1. Standard 3—Genetics
Mendel’s Law 1

2. 3. A multi-cellular organism develops from a single zygote, and its phenotype depends on its genotype, which is established at fertilization. 2

3. 3a. Know how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance (autosomal or X-linked, dominant or recessive. 3

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Sex-Linked Genes
Sex-Linked Genes
The ___ chromosome and the __ chromosomes determine sex.
Genes located on these chromosomes are called ________________ genes.
More than ____ sex-linked genetic disorders have now been mapped to the X chromosome.
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Sex-Linked Genes
X Chromosome
Duchenne muscular dystrophy
Melanoma
The __ chromosome is much smaller than the __ chromosome and appears to contain only a few genes.
X-inactivation center
X-linked severe combined immunodeficiency (SCID)‏
Colorblindness
Hemophilia
Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor
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Sex-Linked Genes
Why are sex-linked disorders more common in males than in females?
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Sex-Linked Genes
For a __________ allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just ______ X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
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Sex-Linked Genes
Colorblindness
Three human genes associated with color vision are located on the ______ chromosome.
In males, a defective version of any one of these genes produces ________________.
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Sex-Linked Genes
Possible Inheritance of Colorblindness Allele
X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles.
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Sex-Linked Genes
Hemophilia
The ___ ______________ also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia.
In ______________, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
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Sex-Linked Genes
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is a _______________ disorder that results in the weakening and loss of skeletal muscle.
It is caused by a defective version of the gene that codes for a muscle protein.
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12. X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in female cells, one ___ chromosome is randomly switched off.
This chromosome forms a dense region in the nucleus known as a Barr body.
Barr bodies are generally not found in males because their single ___ chromosome is still active.
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13. Chromosomal Disorders
What problems does nondisjunction cause?
The most common error in meiosis occurs when homologous chromosomes fail to separate.
This is known as ________________, which means, “not coming apart.”
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14. Chromosomal Disorders
Nondisjunction
Homologous chromosomes fail to separate.
Meiosis I:
Nondisjunction
Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome.
Meiosis II
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15. Chromosomal Disorders
Down Syndrome
If ______ copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome.
Down syndrome involves three copies of chromosome _____.
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16. Chromosomal Disorders
Down Syndrome Karyotype
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake
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17. Chromosomal Disorders
Sex Chromosome Disorders
In females, nondisjunction can lead to Turner’s syndrome.
A female with Turner’s syndrome usually inherits only one _____ chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
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18. Chromosomal Disorders
In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,_________).
The extra X chromosome interferes with meiosis and usually prevents these individuals from _______________.
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