Define and distinguish sex-linked, sex-limited and sex-influenced characters

In genetics, traits can be influenced or expressed differently depending on the sex of the individual. Some traits are linked to sex chromosomes, while others are affected by hormonal or physiological differences between males and females. To describe these traits more precisely, geneticists use three main terms: sex-linked, sex-limited and sex-influenced traits. Although these terms may sound similar, they refer to different types of genetic expression related to sex. Understanding the distinction among these three is important for grasping how certain traits are inherited and expressed differently in males and females.

1. Sex-linked characters:

These are traits controlled by genes that are located on the sex chromosomes, usually on the X chromosome in humans. Because males have only one X chromosome (XY) and females have two (XX), the pattern of inheritance and expression is different in both sexes. Most sex-linked traits are X-linked, and very few are Y-linked. An example is haemophilia and red-green colour blindness, which are X-linked recessive traits.

2. Sex-limited characters:

These are traits that are controlled by autosomal genes, meaning they are not located on sex chromosomes, but they are expressed only in one sex, due to the presence of sex hormones or other physiological differences. These traits are inherited by both sexes but are phenotypically visible only in one. For example, milk production in female mammals and beard growth in human males.

3. Sex-influenced characters:

These are also controlled by autosomal genes, but their expression is influenced by the sex of the individual. In this case, a gene may behave as dominant in one sex and recessive in the other. This means the same genotype can produce different phenotypes in males and females. An example is pattern baldness in humans, which behaves as dominant in males and recessive in females.

Differences Between Sex-Linked, Sex-Limited and Sex-Influenced Characters

The following comparison highlights how these three types of traits differ from each other across multiple biological aspects:

1. Based on chromosomal location:

Sex-linked traits are genes found on the sex chromosomes, mostly on the X chromosome. Their inheritance depends on the sex of the individual because males and females have different sex chromosomes.

Sex-limited traits are controlled by genes on autosomes (non-sex chromosomes) but show up only in one sex, usually because of hormones that switch these genes on or off in that sex.

Sex-influenced traits are also on autosomes, but their expression changes depending on whether the individual is male or female, due to differences in hormones.

2. Based on expression in sexes:

Sex-linked traits are expressed more commonly in males, because they have only one X chromosome. For example, a male with one recessive allele for haemophilia on X will express the trait.

Sex-limited traits are expressed only in one sex, even though both sexes may carry the gene. For example, milk production appears only in females.

Sex-influenced traits are expressed in both sexes, but differently. For example, baldness is more frequent in males because the gene behaves dominantly in them but recessively in females.

3. Based on hormonal control:

Sex-linked traits are generally not regulated by hormones but depend on the presence of X or Y chromosomes.

Sex-limited traits are directly regulated by sex hormones, such as estrogen and testosterone. For example, testosterone promotes beard growth, a sex-limited male trait.

Sex-influenced traits are partially regulated by hormones, which influence gene dominance. For example, testosterone enhances the expression of baldness.

4. Based on inheritance pattern:

Sex-linked traits follow non-Mendelian patterns, especially criss-cross inheritance (mother to son, father to daughter).

Sex-limited traits follow typical Mendelian autosomal inheritance, but phenotypically expressed in only one sex.

Sex-influenced traits also follow autosomal Mendelian inheritance, but expression is modified by sex.

5. Based on genotype-to-phenotype relationship:

In sex-linked traits, even a single recessive allele on the X chromosome can cause the trait in males.

In sex-limited traits, the genotype may be present in both sexes, but phenotype appears only in one sex. For example, male and female birds may carry the same genotype for feather pattern, but expression is sex-limited.

In sex-influenced traits, same genotype shows different dominance in males and females. For example, a heterozygous male (Bb) for baldness will show baldness, but a heterozygous female (Bb) may not.







Comments

Post a Comment

Popular posts from this blog

What is the difference between the classical concept and the modern concept of genes?

The concept of the gene has evolved from a simple unit of heredity to a complex molecular entity. During  Mendel's  time, genes were understood only through the inheritance of physical traits. This formed the  classical concept.  With the discovery of DNA, its double helix structure and the development of molecular biology, the gene is now defined by its chemical structure and functional properties. This is known as the  modern concept. For understanding the difference between the classical and modern concept of genes, the following criteria are used: 1. Based on Definition or Concept Classical Concept:  Gene was defined as an abstract unit of heredity responsible for controlling a  single trait.  It was known through breeding experiments and inheritance patterns. Modern Concept:  A gene is a segment of DNA that contains the information to produce a functional product, either a protein or an RNA. It is both a structural and functional unit of...
Read more

Why is the frequency of double crossover overly low?

The frequency of double crossover is usually much lower than expected. This is mainly due to a natural genetic mechanism called interference, which controls the distribution of crossover events during meiosis. Crossover is essential for genetic recombination but is also tightly regulated to prevent instability in the genome. The lower frequency of double crossovers can be explained by the following reasons: 1. Physical Constraints of Chromosomes Chromosomes have a limited length and physical structure. When a crossover happens at one region of a chromosome, the local chromatin structure and spatial arrangement become less favorable for another crossover nearby. This physical limitation reduces the chance of two crossovers occurring very close to each other on the same chromosome segment. 2. Crossover Interference One of the main reasons for reduced double crossovers is the phenomenon called  interference.  Interference is the effect where the occurrence of one crossover decrea...
Read more

What is gene mapping? How do the linked genes help in gene mapping?

