Download Sex-Linked Inheritance

BIOLOGY 1
STANDARD 4.0
LESSON 4.6
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Genetics and Biotechnology
The student will investigate the concepts of genetics and heredity,
different methods of reproduction, patterns of inheritance, and
genetic disorders; as well as, explore and evaluate the DNA
technologies from both a scientific and ethical perspective.
Lesson 9 - Examine modes of inheritance involving sex-linkage
and interpreting pedigree charts
Proceed to the Sex-Linked Genes Tutorial #1 for more information regarding
sex-linked traits. Also visit Sex-Linked Genes Tutorial #2 for genetic problem
illustrations of sex-linked traits
Sex-Linked Inheritance
(Courtesy: Prentice Hall Science Explorer, Cells and Heredity, 2000)
Some human traits occur more often in one sex than the other. The genes for
these traits are often carried on the sex chromosomes. Genes on the X and Y
chromosomes are often called sex-linked genes because their alleles are passed
from parent to child on a sex chromosome. Traits controlled by sex-linked genes
are called sex-linked traits.
Like other genes, sex-linked genes can have dominant and recessive alleles.
Recall that females have two X chromosomes, whereas males have one X
chromosome and one Y chromosome. In females, a dominate allele on one X
chromosome will mask a recessive allele on the other X chromosome. The
situation is not the same in males, however. In males there is no matching allele
on the Y chromosome to mask, or hide, the allele on the X chromosome. As a
result, any allele on the X chromosome - even a recessive allele - will produce
the trait in a male who inherits it. Because males have only one X chromosome,
males are more likely than females to have a sex-linked trait that is controlled by
a recessive allele.
Sex Chromosomes and Linked Genes
A normal human cell (autosome) contains 46 chromosomes. 22 pairs of regular
chromosomes and one pair of sex chromosomes. These sex chromosomes are
designated by the letters ________ and _________. A human male contains an
__________ and a __________, while the female contain two ___________'s.
During the reproductive process there is a fifty - fifty chance that a male or
female will be born. see below:
The X chromosome is the larger of the two. If the Y chromosome is not present
the offspring will not develop male characteristics. In fact the offspring will be a
female suffering from __________________ syndrome (XO).
If a male receives and extra X, it will become a male with
___________________________ syndrome (XXY). If an organisms receives
additional chromosomes, other than the sex chromosomes, a variety of
abnormalities will occur. One example of this is ________________________
Syndrome. Here an extra chromosome 21 is found, allowing the individual to
develop an abnormal condition.
Certain other abnormal conditions may develop in offspring from certain genes
attached to the sex chromosome X. These are called sex-linked genes.
Examples of this are the bleeders disease called _______________________
and __________________________________.
Hemophilia is called the bleeders disease. Here a person could bleed to death if
the bleeding is not stopped. People with this disease inherited it from their
parents. There is no cure for the disease, but an infected person can receive the
chemical that is missing to complete the clotting process.
Phenotypes and Genotypes for blood clotting.
Phenotypes
(male)
Normal
Hemophiliac
Genotypes
(male)
XHY
X hY
Phenotypes
(female)
Normal
Carrier
Hemophiliac
Genotypes
(female)
XHXH
XHXh
XhXh
1. What is a carrier?
________________________________________________________________
2. Is there such a person called a carrier male? _____________. Explain.
________________________
3. Mate a carrier female with a normal male in the space below.
X X
X XX XX
Y XY XY
a). What are the chances of these parents having normal sons?
_______________.
b). What are the chances of these parents having normal daughters?
___________________.
Colorblindness is inherited by the same process. Here the person infected with
the disease cannot distinguish red colors from green colors.
4. Mate a colorblind male with a normal female in the space below.
X X
X XX XX
Y XY XY
a). What are the chances of these parents having normal sons?
_______________.
b). What are the chances of these parents having normal daughters?
___________________
Please use the following links to study sex-linked traits
Colorblindness tutorial
Chromosomes carry genes
Use the following link to understand Pedigree Charts
Mendel's Laws apply to Humans
After reading the first page, click on “animation” at the bottom of the screen.
Use the right arrow to scroll through the animation. When finished answer the
questions under “problems” at the bottom of the page.