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How Does Gene Linkage Affect Inheritance?

  • If genes are located on the same chromosome, they are said to be linked.

  • In Mendelian genetics, alleles assort independently during meiosis, but linked genes do not.

    • Gene linkage can be demonstrated by using a test cross.

    • Results that differ from those expected in gene linkage can be explained by crossing over.

  • The frequency of crossing over has been used to construct linkage maps.

  • Genes that are on the same chromosome as those that determine sex are said to be sex-linked.

    • The chromosomal basis for sex determination is relatively simple in most organisms.

    • Color blindness is an example of a sex-linked gene in humans.

    • In mammals, one X chromosome of females is inactivated during development.

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What Disorders Are Caused By Gene Defects and Chromosome Abnormalities?

  • Genetic disorders can be caused by defective alleles; some examples in humans are as follows.

    • Hemophilia is a good example of an X-linked recessive disorder.

    • Cystic fibrosis is a serious genetic disorder caused by an autosomal recessive allele.

    • Sickle-cell disease results from a defective gene with incomplete dominance.

    • Many human abnormalities, such as polydactyly are caused by dominant alleles.

  • Abnormalities in chromosome number can occur due to faulty meiotic divisions.

    • Aneuploidy of autosomal chromosomes can cause problems such as Down syndrome.

    • Aneuploidy of the sex chromosomes may alter sexual development.

    • Polyploidy is fairly common in the plant kingdom, but is rare in animals.

  • Alterations in chromosome structure can also cause problems.

  • Genetic testing can detect many genetic defects before birth.
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