Transcript Biology~Chapter 12

Inheritance Patterns
& Human Genetics
Biology~Chapter 12
I. Chromosomes & Inheritance
A. Thomas Hunt Morgan- early 1900’s
-experiments with fruit flies
-observed 4 pairs of chromosomes
-noticed that 3 pairs were the same in males
& females but that 1 pair was different.
- called these “sex chromosomes”.
B. Autosomes & Sex Chromosomes
• Sex chromosomes- contain genes that
determine the gender of an individual.
• Autosomes- the remaining pairs of
chromosomes that do not directly determine
sex.
• In mammals
- 2 X chromosomes= FEMALE (XX)
- 1 X + 1 Y is a MALE (XY)
C. Males
determine the sex
of the offspring
-each sperm has an equal chance of having
an X or a Y
-however- the only option for eggs is to
receive an X chromosome.
1:1 male to female ratio
II. Linked & Sex Linked
Genes
–are pairs of genes that tend to
be inherited together.
A. Linked genes
• Genes which are close together
on same chromosome.
• Linked genes do not exhibit
Mendel’s law of independent
assortment
B. Sex-Linked traits
• are traits that are coded for by alleles
on a sex chromosome.
• Genes found on the X chromosome are X-linked
genes
• Since the X chromosome is larger- there are more
X-linked than Y- linked traits.
NOTE: Since males have only 1 X- a male who carries
the recessive allele will show the X-linked trait.
III. Mutations
A. Mutation -a change in the nucleotide-base
sequence of a gene or DNA
1. Germ cell- occurs in gametes
- do not affect the organism itself
2. Somatic Cell-occur in organism’s body cells
- may affect the organism (ex-cancer)
- NOT inherited
3. Lethal Mutations-cause death, often before birth
4. Beneficial Mutations- result in phenotypes that are
beneficial .
B. Types of Chromosome Mutations
1. Deletion- loss of a piece of chromosome due to
breakage.
2. Inversion – a chromosome piece breaks off, flips
around backwards & reattaches
3. Translocation – a piece of chromosome breaks off
& attaches to a non-homologous chromosome
4. Non-Disjunction – a chromosomes fails to separate
from its homologue during meiosis.
Note human disorders that follow on next few slides:
Examples of Chromosomal
Mutations: Deletion & Inversions
http://staff.jccc.net/PDECELL/evolution/mutations/mutation.html
An example of a human disorder
with a deletion error:
• Cri du chat syndrome– missing part of chromosome 5– cry of infants is similar to that of a meowing
kitten, due to problems with the larynx and
nervous system.
– About 1/3 of children lose the cry by age 2.
Types of non disjunction:
• Trisomy- an extra chromosome leads to an individual
with an extra chromosome in every cell of his/her body.
• Monosomy- is a deficiency in number of chromosomes
and is defined as only one copy of a chromosome that is normally
present in two copies. When fertilized, the outcome is 45
chromosomes in total. Monosomies are less likely to survive when
compared to trisomies.
What causes non-disjunction?
• The cause of non-disjunction is unknown. Nondisjunction seems to be a chance event. Nothing
that an individual does or doesn't do during their
reproductive years can cause these chromosomal
changes. We do know that non-disjunction
occurs more frequently in the eggs of women as
they get older.
Non- Disjunction Disorders with
an extra chromosome
(these folks have 47 instead of the
normal 46 chromosomes in people):
• Down syndrome- extra 21
• Edward’s Syndrome- extra 18
• Patau syndrome – extra 13
Examples Non disjunction:
• Extra #21= Down’s
Syndrome
• Extra #18= (most don’t live
beyond 1st 7 months)
This girl is now 6.)
http://starbulletin.com/96/03/25/news/story2.html
Sex-chromosome abnormalities may
also be caused by non-disjunction.
• Klinefelters Syndrome- XXY ( extra X)
• Turners Syndrome – only 1 X- missing a second
X (XO)
• Super males XYY
• Any combination (up to XXXXY) produces maleness.
Males with more than one X are usually underdeveloped
and sterile.
• XXX and XO women are usually sterile
C. Types of Gene Mutation
1. Point Mutation- the substitution, addition or
removal of a single nucleotide, occurs within
a single gene or segment of DNA
2. Substitiution- one nucleotide replaces
another
3. Frameshift Mutation- if some nucleotides
are deleted- whole segment moves
4. Insertion Mutation – one or more
nucleotides added- also causes a frameshift
• Which type of gene mutation do
you think would cause the most
serious errors?
– Point mutation
– Substitution
– Frameshift
Example:
substitution error
• Just 1 Amino Acid is
a Substitution
ERROR in Sickle
Cell Hemoglobin
• Causes Hb to be
sickle shaped instead
of round- can’t fit
into red blood cells &
changes their shape
too.
Sickle Cell - Disease or Trait:
• Normally, a person inherits two genes (one from
each parent) that produce normal hemoglobin
(hemoglobin A).
• A person with sickle cell trait inherits one
normal beta-globin gene (hemoglobin A) and one
defective gene (hemoglobin S)
(this can actually be a good thing – it provides
some protection from malaria, which infects
RBCs!)
• A person with sickle cell disease inherits 2 bad
copies- only makes HgS
IV. Human Genetics
A. Inheritance of Traits
1. Pedigrees - be able to draw & read these!
2. Patterns- we learn about genetic disease by looking
at patterns of inheritance over several generations.
B. Genetic Traits & Disorders
1.
2.
3.
4.
5.
6.
7.
