with two chromosome 21 copies and one gamete with none

Fertilisation by gamete with two chromosome 21 creates trisomic zygote

**Phenotypic characteristics:**

**Physical features:** Downward slanting eyes, low-set ears, broad face, short stature, floppy muscle tone (hypotonia)

**Cardiac defects:** Atrial/ventricular septal defects (present in ~40% of cases)

**Gastrointestinal defects:** Duodenal atresia, tracheoesophageal fistula

**Intellectual disability:** Moderate to severe; IQ typically 30-70

**Distinctive traits:** Single transverse palmar crease (single line across palm), characteristic facial features, speech difficulties

**Other features:** Increased susceptibility to infections, higher risk of leukemia, early-onset Alzheimer's disease, hearing problems

**Life expectancy:** Modern medical care has increased life expectancy to ~50-60 years

**Maternal age effect:** Risk increases significantly with maternal age (1/1500 at age 20; 1/100 at age 40), suggesting increased non-disjunction with age

#### **Turner Syndrome (Monosomy X or 45,X):**

**Cause:** **Absence of one X chromosome** in females (monosomy X)

**Karyotype:** 45,X (only 45 chromosomes; missing second sex chromosome)

**Occurrence:** ~1 in 2500 female births

**Genetic basis:**

Results from non-disjunction in either parent's meiosis

One parent produces gamete with no X; fertilisation by normal gamete creates monosomic zygote

Often results from post-zygotic loss of one X chromosome (mosaicism; some cells 46,XX and some 45,X)

**Phenotypic characteristics:**

**External features:** Short stature (typically 4-6 inches below average), webbed neck, broad flat chest, low posterior hairline, shield-shaped chest

**Skeletal abnormalities:** Increased carrying angle of elbows, short metacarpals

**Gonadal dysgenesis:** Streak gonads (non-functional); absent menstruation; female reproductive organs present but non-functional

**Cardiovascular defects:** Bicuspid aortic valve, coarctation of aorta (narrowing), hypertension

**Renal abnormalities:** Kidney dysgenesis or hypoplasia

**Hearing problems:** Especially high-frequency loss

**Intelligence:** Usually normal cognitive development (unlike Down syndrome)

**Secondary sexual characteristics:** Absent without hormone replacement therapy (estrogen/progesterone treatment)

**Infertility:** Usually sterile due to gonadal dysgenesis; however, egg donation with IVF can enable pregnancy

**Life expectancy:** Near-normal with medical management; cardiac complications are major health concern

#### **Klinefelter Syndrome (47,XXY):**

**Cause:** **Extra X chromosome in males** (trisomy of sex chromosome)

**Karyotype:** 47, XXY (47 chromosomes; extra X chromosome)

**Occurrence:** ~1 in 500-1000 male births; most common sex chromosome disorder in males

**Genetic basis:**

Results from non-disjunction during meiosis I or II in either parent

Usually maternal origin (non-disjunction of X chromosomes)

Creates XY sperm that fertilises normal X egg, or normal Y sperm fertilises XX egg

**Phenotypic characteristics:**

**Physical features:** Tall stature, long legs, gynecomastia (breast development; ~50% of cases), small/firm testes, reduced facial/body hair, infertile

**Sexual characteristics:** Reduced testosterone production; hypogonadism

**Behavioral/cognitive:** May have learning difficulties, speech/language delays; some cases associated with increased risk of behavioral problems

**Reproductive:** Azoospermia (no sperm production); infertility; however, sperm can sometimes be retrieved from testes for assisted reproduction

**Other features:** Increased risk of type 2 diabetes, osteoporosis, metabolic syndrome

**Hormone replacement:** Testosterone therapy can improve secondary sexual characteristics