There are many more X-linked conditions than Y-linked conditions, since humans have several times as many genes on the X chromosome than the Y chromosome. Only females are able to be carriers for X-linked conditions; males will always be affected by any X-linked condition, since they have no second X chromosome with a healthy copy of the gene. As such, X-linked recessive conditions affect males much more commonly than females.
In X-linked recessive inheritance, a son born to a carrier mother and an unaffected father has a 50% chance of being affected, while a daughter has a 50% chance of being a carrier, however a fraction of carriers may display a milder (or even full) form of the condition due to a phenomenon known as skewed X-inactivation, in which the normal process of inactivating half of the female body's X chromosomes preferably targets a certain parent's X chromosome (the father's in this case). If the father is affected, the son will not be affected, as he does not inherit the father's X chromosome, but the daughter will always be a carrier (and may occasionally present with symptoms due to aforementioned skewed X-inactivation).
In X-linked dominant inheritance, a son or daughter born to an affected mother and an unaffected father both have a 50% chance of being affected (though a few X-linked dominant conditions are embryonic lethal for the son, making them appear to only occur in females). If the father is affected, the son will always be unaffected, but the daughter will always be affected. A Y-linked condition will only be inherited from father to son and will always affect every generation.
Illustration of some X-linked heredity outcomes (A) the affected father has one X-linked dominant allele, the mother is homozygous for the recessive allele: only daughters (all) will be affected. (B) the affected mother is heterozygous with one copy of the X-linked dominant allele: both daughters and sons will have 50% probability to be affected. (C) the heterozygous mother is called "carrier" because she has one copy of the recessive allele: sons will have 50% probability to be affected, 50% of unaffected daughters will become carriers like their mother.[2]
An example pedigree chart of the inheritance of a sex-linked disorder
Each child of a mother affected with an X-linked dominant trait has a 50% chance of inheriting the mutation and thus being affected with the disorder. If only the father is affected, 100% of the daughters will be affected, since they inherit their father's X chromosome, and 0% of the sons will be affected, since they inherit their father's Y chromosome.
There are fewer X-linked dominant conditions than X-linked recessive, because dominance in X-linkage requires the condition to present in females with only a fraction of the reduction in gene expression of autosomal dominance, since roughly half (or as many as 90% in some cases) of a particular parent's X chromosomes are inactivated in females.
Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder, although differences in X chromosome inactivation can lead to varying degrees of clinical expression in carrier females since some cells will express one X allele and some will express the other. All males possessing an X-linked recessive mutation will be affected, since males have only a single X chromosome and therefore have only one copy of X-linked genes. All offspring of a carrier female have a 50% chance of inheriting the mutation if the father does not carry the recessive allele. All female children of an affected father will be carriers (assuming the mother is not affected or a carrier), as daughters possess their father's X chromosome. If the mother is not a carrier, no male children of an affected father will be affected, as males only inherit their father's Y chromosome.
The incidence of X-linked recessive conditions in females is the square of that in males: for example, if 1 in 20 males in a human population are red–green color blind, then 1 in 400 females in the population are expected to be color-blind (1/20)*(1/20).
Fur color in domestic cats: the gene that causes orange pigment is on the X chromosome; thus a Calico or tortoiseshell cat, with both black (or gray) and orange pigment, is nearly always female.
It is important to distinguish between sex-linked characters, which are controlled by genes on sex chromosomes, and two other categories.[5]
Sex-influenced traits
Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body.[6] Even in a homozygous dominant or recessive female the condition may not be expressed fully. Example: baldness in humans.
Sex-limited traits
These are characters only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes.[6] Examples: female sterility in Drosophila; and many polymorphic characters in insects, especially in relation to mimicry. Closely linked genes on autosomes called "supergenes" are often responsible for the latter.[7][8][9]
^Morgan T.H. 1910. Sex-limited inheritance in Drosophila. Science32: 120–122
^Doncaster L. & Raynor G.H. 1906. Breeding experiments with Lepidoptera. Proceedings of the Zoological Society of London. 1: 125–133
^Zirkle, Conway (1946). The discovery of sex-influenced, sex limited and sex-linked heredity. In Ashley Montagu M.F. (ed) Studies in the history of science and learning offered in homage to George Sarton on the occasion of his sixtieth birthday. New York: Schuman, p167–194.
