Download 7th Grade Science Notes

7th Grade Science Notes
May 13, 2010
Human autosomal (body) traits:
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Humans have a wide variety of phenotypes (how we look) because of the many ways
genes can combine (our genotypes) on our 46 chromosomes.
The first 22 pairs of chromosomes are called “autosomal” chromosomes.
All autosomal traits are either controlled by single genes or multiple genes.
Single gene traits usually have only two versions or alleles, one is dominant and one is
recessive.
Sometimes there are multiple alleles (3 or more versions) for a single gene trait. You
will still only have one spot on your chromosome pair that controls that trait but there
are 3 or more alleles that can fit in that spot. An example is human blood types. There
are 3 alleles for the blood type gene – A, B, or O. A and B are equally dominant but
blood type O is recessive.
Some traits are controlled by more than one gene. Each gene can have many alleles
that interact with the other genes to produce a large number of phenotypes. An
example of this type of trait is skin color, eye color, and height.
Human sex-linked traits:
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Chromosome pair #23, the X and Y, are called the “sex” chromosomes.
Genes that are on these chromosomes are called “sex-linked” genes.
Each male carries an X and a Y chromosome. Each female carries two X chromosomes.
If a disease or abnormality occurs on the X chromosome, it will always be expressed in
the male because they have only one X. It may not be expressed in the female because
they have two X’s and a “good” X will override the “bad” or abnormal X.
If a female has an abnormal gene on one of her X chromosomes, she is called a carrier
of that trait. This is because she can carry or pass it on to her offspring without having
the disease herself.
Two examples of sex-linked disorders are color blindness and hemophilia. Both
disorders are on the X chromosome so females only have the disorder if it is on both of
their X chromosomes. Otherwise, they are carriers.
Males will show colorblindness or hemophilia more often because they only have one X
chromosome.
Pedigrees
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A pedigree is a chart showing the inheritance pattern of a trait through several
generations of a family.
The pedigree of Britain’s Royal family beginning with Queen Victoria helped geneticists
determine that hemophilia was a sex-linked disorder to the X chromosome.
Do “Pedigrees of the Rich and Famous” worksheet in class and go over the symbols.