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General Biology II: Hemophilia: a sex-linked disorder

General Biology II
Hemophilia: a sex-linked disorder
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Reference Information
  6. The Process of Science
  7. 3. Biological Molecules
  8. 4. Structure of DNA
  9. 5. DNA Replication
  10. 6. Protein Synthesis
    1. 6.1 What are proteins and what do they do?
    2. 6.2 What is a gene?
    3. 6.3 How do genes direct the production of proteins?
    4. 6.4 Transcription: from DNA to mRNA
    5. 6.5 Eukaryotic RNA Processing
    6. 6.6 Translation
    7. 6.7 The Genetic Code
    8. Optional Section - Micropigs
  11. 7. Mutations
    1. How Gene Mutations Occur
    2. Intro to Genetic Disorders
    3. Do all gene affect health and development?
    4. Types of Mutations
    5. Changes in Numbers of Genes
    6. Changes in Chromosome Number
    7. Complex Multifactorial Disorders
    8. Genetic Predispositions
    9. Genetics and Statistics
  12. Gene Regulation
    1. 8.1 Prokaryotic versus Eukaryotic Gene Expression
    2. 8.2 What is the epigenome?
    3. 8.3 Alternative RNA splicing
  13. 9. Biotechnology
    1. 9.1 Manipulating Genetic Material
    2. 9.2 Cloning
    3. 9.3 Genetic Engineering
    4. 9.4 Biotechnology in Medicine and Agriculture
    5. 9.5 Genomics and Proteomics
    6. 9.6 Applying Genomics
    7. 9.7 Proteomics
  14. 10. Cell Division - Binary Fission and Mitosis
    1. 10.1 Prokaryotic Cell Division
    2. 10.2 Eukaryotic Cell Division
    3. 10.3 Control of the Cell Cycle
    4. 10.4 Cancer and the Cell Cycle
  15. 11. Meiosis
    1. 11.1 Sexual Reproduction
    2. 11.2 Overview of Meiosis
    3. 11.3 Interphase
    4. 11.4 Meiosis I
    5. 11.5 Meiosis II
    6. 11.6 Comparing Meiosis and Mitosis
    7. 11.7 Errors in Meiosis
  16. 12. Patterns of Inheritance
    1. 12.1 Mendelian Genetics
    2. 12.2 Garden Pea Characteristics Revealed the Basics of Heredity
    3. 12.3 Phenotypes and Genotypes
    4. 12.4 Monohybrid Cross and the Punnett Square
    5. 12.5 Laws of Inheritance
    6. 12.6 Extensions of the Laws of Inheritance
    7. 12.7 Multiple Alleles
    8. 12.8 Sex-Linked Traits
    9. 12.9 Linked Genes Violate the Law of Independent Assortment
    10. 12.10 Epistasis
  17. Genetics: Dog Coat Color
    1. Introduction to Genetics
    2. Pedigrees and Punnett Squares
    3. Black fur color: a dominant trait
    4. Yellow fur color: a recessive trait
    5. Epistasis: the relationship between black, brown, and yellow fur
    6. Brindle color: partial dominance and epistasis
    7. Incomplete dominance: when traits blend
    8. White spotting: When there's more than two alleles
    9. Hemophilia: a sex-linked disorder
    10. Overall phenotypes: putting it all together
    11. Additional complexity
    12. It's not all in the genes

Hemophilia: a sex-linked disorder

So far, all the genes we have discussed have had two copies present in all individuals. This is because the individual inherited one from the male parent’s haploid gamete and one from the female parent’s haploid gamete. The two gametes came together during fertilization to produce a diploid individual. There is, however, one exception to this: genes which are present on the sex chromosomes.

In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes – one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked.

The X chromosome is one of two sex chromosomes. Humans and most mammals have two sex chromosomes, the X and Y. Females have two X chromosomes in their cells, while males have X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring since the female can only give an X chromosome to the offspring.

The X chromosome is one of two sex chromosomes. Humans and most mammals have two sex chromosomes, the X and Y. Females have two X chromosomes in their cells, while males have X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring.
Figure 24: A diagram showing the autosomal and sex chromosomes. Remember that in a diploid cell, there would be two copies of each autosomal chromosome present. (Credit: Darryl Lega, NHGRI)

Most sex-linked genes are present on the X chromosome simply because it is much larger than the Y chromosome. The X chromosome spans about 155 million DNA base pairs and represents approximately 5 percent of the total DNA in cells. The X chromosome likely contains 800 to 900 genes. In contrast, the Y chromosome has approximately 59 million base pairs and only 50-60 genes. Sex is determined by the SRY gene, which is located on the Y chromosome and is responsible for the development of a fetus into a male. This means that the presence of a Y chromosome is what causes a fetus to develop as male. Other genes on the Y chromosome are important for male fertility.

Hemophilia is a bleeding disorder that slows the blood clotting process. People with this condition experience prolonged bleeding or oozing following an injury, surgery, or having a tooth pulled. In severe cases of hemophilia, continuous bleeding occurs after minor trauma or even in the absence of injury (spontaneous bleeding). Serious complications can result from bleeding into the joints, muscles, brain, or other internal organs. Milder forms of hemophilia do not necessarily involve spontaneous bleeding, and the condition may not become apparent until abnormal bleeding occurs following surgery or a serious injury.

The major types of this condition are hemophilia A (also known as classic hemophilia or factor VIII deficiency) and hemophilia B (also known as Christmas disease or factor IX deficiency). Although the two types have very similar signs and symptoms, they are caused by mutations in different genes.

Hemophilia A and hemophilia B are inherited in an X-linked recessive pattern. The genes associated with these conditions are located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, it is very rare for females to have hemophilia. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

12.xlinkrecessive
Figure 25: X-linked recessive inheritance. (Credit: U.S. National Library of Medicine)
12.xlinkedpunnett
Figure 25: If a carrier female and a normal male produce offspring, there is a 25% total chance that they will have a child with hemophilia. None of their daughters will have the disease (although all will be carriers). Half their sons will be hemophiliacs.

In X-linked recessive inheritance, a female with one altered copy of the gene in each cell is called a carrier. Carrier females have about half the usual amount of coagulation factor VIII or coagulation factor IX, which is generally enough for normal blood clotting. However, about 10 percent of carrier females have less than half the normal amount of one of these coagulation factors; these individuals are at risk for abnormal bleeding, particularly after an injury, surgery, or tooth extraction.

Colorblindness is another example of a sex-linked trait in humans. The genes that produce the photopigments necessary for color vision are located on the X chromosome. If one of these genes is not functional because it contains a harmful mutation, the individual will be colorblind. Men are much more likely than women to be colorblind: up to 100 times more men than women have various types of colorblindness (http://www.colour-blindness.com/general/prevalence/).

12.Ishihara_compare_1
Figure 26: A test image for color-blindness as seen by someone with normal color vision and several types of colorblindness. (Credit: Sakurambo)

References:

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:zLLYW2hj@5/Extensions-of-the-Laws-of-Inhe

“X chromosome” by Genetics Home Reference: Your Guide to Understanding Genetic Conditions, National Institutes of Health: U.S> National Library of Medicine is in the Public Domain

“Y chromosome” by Genetics Home Reference: Your Guide to Understanding Genetic Conditions, National Institutes of Health: U.S> National Library of Medicine is in the Public Domain

“Hemophilia” by Genetics Home Reference: Your Guide to Understanding Genetic Conditions, National Institutes of Health: U.S> National Library of Medicine is in the Public Domain

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Copyright © 2016 by Lisa Bartee and Christine Anderson. Mt Hood Community College Biology 102 by Lisa Bartee and Christine Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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