👥 Sex-Linked Inheritance, Pedigree Analysis & Human Genetics
Study X-linked and Y-linked inheritance patterns for CUET Agriculture. Color blindness, haemophilia, pedigree analysis and human genetics.
Sex-Linked Inheritance
When a gene is located on a sex chromosome (X or Y), its inheritance pattern is called sex-linked inheritance. Sex-linked traits are inherited through both males and females but their expression differs between sexes because males and females have different sex chromosome compositions.
Types of Sex Linkage
(1) X-Linked Inheritance
Traits whose genes are located on the X chromosome. X-linked traits are inherited by both males and females, but expression is different because of the dosage difference:
- Females have 2 X chromosomes → can be homozygous (XX) or heterozygous (carrier, X^A X^a)
- Males have 1 X chromosome → are always hemizygous (express whatever allele is present on their single X)
This means that for recessive X-linked traits, males are much more commonly affected because they only need one copy of the recessive allele, while females need two copies.
(i) Eye Colour in Drosophila
Morgan's classic experiment on eye colour in Drosophila demonstrated X-linked inheritance. If (+) represents the wild-type red eye gene and (W) represents the white eye gene:
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Sex-Linked Inheritance
When a gene is located on a sex chromosome (X or Y), its inheritance pattern is called sex-linked inheritance. Sex-linked traits are inherited through both males and females but their expression differs between sexes because males and females have different sex chromosome compositions.
Types of Sex Linkage
(1) X-Linked Inheritance
Traits whose genes are located on the X chromosome. X-linked traits are inherited by both males and females, but expression is different because of the dosage difference:
- Females have 2 X chromosomes → can be homozygous (XX) or heterozygous (carrier, X^A X^a)
- Males have 1 X chromosome → are always hemizygous (express whatever allele is present on their single X)
This means that for recessive X-linked traits, males are much more commonly affected because they only need one copy of the recessive allele, while females need two copies.
(i) Eye Colour in Drosophila
Morgan's classic experiment on eye colour in Drosophila demonstrated X-linked inheritance. If (+) represents the wild-type red eye gene and (W) represents the white eye gene:
| Genotype | Phenotype |
|---|---|
| X^+ X^+ | Red-eyed female (Homozygous) |
| X^+ X^W | Red-eyed female (Heterozygous/Carrier) |
| X^W X^W | White-eyed female (Homozygous) |
| X^+ Y | Red-eyed male (Hemizygous) |
| X^W Y | White-eyed male (Hemizygous) |
- Female eye colour can be homozygous or heterozygous
- Male eye colour is always hemizygous — males cannot be carriers
- For recessive X-linked traits, males are more commonly affected because they only need one copy of the recessive allele
NOTE
This was the first experimental demonstration of sex-linked inheritance. Morgan's work on Drosophila eye colour earned him the Nobel Prize and established the foundation for understanding sex-linked traits in humans.
(ii) Haemophilia (Bleeder's Disease)
Haemophilia is one of the most well-known X-linked recessive disorders. It is a blood clotting disorder where affected individuals bleed excessively even from minor injuries.
- Haemophilia gene is X-linked recessive
- Discovered by John Otto
- Inheritance follows a criss-cross pattern (father → daughter carrier → grandson affected)
Types of Haemophilia:
| Type | Deficient Factor | Details |
|---|---|---|
| Haemophilia A | Factor VIII (Anti-haemophilic globin/Factor) | Most common type; X-linked |
| Haemophilia B | Factor IX (PTC - Plasma Thromboplastin Component) | Christmas disease; X-linked |
| Haemophilia C | Factor XI (PTA - Plasma Thromboplastin Antecedent) | Autosomal (not sex-linked) |
Genotypes:
| Genotype | Phenotype |
|---|---|
| X^H X^H | Normal female |
| X^H X^h | Carrier female (phenotypically normal but carries one defective allele) |
| X^h X^h | Haemophilic female (rare, usually lethal) |
| X^H Y | Normal male |
| X^h Y | Haemophilic male |
WARNING
X^h X^h females are extremely rare because they would need a haemophilic father (X^h Y) AND a carrier mother (X^H X^h). Homozygous haemophilic females often do not survive, which is why haemophilia is almost exclusively seen in males.
