Sex linkage

SEX LINKAGE


Recap Mendel’s law of inheritance

  • Law of dominance

  • Law of independent assortment

  • Law of segregation

NON-MENDELIAN LAWS OF INHERITANCE 


1. INCOMPLETE DOMINANCE 

  • The offspring's phenotype is a blend of the phenotypes of both parents.

2. CODOMINANCE 

  • Where both alleles of a gene are fully expressed in the phenotype of a heterozygous individual.

3. MULTIPLE ALLELES 

  • When more than two alleles exist for a single gene within a population.

4. POLYGENES 

  • The interaction between genes and environmental factors where the environment influences the expression of genes. This is also known as gene-environment interaction.

5. SEX LINKAGE 

  • Traits determined by genes located on sex chromosomes (X or Y). These traits often show different inheritance patterns between males and females.


A karyotype is a visual presentation of the appearance and number of chromosomes of an individual. 

TYPES OF INHERITANCE OF SEX TRAITS 

  • SEX-LINKED TRAITS 

  • SEX-INFLUENCED TRAITS 

  • SEX-LIMITED TRAITS


SEX-LINKED TRAITS

  • Traits that are carried or found in the sex chromosomes are called sex-linked traits. Genes on the X chromosomes are called X-linked genes and those in the Y chromosome are called Y-linked genes.

  • X-chromosome is a bit bigger and longer than the Y chromosome, therefore, has more genes found in it. 

SEX INFLUENCED TRAITS

  • Are autosomal qualities that are impacted by sex. Specifically sex hormones, such as testosterone in males and estrogen and progesterone in females. Although these characteristics are present in both sexes, the degree and frequency of phenotypic expression differ by sex. 


WHEN DO SEX INFLUENCED TRAITS OCCUR? 

  • EARLY DEVELOPMENT -  PUBERTY - ADULTHOOD - AGING

WHY DOES IT OCCUR? 

  • Sex-influenced traits develop from the interaction between autosomal genes and sex hormones. These traits can be expressed differently in males and females due to hormonal influences. 

FACTORS

Hormonal Regulation: Hormones like testosterone, estrogen, and progesterone can activate or suppress certain genes, causing traits to appear differently between the sexes.

  • PATTERN BALDNESS 

    • Males with higher testosterone levels have a dominant baldness gene, whereas females with lower testosterone levels have a recessive baldness gene.


Evolutionary Adaptation: Some of these traits may have evolved because they provided specific advantages to one sex, enhancing reproductive success or survival.

  • MALE PEACOCKS

    • In peacocks, males have colorful tail feathers that they display to attract females, influenced by the hormone testosterone.


Genetic Dominance and Recessiveness: In sex-influenced traits, a gene may act dominantly in one sex but recessively in the other, influenced by hormonal interaction with genetic pathways. This results in different trait expressions in males and females.

  • RATIO OF RING AND INDEX FINGER 

    • This finger length ratio is considered a sex-influenced trait because it varies between the sexes based on hormonal differences. 


SEX LIMITED TRAITS

  • These are traits that are expressed only in one of the sexes. 

Controlled by: autosomes


DIFFERENCE WITH OTHER TRAITS

  • Sex-linked traits are linked to X and Y chromosomes. 

  • Sex-limited traits are on regular (autosomal) chromosomes, but they depend on hormones to appear in only one sex. 

Example:

  • Both men and women have the genes for growing beards but only men grow beards because of the effect of the male sex hormones (testosterone).

  • Milk production depends on hormones like prolactin and oxytocin, which are present in higher amounts in females, especially after giving birth.