Chapter 14- Human Genetics

Copyright Pearson Prentice Hall

Biology Biology


14–1 Human Heredity14-1 Human Heredity


Human Chromosomes

Cell biologists analyze chromosomes by looking at karyotypes.Cells are photographed during mitosis.Scientists then cut out thechromosomes from the photographs and group them together in pairs.


A picture of an individual’s chromosomes, arranged in homologous pairs, is called a karyotype.

HumanKaryotype


Human Chromosomes

Humans have 46 chromosomes. Two are called sex chromosomes, because they determine an individual's sex.•Females have two X chromosomes.

•Males have one X chromosome and one Y chromosome.


Human Chromosomes

The other 44 chromosomes are known as autosomal chromosomes, or autosomes.


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).


Human Chromosomes

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



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.


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 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 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.


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.


Blood Group GenesHuman 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.


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.


Human Genes

ABO blood group•There are three alleles for this gene, IA, IB, and i.

•Alleles IA and IB are codominant.


Human Genes

Individuals with alleles IA and IB produce both A and B antigens, making them blood type AB.


Human Genes

The i allele is recessive. Individuals with alleles IAIA or IAi produce only the A antigen, making them blood type A.


Human Genes

Individuals with IBIB or IBi alleles are type B.


Human Genes

Individuals who are homozygous for the i allele (ii) produce no antigen and are said to have blood type O.



Recessive AllelesThe 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.



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.



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.



Sickle Cell Disease is characterized by the bent and twisted shape of the red blood cells.


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.


There are three phenotypes associated with the sickle cell gene.

Homozygous dominant- normalHeterozygous dominant- healthy with malaria resistanceHomozygous recessive- sickle cell

Sickle cell alleles are considered codominant.


Malaria and the Sickle Cell Allele

Regions where malaria is common

Regions where the sickle cell allele is common

Because the sickle cell allele gives resistance to malaria it persists in areas where malaria is a problem.


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.


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.


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.


From Gene to Molecule

Normal CFTR is a chloride ion channel in cell membranes.Abnormal CFTR cannot be transported to the cell membrane.


The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus.

These diseases are autosomal recessive because you only need one functional gene to make a good protein.FF Ff ff


Makes good

protein

Makes some good protein

and some that doesn’t work

Makes protein that doesn’t

work


14–2 Human Chromosomes14-2 Human Chromosomes


Sex-Linked Genes•The X chromosome and the Y

chromosomes determine sex.

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

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


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

MelanomaX-inactivation center

X-linked severe combined immunodeficiency (SCID)

ColorblindnessHemophilia

Y Chromosome

Testis-determiningfactor


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.


ColorblindnessThree 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.


Possible Inheritance of Colorblindness Allele


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.


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.

Recessive traits from genes on the X-chromosome, like color blindness and hemophilia, are much more common in males.


X-Chromosome Inactivation•British geneticist Mary Lyon discovered that in female cells, one X chromosome is randomly switched off.

•The inactive X 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.


GENETIC DISORDERS caused byNON-DISJUNCTION

•The most common error in meiosis occurs when homologous chromosomes fail to separate.





•This is known as nondisjunction, which means, “not coming apart.”

•nondisjunction causes abnormal numbers of chromosomes


Down Syndrome

•If two 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 21.


Chromosomal DisordersDown 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


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.


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.

The X chromosome contains genes necessary for survival.


14–3 Human Molecular Genetics14-3 Human Molecular Genetics


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.


•DNA fingerprinting analyzes the DNA repeats which make a unique pattern for each individual.

•Only identical twins are genetically identical.

•DNA samples can be obtained from blood, sperm, and hair strands with tissue at the base.


Human DNA Analysis

Chromosomes contain large amounts of DNA called repeats that do not code for proteins. This DNA pattern varies from person to person.

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Restriction enzymes are used to cut the DNA into fragments containing genes and repeats.


DNA fragments are separated using gel electrophoresis.Fragments containing repeats are labeled. This produces a series of bands—the DNA fingerprint.


DNA Fingerprint


In 1990, scientists in the United States and other countries began the Human Genome Project.The Human Genome Project is an ongoing effort to analyze the human DNA sequence.In June 2000, a working copy of the human genome was essentially complete.


Research groups are analyzing the DNA sequence, looking for genes that may provide clues to the basic properties of life.Biotechnology companies are looking for information that may help develop new drugs and treatments for diseases.


A Breakthrough for Everyone•Data from publicly supported research

on the human genome have been posted on the Internet on a daily basis.

•You can read and analyze the latest genome data.


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.


Viruses are often used because of their ability to enter a cell’s DNA.Virus particles are modified so that they cannot cause disease.

Normal hemoglobin gene

Genetically engineered virus


A DNA fragment containing a replacement gene is spliced to viral DNA.

Chromosomes

Bone marrow cell

Nucleus

Genetically engineered virus


The patient is then infected with the modified virus particles, which should carry the gene into cells to correct genetic defects.


14–1

A chromosome that is not a sex chromosome is know as a(an)

a. autosome.

b. karyotype.

c. pedigree.

d. chromatid.


14–1

Whether a human will be a male or a female is determined by which

a. sex chromosome is in the egg cell.

b. autosomes are in the egg cell.

c. sex chromosome is in the sperm cell.

d. autosomes are in the sperm cell.


14–1

Mendelian inheritance in humans is typically studied bya. making inferences from family

pedigrees. b. carrying out carefully controlled

crosses. c. observing the phenotypes of individual

humans. d. observing inheritance patterns in other

animals.


14–1

An individual with a blood type phenotype of O can receive blood from an individual with the phenotype

a. O.

b. A.

c. AB.

d. B.


14–1

The ABO blood group is made up of

a. two alleles.

b. three alleles.

c. identical alleles.

d. dominant alleles.


14–2

The average human gene consists of how many base pairs of DNA?

a. 3000

b. 300

c. 20

d. 30,000


14–2

Which of the following genotypes indicates an individual who is a carrier for colorblindness?

a. XCX

b. XCXc

c. XcY

d. XCY


14–2

Colorblindness is much more common in males than in females because

a. the recessive gene on the male’s single X chromosome is expressed.

b. genes on the Y chromosome make genes on the X chromosome more active.

c. females cannot be colorblind.

d. colorblindness is dominant in males and recessive in females.


14–2

The presence of a dense region in the nucleus of a cell can be used to determine the

a. sex of an individual.

b. blood type of an individual.

c. chromosome number of an individual.

d. genotype of an individual.


14–2

Nondisjunction occurs during

a. meiosis I.

b. mitosis.

c. meiosis II.

d. between meiosis I and II.


14–3

DNA fingerprinting analyzes sections of DNA that have

a. Little or no known function but are identical from one individual to another.

b. little or no known function but vary widely from one individual to another.

c. a function and are identical from one individual to another.


14–3

DNA fingerprinting uses the technique of

a. gene therapy.

b. allele analysis.

c. gel electrophoresis.

d. gene recombination.


14–3

Repeats are areas of DNA that

a. do not code for proteins.

b. code for proteins.

c. are identical from person to person.

d. cause genetic disorders.


14–3

Data from the human genome project is available a. only to those who have sequenced the

DNA. b. to scientists who are able to understand the

data. c. by permission to anyone who wishes to do

research. d. to anyone with Internet access.


14–3

Which statement most accurately describes gene therapy? a. It repairs the defective gene in all cells of the

body. b. It destroys the defective gene in cells where

it exists. c. It replaces absent or defective genes with a

normal gene. d. It promotes DNA repair through the use of

enzymes.

Education

Chapter 14- Human Genetics