Name 2 Genetic Diseases.

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14-1 Human Heredity

14–1 Human Heredity

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Human ChromosomesCell biologists analyze chromosomes by

looking at karyotypes: a picture of chromosomes.

A karyotype is made by:

1. Photographing cells during mitosis.

2. Cutting out chromosomes

3. Grouping them together in pairs.

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Human Chromosomes

Humans have 46 chromosomes.

2 Sex chromosomes determine an individual's sex (X and Y).

X= Female

Y = Male

44 chromosomes autosomal

chromosomes.

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Human Chromosomes

Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis.

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Pedigree Charts 

A pedigree chart shows the relationships within a family.

Genetic counselors analyze pedigree charts to infer the genotypes of family members.

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Human Traits

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A circle representsa female.

A horizontal line connecting a male and a female represents a marriage.

A shaded circle or square indicates that a person expresses the trait.

A square representsa male.

A vertical line and a bracket connect the parents to their children.

A circle or square that is not shaded indicates that a person does not express the trait.

Genes and the Environment 

Most human traits are impossible to associate with 1 gene.

Traits, such as the shape of your eyes or ears, are polygenic.

Personal traits are only partly governed by genetics. The rest is environment.

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Human Genes

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Human Genes

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Human Genes

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Sickle Cell is found in 1 in 500 African Americans.

People who are heterozygous for the sickle cell allele are generally healthy and resistant to malaria.

Malaria and the Sickle Cell Allele

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Regions where malaria is common

Regions where the sickle cell allele is common

14-2 Human Chromosomes

14–2 Human Chromosomes

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Sex-Linked Genes

•Genes located on these chromosomes are called sex-linked genes.

•More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. 

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Sex-Linked Genes• The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.

Duchenne muscular dystrophy

X Chromosome

Melanoma

X-inactivation center

X-linked severe combined immunodeficiency (SCID)

Colorblindness

Hemophilia

Y Chromosome

Testis-determiningfactor

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Sex-Linked Genes

• For a recessive 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 one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.

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Sex-Linked GenesAn example: Colorblindness• Three human genes associated with color vision are located on the X chromosome. 

• In males, a defective version of any one of these genes produces colorblindness.

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Sex-Linked Genes• Possible Inheritance of Colorblindness Allele

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Sex-Linked Genes

An example: Hemophilia• The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia. 

• In hemophilia, 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 GenesAn Example: Duchenne Muscular Dystrophy

• Duchenne muscular dystrophy is a sex-linked 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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X-Chromosome Inactivation

• X-Chromosome Inactivation• British geneticist Mary Lyon discovered that in female cells, one X 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 X chromosome is still active.

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Chromosomal Disorders

• Nondisjunction: the most common error in meiosis occurs when homologous chromosomes fail to separate.

• If nondisjunction occurs, abnormal numbers of chromosomes find their way into gametes, and a disorder of chromosome numbers may result.

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Chromosomal Disorders• Nondisjunction Homologous

chromosomes fail to

separate.

Meiosis I:Nondisjunction

Meiosis II

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Chromosomal Disorders•An Example: Down Syndrome

• If two copies of a chromosome fail to separate during meiosis, an individual may be born with 3 copies of a chromosome. 

•Down syndrome involves three copies of chromosome 21.

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Chromosomal Disorders• Down syndrome produces mild to severe mental retardation.

• It is characterized by:• increased susceptibility to many diseases 

• higher frequency of some birth defects

Down Syndrome Karyotype

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Chromosomal DisordersAn Example: Sex Chromosome Disorders

• In females, nondisjunction can lead to Turner’s syndrome. 

• A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).

•Women with Turner’s syndrome are sterile.

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Chromosomal Disorders

• In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY). 

• The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.