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X-Linked Traits Biology Unit 6 Velekei Genetics of Sex In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develop as a female: XX gene redundancy, like autosomal chromosomes an X & Y chromosome X Y X XX XY X XX XY develop as a male: XY no redundancy 50% female : 50% male Genes on sex chromosomes Y chromosome few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones many effects = pleiotropy! X chromosome other genes/traits beyond sex determination mutations: hemophilia Duchenne muscular dystrophy color-blindness Human X chromosome Sex-linked usually means “X-linked” more than 60 diseases traced to genes on X chromosome Duchenne muscular dystrophy Becker muscular dystrophy Chronic granulomatous disease Retinitis pigmentosa-3 Norrie disease Retinitis pigmentosa-2 Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Adrenal hypoplasia Glycerol kinase deficiency Ornithine transcarbamylase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Androgen insensitivity Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Immunodeficiency, X-linked, with hyper IgM Lymphoproliferative syndrome Albinism-deafness syndrome Fragile-X syndrome Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation PRPS-related gout Lowe syndrome Lesch-Nyhan syndrome HPRT-related gout Hunter syndrome Hemophilia B Hemophilia A G6PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked Sex-linked Genes Genes on the X chromosome are called “sex- linked”, because they expressed more often in males than in females There are very few genes on the Y chromosome. Since males only have one X chromosome, all genes on it, whether dominant or recessive, are expressed. Sex-linked Genes In contrast, a mutant gene on an X chromosome in a female is usually covered up by the normal allele on the other X. Most mutations are recessive. So, most people with sex-linked genetic conditions are male. Another fact about sex-linked genes. Males produce ½ their sperm with their X chromosome, and half with their Y chromosome. The X-bearing sperm lead to daughters and the Y-bearing sperm lead to sons. So, sons get their only X from their mothers, and the father’s X goes only to daughters. The Y chromosome is passed from father to son. Why can females have 2 copies of the X chromosome, when males only have 1? Answer: In each cell one of the X chromosomes ‘turns off’. This turned off chromosome is known as a Barr body. The effect of Barr bodies can be seen in Calico colored cats. X-inactivation Female mammals inherit 2 X chromosomes one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random patchwork trait = “mosaic” patches of black XH XH Xh tricolor cats can only be female Xh patches of orange Colorblindness We have 3 color receptors in the retinas of our eyes. They respond best to red, green, and blue light. Each receptor is made by a gene. The blue receptor is on an autosome, while the red and green receptors are on the X chromosome (sex-linked). Colorblindness Most colorblind people are males, who have mutated, inactive versions of either the red or the green (sometimes both) color receptors. Most females with a mutant receptor gene are heterozygous: the normal version of the receptor genes gives them normal color vision. Colorblind Test! You will see circles with many colors of dots The dot pattern makes up a number What number do you see? With Color Vision: This one you can even see in black and white Color Blind Test What number do you see? Color Blind Test What number do you see? This what you would see if you were color blind What number do you see? Color Blind Test What number do you see? Color Blind Test What number do you see? Color Blind Test What number do you see? Color Blind Test What number do you see? With color vision you see this: But if you were red-green colorblind…. You would see the #: 5 What do the colorblind see? Types of Colorblindness NORMAL RED YELLOW GREEN CYAN BLUE MAGENTA PROTAN: DEUTERAN: TRITAN: Red Blind Green Blind Blue Blind Types of Colorblindness – Normal Protanopia: no red No color vision Deuteranopia: no green Tritanopia: no blue How to write Alleles for X-Linked Traits Women: Normal: XBXB Carrier: XBXb Colorblind: XbXb Men: Normal: XBY Colorblind: XbY Hemophilia Hemophilia is a disease in which the blood does not clot when exposed to air. People with hemophilia can easily bleed to death from very minor wounds. Hemophilia is another sex-linked trait. Hemophilia is treated by injecting the proper clotting proteins, isolated from the blood of normal people. In the early 1980’s, the blood supply was contaminated by HIV, the AIDS virus, and many hemophiliacs contracted AIDS at that time. Small cuts, scrapes and bruises can be life threatening 1 in 10, 000 males 1 in 100,000,000 females sex-linked recessive Hemophilia Hh XHXhx HH XH Y XH female / eggs male / sperm XH XH