Sex-Linked Traits

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Karyotype Studies (not being tested on) Karyotype—micrograph in which the pairs of homologous chromosomes are arranged in decreasing size Images of chromosomes stained during metaphase Chromosomes are arranged in decreasing size to produce a micrograph

Telomeres Telomere caps consist of DNA associated with proteins Serves a protective function for the structure of the chromosome They might also be involved in aging and cancer

Nondisjunction (not being tested on) Cell division during which sister chromatids fail to separate properly If this occurs during Meiosis I or II then the resulting gametes will not have the correct number of chromosomes Down syndrome: nondisjunction occurred on chromosome 21. The result is short stature, heart defects, and mental disability

Chromosome Inactivation Barr bodies: the inactivated X chromosome in females One X chromosome is randomly inactivated in an early embryonic cell Example: Tortoise shell cats The gene for coat color is on the X chromosome. When the cat is heterozygous in coat color (black/orange) it is born with patches of different colored fur on its body.

Sex-Linked Traits Traits are controlled by genes located on the sex chromosomes Humans: 22 homologous pairs of matching chromosomes called autosomes, and one pair of sex chromosomes (XX or XY) There are genes found on the X chromosome that are not found on the Y chromosome Inheritance of traits determined by these genes is therefore linked to the sex of the individual

X-linked Traits Genes located on the X chromosome that control traits Since males have only 1 X chromosome they are affected by recessive X-linked traits Females are less likely to express a recessive trait because she has two X chromosomes. One of the X chromosomes can mask the affect of the trait.

Sex Determination Sex chromosomes determine an individual’s gender Autosomes are all the other chromosomes that determine everything else. (i.e. Body cells) The Y chromosome determines the sex of the organism XX=Female; XY=Male

When completing crosses… When writing genotypes: Put the trait as a superscript to the sex chromosome XRXR; XRY Capital letter represents a dominant allele and lowercase letter represents a recessive allele

Examples (to name a few) Red-Green Color Blindness Hemophilia

Ex: Hemophilia Recessive sex-linked disorder characterized by delayed clotting of the blood. Very rare in females because she would need to have both X chromosomes with the recessive trait.

Types of Genotypes XHXH (female with normal blood clotting) XHXh (female with normal blood clotting, but is a carrier of the trait) XhXh (female with hemophilia) XHY (male with normal blood clotting) XhY (male with hemophilia)

Ex: Red-Green Color Blindness This is a recessive X-linked trait. A male only needs one copy of this allele in order to be colorblind A female would need to have two copies of the recessive allele. Thus it is very rare to find a color blind female