Sex and Heredity: Modes of Inheritance and Sex Dimorphism

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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. Male moose with antlers Male moose competing with antlers Example: Peacocks and butterflies show visible differences between sexes. Male and female peafowl Male and female butterflies

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.

Human karyotype showing X and Y chromosomes Diagram of X and Y chromosomes

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.

Normal and color-blind vision Color blindness test Pedigree for X-linked recessive inheritance

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

Punnett square for sex-linked inheritance

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

Pedigree for X-linked recessive inheritance Pedigree for X-linked dominant inheritance

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.