Sex-Limited Character in Biology: Explained with Examples

Sex-Limited Genes are found in both sexes of sexually reproducing animals, but are expressed in only one and thus have no penetrance in another, or are simply 'turned off' in the other. In other words, although possessing the same genotype, Sex-Limited Genes cause the two sexes to exhibit different behaviours or phenotypes. This term refers to autosomal qualities only, and therefore should not be mistaken with sex-linked features, which refer to genetic variations on the chromosomes of the sexes. Sex-restricted genes are separated from sex-influenced genes, which have distinct expression patterns in each sex.

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Genetics

Storage Effect: Because Sex-Limited Genes exist in both sexes however are expressed only in one, the unexpressed genes are protected from the selection. If talking about a short-term scale, this implies that within one generation, selection would only affect the sex which expresses the sex-limited trait(s) of significance. Because these features are buried (unexpressed) in the genes of the opposite sex, the left portion of the gene pool for such traits would be unaffected by selection. The phenomenon is known as the 'storage-effect,' even though a fraction of the alleles for all these sex-limited features is masked from the selection.

Effects of Sexual Antagonism: When two species' optimal fitness strategies for reproduction collide, sexual antagonism arises. A famous example of opposing optimal strategies is multiple matings. Males could profit from much more regular matings since they have a lesser investment amount in reproduction. Females, on the other hand, put far more effort into reproduction and could be threatened, damaged, or even killed as a result of many matings.

Effects on Animal Behavior: Animal behavior is so broad that it appears in practically every primary literature containing live animals in some form. Although the instances above involve parts of animal behaviour, a more overt instance of it in regard to sex-limited features and other example of sex limited traits in animals are given in Teplitsky et al. research (2010) focusing on red-billed gull breeding time. The subject of this study is breeding time, which is a part of reproductive Biology. Reproduction and sexual activity are two important components of animal behaviour that are universally manifested in some form across the animal kingdom. 

Sex Linked Character:

When a gene mutation (allele) is found on a sex chromosome (allosome) instead of a non-sex chromosome (allosome), sex linked characterises the sex-specific patterns of inheritance and presentation (autosome). These are referred to as X-linked dominant, X-linked recessive, and Y-linked in humans. The inheritance and presentation of any and all three are different based on the parent's and child's sex. The inheritance and presentation among all three are different based on the parent's and child's sex. This distinguishes them from autosomal dominance and recessiveness in a significant way.

Because humans have a few times as many genes on the X chromosome as the Y chromosome, there seem to be significantly more X-linked disorders than Y-linked disorders. Because they lack a second X chromosome with a good and healthy copy of the gene, solely females can be carriers for X-linked illnesses; males would always be impacted by an X-linked condition. As a result, males are affected far more frequently than females by X-linked recessive diseases. 

A son born to a mother who is the carrier and a father who is unaffected has a 50% chance of being affected, whereas a daughter seems to have a 50% chance of becoming a carrier in X-linked recessive inheritance. Due to a phenomenon called skewed X-inactivation, wherein the usual process of inactivating 50 percent of the female body's X chromosomes preferentially targets a specific parent's X chromosome (in this example, the father's), a fraction of carriers might show a milder (or perhaps even full) form of the illness. Because the son does not inherit the father's X chromosome, if the father is sick, the son would not be impacted, but the daughter has always been a carrier.

X-Linked Dominant Inheritance

Each woman who has an X-linked dominant characteristic has a 50% probability of having the mutation and consequently developing the condition. If just the father is impacted, all of the daughters would be affected because they receive their father's X chromosome, but no sons will indeed be affected because they acquire their father's Y chromosome.

Because around half (or as much as 90% in some situations) of a specific parent's X chromosomes are inactivated in females, X-linked dominant conditions are far less common than X-linked recessive disorders.

Examples:-

• Alport Syndrome: 

Alport syndrome is a hereditary illness characterized by glomerulonephritis, end-stage renal disease, and hearing problems that affect 1 in 5,000-10,000 children. Alport syndrome can damage the eyes as well, though the alterations normally do not impede vision unless the lens changes later in life. Urine with blood is a common occurrence. Proteinuria is a symptom of advanced renal disease.

Coffin-Lowry syndrome: Coffin-Lowry syndrome is an X-linked dominant genetic condition that causes significant mental disorders, and also heart abnormalities, kyphoscoliosis, growth abnormalities, and hearing and visual abnormalities.