Patterns of Sex-Linked Inheritance
(a) Criss-Cross Inheritance
In this pattern, an X-linked gene is passed from a parent to offspring of the opposite sex in the next generation, and then to the original sex through grandchildren. The gene "criss-crosses" between sexes across generations.
Types:
| Pattern | Transmission | Description |
|---|---|---|
| Diagenic / Diagynic | Father → Daughter → Grandson | Father's X-linked trait appears in grandson through carrier daughter |
| Diandric | Mother → Son → Grand-daughter | Mother's X-linked trait appears in grand-daughter through son |
(b) Non Criss-Cross Inheritance
The sex-linked trait is passed to offspring of the same sex as the parent — no crossing between sexes:
| Pattern | Transmission | Description |
|---|---|---|
| Hologenic / Hologynic | Mother → Daughter → Grand-daughter | Only through females |
| Holandric | Father → Son → Grandson | Only through males (Y-linked) |
Sex-Limited Characters
These are traits that are different from sex-linked traits — they involve genes on autosomes, not sex chromosomes:
- Traits that are present in only one sex, even though the genes are present in both sexes
- The genes are on autosomes, not sex chromosomes
- Expression is limited to one sex due to hormonal differences
Examples of secondary sexual characters:
- Beard growth in males — the gene is present in both sexes but expressed only in males due to testosterone
- Milk production in females — gene present in both sexes but expressed only in females due to estrogen and prolactin
- Horns in cattle: gene present in both males and females but expressed differently (e.g., horns only in male sheep)
TIP
Sex-limited = trait appears in only ONE sex. Sex-linked = gene is on a sex chromosome. Do not confuse these two concepts — they are fundamentally different.
Sex-Influenced Characters
- Traits whose expression differs between males and females, but the trait can appear in both sexes
- The gene is located on autosomes (somatic chromosomes)
- The same genotype produces different phenotypes in males and females
- This is due to hormonal differences affecting gene expression
Example: Male Pattern Baldness
B = gene for baldness
| Genotype | Male | Female |
|---|---|---|
| bb | Not bald | Not bald |
| Bb | Bald | Not bald |
| BB | Bald | Bald |
- Bb is dominant in males (bald) but recessive in females (not bald)
- This is because testosterone in males promotes baldness expression in Bb heterozygotes
- BB causes baldness in both sexes
NOTE
In sex-influenced traits, the dominance relationship changes depending on the sex of the individual. The allele B behaves as dominant in males but recessive in females. This is entirely due to hormonal environment, not the chromosomal location of the gene.
Human Genetics
Why Humans Are Difficult Subjects for Genetic Studies
Unlike Drosophila or pea plants, humans present unique challenges for genetic research:
- Small family size — few offspring per generation (unlike pea's hundreds of seeds)
- Long generation time — cannot use in laboratory experiments spanning multiple generations quickly
- Longer lifespan — takes many years to study multiple generations
- Low offspring number — statistical analysis is difficult with small sample sizes
- Humans are heterozygous for most traits — both homozygous and heterozygous individuals are common
- Controlled crosses are not possible — cannot plan matings experimentally (ethical restrictions)
Eugenics
- The study of improving human genetics through selective breeding
- Term coined by Sir Francis Galton (1883)
- Based on the Greek word "Eugene" meaning "well-born"
- Galton studied hereditary traits at a scientific level and proposed improving the human race through genetic selection
- Called the "Founder of Eugenics"
Methods Used in Human Genetic Studies
Since controlled crosses are not possible in humans, alternative methods are used:
- Pedigree analysis — tracing hereditary traits through family trees (most important method)
- Twin studies — comparing traits in monozygotic (identical) vs dizygotic (fraternal) twins
- Population studies — studying gene frequencies in large populations
- Cytogenetic studies — analyzing chromosomes for abnormalities
Common Autosomal Characters in Humans
| Character | Dominant | Recessive |
|---|---|---|
| Eye colour | Brown (Black) | Blue |
| Ear lobes | Free (unattached) | Attached |
| Hair | Curly hair | Straight |
| Cheeks (Dimples) | Dimpled | Normal (no dimples) |
| Tongue rolling | Roller | Non-Roller |
| PTC paper tasting | Tasters | Non-tasters |
| Skin Pigmentation | Normal | Albino |
TIP
For CUET, remember these dominant-recessive pairs. Questions often ask which trait is dominant — free ear lobes, brown eyes, curly hair, dimples, and tongue rolling are all dominant.
