Transcript Ch. 14 - The Human Genome

The Human Genome
Chapter 14
Human Heredity
14-1
NOVA: Cracking the Code of Life

Chapter 1: Instructions for Making a
Human Being
Human Chromosomes
A karyotype is
a picture of
chromosomes from a
cell arranged in
homologous pairs.
Human Chromosomes

Humans have 46 chromosomes.
◦ 44 autosomes
◦ 2 sex chromosomes
 Normal female: 46 XX
 Normal male: 46 XY
 Sperm:
23 X or 23 Y
 Egg:
23 X
Human Chromosomes
Human Chromosomes

What determines the sex of a child?
 The sperm……
Human Traits

To study the inheritance of human traits
genetic counselors use a pedigree
chart.
 From this, geneticists can infer genotypes of family
members.
Human Traits

Draw a pedigree chart.
Human Traits

Associating an observed human trait with
a gene is difficult.
 Many human traits are polygenic.
 The environment influences the expression of a
trait.
Human Genes

Humans have 4 blood types caused by 3
alleles.
◦ A & B – codominant
◦ O – recessive
 Types: A, B, AB, O

Blood types must be matched for a safe
transfusion.
 Draw blood type chart…………………..
Human Genes

Rh factors represent another group of
antigens found on some red blood cells.
◦ Rh+
◦ Rh-
= present.
= absent.
Human Genes

Most genetic disorders are caused by
recessive alleles.
 To have the disorder an individual must inherit two
recessive alleles.
 Ex: PKU, Tay-Sachs, Cystic Fibrosis, Albinism
Figure 14-8 The Cause of Cystic Fibrosis
Cystic Fibrosis
Chromosome #
7
CFTR
gene
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.
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.
Human Genes

A few are caused by dominant alleles.
 Only one allele needs to be inherited to be
expressed.
 Ex: Huntington’s disease, Dwarfism
Human Genes

Sickle cell anemia is caused by a
codominant allele.
 One normal allele and one abnormal allele are
inherited for making the protein hemoglobin.
 The sickle cell allele is inherited among many African
Americans.
 Carrying this allele produces resistance to the Malaria
parasite.
Sickle Cell
Anemia
NOVA: Cracking the Code of Life

Chapter 2: Getting the Letters Out
Human Chromosomes
14-2
Human Genes and Chromosomes

Only 2% of your DNA functions as genes.
Human Genes and Chromosomes

Genes located on the same chromosome
are linked.
◦ They tend to be inherited together.
◦ They can be separated by crossing-over
during meiosis.
Crossing-Over
Sex-Linked Genes
Sex-linked genes are located on the sex
chromosomes.
 More genes are on the X than the Y.

Sex-Linked Genes

Genes on the X chromosome are always
expressed in males even if recessive.
◦ Males get sex-linked disorders more often
than females.
 Colorblindness, hemophilia, muscular dystrophy
X-Chromosome Inactivation

In females, one X chromosome is
randomly switched off.
◦ Calico cats are always female.
X-Chromosome Inactivation

The switched off X chromosome
becomes a dense region in the nucleus
known as a Barr body.
 Barr bodies aren’t found in males.
Olympics and Barr Bodies

Sex testing was introduced in competitive
sports in the mid-1960s, amid rumor that
some competitors in women's events
were not truly female - especially two
Soviet sisters who won gold medals at the
1960 and 1964 Olympics, and who
abruptly retired when gender verification
testing began.
Olympics and Barr Bodies

The first tests, at the European
Championships in 1966 and the PanAmerican Games in 1967, required female
competitors to undress before a panel of
doctors. Other methods used during this
period included manual examination or
close-up scrutiny of the athlete's genital
region.
Olympics and Barr Bodies

When athletes complained that these tests were
degrading, the IOC at the Mexico City Olympics
in 1968 introduced genetic testing in the form of
a sex chromatin (Barr body) analysis of cells from
a buccal smear. The procedure was further
modified at the Barcelona games, using the
polymerase chain reaction to amplify
the DNA extracted from a specimen to allow
detection of a Y chromosome gene, SRY, that
codes for male determination.
 Journal of the American Medical Association, July 17, 1996, vol. 276, no.
3, pp. 177-178
Chromosomal Disorders

Caused by nondisjunction during
meiosis.
◦ Homologous chromosomes fail to separate
resulting in sex cells with one too many or
one too few chromosomes.
Nondisjunction
Section 14-2
Non disjunction
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Chromosomal Disorders
Down’s syndrome = 47,XX or 47,XY
 Turner’s syndrome = 45,X
 Klinefelter’s syndrome = 47,XXY

Down’s Syndrome
Turner’s Syndrome
Kleinefelter’s Syndrome
Human Molecular Genetics
14-3
Human DNA Analysis
Even though the human genome is over 6
billion bases long, genetic tests exist for
detecting the presence of defective
recessive genes.
 Prospective parents can now be tested to
determine if they carry recessive alleles
for hundreds of disorders.
 Is this a good idea?

DNA Fingerprinting
All humans share the same genes.
 The 98% of DNA that is not encoded in
genes is what makes us different from
each other.
 DNA fingerprinting makes use of this
difference to identify individuals.

DNA Fingerprinting4-18 DNA
Fingerprinting
Restriction enzyme
Chromosomes contain large
amounts of DNA called repeats
that do not code for proteins.
This DNA varies from person to
person. Here, one sample has 12
repeats between genes A and B,
while the second sample has 9
repeats.
Restriction enzymes are used to
cut the DNA into fragments
containing genes and repeats.
Note that the repeat fragments
from these two samples are of
different lengths.
The DNA fragments are separated
according to size using gel
electrophoresis. The fragments
containing repeats are then labeled
using radioactive probes. This
produces a series of bands—the
DNA fingerprint.
The Human Genome Project

In 2000, the entire human genome was
sequenced.
◦ Dr. Francis Collins, Dr. Eric Lander and Dr.
Craig Venter (NOVA – Cracking the Code of
Life)

Advances in technology allowed this to
happen in several years rather than the
predicted 20.
Rapid Sequencing
Gene
Sequence
Promoter
Start
signal
Gene
Stop
signal
Gene Therapy

The process of changing a gene that
causes a disorder.
◦ An absent or defective gene is replaced by a
normal functioning gene.

Viruses are often used to deliver
functional genes to affected cells.
Gene Therapy
Normal hemoglobin gene
Bone
marrow cell
Nucleus
Chromosomes
Genetically engineered virus
Bone
marrow
Ethical Issues…
Genetic cures
 Designer babies
 Medical costs
 Research costs
