Non-Mendelian Genetics: Sex-Linked, Sex-Influenced & Sex-Limited Traits
Quick Recap — Previously Covered Non-Mendelian Patterns
Incomplete Dominance
Dominant allele not completely masks the recessive.
Offspring phenotype is intermediate (e.g.
red RR × white rr → pink Rr flowers).
Co-Dominance
Both alleles expressed equally; no blending.
Classic example: human AB blood type (both A and B surface antigens present).
Multiple Alleles
Single gene possesses > 2 possible alleles in the population (e.g. ABO blood group has I^A, I^B, i).
Multiple Alleles: Definition & Core Idea
A single gene can have more than two alternative forms (alleles) present in a population.
Individual organisms still inherit exactly two alleles, one from each parent.
Presence of several alleles in the gene pool → expanded range of phenotypes.
Classic classroom illustration: human ABO blood-group system.
Human ABO Blood-Group System
Controlled by one gene with three alleles: A, B, and i (also called O; recessive).
Phenotypic outcomes (blood types): A,\;B,\;AB,\;O (total 4).
When determining blood type, the alleles interact via codominance and complete dominance:
A and B are codominant with each other (both expressed if present together).
i is recessive to both A and B.
Possible genotype ⇢ phenotype pairs:
AA or Ai ⇒ Type A
BB or Bi ⇒ Type B
AB ⇒ Type AB
ii ⇒ Type O
Antigens ("ID Cards") on Red Blood Cells
Antigen = surface molecule that labels the cell as "self."
Distribution by type:
Type A: has A antigens
Type B: has B antigens
Type AB: has A + B antigens (both expressed because of codominance)
Type O: no antigens
Antibodies ("Security Guards") in Blood Plasma
The immune system produces antibodies that target foreign antigens.
Distribution by type:
Type A: produces anti-B antibodies
Type B: produces anti-A antibodies
Type AB: no antibodies (can accept any type → universal recipient)
Type O: produces anti-A & anti-B antibodies (can donate to any type → universal donor but can receive only from O)
Transfusion Compatibility Snapshot
Donor O → Everyone (no antigens to provoke reaction).
Recipient AB ← Everyone (has no antibodies).
Mismatching antigen/antibody combinations trigger immune reactions → agglutination (clumping) & potential medical crisis.
Broader Relevance of Multiple Alleles in Humans
Hair color: numerous alleles create shades from light blond through brown to black & red.
Hair texture: straight ⇢ wavy ⇢ curly variation produced by interacting alleles.
Eye color & shape: multiple alleles yield blue, green, brown hues and diverse ocular shapes.
Skin complexion: continuum from lighter to darker pigmentation controlled by several alleles.
Take-home point: polyallelic inheritance underlies much of human phenotypic diversity beyond simple dominant/recessive examples.
Key Takeaways
A population-level view is essential: while you carry 2 alleles/gene, the species can carry >2.
ABO blood groups elegantly demonstrate codominance (A & B) and recessiveness (i).
Medical implications: knowing antigens & antibodies is critical for safe blood transfusions & organ transplants.
Similar multi-allelic patterns drive variability in many everyday traits, underscoring the richness of genetic inheritance.
Human Karyotype & Sex Determination
Humans: 2n = 46 chromosomes.
22 pairs autosomes (somatic)
1 pair sex chromosomes (gonosomes)
Sex chromosome combinations
Male: 44 \text{ autosomes} + XY
Female: 44 \text{ autosomes} + XX
Fertilization probabilities
Sperm carries either X or Y
Egg always carries X
Therefore P(\text{male}) = 50\%, P(\text{female}) = 50\%
Sex-Linked Traits (X-Linked & Y-Linked)
Definition: Traits whose controlling allele lies on a sex chromosome.
Typically recessive; expression differs between sexes due to chromosome counts.
X-Linked
Allele located on X chromosome.
Examples: Color blindness, Hemophilia.
Y-Linked
Allele on Y chromosome ⇒ expressed only in males.
Example: Hypertrichosis (hairy ears).
Key implication
Males (XY) possess single X ⇒ any recessive X-linked allele is unmasked.
Females (XX) can be carriers if only one X carries the allele.
X-Linked Example 1: Color Blindness
Biology & Significance
Inability to distinguish certain colors (commonly red-green).
Caused by recessive allele x^c on the X chromosome.
Female Genotypes & Phenotypes
X^C X^C → normal vision
X^C x^c → carrier, normal vision (trait masked)
x^c x^c → color-blind
Male Genotypes & Phenotypes
X^C Y → normal
x^c Y → color-blind (only one X needed)
Sample Problem • Normal Female × Color-Blind Male
Parental Genotypes
Mother (normal, non-carrier assumed): X^C X^C
Father (color-blind): x^c Y
Punnett Square Outcome
Offspring genotypes:
X^C x^c (carrier daughters) ×2, X^C Y (normal sons) ×2
Ratio 2:2 ⇒ simplified 1:1.
Phenotypes
50\% carrier females, 50\% normal males.
0\% color-blind daughters or sons in this cross.
X-Linked Example 2: Hemophilia
Condition Overview
Blood fails to clot efficiently; minor cuts can be fatal.
Recessive allele x^h on X chromosome.
