Chapter 12
Human Genetics
Modern Study of Genetics
• 1882: Flemming observed threadlike
chromosomes in the nuclei of dividing cells
• 1887: Weismann suggested that meiosis
halves the number of chromosomes when
gametes are made
– 1900 Mendel work was finally appreciated
especially his view that diploid cells have two
units for the trait
– units segregate during gamete formation
Chromosomes and Inheritance
• Genes are units of information about
inheritable traits
• Diploid organisms possess pair of
homologous chromosomes, which are
alike in length, shape, and gene sequence
• Alleles are slightly different molecular
forms of the same gene, which are suffled
during meiosis
Continue…
• Crossing over between chromosomes
results in genetic recombination
• Independent assortment: refers to the
random alignment of each pair of
homologous chromosomes at metaphaseI
of meiosis , which results in new
combinations of genes in offspring
• Chromosome’s structure may change
during mitosis or meiosis
Section 12.2
• You will have questions on your test – be
sure you go back and look at that section
Sex Determination
• Each human egg will contain 22 autosome
plus one X but sperm will carry 22
autosomes plus either an X or Y
– X bearing Egg plus X bearing sperm produces
female offspring
– X bearing egg plus Y bearing sperm produces
male offspring
Continue…
• X Chromosomes obviously codes for
sexual traits, but it also carries many
genes for nonsexual traits
– Y chromosomes carries a male determining
(SRY) gene which leads to formation of the
testes
– Absence of the male gene in females results
in formation of ovaries
Linked genes
• Linked genes on specific chromosomes
are referred to as linkage group
• Thomas Hunt Morgan: fruit flies
experiment confirmed the location on a
chromosome
– Some of the most intriguing linkage are those
of X-linked and Y-linked genes
• Analysis lead to the result of linkage
groups and gene locations
Human Genetics
• Is very difficult to study
– We live in very diverse population and
variable conditions
– Mate by chance and may or may not choose
to reproduce
– Small family size is not sufficient for
meaningful statistical analysis
Pedigrees
• A pedigree is a chart that shows genetic
connections among individuals
• Provides data on inheritance patterns
through several generations
• Knowledge of probability and Mendelian
inheritance patterns is used in analysis of
pedigrees to yield clues to a trait genetic
basis
Human Genetic Disorder
• Genetic abnormality- is a condition that is
deviation from the usual, or average and is
not life threatening
• Genetic disorder- is more appropriately
used to describe conditions that cuase
medical problems
• Genetic disease- applied to those
instances where a person’s genes
increase susceptibility to infection or
weakens the response to it.
Autosome
• any chromosome other than the X & Y
(humans have 22 pairs of autosome + 1
pair of sex chromosomes)
Autosomal Recessive Inheritance
• Characteristics of this condition are:
– Either parent can carry the recessive allele on an
autosome
– Heterozygotes are symptom-free – Homozygous are
affected
– 2 Heterozygous parents
• 50% producing a heterozygous child
• 25% homozygous recessive
• Galactosemia (inability to metabolize lactose)
– Single gene mutations prevents the manufacturing of
an enzyme needed in the conversion pathyway
Autosomal Recessive
Examples of Autosomal Recessive
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Tay - Sachs Disease
More common among Jewish
populations
Young children begin showing signs
of slowed development
Severe impairment and death
Cystic Fibrosis
More common in Caucasion
Mucus in respiratory tract, difficulty
breathing
extreme salty sweat
Mucus may cause secondary
infections
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Phenylketonuria (PKU)
Lack enzyme for normal
metabolism
Phenylalanine builds up and
causes brain damage
Newborns are routinely tested
Changes in diet lead to normal life
Sickle Cell Disease
More common in Africans
(African-Americans)
Causes blood to be sickle shaped
Affects oxygen flow to organs,
causing weakness, pain, anemia,
etc
Heterozygotes are resistant to
malaria
Autosomal Dominant Inheritiance
• The dominant allele is nearly always
expressed and if it reduces the chance of
surviving or reproducing, its frequency
should decrease; nonreproductive effects,
and postreproductive.
• If a parent is heterozygous and other
parent is homozgyous recessive there is a
50% chance that any child will be
heterozygous
Autosomal Dominant
Examples
• Neurofibromatosis
• Tumors form under skin
and
• Can cause skeletal
deformities, blindness
• Huntington Disease
• Neurological disorder,
progressive degeneration
of the brain
• Symptoms appear later in
life
• Achondroplasia
• Common form of
Dwarfism
• Short arms and legs,
normal torso
• Homozygotes (AA) do not
survive
Examples
• Huntington Disease: serious degeneration
of the nervous system with an onset from
age 40 onward, which time the gene has
usually been passed on to offspring
• Achondroplasia (dwarfism) Benign
abnormality that does not affect the person
reproductively
X-linked Recessive Inheritance
• Characteristics of this condition are
– Mutated gene only on the X chromosome
– Males: are either normal or afflicted
– Female: can be normal, carrier, and afflicted
• Examples:
– Hemophilia A having the inability to clot blood
– Fragile X syndrome defective X chromosomes
that produces a faulty protein that results in
retarded brain development
Continue…
• Some genes are located on the X chromosome.