Gene mapping is the method used to determine the location of genes on a chromosome and the distance between them. It helps in identifying the exact position of a gene responsible for a particular trait or disease. The concept started with the work of  Thomas Hunt Morgan  in the early 1900s when he studied Drosophila melanogaster (fruit fly) and observed that some traits are inherited together. This was because the genes responsible for those traits were located close to each other on the same chromosome. This phenomenon is known as  linkage. There are two main types of gene mapping: 1. Genetic Mapping (Linkage Mapping): Genetic mapping uses the frequency of recombination or crossing over between genes to estimate their distance on a chromosome. It gives a  relative position of genes rather than their exact physical location. 2. Physical Mapping Physical mapping uses molecular biology techniques to determine the exact nucleotide sequence of DNA and the exact physical ...
Read more

The First Living Molecule

Image
The question of which molecule can be considered the "first living molecule" is complex and doesn't have a definitive answer. This depends on whether we are discussing the molecule that played a key role in the  origin of life  or the  first organic molecules  that formed chemically on early Earth. While  RNA is considered crucial in the emergence of life,   amino acids were likely the first molecules to appear in Earth's primordial environment.  Understanding these distinctions clarifies the discussion about whether RNA or amino acids came first. Introduction: 1. According to the Origin of Life: In the context of the origin of life, the first molecule is likely  RNA.  RNA's ability to store genetic information and catalyze its own replication (acting as both a genetic material and an enzyme) gives it a key role in early life. The  RNA World Hypothesis  suggests that RNA existed before amino acids or proteins, as it could facilitat...
Read more

What is the difference between regulatory gene and structural gene?

Genes are basic units of heredity that carry instructions for the development and functioning of living organisms. Among various types of genes,  regulatory genes  and  structural genes  are essential for the proper functioning of cells, but their roles are quite different. Regulatory genes mainly control other genes, while structural genes directly code for proteins that form the body's structures or perform specific functions. To understand their differences clearly, we will compare them based on important criteria. 1. Based on Function The main function of  regulatory genes  is to control or regulate the expression of other genes. They produce regulatory proteins, such as repressors or activators, that influence whether structural genes are switched on or off. This control mechanism ensures that genes are expressed only when needed. In contrast,  structural genes  code directly for proteins or RNA molecules that are involved in building cellula...
Read more

Subatomic Particles

Image
Atoms are the basic units of matter and are made up of smaller components called subatomic particles. There are many types of subatomic particles known to science, but in the context of basic atomic structure, only three are considered most important: electrons, protons and neutrons. These three particles differ in their location, charge and mass. Together, electrons, protons and neutrons form the complete structure of atoms. Their arrangement and interaction define the atom's properties, chemical behavior, and participation in physical and chemical processes. These subatomic particles laid the foundation of modern atomic theory and quantum chemistry. 1. Electron Electrons are negatively charged subatomic particles. They are extremely small in mass and are found outside the nucleus of the atom in specific regions called orbitals or shells. The charge of an electron is −1  and its mass is approximately 1/1836 of a proton or neutron, which is around 9.1 × 10⁻³¹ kg, meaning it is n...
Read more

Differentiate between linked genes and unlinked genes?

Genes are specific sequences of DNA that code for proteins and determine traits in an organism. During the study of chromosomal theory of  inheritance, scientists found that not all genes behave the same way. Some genes tend to be inherited together while others assort independently. Based on this behavior, genes are divided into two types:  linked genes and unlinked genes.  This concept is very important in genetics because it helps in understanding how traits are passed on and how gene positions can be mapped on chromosomes. These differences are explained based on specific criteria: 1. Based on Chromosomal Location Linked genes  are located close to each other on the same chromosome. Because of their close physical proximity, they usually move together during meiosis and are inherited as a group. For example: In Drosophila melanogaster (fruit fly), the genes for eye color and wing shape are located close to each other on the X chromosome. Unlinked genes  are ...
Read more

Can marriage between two albinos produce children with normal pigmentation?

Image
Albinism is a genetic condition where a person lacks melanin, the pigment responsible for the colour of skin, hair and eyes. It is usually inherited in an autosomal recessive manner. The most common forms of albinism, such as oculocutaneous albinism (OCA), are caused by mutations in genes like TYR, OCA2, TYRP1 etc. For a person to show albinism, they must inherit two copies of the defective allele, one from each parent. This means the genotype of an albino individual is generally homozygous recessive (aa). If both parents are albino due to mutations in the same gene, then their genotype would be aa, and the only gametes they can produce will carry the a allele. When we perform a cross: Parent 1: aa × Parent 2: aa Gametes: a a All children will also be aa, hence all children will be albino. Therefore, if both parents are albino due to the same gene, they cannot produce a normally pigmented child. However, in rare cases, albinism can result from mutations in different genes in e...
Read more

Give two examples of gene interaction resulting in the formation of structural proteins

Gene interaction refers to a situation where two or more genes influence the  same trait.  In the case of structural proteins, sometimes the final functional protein is not made from a single gene product but is the result of the combination of different polypeptides produced by different genes. Such interaction is especially important in the formation of complex structural proteins that require the association of multiple chains to become functional. Two good examples of this kind of gene interaction are seen in  haemoglobin and MHC (Major Histocompatibility Complex) molecules. 1. Haemoglobin (HbA) Haemoglobin is the oxygen-carrying protein found in red blood cells. The adult type of haemoglobin, called  HbA,  is a tetramer made up of two  alpha-globin chains and two beta-globin chains.  These chains are coded by different genes: The alpha-globin gene is located on  chromosome 16. The beta-globin gene is located on  chromosome 11. Both these...
Read more