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Biology
Biology
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14–1 Human Heredity
EQ: How is genetics used to determine characteristics and treat disease?
14-1 Human H
Photo credit: Richard Hutchings/Photo Researchers, Inc.
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Human Chromosomes
Human Chromosomes
Cell biologists analyze chromosomes by looking at karyotypes.
Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs.
A picture of chromosomes arranged in this way is known as a karyotype.
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Human Chromosomes
Human Karyotype
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
How is sex determined?
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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
Human Traits
In order to apply Mendelian genetics to humans, biologists must identify an inherited trait controlled by a single gene.
They must establish that the trait is inherited and not the result of environmental influences.
They have to study how the trait is passed from one generation to the next.
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Human Traits
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
A square represents
a male.
A circle represents
a female.
A vertical line and a bracket connect the parents to their children.
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 circle or square that is not shaded indicates that a person does not express the trait.
This drawing shows what the symbols in a pedigree represent.
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Human Traits
Genes and the Environment 
Some obvious human traits are almost impossible to associate with single genes.
Traits, such as the shape of your eyes or ears, are polygenic, meaning they are controlled by many genes.
Many of your personal traits are only partly governed by genetics.
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Human Genes
Human Genes
The human genome includes tens of thousands of genes.
In 2003, the DNA sequence of the human genome was published.
In a few cases, biologists were able to identify genes that directly control a single human trait such as blood type.
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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, so individuals who are Rh+/Rh+ or Rh+/Rh are said to be Rh-positive.
Individuals with 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.
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Human Genes
Individuals with alleles IA and IB produce both A and B antigens, making them blood type AB.
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Human Genes
The i allele is recessive.
Individuals with alleles IAIA or IAi produce only the A antigen, making them blood type A.
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Human Genes
Individuals with IBIB or IBi alleles are type B.
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Human Genes
Individuals who are homozygous for the i allele (ii) produce no antigen and are said to have blood type O.
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Human Genes
Recessive Alleles
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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Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
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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23. X Chromosome
Duchenne muscular dystrophy
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Melanoma
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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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 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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Human DNA Analysis
Human DNA Analysis
There are roughly 6 billion base pairs in your DNA.
Biologists search the human genome using sequences of DNA bases.
Genetic tests are available for hundreds of disorders.
DNA testing can pinpoint the exact genetic basis of a disorder.
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Gene Therapy
In gene therapy, an absent or faulty gene is replaced by a normal, working gene.
The body can then make the correct protein or enzyme, eliminating the cause of the disorder.
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