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Biology
Biology
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14–1 Human Heredity
14-1 Human Heredity
Photo credit: Richard Hutchings/Photo Researchers, Inc.
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Human Chromosomes
A picture of chromosomes arranged in pairs is known as a karyotype.
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Human Chromosomes
These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc.
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Human Chromosomes
Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex.
Females have two copies of an X chromosome.
Males have one X chromosome and one Y chromosome.
The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes.
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Human Chromosomes
All human egg cells carry a single X chromosome (23,X).
Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y).
About half of the zygotes will be 46,XX (female) and half will be 46,XY (male).
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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.
In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male).
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Human Traits
In order to apply Mendelian genetics to humans
identify an inherited trait controlled by a single gene.
establish that the trait is inherited and not the result of environmental influences.
study how the trait is passed from one generation to the next.
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Human Traits
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
This drawing shows what the symbols in a pedigree represent.
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Human Genes
Blood Group Genes
Human blood comes in a variety of genetically determined blood groups.
A number of genes are responsible for human blood groups.
The best known are the ABO blood groups and the Rh blood groups.
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Human Genes
The Rh blood group is determined by a single gene with two alleles—positive and negative.
The positive (Rh+) allele is dominant
Rh+/Rh+ or Rh+/Rh - Rh-positive.
two Rh- alleles are said to be Rh-negative.
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Human Genes
ABO blood group
There are three alleles for this gene, IA, IB, and i.
Alleles IA and IB are codominant.
i is recessive – ii individuals are type O
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Human Genes
The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype.
Many disorders are caused by autosomal recessive alleles.
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Human Genes
This table shows the major symptoms of some well-known genetic disorders.
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Human Genes
Dominant Alleles
The effects of a dominant allele are expressed even when the recessive allele is present.
Two examples of genetic disorders caused by autosomal dominant alleles are achondroplasia and Huntington disease.
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Human Genes
This table shows the major symptoms of some well-known genetic disorders.
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Human Genes
Codominant Alleles
Sickle cell disease is a serious disorder caused by a codominant allele.
Sickle cell is found in about 1 out of 500 African Americans.
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Human Genes
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From Gene to Molecule
From Gene to Molecule
How do small changes in DNA cause genetic disorders?
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From Gene to Molecule
In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder.
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From Gene to Molecule
The most common allele that causes cystic fibrosis is missing 3 DNA bases.
As a result, the amino acid phenylalanine is missing from the CFTR protein.
Cystic fibrosis is usually caused by the deletion of three bases in the DNA of a single gene. As a result, the body does not produce normal CFTR, a protein needed to transport chloride ions. Cystic fibrosis causes serious digestive and respiratory problems.
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From Gene to Molecule
Normal CFTR is a chloride ion channel in cell membranes.
Abnormal CFTR cannot be transported to the cell membrane.
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From Gene to Molecule
The cells in the person’s airways are unable to transport chloride ions.
As a result, the airways become clogged with a thick mucus.
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From Gene to Molecule
Sickle Cell Disease
Sickle cell disease is a common genetic disorder found in African Americans.
It is characterized by the bent and twisted shape of the red blood cells.
These red blood cells contain the abnormal hemoglobin characteristic of sickle cell disease. Photo credit: ©Omikron/Photo Researchers, Inc.
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From Gene to Molecule
There are three phenotypes associated with the sickle cell gene.
An individual with both normal and sickle cell alleles has a different phenotype—resistance to malaria—from someone with only normal alleles.
Sickle cell alleles are thought to be codominant.
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From Gene to Molecule
Malaria and the Sickle Cell Allele
The map on the left shows where malaria is common. The map on the right shows regions where people have the sickle cell allele.
Regions where malaria is common
Regions where the sickle cell allele is common
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