Pedigree Analysis
Pedigree analysis involves studying family histories (pedigrees) to trace the inheritance pattern of specific traits across generations. It is the most important tool for human genetics because controlled crosses are not possible in humans.
Standard Pedigree Symbols
| Symbol | Meaning |
|---|---|
| Square (□) | Male |
| Circle (○) | Female |
| Horizontal line connecting □ and ○ | Marriage/Mating |
| Vertical line from couple to children | Parent-offspring |
| Children shown left to right | Oldest to youngest (chronological order) |
| Diamond (◊) | Sex unspecified |
| Filled square/circle (▪/•) | Affected individual |
| Half-filled square/circle | Heterozygous carrier (for autosomal trait) |
| Circle with central dot | Carrier female for sex-linked recessive trait |
| Diagonal line through symbol | Deceased |
| Double horizontal line | Consanguineous marriage (between relatives) |
| Number inside symbol (e.g., 5) | 5 unaffected offspring of that sex |
Patterns of Inheritance in Pedigrees
Recognizing these patterns is essential for CUET. Each type of inheritance has distinctive features:
(a) Autosomal Dominant
- Affected individuals appear in every generation (no skipping)
- Affected individual has at least one affected parent
- Both males and females equally affected
- Unaffected parents do not have affected children (usually)
(b) Autosomal Recessive
- Trait may skip generations (carriers are unaffected)
- Affected individuals can have unaffected parents (both carriers)
- Both males and females equally affected
- Consanguineous marriages increase the chance of expression
(c) X-Linked Recessive
- More males affected than females
- Affected males usually have carrier mothers
- Never father-to-son transmission (sons get Y from father, not X)
- Carrier females pass the trait to ~50% of sons
- Follows criss-cross inheritance pattern
(d) X-Linked Dominant
- Affected fathers pass to all daughters (never to sons)
- Affected mothers pass to ~50% of children regardless of sex
- More females affected than males
IMPORTANT
Quick identification guide:
- Equal in both sexes + every generation → Autosomal Dominant
- Equal in both sexes + skips generations → Autosomal Recessive
- More males + never father-to-son → X-Linked Recessive
- More females + affected father gives to all daughters → X-Linked Dominant
Golden Key Points
| Point | Detail |
|---|---|
| Chromosomal theory | Sutton & Boveri (1902) |
| Linkage coined by | Morgan (studied in Drosophila) |
| Linkage group number | = Haploid chromosome number (n) |
| Complete linkage | No recombinants; found in male Drosophila |
| Incomplete linkage | Recombination <50% |
| 1 cM (centiMorgan) | = 1% recombination frequency |
| First genetic map | Sturtevant (Morgan's student) |
| XX-XY type | Humans, Drosophila (Lygaeus type) |
| ZW-ZZ type | Birds (female heterogametic) |
| XX-XO type | Grasshopper (Protenor type) |
| Haploid-Diploid | Honey bees (Queen 2n, Drone n) |
| Genic balance theory | C.B. Bridges (Drosophila) |
| Sex index = X/A | 1.0 = Female, 0.5 = Male |
| Barr body | Inactivated X chromosome (count = X - 1) |
| Haemophilia discovered by | John Otto |
| Colour blindness | Horner; Ishihara charts for diagnosis |
| Hypertrichosis | Y-linked (Holandric) |
| Sex-limited traits | One sex only (autosomal genes, hormonal control) |
| Sex-influenced traits | Different expression in two sexes (e.g., baldness) |
| Eugenics | Sir Francis Galton (1883) |
Practice Questions (Beginner's Box)
- The law of independent assortment is an exception for:
- (1) Dominance (2) Incomplete dominance (3) Segregation (4) Linkage
