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

1. Parental Genotypes

   • Mother (normal, non-carrier assumed): $X^C X^C$

   • Father (color-blind): $x^c Y$

2. Punnett Square Outcome

   • Offspring genotypes:
     $X^C x^c$ (carrier daughters) ×2, $X^C Y$ (normal sons) ×2
     Ratio $2:2$ ⇒ simplified $1:1$.

3. 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

1. Parents

   • Mother: $X^H x^h$

   • Father: $X^H Y$

2. 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)

3. Genotypic Ratio $1:1:1:1$

4. 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.

Sample Problem • Heterozygous Not-Bald Female × Homozygous Bald Male

1. Parents

   • Female: $B b$

   • Male: $b b$

2. 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).

3. 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.