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Transcript
Sex Linked / "X" Linked Genetics Recall: Autosomes = the chromosomes other than the sex chromosomes. Human cells contain 23 pairs of chromosomes. There are 22 pairs of autosomes and 2 sex chromosomes ("X" and "Y") in the nuclei. human karyotype Sex/Gender chromosomes = chromosomes that determine the gender of an organism. In humans, females have two X chromosomes (genotype = XX). In humans, males have one X and one Y chromosome (genotype = XY). The "X" chromosomes is the larger chromosome. The "Y" chromosome is the smaller chromosome. The X chromosome carries hundreds of genes but only a few of these have anything to do directly with gender. Chromosomal Determination of Sex In other organisms there are different methods of determining the gender of an individual. In some reptiles, the temperature of the eggs as they are forming will trigger the gender of the individuals. In humans the father's gamete (Sperm) determines the sex of offspring! X is found in the ovum X or Y is found in the sperm The offspring is either XX or XY Sex Linked / "X" Linked Genetics Genes that are carried by sex chromosomes are said to be sex linked or X-linked traits. In the case of X-linked genetics, the gene is found on chromosome X so we show the X or Y chromosome with a superscript for the allele. The "a" recessive allele will be expressed in his phenotype. We use the symbol Xa. The "a" recessive allele will not be expressed in her phenotype. A We use the symbol X . Are you colour-blind? Normal 8 Red-green deficiencies 3 Red-green deficiencies read 17 Total colour blindness should not read any numeral Ex 1: A man who has normal vision is married to a woman who is a carrier for red-green colour blindness (her father was red-green colour blind). a) What is the probability that their children are red-green colour blind? b) What is the probability that their sons are red-green colour blind? c) What is the probability that their daughters are red-green colour blind? = Normal Vision, = Red-Green colour blindness Genotypes of parents: Father = , Mother = _____ of their children would be red-green colour blind. _____ of their sons would be red-green colour blind. _____ of their daughters would be red-green colour blind. XN Y Xn Facts about X-Linked Traits: - Almost all the genes on the X have no counterpart on the Y chromosome - Fathers can not pass X-linked alleles to their sons. - Father WILL pass X-linked alleles to their daughters. - Mothers can pass sex-linked alleles to both sons and daughters. X-linked recessive traits are primarily expressed in the phenotype of males. (men only have one X chromosome and there are no corresponding gene on the Y chromosome in most cases) X-linked recessive traits in females are often masked in their phenotype by a dominant normal allele on the other chromosome. This explains why women are frequently carriers of X-linked traits but rarely have them expressed in their own phenotypes. Sex Linked Disorders There are about 1100 human X-linked genes. Most of them code for something other than female anatomical traits. Many of the non-sex determining X-linked genes are responsible for abnormal conditions such as ... * hemophilia * red-green colour blindness * congenital night blindness, * duchene muscular dystrophy Other Sex-Linked Genetic Disorders: Sex-chromosome abnormalities may also be caused by nondisjunction of one or more sex chromosomes. Any combination (up to XXXXY) produces maleness. Males with more than one X are usually underdeveloped and sterile. XXX and X women are known, although in most cases they are sterile. Baldness in humans is a dominant, sex-influenced trait. This gene is on the autosomes, not the sex chromosomes, but how it is expressed is influenced by the person’s sex (due to hormones present, etc.). A man who is BB or Bb will be bald and will be non-bald only if he is bb. A woman will only be bald if she is BB and non-bald if she is Bb or bb (it’s almost like baldness is dominant in males and recessive in females). Actually, because of the influence of other sex-related factors, most women who are BB never become totally bald like men do, but rather, their hair becomes “thin” or sparse. Y-linked traits Y-linked traits are controlled by alleles on the Y chromosome. Y-linked traits are known as holandric traits as they are “wholly male”. Do the words homozygous or heterozygous apply to Y-linked traits? NO! Because there is only ever one Y chromosome present at a time! Are females affected by Y-linked traits? NO! Because females don’t carry Y chromosomes! An example of a Y-linked trait is HAIRY EARS! All of the sons of an affected male will display this Y-linked trait. Ex 2: A woman who has X- linked hypophosphatemia, which affects bone development. She marries a man with normal bone structure. What is the probability that the woman and her husband will have a child with the disorder?