Genotype Key
Female: X^H X^H (normal), X^H x^h (carrier), x^h x^h (hemophilic).
Male: X^H Y (normal), x^h Y (hemophilic).
Sample Problem • Carrier Female × Normal Male
Parents
Mother: X^H x^h
Father: X^H Y
Punnett Square Result
X^H X^H (normal daughter)
X^H x^h (carrier daughter)
X^H Y (normal son)
x^h Y (hemophilic son)
Genotypic Ratio 1:1:1:1
Phenotypic Percentages
25\% normal female
25\% carrier female
25\% normal male
25\% hemophilic male
Sex-Influenced Traits
Locus on an autosome, yet expression modulated by sex hormones.
Trait usually recessive but manifests differently between sexes.
Example: Pattern Baldness
Allele representation
B = non-bald (dominant)
b = bald (recessive)
Hormone Influence: Testosterone heightens expression; males need only one b to be bald.
Sex | Genotype | Phenotype |
|---|---|---|
Female | BB | not bald |
Bb | not bald (carrier) | |
bb | bald | |
Male | BB | not bald |
Bb | bald | |
bb | bald |
Sample Problem • Heterozygous Not-Bald Female × Homozygous Bald Male
Parents
Female: B b
Male: b b
Punnett Outcomes
Offspring genotypes: B b, B b, b b, b b (ratio 1:1).
Male progeny: 50\% bald (Bb or bb both bald), 50\% not bold? Wait: For males, Bb = bald. For clarification: all sons with Bb or bb will be bald.
Female progeny: 50\% carriers (Bb, not bald), 50\% bald (bb).
Overall % Baldness depends on sex distribution, but genetically 50\% of all offspring carry bb.
(Teacher’s original slide only asked for % bald; after full count in a 4× Punnett square, 75 % of male offspring and 50 % of total offspring would be bald; ensure to revisit with actual square in study practice.)
Sex-Limited Traits
Expressed in only one sex even though genes exist in both.
Example — Lactation in Cattle
Gene L (lactation) dominant over l (non-lactation).
Females:
LL or Ll → produce milk
ll → do not lactate
Males: Regardless of genotype (LL, Ll, ll) → never lactate.
Key Take-aways
Differential expression due to anatomy, physiology, or hormones, not chromosome count alone.
Comparative Summary & Practical Implications
Sex-Linked: chromosome-based, often medical (color blindness, hemophilia).
Pedigree analysis crucial for genetic counseling; sons of carrier mothers at higher risk.
Sex-Influenced: hormone-dependent expressivity (baldness).
Same genotype, different phenotype across sexes; illustrates gene–environment (hormonal) interplay.
Sex-Limited: phenotype appears in only one sex (lactation).
Important in livestock breeding: bulls carry lactation genes that affect the dairy potential of daughters.
Ethical/medical considerations
Carrier detection & counseling can reduce incidence of severe X-linked disorders.
Understanding sex-influenced patterns informs personalized medicine (e.g., androgen-related traits, drug metabolism differences).
Quick Recap
Incomplete Dominance: Dominant allele is not completely masking the recessive one, resulting in an intermediate phenotype (e.g., red RR × white rr
→ pink Rr flowers).
Co-Dominance: Both alleles are expressed equally without blending (e.g., human AB blood type).
Multiple Alleles: A single gene possesses more than two possible alleles within a population (e.g., ABO blood group has I^A, I^B, i).
Human Karyotype & Sex Determination
Humans have 2n = 46 chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females, XY for males).
Sperm carries either an X or Y chromosome, while eggs always carry an X, leading to a 50\% probability for male or female offspring.
Sex-Linked Traits (X-Linked & Y-Linked)
Traits whose controlling allele lies on a sex chromosome, typically recessive, and express differently between sexes.
X-Linked: Allele located on the X chromosome (e.g., color blindness, hemophilia). Males (XY) exhibit recessive X-linked traits if present, as they have only one X. Females (XX) can be carriers.
Y-Linked: Allele on the Y chromosome, expressed exclusively in males (e.g., hypertrichosis).
Example: In a cross between a normal female (X^C X^C) and a color-blind male (x^c Y), all daughters will be carriers (X^C x^c) and all sons will be normal (X^C Y). For hemophilia, a carrier female (X^H x^h) crossed with a normal male (X^H Y) can produce normal, carrier, and affected offspring in a 1:1:1:1 genotypic ratio.
Sex-Influenced Traits
These traits have their locus on an autosome, but their expression is modulated by sex hormones.
Example: Pattern baldness, where the recessive baldness allele (b) is more pronounced in males due to testosterone. A male with genotype Bb will be bald, while a female with the same genotype Bb will not.
Sex-Limited Traits
Expressed in only one sex, despite the genes being present in both sexes.
Example: Lactation in cattle. Only females produce milk, regardless of the male's genotype, due to anatomical and physiological differences rather than chromosome count alone.
Comparative Summary & Practical Implications
Sex-Linked: Chromosome-based traits (X or Y), often related to medical conditions, requiring pedigree analysis for counseling.
Sex-Influenced: Hormone-dependent expression, leading to different phenotypes in males and females with the same genotype.
Sex-Limited: Phenotype appears in only one sex due to sex-specific biological characteristics.