sex linked / x linked genetics - wikispaces - genetics sex...genes that are carried by either sex...
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Autosomes = the chromosomes other than the sex chromosomes.
Each member of an autosome pair (in diploid organisms) is of similar length and in the genes it carries.
Human cells contain 23 pairs of chromosomes.There are 22 pairs of autosomes and 2 sex chromosomes ("X" and "Y") in the nuclei.
karyotype = picture of the set of chromosomes for an individual
Sex Linked / "X" Linked Genetics
Autosomal Dominant Inheritance Autosomal Recessive Inheritance
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Sex Linked / "X" Linked GeneticsSex/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.
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Sex Linked / "X" Linked Genetics
Only men inherit Y chromosomes so they are the only ones to inherit Ylinked traits. Both men and women can get the Xlinked traits since both inherit X chromosomes.
It is the father's gamete (sperm) that determines the gender of the offspring.The sperm donates either an X allele or a Y allele while the egg donates an X allele.
Gametes are the sex cells (egg or sperm) that carry only one allele.
A gamete is a "haploid" (alleles in singles) set of genes for an egg or a sperm.
X is found in the ovum X or Y is found in the sperm
The offspring is either XX or XY
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Sex Linked / "X" Linked Genetics
Genes that are carried by either sex chromosome are said to be sex linked or Xlinked traits.
In the case of Xlinked 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 not be expressed in her phenotype.
We use the symbol XA.
The "a" recessive allele will be expressed in his phenotype.
We use the symbol Xa.
Sex Linked / "X" Linked Genetics
The X allele in the normal form is symbolized by ...
an "XN" when the normal form of the gene is dominant
an "Xn" when the normal form of the gene is recessive
Genotypes of parents:
Father = XNY Mother = XNXN
Father = XnY Mother = XNXn
Mother = XnXn
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Sex Linked / "X" Linked Genetics
There are about 1100 human Xlinked genes.
Most of them code for something other than female anatomical traits.
Many of the nonsex determining Xlinked genes are responsible for abnormal conditions such as ...
* hemophilia * redgreen colour blindness * congenital night blindness, * duchene muscular dystrophy
Are you colourblind?
Normal 8Redgreen deficiencies 3
Redgreen deficiencies read 17Total colour blindness should not read any numeral
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Facts about XLinked Traits: Almost all the genes on the X have no counterpart on the Y chromosome Fathers can not pass Xlinked alleles to their sons. Father WILL pass Xlinked alleles to their daughters. Mothers can pass sexlinked alleles to both sons and daughters. If a sex linked trait is due to a recessive allele: * A female will express the phenotype only if she is homozygous recessive. * A male with one recessive allele from his mother will express the phenotype.
The inheritance of these genes on the X chromosome follows special rules because males have only a single X chromosome.
Xlinked recessive traits are primarily expressed in the phenotype of males. This is due to the fact that men only have one X chromosome and there are no corresponding gene on the Y chromosome in most cases.
Xlinked 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 Xlinked traits but rarely have them expressed in their own phenotypes.
Colour Vision:
Colour vision is due to a gene found on the X chromosome (not on the Y chromosome).Having one bad copy of this gene in a male (XY) guarantees a colour blind phenotype.
Should a heterozygous females for colourblindness be able to see colour?
Suppose: XN= colour vision and Xn= redgreen colour blind vision
The retina of a heterozygous (XNXn) female will have some cells with the XN allele inactivated and other cells with the Xn allele inactivated.
A heterozygous carrier of redgreen colour blindness has some colour blind cells in her retina. The noncolor blind cells enable her to see colour.
Sex Linked / "X" Linked Genetics
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Sample Genetics Problem:A man who has normal vision is married to a woman who is a carrier for redgreen colour blindness (her father was redgreen colour blind). a) What is the probability that their children are redgreen colour blind?b) What is the probability that their sons are redgreen colour blind?c) What is the probability that their daughters are redgreen colour blind?
= Normal Vision, = RedGreen colour blindness
Phenotypes of parents: Father = , Mother =
_____ of their children would be redgreen colour blind.
_____ of their sons would be redgreen colour blind.
_____ of their daughters would be redgreen colour blind.
XN XnY
Sex Linked / "X" Linked GeneticsSexLinked Genetic Disorders:
Sexchromosome 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.
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Baldness in humans is a dominant, sexinfluenced 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 nonbald only if he is bb. A woman will only be bald if she is BB and nonbald 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 sexrelated factors, most women who are BB never become totally bald like men do, but rather, their hair becomes “thin” or sparse.
If two parents are heterozygous for baldness, what are the chances of their children being bald? Use a Punnett square to illustrate this.
Note: because the sex of a person does make a difference in how the gene is expressed, you need to set this up as a dihybrid cross to account for the sex of the children.
Ylinked traits
Ylinked traits are controlled by alleles on the Y chromosome.
Ylinked traits are known as holandric traits as they are “wholly male”.
Do the words homozygous or heterozygous apply to Ylinked traits?
NO! Because there is only ever one Y chromosome present at a time!
Are females affected by Ylinked traits?
NO! Because females don’t carry Y chromosomes!
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An example of a Ylinked trait is HAIRY EARS!
All of the sons of an affected male will display this Ylinked trait.
Practice Problem
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?