GN 301 Module 5: Sex Linkage

Sex Ratios: Numerical values and when they are measured

  • Primary Sex Ratio: the ratio of males to females at conception
    • Based on numbers of XY to XX individuals since we cannot see male/female characteristics at conception
    • 120 males conceived to 100 females (1.2 : 1)
  • Secondary Sex Ratio: the ratio of males to females at birth
    • 105 males to 100 females
    • 106:100 Caucasian Americans
    • 103:100 African Americans
    • 105:100 world-wide
  • Tertiary Sex Ratio: the ratio of male to female any time after birth
    • Age 20: 100 males to 100 females (even)
    • Age 50: 87 males to 100 females
    • Age 87: 50 males to 100 females
    • Age 100: 20 males to 100 females

Sex Chromosomes

  • Importance of “Highly Conserved”

Highly conserved sex chromosomes are important because they’ve changed very little over many years and have conserved a lot of the same properties.

  • Which is more highly conserved – X or Y?

The X chromosome is highly conserved from an evolutionary point of view, meaning they have a similar if not the same function amongst species

When we have conserved sequences, they must be exactly right in order to maintain their function.

Definitions
  • Holandric: Genes on the Y chromosome that are passed down directly from father to son.
  • Hemizygous: An individual only has 1 copy of a genetic locus.

Example: Hemophilia

Hemophilia: An X-linked recessive disorder where clotting factor VIII is defective.

  • Genotypes and Phenotypes:
    • Female: HH, Hh, or hh. Only hh is hemophilia.
    • Male: HY or hY. Males have a greater chance of getting hemophilia because they only have one X chromosome, while females have 2 to balance it out.
  • A woman with hemophilia marries a man without hemophilia. What proportion of their children will have hemophilia?

All of their sons will have hemophilia and none of their daughters.

hh
HHhHh
YhYhY

The Y chromosome signals a male, who only inherits the recessive h. Meanwhile, the daughters will inherit the dominant H and not get hemophilia.

Sample Problem:

The shaded trait is on the X chromosome. Did Dad pass the affected trait to his son?

Example: Red-Green Color Blindness (an X-linked trait)

  • Which allele is dominant? How do you know?:
    • Being not color blind is dominant. We know this because the color blind allele skips a generation but presents itself in the grandson of someone who was colorblind. If being colorblind were dominant, this wouldn’t be possible.

Holandric Inheritance example: Hypertrichosis of the Ears (Y-linked)

 4 generation pedigree. Generation 1: Shaded Male (I-1) married to unshaded female (I-2). Union produces three children: II-2, II-3 and II-5. Generation 2: Unshaded female (II-1) married to II-2 (shaded male). Union produces 2 children III-1 (shaded male) and III-2 (unshaded male). II-3 (shaded male) married to II-4 (unshaded female). Union produces two children: III-3 and III-4 both are shaded males. III-4 mararies unshaded female (III-5) and union produces 1 child, IV-1 (shaded male)

Pedigrees where more than one type of inheritance is possible:

Sometimes more than one mode of inheritance can be possible for a shaded trait in a pedigree. To figure out which mode(s) are possible, you need to see if you can put genotypes on the pedigree that are consistent with each mode of inheritance.

 Which method(s) of inheritance are consistent with the inheritance pattern of the shaded trait? (Circle all that apply)a. X-linked dominant

It is not Y-linked because the trait is presented in a female. It is also not dominant because neither of her parents had the trait. It is also not X-linked because the male sibling doesn’t have the trait. Therefore, it must be autosomal recessive. \n

 Which method(s) of inheritance are consistent with the inheritance pattern of the shaded trait? (Circle all that apply)a. X-linked dominant

It is not a holandric trait because it was not presented in the father, so the mother must have given it to her son. It is not a dominant trait either because it is not presented in his parents. Therefore, it must be X-linked recessive or autosomal recessive.

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