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    why are women often carriers of x-linked traits but rarely affected by them?


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    Chromosomes Flashcards

    Study with Quizlet and memorize flashcards terms like Chromosome, Chromosome Function, Sex Linkage and more.



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    are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made up of "Deoxyribonucleic acid" or DNA.

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    Chromosome Function

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    Keeps DNA tightly wrapped so they remain intact, are accurately copied, while being evenly distributed among cells.

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    Biology 1st Edition

    Kenneth R. Miller, Levine

    2,470 explanations

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    Christy C. Hayhoe, Doug Hayhoe, Jeff Major, Maurice DiGiuseppe

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    Terms in this set (40)


    are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made up of "Deoxyribonucleic acid" or DNA.

    Chromosome Function

    Keeps DNA tightly wrapped so they remain intact, are accurately copied, while being evenly distributed among cells.

    Sex Linkage

    is the phenotypic expression of an allele that is dependent on the gender of the individual and is directly tied to the sex chromosomes.


    is a condition in which the ability of the blood to clot is severely reduced, causing the sufferer to bleed severely from even a slight injury. (No clot so bleeds out)

    Why are women often carriers of X linked traits but rarely affected by them?

    Because females have two X chromosomes where either can be active or inactive.

    X-linked recessive inheritance

    is a type of inheritance where the mutation in a gene on the X chromosome causes the phenotype to be expressed in males since they have one X chromosome and women have two X chromosomes.

    Carrier females who have only one copy of the mutation do not usually express the phenotype.

    Why only males for Hemophilia?

    Just one altered copy is enough to cause the condition, since they only have that one X.

    Can females ever get the hemophilia disorder?

    Yes, it is rare but both X chromosomes are affected or one is affected and the other is missing or inactive.


    Is the particular array of chromosomes of an organism.


    Similarity in sequence of a protein or nucleic acid between organisms of the same and different species.

    PAR1 & PAR2 are?

    Pseudoautosomal Regions

    PAR1 is necessary?

    In male meiosis to get proper segregation of the chromosomes.

    Coagulation Cascade

    is a hemophilia mechanism where clotting proteins like VII and IX circulating in the blood are activated on the surface of the platelets to form a mesh-like fibrin clot and these proteins work like dominos in a chain reaction.


    Is a process by which one of the copies of the X chromsome present in female mammals is inactivated. The active X chromosome is silenced because of how it was packaged. The inactive structure is called "heterochromatin"

    Human Mosaicism

    Occurs when there is a presence of two or more populations of cells with different genotypes in one individual, who has developed from a single fertilized egg.

    Mosaicism in males is?

    X-Linked Rett Syndrome

    Is a neurological disorder that occurs in girls and leads to severe impairments in speech, walking, eating, and breathing.

    Why does Rett Syndrome mostly affect girls?

    Girls have two X chromosomes in every cell. If they have Rett syndrome, some of the cells will use the defective gene. Other cells will use the healthy genes which will help to make up for the cells using the defective gene.

    Why does Rett Syndrome not affect boys?

    However, boys only have one X chromosome in every cell. They lack the extra X chromosome that can protect their bodies from being completely overcome by the disorder. Therefore boys with the cell mutation that causes Rett syndrome often die before or shortly after birth.

    Sex determination in Birds:

    Female is ZW (XY) is heterogametic

    Male is ZZ (XX) is homogametic

    They are flipped^

    Sex Determination in Alligators:

    Female if < 34°C Male if > 34°C

    Sex Determination in Grouper Fish:

    Size: biggest female in group becomes male.

    Chromosomal Vs. Genetic Disorder

    Chromosomal Disorders are not passed from one generation to the next, like Genetic disorders, their disorders are caused by changes in their number of chromosomes from disruptions in meiosis.

    Sickle Cell Anemia

    is a disorder of the blood caused by an inherited abnormal hemoglobin. The abnormal hemoglobin causes distorted (sickled) red blood cells. The sickled red blood cells are fragile and prone to rupture.