Females receive two alleles for these genes, but
males only receive one.
• When doing a punnet square, use large X's and
Y's to denote male and female, use superscript
letters to designate the alleles
• If you are trying to determine if an allele is sexlinked, and which is dominant, a reciprocal cross
is performed White eyed male x red eyed female
| Red eyed male x white eyed female
• XrY x XRXR | XRY x XrXr
Human Sex-Linked Disorder
• Colorblindness
• Red-green colorblindness makes it difficult for the
person to discern the two colors (test yourself!)
• If the parent is a male, the genotype is automatically
known. A colorblind male has to be b, since he only
has one allele and colorblindness is recessive. A
normal male must then be B
• Females can be heterozygous for the colorblindness
trait - they are called carriers. A female can be BB normal, Bb - carrier, or bb - colorblind
• The following shows a cross between a normal man
and a woman who is a carrier.
Continue…
• Muscular Dystrophy
• characterized by the
wasting away of muscles
• life expectancy: ~20 yrs
• females can be carriers
for the disease, where
they pass the disease to
their sons only
• Hemophilia
• also known as
"bleeder's disease"
• blood does not clot
properly
• disease was present
in the royal family,
starting with Queen
Victoria
Continue…
• Fragile X Syndrome
• caused by triplet repeats in a gene on the X
chromosome
• causes mental retardation
• named because the X chromosome had an odd
appearance - the tip of the chromosome seemed
to be attached only by a small thread
• the number of repeats of the gene determines
the severity of the disease
Problems:
• Problems:
1. What is the chance that a woman with
hemophilia will have a child with hemophilia?
(What sex would the child need to be?)
• 2. In a cross where a brown-haired female is
crossed with a black-haired male, all the male
offspring have brown hair and all the female
offspring have black hair. What must be the
genotypes of the parents? Which allele is
dominant?
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Continue Problems….
• 3. In drosophila (fruit fly), the allele for eye
color is located on the X chromosome,
where red eyes is dominant to white eyes.
Show the cross of a heterozygous redeyed female and a white eyed male.
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4. A man who is colorblind marries a
woman whose father is colorblind. What is
the chance that they will have colorblind
children?
Progeria
• Read Focus on Health
• A genetic disorder in which a individual
rapidly ages
Karyotype A picture of a person's chromosomes,
arranged by size and grouped into homologous pairs.
Chromosomes Changes
• Duplication: when a gene sequence is in
excess of the normal amount; apparently
this true region that code for polypeptides
of hemoglobin and is not harmful
Chromosomes Changes
• Inversion: alters the position and
sequence of the genes so that gene order
is reversed
Chromosomes Changes
• Translocation: occurs when a part of one
chromosome transferred to a
nonhomlogous chromosome
– Leukemia in which segment number 9 is
attached to chromosome 22
Chromosomes Changes
• Deletion: is loss of a chromosome
segment as when a terminal segment is
lost or when viruses, chemical or
irradiation cause breaks in a chromosome
region Example: Cri-du-chat (deletion of
chromosome of 5
Changes in Chromosome Number
• Aneuploidy is a condition in which the
gametes or cells of an affected individual
end up with one extra or one less
chromosome than is normal
Continue..
• Polyploidy is the presence of three or
more of each type of chromosome in
gametes or cells
– Common in plants but fatal in humans
• A chromosome number can change during
mitotic or meiotic cell division or during the
fertilization process
• Tetraploid germ cells can result if
cytoplasmic division does not follow
normal DNA replication or mitosis
Nondisjunction
• At anaphase I or anaphase II frequently
results in a change in chromosome
number
– If gamete with an extra chromosome (n+1)
joins a normal gamete at fertilization the
diploid cell will be 2n+1, this condition is
called trisomy
– If an abnormal gamete is missing a
chromosome, the zygote will be 2n -1
monosomy
Down Syndrome
• Down syndrome results from trisomy 21
– 1 in 1,100 liveborns in North America are
affected
– Mental Retardation
– Heart Defects
– Frequently occur with women over the age of
35
Changes in Sex Chromosomes
Females
• Turner Syndrome
– Whose cells have only one X chromosomes
(XO)
– Infertile
– Shorter life expectancy
– Premature aging
– 75% of the cases are due to nondisjunction in
the father
Changes in Sex Chromosomes
Females
• XXX Condition
– Females inherits 3, 4, or 5 chromosomes
– Girls are taller and slimmer than average
– Fertile
– Fall in the normal range of appearance and
social behavior
Changes in Sex Chromosomes
Males
• Klinefelter Syndrome
– Nondisjunction results of an extra X
chromosome (XXY)
– 67% results from a nondisjunction in the
mother
– Sterile
– Slight Mental Retardation
– Body Feminization
Changes in Sex Chromosomes
Males
• XYY
– Extra Y chromosome in these males
– Fertile
– Taller than average
– Slightly mental retarded
– Erroneous correlation have linked these
persons with predisposition to crime