Y XH XH XHY XH Xh Xh XH Xh XH Xh carrier Xh Y disease XHY Y Common amongst royalty in Europe Queen Victoria = Carrier Sex-Influenced Traits Some traits appear to be specific to one sex, but are not sex-linked: their genes are not on the X chromosome. It is sex-influenced. The best human example is male pattern baldness. Baldness is dominant in males: heterozygotes and homozygotes both become bald. In females, baldness is recessive: only homozygotes (which are relatively rare) become bald. Also, females tend to lose hair more evenly than men, giving a sparse hair pattern rather than completely baldness. Errors of Meiosis Chromosomal Abnormalities Chromosomal abnormalities Incorrect number of chromosomes nondisjunction chromosomes don’t separate properly during meiosis breakage of chromosomes deletion duplication inversion translocation Nondisjunction Problems with meiotic spindle cause errors in daughter cells homologous chromosomes do not separate properly during Meiosis 1 sister chromatids fail to separate during Meiosis 2 too many or too few chromosomes 2n n-1 n n+1 n Alteration of chromosome number error in Meiosis 1 error in Meiosis 2 all with incorrect number 1/2 with incorrect number Nondisjunction Baby has wrong chromosome number trisomy cells have 3 copies of a chromosome monosomy cells have only 1 copy of a chromosome n+1 n n-1 n trisomy monosomy 2n+1 2n-1 Down syndrome Trisomy 21 3 copies of chromosome 21 1 in 700 children born in U.S. Chromosome 21 is the smallest human chromosome but still severe effects Frequency of Down syndrome correlates with the age of the mother Sex chromosomes abnormalities Human development more tolerant of wrong numbers in sex chromosome But produces a variety of distinct syndromes in humans XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female Klinefelter’s syndrome XXY male one in every 2000 live births have male sex organs, but are sterile feminine characteristics some breast development lack of facial hair tall normal intelligence Klinefelter’s syndrome Jacob’s syndrome male XYY Males 1 in 1000 live male births extra Y chromosome slightly taller than average more active normal intelligence, slight learning disabilities delayed emotional maturity normal sexual development Trisomy X XXX 1 in every 2000 live births produces healthy females Why? Barr bodies all but one X chromosome is inactivated Turner syndrome Monosomy X or X0 1 in every 5000 births varied degree of effects webbed neck short stature sterile Changes in chromosome structure error of replication deletion loss of a chromosomal segment duplication repeat a segment inversion error of crossing over reverses a segment translocation move segment from one chromosome to another A few oddities It is possible to be XY and female. Two ways this can happen: 1. the SRY gene can be inactivated by a mutation. If SRY doesn’t work, testes don’t develop and the embryo develops as a normal female. 2. In a condition called “androgen insensitivity”, the person is XY with a functional SRY gene, but her cells lack the testosterone receptor protein, so the cells don’t ever get the message that the testosterone is sending. Testes develop in the abdominal cavity, and no ovaries, fallopian tubes, or uterus develop. At puberty, the internal testes secrete testosterone, which gets converted into estrogen and the body develops as a normal (but sterile) adult female. Hermaphrodites ?!? Hermaphrodite: An individual that has all female reproductive parts, and all male reproductive parts No such thing in Humans Hermaphrodites In some cases, the cells respond a little bit to testosterone produced by the testes. The embryo develops with ambiguous genitalia, neither completely male not completely female. Another condition, congenital adrenal dysplasia, causes the adrenal glands to produce an abnormally large amount of testosterone in a female embryo, This can also cause development of ambiguous genitalia. Another rare condition: a chimera occurs when two separate embryos fuse together. This can result in a person with some XX cells and some XY cells. This condition is extremely rare: more people say they have it than actually do. Twins 2% of births Monozygotic (Identical) 30% of twins A single zygote splits into two. This happens between 1 to 9 days after the zygote forms. The twins share the same genome Dizygotic (Fraternal) 70% of twins Two separate eggs are fertilized with two separate sperm. Two totally independent zygotes are created. The twins have different genomes Conjoined twins – very rare (1 in 200,000) Identical twins who fail to completely separate after the 13th day after fertilization This may be due to the fusion, or incomplete separation of zygotes May be two fully formed individuals connected at various locations, or rarely, parasitic twins, where one is much smaller and less formed, or even completely contained.