- Answer: (4)
Explanation
Linkage is the exception to the law of independent assortment. When genes are on the same chromosome (linked), they do NOT assort independently — they tend to be inherited together. Mendel's law of independent assortment only applies to genes on different chromosomes.-
Sex-linked gene was experimentally demonstrated by:
- (1) Morgan (2) Muller (3) Mendel (4) Johanssen
- Answer: (1)
-
X-linked recessive trait is typically expressed in:
- (1) Males (2) Females (3) Males and females equally (4) Not typically expressed
- Answer: (1)
-
In maize, if 10 linkage groups are present, how many linkage groups will be found:
- (1) 05 (2) 10 (3) 20 (4) 40
- Answer: (2) 10
Explanation
The number of linkage groups equals the haploid chromosome number (n). Maize has 2n = 20, so n = 10 linkage groups. The number of linkage groups is a fixed property of the species.- In birds, sex determination method shows:
- (1) Male heterogametic (2) Both heterogametic (3) Female heterogametic (4) Both homogametic
- Answer: (3)
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Sex-linked inheritance | Gene on sex chromosome (X or Y); expression differs between sexes |
| X-linked inheritance | Gene on X chromosome; females homozygous or heterozygous (carrier); males always hemizygous |
| Males more affected (X-linked recessive) | Males need only one copy of recessive allele; females need two copies |
| Drosophila eye colour (Morgan) | First demonstration of sex-linkage; red eye (X^+) dominant over white (X^W) |
| Haemophilia | X-linked recessive; discovered by John Otto; follows criss-cross inheritance |
| Haemophilia A | Deficiency of Factor VIII; most common type |
| Haemophilia B | Deficiency of Factor IX (Christmas disease) |
| Haemophilia C | Deficiency of Factor XI; autosomal (not sex-linked) |
| Colour blindness | X-linked recessive; discovered by Horner; diagnosed using Ishihara Charts |
| Types of colour blindness | Protanopia (can't see red), Deuteranopia (can't see green) |
| Colour blindness frequency | Males: ~8%; Females: ~0.4% |
| Y-linked (Holandric) | Only father → son → grandson; never in females Examples: Hypertrichosis, TDF/SRY gene |
| Criss-cross inheritance | X-linked gene crosses between sexes across generations |
| Diagenic (Diagynic) | Father → Daughter (carrier) → Grandson |
| Diandric | Mother → Son → Grand-daughter |
| Hologenic (Hologynic) | Mother → Daughter → Grand-daughter |
| Holandric | Father → Son → Grandson (Y-linked) |
| Sex-limited characters | Expressed in only one sex; genes on autosomes; hormonal control Examples: beard (males), milk production (females) |
| Sex-influenced characters | Different expression in males vs females; autosomal genes Example: baldness — Bb = bald in males, not bald in females |
| Human genetics challenges | Small family, long generation time, no controlled crosses |
| Eugenics | Coined by Sir Francis Galton (1883); study of improving human genetics |
| Pedigree analysis | Tracing traits through family trees; most important human genetics method |
| Autosomal dominant pattern | Every generation; both sexes equally affected |
| Autosomal recessive pattern | Skips generations; consanguinity increases risk |
| X-linked recessive pattern | More males; never father-to-son; carrier mothers |
| X-linked dominant pattern | Affected fathers → all daughters; more females affected |
| Common dominant human traits | Free ear lobes, brown eyes, curly hair, dimples, tongue rolling, PTC tasters |
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