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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
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
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
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 circle represents
a female.
A square represents
a male.
A horizontal line connecting a male and a female represents a marriage.
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.
A shaded circle or square indicates that a person expresses 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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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
Cystic Fibrosis
Cystic fibrosis is caused by a recessive allele.
Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways.
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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
Hemoglobin is the protein in red blood cells that carries oxygen.
In the sickle cell allele, just one DNA base is changed.
As a result, the abnormal hemoglobin is less soluble than normal hemoglobin.
Low oxygen levels cause some red blood cells to become sickle shaped.
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From Gene to Molecule
People who are heterozygous for the sickle cell allele are generally healthy and they are resistant to malaria.
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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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14–1
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14–1
A chromosome that is not a sex chromosome is know as a(an)
autosome.
karyotype.
pedigree.
chromatid.
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14–1
Whether a human will be a male or a female is determined by which
sex chromosome is in the egg cell.
autosomes are in the egg cell.
sex chromosome is in the sperm cell.
autosomes are in the sperm cell.
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14–1
Mendelian inheritance in humans is typically studied by
making inferences from family pedigrees.
carrying out carefully controlled crosses.
observing the phenotypes of individual humans.
observing inheritance patterns in other animals.
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14–1
An individual with a blood type phenotype of O can receive blood from an individual with the phenotype O. A.
AB. B.
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14–1
The ABO blood group is made up of
two alleles.
three alleles.
identical alleles.
dominant alleles.
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43. END OF SECTION