Sex-Linked Genes And Disorders

Introduction

Human somatic cells, including zygotes, have _________ pairs of chromosomes (total). The first 22 paris of these are called _______________. The last pair are called ___________ chromosomes.

Human females (symbol = _______) are written as: ____________. Human males (symbol = _______) are written as: __________.

Gametes produced by females carry only the ______ sex chromosome. Gametes produced by males carry either the _______ or _______ sex chromosome. This is in addition to __________ (#) other autosomes.


Sex-Linked Disorders

Recall that non-_____________________ disorders can occur in which gametes receive more or less than the proper number of sex chromosomes (or autosomes). A _____________________ chart can often be used to identify these disorders before a baby is born. Complete the table below:

Sex Chromosomes
Phenotype
XX
 
XY
 
XXX
 
XO
 
XXY
 
XYY
 
YO
 

The sex chromosomes contain genes (stretches of __________) which code for some traits such as production of female sex hormones (_______________), male sex hormones (__________________), etc. They also carry genetic disorders such as _________________ (blood fails to clot properly), colour blindness, high blood pressure (______________________), muscular dystrophy, etc.

Because a female has 2 X chromosomes, she has 2 sex-linked genes (one on each X chromosome). But a male can only have 1 sex-linked gene because the Y chromosome does not carry a copy of the genes located on the X chromosome. Therefore, males often suffer from more sex-linked disorders. Also, sex-linked disorders are usually caused by _______________ (not dominant) genes.


Example: Hemophilia (recessive sex-linked disorder)

Normal female = _____________Carrier female = ______________
Normal male = ______________Male with hemophilia = _____________

Because the male only has sex-linked genes on the ________ chromosome, there is nothing on the ______ chromosome to mask the disorder on the _______ chromosome. So if a male has even 1 copy of the disordered gene, he suffers from the disorder. A female can carry 1 disordered gene on one _______ chromosome but still have a normal gene to mask it on the other ________ chromosome. A "carrier female" usually doesn't suffer from the disorder, she just carries it and can pass it on .... usually to her SONS as you'll soon see!


How Disorders Can Skip a Generation

You've all heard how some disorders can skip a generation and then show up. Here's how it works!


The Dash Method

You can use the dash method to figure out someone's genotype if you know the genotypes of the offspring.


Dominant Sex-Linked Disorders

So what happens if a sex-linked disorder is caused by a dominant gene instead of a recessive gene? The presence of a single dominant gene on the X chromosome causes the disorder. Let's "see" if you can do the problem below...


Try my "Drag-and-Drop Genetics" and "Java Genetics" problems. Go to the zeroBio, Gr. 11 Biology page under Unit 2 for the Web links.