• Fragile X Syndrome (FXS): 

The genetic condition Fragile X syndrome (FXS) is characterized by mild-to-moderate intellectual impairment. Males have a lower average IQ than females, with roughly two-thirds of those affected being intellectually handicapped. A long and narrow face, flexible fingers, huge ears, and huge testicles are all possible physical characteristics. Around a third of all those affected show autism symptoms including social interaction issues and speech delays. Seizures occur in roughly 10% of people with hyperactivity. Males are much more affected than females in most cases.

• Hypoparathyroidism: 

Hypoparathyroidism is a condition in which the parathyroid glands' function is impaired due to a lack of parathyroid hormone synthesis. Low calcium levels in the blood can cause muscle cramping and twitching, as well as tetany (involuntary muscle spasm) and a variety of other symptoms. It's an illness that only a few people have ever heard of. It could be hereditary, however, it is also seen after thyroid or parathyroid gland surgery, and this can be triggered by immune system injury as well as a variety of other less common causes. Blood tests as well as other investigations, including genetic testing, are used to make the diagnosis, based on the results. Calcium and vitamin D supplementation are the most common treatments for hypoparathyroidism.

X-linked Recessive Inheritance

Carriers are females that have one X-linked recessive mutation and do not show medical symptoms of the condition. However, changes in X chromosome inactivation might contribute to varying degrees of clinical expression in carriers, as certain cells express one X allele while others express another. Because males have only one X chromosome and hence only a single copy of X-linked genes, all males with an X-linked recessive mutation would be impacted. If the father does not contain the recessive allele, all offspring of a carrier mother have a 50% risk of obtaining the mutation. Because daughters inherit their father's X chromosome, all-female children of an affected father would be carriers (provided the mother is neither affected nor a carrier). Because boys only inherit their father's Y chromosome, unless the mother would not be a carrier, none of the male children of an affected father would be impacted.

The incidence of X-linked recessive disorders in women seems to be the square of the incidence in men: for instance, if 1 in 20 men in a human community becomes colourblind, then 1 in 400 women in the group are likely to be colourblind (1/20)*(1/20)

Examples:

• Aarskog–Scott syndrome

Short stature, facial deformities, skeletal and genital malformations are all symptoms of Aarskog–Scott syndrome, a rare X-linked disorder. Males are more likely to be affected by this illness, however, females may experience modest symptoms.

• Adrenoleukodystrophy

ALD (adrenoleukodystrophy) is a disease caused by a mutation on the X chromosome. A deficiency in the very long sequence of fatty acids transporter in peroxisomes leads to fatty acid accumulation that results in damage to the myelin sheath of the nerves, leading to hyperactivity and seizures. Difficulties in talking, hearing and interpreting verbal instructions are among the other signs and symptoms.

• Color Blindness 

Color blindness (color vision deficit) is a condition in which a person's ability to see color or color distinctions is impaired. This could make it difficult to do things like pick ripe fruit, dress appropriately, and read traffic lights. Some educational tasks may be more challenging if you are color blind. However, most color-blind persons adapt, and problems are usually mild.

• Fabry Disease

Fabry disease, commonly referred to as Anderson–Fabry disease is a rare genetic disorder that affects the heart, kidneys, and skin. Lysosomal storage diseases are a group of disorders that include Fabry disease.

Other Example of Sex Limited Traits in Animals:-

• One of the first sex-linked genes recognized was white eyes in Drosophila melanogaster flies.

• The gene which creates orange pigment in domestic cats is found on the X chromosome, therefore a Calico or tortoiseshell cat is both black (or gray) and the orange pigment is almost usually female.

• Leonard Doncaster identified the "lacticolor" X-linked recessive gene in the moth Abraxas grossulariata, which was the first sex-linked gene ever identified.

Related Terms:

It's important to draw a distinction between sex-related features, that are regulated by genes on sex chromosomes, and two additional types of features.

Sex-influenced Traits- Traits that are influenced or conditioned upon whether they occur in a male or female body are known as sex-influenced or sex-conditioned traits. The condition would not be completely expressed in a homozygous dominant or recessive female. Human baldness is an illustration.

Sex-limited Traits- Characters that can just be expressed in single sex. Genes on both the autosomal or sex chromosomes can produce them. Female sterility in Drosophila, as well as many polymorphic traits in insects, particularly in relation to mimicry, are instances. The latter is frequently caused by "supergenes," which are strongly linked genes on autosomes.

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