^ abKing R.C; Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press. ISBN0-19-530761-5
^Mallet J.; Joron M. (1999). "The evolution of diversity in warning color and mimicry: polymorphisms, shifting balance, and speciation". Annual Review of Ecology and Systematics. 30: 201–233. doi:10.1146/annurev.ecolsys.30.1.201.
^Ford E. B. (1965) Genetic polymorphism. p17-25. MIT Press 1965.
^Joron M, Papa R, Beltrán M, et al. (2006). "A conserved supergene locus controls colour pattern diversity in Heliconius butterflies". PLOS Biol. 4 (10): e303. doi:10.1371/journal.pbio.0040303. PMC1570757. PMID 17002517.
May 06, 2023
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This article is about sex linked inheritance For hybrid chickens with sexually differentiated hatchling color see Sex link Sex linked describes the sex specific reading patterns of inheritance and presentation when a gene mutation allele is present on a sex chromosome allosome rather than a non sex chromosome autosome In humans these are termed X linked recessive X linked dominant and Y linked The inheritance and presentation of all three differ depending on the sex of both the parent and the child This makes them characteristically different from autosomal dominance and recessiveness Experimental cross performed by Thomas Hunt Morgan illustrating the X linked inheritance of white eyed mutation in fruit flies 1 There are many more X linked conditions than Y linked conditions since humans have several times as many genes on the X chromosome than the Y chromosome Only females are able to be carriers for X linked conditions males will always be affected by any X linked condition since they have no second X chromosome with a healthy copy of the gene As such X linked recessive conditions affect males much more commonly than females In X linked recessive inheritance a son born to a carrier mother and an unaffected father has a 50 chance of being affected while a daughter has a 50 chance of being a carrier however a fraction of carriers may display a milder or even full form of the condition due to a phenomenon known as skewed X inactivation in which the normal process of inactivating half of the female body s X chromosomes preferably targets a certain parent s X chromosome the father s in this case If the father is affected the son will not be affected as he does not inherit the father s X chromosome but the daughter will always be a carrier and may occasionally present with symptoms due to aforementioned skewed X inactivation In X linked dominant inheritance a son or daughter born to an affected mother and an unaffected father both have a 50 chance of being affected though a few X linked dominant conditions are embryonic lethal for the son making them appear to only occur in females If the father is affected the son will always be unaffected but the daughter will always be affected A Y linked condition will only be inherited from father to son and will always affect every generation The inheritance patterns are different in animals that use sex determination systems other than XY In the ZW sex determination system used by birds the mammalian pattern is reversed since the male is the homogametic sex ZZ and the female is heterogametic ZW In classical genetics a mating experiment called a reciprocal cross is performed to test if an animal s trait is sex linked A B C Illustration of some X linked heredity outcomes A the affected father has one X linked dominant allele the mother is homozygous for the recessive allele only daughters all will be affected B the affected mother is heterozygous with one copy of the X linked dominant allele both daughters and sons will have 50 probability to be affected C the heterozygous mother is called carrier because she has one copy of the recessive allele sons will have 50 probability to be affected 50 of unaffected daughters will become carriers like their mother 2 Contents 1 X linked dominant inheritance 1 1 Examples 2 X linked recessive inheritance 2 1 Examples 3 Y linked 4 Sex linked traits in other animals 5 Related terms 5 1 Sex influenced traits 5 2 Sex limited traits 6 See also 7 ReferencesX linked dominant inheritance EditMain article X linked dominant inheritance An example pedigree chart of the inheritance of a sex linked disorder Each child of a mother affected with an X linked dominant trait has a 50 chance of inheriting the mutation and thus being affected with the disorder If only the father is affected 100 of the daughters will be affected since they inherit their father s X chromosome and 0 of the sons will be affected since they inherit their father s Y chromosome There are fewer X linked dominant conditions than X linked recessive because dominance in X linkage requires the condition to present in females with