Sex and Heredity: Modes of Inheritance and Sex Dimorphism

Sex Dimorphism and Modes of Inheritance

Sex Dimorphism

Sex dimorphism refers to differences in appearance or traits between males and females of a species, beyond primary sexual characteristics. These differences can be subtle or pronounced, and are often genetically determined.

• Sexually Dimorphic Traits: Traits that differ between males and females, such as coloration, size, or presence of structures like antlers.

• Primary vs. Secondary Sexual Characteristics: Primary characteristics are directly involved in reproduction, while secondary characteristics are traits like plumage or antlers.

Example: Male moose have antlers, which are used for attracting females and competing with other males. Example: Peacocks and butterflies show visible differences between sexes.

Types of Sexually Dimorphic Traits

• Sex Influenced Traits: Expression is affected by the sex of the individual, but both sexes can carry the alleles. These traits are often autosomal, not sex-linked.

• Sex Limited Traits: Only expressed in one sex, even though both sexes can carry the alleles. Example: Antlers in moose are only expressed in males.

Sex Linkage and Chromosomal Inheritance

Sex Linkage

Sex linkage describes inheritance patterns resulting from genes located on sex chromosomes (X or Y in mammals, Z or W in birds).

• Sex Chromosomes: Presence of sex chromosomes allows for sex-linked inheritance. Without sex chromosomes, traits cannot be sex-linked.

• X-linked Traits: Most sex-linked traits are X-linked, as the X chromosome contains many genes.

• Y-linked Traits: Traits inherited only through the Y chromosome, passed from father to son.

Inheritance Patterns of Sex-Linked Traits

• X-linked Recessive: More common in males due to hemizygosity (only one X chromosome). Sons do not inherit the trait from their father. Example: Color blindness.

• X-linked Dominant: Affected fathers pass the trait to all daughters, but not to sons. Both sexes can be affected, but inheritance patterns differ.

• Y-linked (Holandric): Only males are affected; trait is passed directly from father to son.

Key Clues for Identifying Sex-Linked Traits

• X-linked Recessive:

  • Common in males

  • Trait can skip generations

  • Affected sons inherit the trait from their mother

  • Affected daughters must have an affected father and a carrier or affected mother

• X-linked Dominant:

  • Affected fathers pass the trait to all daughters

  • Inheritance patterns differ in reciprocal crosses

• Y-linked:

  • Only males affected

  • Trait passed from father to son

Predicting Outcomes for Sex-Linked Crosses

Punnett Squares for Sex-Linked Traits

Punnett squares can be used to predict genotypic and phenotypic ratios for crosses involving sex-linked traits.

• For XX/XY systems (humans):

  • Daughters inherit one X from each parent

  • Sons inherit X from mother and Y from father

• For ZZ/ZW systems (birds):

  • Males are ZZ, females are ZW

Pedigree Analysis and Practice Problems

Pedigree Analysis

Pedigrees are used to track inheritance patterns in families and identify modes of inheritance.

• Filled symbols represent affected individuals

• Patterns can help distinguish between autosomal and sex-linked traits

Practice Problem: X-linked Recessive Disorder

Example: Ornithine transcarbamylase deficiency (OTD)

• Father has OTD (dY), mother is homozygous wild-type (DD)

• Daughter's genotype: Dd

• Son with unaffected father: 50% chance of OTD

• Daughter with unaffected father: 50% chance of being carrier, 0% chance of OTD

• Father genotype for affected daughter: dY

• Proportion of affected daughters and sons: 50% each

Summary Table: Modes of Inheritance

Mode	Pattern	Key Clues
Autosomal Dominant	Affected in every generation	Unaffected parents cannot have affected child
Autosomal Recessive	Can skip generations	Affected child can have unaffected parents
X-linked Dominant	Affected fathers pass trait to all daughters	Both sexes affected, but patterns differ
X-linked Recessive	More common in males	Affected sons inherit from mother
Y-linked	Only males affected	Trait passed father to son

Summary and Study Guide

• Discriminate between sex influenced, sex limited, and sex-linked traits

• Use pedigree analysis to infer modes of inheritance

• Predict outcomes for crosses involving sex-linked traits

• Identify key clues for different inheritance patterns

Additional info: These notes cover content relevant to Ch. 3 (Cell Division and Chromosome Heredity), Ch. 4 (Gene Interaction), and Ch. 5 (Genetic Linkage and Mapping in Eukaryotes), focusing on sex dimorphism, sex linkage, and inheritance patterns.