Chapter 12
Inheritance Patterns and Human Genetics
Early Work
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In the early 1900’s Thomas Hunt Morgan began studying fruit flies(Drosophila
melanogaster)
He noticed they have 4 pairs of chromosomes. 3 of the pairs were identical in males and
females.
The fourth pair were identical in females, now called X chromosomes.
The fourth pair in males were 1 X and a much shorter one now called Y.
X and Y chromosomes are now known as the sex chromosomes.
Sex Chromosomes and Autosomes
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Sex chromosomes contain genes that determine the sex of an organism.
o They also contain genes that have nothing to do with sex of the organism.
Autosomes contain genes that are not directly related to sex determination.
Sex Determination
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Males determine the sex of the child.
Females have 2 X chromosomes, males have an X and a Y.
There is a 50 % chance a child will be male and 50% chance the child will be female.
In male mammals the Y chromosome contains a gene called SRY(Sex determining Region
Y).
The gene codes for a protein that allows the gonads to develop as testes in an embryo.
Females lack the gene so the gonads develop as ovaries.
Sex Linked Genes and Traits
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These are genes located on either the X or the Y chromosome.
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The X chromosome is larger so there are more X linked traits.
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Because males have only one X chromosome, a male that carries a recessive allele on an
X chromosome will exhibit that trait.
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SRY is a Y linked trait.
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X linked traits that are recessive and not good.
o Colorblindness- can’t distinguish between certain colors
o Hemophilia- lack ability to clot blood
o Muscular dystrophy- weakens and destroys muscle tissue
o Icthyosis simplex- scaly skin
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SRY is a Y linked disorder.
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Genotypes of X linked traits:
o Females
Males
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XHXH-normal
XHY-normal
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XHXh-carrier
XhY-has disorder
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XhXh-has disorder
Linked Genes- pairs of genes that tend to be inherited together
Mutations- a change in the nucleotide base sequence
1. Germ cell mutation- occurs in an organisms gametes and do not affect the
organism itself, but can be passed on to the offspring
2. Somatic cell mutations- take place in an organisms body cells and therefore
affect the organism. Somatic cell mutations can not be inherited. Certain types
of skin cancer and leukemia are examples.
3. Lethal mutations- cause death, often before birth
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Some mutations are beneficial to the organism. They have a better chance at survival
and reproduction, and therefore have an evolutionary advantage.
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Mutations provide the variation upon which natural selection acts.
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Mutations can involve an entire chromosome or a single DNA nucleotide.
Chromosome mutations- involve changes in the structure of a chromosome, or the loss or
gain of a chromosome.
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There are four types of chromosome mutations:
1. Deletion-the loss of a piece of chromosome due to breakage.
2. Inversion- a piece of a chromosome breaks off, flips around backwards, and
reattaches.
3. Translocation- a piece of one chromosome breaks off and reattaches to a
nonhomologous chromosome
4. Nondisjunction- a chromosome fails to separate from its homologue during
meiosis. A gamete receives an extra copy of a chromosome and another
gamete receives no copies.
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Down syndrome is an example of nondisjunction. There are three
copies of chromosome 21.
Gene mutations- the addition, subtraction, or removal of a single nucleotide is a point
mutation, which is the a change that occurs within a single gene .
1. Substitution- one nucleotide replaces another. It can change the amino acid
being used during protein synthesis.
2. Deletion- one or more nucleotides are lost. This can cause an incorrect grouping
of the remaining codons called a frameshift mutation.
3. Insertion mutation- one or more nucleotides are added to a gene which can lead
to a frameshift mutation.