    Why is the S allele so prevalent in sickle cell & malaria?

    The S allele is required for disease expression.

    Explain the heterozygote advantage with regards to sickle-cell anemia:

    Malaria Parasite causes the RBC to rupture plasmodium so they cannot reproduce.

    Sickle Cell trait resists?


    XXY Characteristics:

    You end up having a problem with weight, reach puberty late, some breast development, taller than the average man and your sterile. No sperm.

    XO Characteristics:

    Source : quizlet.com

    Biological Basis of Heredity: Sex Linked Genes

    A particularly important category of genetic linkage has to do with the X and Y sex chromosomes.  These not only carry the genes that determine male and female traits but also those for some other characteristics as well.  Genes that are carried by either sex chromosome are said to be sex linked.

    Men normally have an X and a Y combination of sex chromosomes, while women have two X's.  Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits.  Men and women can get the X-linked ones since both inherit X chromosomes.

    Sex cell inheritance patterns

    for male and female children

    X-linked recessive traits that are not related to feminine body characteristics are primarily expressed in the observable characteristics, or phenotype , of men.  This is due to the fact that men only have one X chromosome.  Subsequently, genes on that chromosome not coding for gender are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome in most cases.  In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other.  This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes.

    The "a" recessive allele

    will be expressed in his



    The "a" recessive allele

    will not be expressed in

    her phenotype male female

    There are about 1,098 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 , Duchenne muscular dystrophy , fragile-X syndrome , some high blood pressure, congenital night blindness, G6PD deficiency, and the most common human genetic disorder, red-green color blindness.  X-linked genes are also responsible for a common form of baldness referred to as "male pattern baldness".

    If a woman is a carrier of an X-linked recessive

    allele for a disorder and her mate does not have

    it, their boys will have a 50% chance of inheriting

    the disorder.  None of their girls will have it, but

    half of them are likely to be carriers.


    If a man has an X-linked recessive disorder and his

    mate does not carry the allele for it, all of their girls

    will be carriers. None of their boys will inherit the

    harmful allele.  Only girls receive X chromosomes

    from their fathers.

    Queen Victoria of England was a carrier of the gene for hemophilia.  She passed the harmful allele for this X-linked trait on to one of her four sons and at least two of her five daughters.  Her son Leopold had the disease and died at age 30, while her daughters were only carriers.  As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain.  By the early 20th century, ten of Victoria's descendents had hemophilia.  All of them were men, as expected.

    Queen Victoria (1819-1901) with her husband and nine children in 1857

    By comparison to the X chromosome, the much smaller Y chromosome has only about 26 genes and gene families.  Most of the Y chromosome genes are involved with essential cell house-keeping activities (16 genes) and sperm production (9 gene families).  Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits.  When any of the 9 genes involved in sperm production are missing or defective the result is usually very low sperm counts and subsequent infertility.  One in six American couples are infertile.  It is now thought that about 1/3 of these couples is unable to have children as a consequence of the male mate not having the necessary sperm producing genes on his Y chromosome.

    Because the Y chromosome only experiences recombination with the X chromosome at the ends (as a result of crossing-over), the Y chromosome essentially is reproduced via cloning from one generation to the next.  This prevents mutant Y chromosome genes from being eliminated from male genetic lines except by inactivation or deletion.  Subsequently, the Y chromosome now has few active genes and mostly contains genetic junk rather than genes.

    Chimpanzees are our closest living relatives.  They have been on a separate evolutionary path from humans for only 6-7 million years.  Subsequently, we still share most of our genes.  We differ by only 1-2% in terms of DNA sequences.  However, the genes on the Y chromosome are a major exception.  The DNA sequences on Y chromosomes of chimpanzees and humans differ by 30%.  This indicates that Y chromosomes have been evolving at a much faster rate than the X and all other chromosomes.

    Source : www2.palomar.edu

    Why are females always carriers for X

    Actually, some women who carry X-linked recessive conditions do have symptoms, and X-inactivation can play a role in this.

