Sex-Linked Traits

  1. Humans have 46 chromosomes in most of their cells.
  2. Sex cells contain 23 pairs of chromosomes.
  3. Inherit half (23) chromosomes from our mother and 23 chromosomes from our father.
  4. Some traits cover up other traits. Having freckles is dominant (F) and not having freckles is recessive (f).
  5. Chromosomes appear in pairs.
  6. Mother has freckles (FF - homozygous) or (Ff - heterozygous). Father does not have freckles (ff).
  7. Punnett square is used to represent all the possible combination of offspring that could be produced by those parents.
  f f
F Ff Ff
F Ff Ff

  1. One pair of chromosomes (the X and the Y) is called the sex chromosomes because they determine the sex of the individual.

  2. Females have two X chromosomes. Males have one X and one Y chromosome.

  3. The X-chromosome is much longer than the Y-chromosome. It contains a number of genes that are not carried on the Y-chromosome.

  4. Genes on the X-chromosome but not the Y-chromosome are called sex-linked genes. (Note: there are some genes located only on the Y-chromosome but for the purposes of this class, we will not include them in our definition of sex-linked genes.)

  5. Colorblindness, Duchene muscular dystrophy, and hemophilia are examples of sex-linked traits in humans. (All are sex-linked recessive traits.)

  6. N = normal vision; n = colorblindness.

  7. Female genotype can be XNXN, XNXn, XnXn. Male genotype can be XNY, XnY

  8. If a woman has normal vision (not a carrier) marries a man who is colorblind, what is the probability they will have a child that is colorblind?

  Xn Y
XN XNXn XNY
XN XNXn XNY
  1. If a woman has normal vision (is a carrier for colorblindness) marries a man who has normal vision, what is the probability they will have a child that is colorblind? What would be the sex of those children?
  XN Y
XN XNXN XNY
Xn XNXn XnY

None of the daughters will be colorblind. Each son has a fifty-fifty chance of being colorblind.

  1. If a woman that is colorblind marries a man who has normal vision, what is the probability they will have a child that is colorblind? What would be the sex of the those children?
  XN Y
Xn XNXn XnY
Xn XNXn XnY

All of the sons will be colorblind.

 

 

 

 

 

 

 

Sex-Linked Trait Problems

  1. If a woman has normal vision (not a carrier) marries a man who is colorblind, what is the probability they will have a child that is colorblind?

     
     
     
  1. If a woman has normal vision (is a carrier for colorblindness) marries a man who has normal vision, what is the probability they will have a child that is colorblind? What would be the sex of those children?
     
     
     
  1. If a woman that is colorblind marries a man who has normal vision, what is the probability they will have a child that is colorblind? What would be the sex of the those children?