only a fraction of the reduction in gene expression of autosomal dominance since roughly half or as many as 90 in some cases of a particular parent s X chromosomes are inactivated in females Examples Edit Alport syndrome Coffin Lowry syndrome CLS Fragile X syndrome Idiopathic hypoparathyroidism Incontinentia pigmenti Rett syndrome RS Vitamin D resistant rickets X linked hypophosphatemia X linked recessive inheritance EditMain article X linked recessive inheritance Females possessing one X linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder although differences in X chromosome inactivation can lead to varying degrees of clinical expression in carrier females since some cells will express one X allele and some will express the other All males possessing an X linked recessive mutation will be affected since males have only a single X chromosome and therefore have only one copy of X linked genes All offspring of a carrier female have a 50 chance of inheriting the mutation if the father does not carry the recessive allele All female children of an affected father will be carriers assuming the mother is not affected or a carrier as daughters possess their father s X chromosome If the mother is not a carrier no male children of an affected father will be affected as males only inherit their father s Y chromosome The incidence of X linked recessive conditions in females is the square of that in males for example if 1 in 20 males in a human population are red green color blind then 1 in 400 females in the population are expected to be color blind 1 20 1 20 Examples Edit Aarskog Scott syndrome Adrenoleukodystrophy ALD Bruton s agammaglobulinemia Color blindness Complete androgen insensitivity syndrome Congenital aqueductal stenosis hydrocephalus Duchenne muscular dystrophy Fabry disease Glucose 6 phosphate dehydrogenase deficiency Haemophilia A and B Hunter syndrome Inherited nephrogenic diabetes insipidus Menkes disease kinky hair syndrome Ornithine carbamoyltransferase deficiency Wiskott Aldrich syndromeY linked EditMain article Y linkage Various failures in the SRY genesSex linked traits in other animals EditWhite eyes in Drosophila melanogaster flies was one of the earliest sex linked genes discovered 3 Fur color in domestic cats the gene that causes orange pigment is on the X chromosome thus a Calico or tortoiseshell cat with both black or gray and orange pigment is nearly always female The first sex linked gene ever discovered was the lacticolor X linked recessive gene in the moth Abraxas grossulariata by Leonard Doncaster 4 Related terms EditIt is important to distinguish between sex linked characters which are controlled by genes on sex chromosomes and two other categories 5 Sex influenced traits Edit Sex influenced or sex conditioned traits are phenotypes affected by whether they appear in a male or female body 6 Even in a homozygous dominant or recessive female the condition may not be expressed fully Example baldness in humans Sex limited traits Edit These are characters only expressed in one sex They may be caused by genes on either autosomal or sex chromosomes 6 Examples female sterility in Drosophila and many polymorphic characters in insects especially in relation to mimicry Closely linked genes on autosomes called supergenes are often responsible for the latter 7 8 9 See also EditX linked dominant inheritance X linked recessive inheritanceReferences Edit Morgan Thomas Hunt 1919 The physical basis of heredity Philadelphia J B Lippincott Company Genetics home reference 2006 genetic conditions illustrations National Library of Medicine Morgan T H 1910 Sex limited inheritance in Drosophila Science 32 120 122 Doncaster L amp Raynor G H 1906 Breeding experiments with Lepidoptera Proceedings of the Zoological Society of London 1 125 133 Zirkle Conway 1946 The discovery of sex influenced sex limited and sex linked heredity In Ashley Montagu M F ed Studies in the history of science and learning offered in homage to George Sarton on the occasion of his sixtieth birthday New York Schuman p167 194 a b King R C Stansfield W D amp Mulligan P K 2006 A dictionary of genetics 7th ed Oxford University Press ISBN 0 19 530761 5 Mallet J Joron M 1999 The evolution of diversity in warning color and mimicry polymorphisms shifting balance and speciation Annual Review of Ecology and Systematics 30 201 233 doi 10 1146 annurev ecolsys 30 1 201 Ford E B 1965 Genetic polymorphism p17 25 MIT Press 1965 Joron M Papa R Beltran M et al 2006 A conserved supergene locus controls colour pattern diversity in Heliconius butterflies PLOS Biol 4 10 e303 doi 10 1371 journal pbio 0040303 PMC 1570757 PMID 17002517 Retrieved from https en wikipedia org w index php title Sex linkage amp oldid 1141157534, wikipedia, wiki, book, books, library,