    But before I get into how that happens, let’s look at how X-inactivation works.

    Why does X-inactivation happen?

    Most genetic information is the same for males and females. Usually, humans have 23 pairs of chromosomes in their cells. Of these, 22 pairs are the same between men and women.

    The difference comes in the last pair of chromosomes. These are also called the sex chromosomes.

    Most men are XY: one X chromosome from Mom and one Y chromosome from Dad.

    Most women are XX: one X chromosome inherited from each parent.

    We only need the information from one X-chromosome to survive. After all, men only have one X-chromosome!

    But women have two X-chromosomes. If both of the X-chromosomes were active, there would be an overload of information from the X-chromosome.

    If you think of genetic information like a recipe, you can see how having some instructions doubled could be a problem.

    Imagine you’re making a cake and you repeat a section of the instructions. If you use twice as much flour, your cake probably won’t turn out the way you planned! In the same way, having twice as much information from the X-chromosome could cause a problem.

    X-inactivation solves this problem. Nearly every cell in a woman’s body shuts down one X-chromosome, leaving just one working version.

    How does X-inactivation work?

    Soon after an egg is fertilized, each cell in the new embryo randomly puts one X-chromosome into a dormant state.

    Once a cell puts an X-chromosome out of commission, it sticks to that decision. Every new cell that comes from it will inactivate the same chromosome.

    The inactive X-chromosome is packed into a small ball and shuffled to the side of the cell. The chromosome is squished so tightly that the cell can’t use it at all – it’s completely inactivated.

    Humans don’t have any obvious signs that show which X-chromosome is active in each cell. But in cats, you can actually see where each X-chromosome is active.

    One of the genes for feline fur color is located on the X-chromosome. A female cat with different color genes on each of her X chromosomes will have tortoiseshell fur! These patches of black and orange hair let you actually see where each X chromosome is active.

    The patches on this tortoiseshell cat are caused by X-inactivation. (Image from Wikimedia)

    We expect that around half of a woman’s cells will use one X-chromosome and half will use the other. However, it isn’t always that simple!

    Sometimes we see that nearly all of a woman’s cells have inactivated the same X-chromosome. There are a few different theories for why this could happen:

    More cells may have randomly chosen to turn off the same chromosome.

    Cells might recognize a problem with one X-chromosome and inactivate the damaged one more often.

    Cells using one X-chromosome might not survive as well, causing cells expressing the other to be more common.

    There are other possibilities as well! Researchers are trying to learn more about how X-inactivation works.

    So what does this mean for women who carry X-linked genetic changes?

    In general, women aren’t affected by X-linked conditions. That’s because X-linked conditions are usually caused when you don’t have any working copies of a gene on the X-chromosome.

    Men only have one X-chromosome to begin with. So they only need one non-working (or mutated) gene to show symptoms of an X-linked condition.

    But, women have an extra X-chromosome! Even if a woman has a mutation on one X-chromosome, she’ll probably have a working version on the other chromosome. Usually, enough of a woman’s cells express the working version of the gene to keep her healthy.

    However, there are some women who have symptoms of X-linked conditions. Often women who have symptoms are more mildly affected than their male relatives. That’s because they have the working copy of X-chromosome helping out.

    But it is possible for women to be nearly as severely affected as males. Women who have more severe symptoms may simply have more cells that inactivated the healthy X-chromosome.

    It can be difficult for female carriers who want to know if they will eventually develop symptoms. Doctors may consider testing some of a woman’s cells to see which X-chromosome is more commonly inactivated.

    However, this doesn’t always predict which women will develop symptoms. There are different patterns of X-inactivation throughout the body and we can’t test every cell!

    Whether or not a female carrier has symptoms, she can pass the non-working X-chromosome on to children. People who think they could be at risk for having (or carrying) an X-linked condition should consider speaking to a doctor or genetic counselor. To learn more about genetic counseling or to find a genetic counselor in your area, visit http://nsgc.org.

    Source : www.thetech.org

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