Polygenic Inheritance
Complex Characters
Multiple Alleles
Incomplete Dominance
X-linked Traits
Sex- Influenced Traits
Single Allele Traits
Inheritance of Traits
•
Pedigree - a diagram that
shows how a trait is inherited over
several generations
• - see diagrams page 241 textbook
Reading a pedigree
• males are represented by squares
• females by circles.
HE
SHE
• An individual who exhibits the trait ,
– (for example, someone who suffers from Marfan syndrome),
is represented by a filled symbol.
• A horizontal line between two symbols is a mating
DAD
MOM
Reading a Pedigree
• The offspring:
– are connected to each other by a horizontal
line above the symbols
– and to the parents by vertical lines.
Example pedigree:
•
•
•
•
•
Is the mom or dad in generation I affected by a trait?
How many offspring are shown in generation II?
How many daughters & sons in generation II?
How many have the trait?
How many offspring does daughter #1 have? How
many have the trait?
Dominant traits
• are passed on to a son or daughter EVEN if
only one parent has it.
• Every affected individual has at least one
affected parent
• Affected individuals who mate with unaffected
individuals have a 50% chance of transmitting
trait to each child
• Two affected
parents may have
unaffected children.
• Recessive pedigrees:
• Where did the trait seen in generation III come from?
• Traits can be passed on to children if both parents,
even if they may seem "normal”, are
recessive trait.
carriers of the
Recessive traits
• are passed on to children from both parents,
although the parents may seem perfectly "normal."
• Characteristics of recessive pedigrees are:
– An individual who is affected may have parents who are
not affected;
– All the children of two affected individuals are affected;
– In pedigrees involving rare traits, the unaffected parents of
an affected individual may be related to each other.
Recessive pedigree:
If individuals 2 + 3 in generation 3 have more childrenwill they be affected?
Can Individual #8 in gen. 3 have any unaffected
children?
Can individual #9 in gen 3 have affected children?
Example pedigree of cystic fibrosis
www.cfscreening.com.au/.../CF/CFInherited.shtml
B. Genetic Traits & Disorders
1. Polygenic Inheritance
- traits influenced by several genes
(most human characteristics)
example- skin color is additive effect of 6
genes
B. Genetic Traits & Disorders
2. Complex Characters
• traits influenced by both genes & environment
• Example- height
B. Genetic Traits & Disorders
3. Multiple Alleles• genes with 3
or more alleles.
– Example- human blood type.
IA, IB, i
• 3 alleles- both IA & IB are dominate over i,
Codes for sugars that are on blood cell surface.
• four different blood typesA, B, AB, O
• What are the 2 genotypes for “type B?
• Which blood type can donate to all the others?
(universal donor?)
• Which blood type makes antibodies (will clot against)
both A & B blood types?
B. Genetic Traits & Disorders
4. Incomplete Dominance
• there is an intermediate phenotype
• Human example- wavy hair is the
intermediate between straight & curly hair
(remember chapter 9 plant example:
- pink 4 o’clock flowers,
In between red & white flowers)
B. Genetic Traits & Disorders
5. X-linked Traits
• Traits that are on the X chromosome.
• Since males only have one X
chromosome- they are affected more
than females
• Females have 2 chances to get a good
copy of the gene but males only get 1
chance.
Examples of X-linked traits
• Hemophilia- blood clotting disorderboys may bleed to death
• Duchene muscular dystrophy -affects
cardiac and skeletal muscle, as well as
some mental functions.
• Fragile-X syndrome- part of X
chromosome has errors- most common
cause of mental retardation in males
• Red-green color blindness- males
cannot see these 2 colors.
B. Genetic Traits & Disorders
6. Sex- Influenced Traits
• males & females show different phenotypes
with same genotype• example- male pattern baldness
7. Single Allele Traits
• trait controlled by 1 allele- Dominant or Recessive
Dominant Examples:
• Achondroplastic dwarfism- dwarf size
• Polydactly is the presence of a sixth digit.
• Huntington's disease (also called Woody
Guthrie's disease)-progressive destruction of brain
cells after age 30
• Marfans Syndrome- progressive connective
tissue disorder. (some cases are new mutations with
no family history)
Single allele recessive traits:
• Albinism lack of pigmentation in skin, hair, and eyes, Homozygous recessive (aa) people make no pigments
• PKU (Phenylketonuria) - lack enzyme to use the
amino acid phenylalanine, buildups breakdown
products leads to mental retardation. 1 in 15,000 infants
has- treat with DIET! (no phenylalanine)
• Tay-Sachs Disease- degeneration of the nervous
system. Children rarely survive past five years of age.
• Maple Syrup Urine Disease (MSUD) –like PKU
amino acids leucine, isoleucine, and valine
More single allele recessive examples:
• Sickel Cell Anemia- single amino acid
substitution in hemoglobin causes lack of
oxygen to body
• Cystic fibrosis- body creates sticky thick
mucus – chloride ions- detected in SWEAT
TEST. - was once considered a fatal
childhood disease- modern treatments can
increase lifespan to 30 years plus.
C. Detecting Genetic Disease
1. Testing
• Amniocetesis- Dr removes some amniotic fluid,
14th-18th week of pregnancy, analyze fluid for proteins
& look at chromosomes- some risk of pregnancy loss
• Chorionic Villi Sampling- clip cells from placental
area- 8-14th week pregnancy
2. Treatments- vary with disease
List of Human genetic disorders- Pick 1 for your project.
•
•
•
•
Huntington’s Disease
Down’s Syndrome
Cystic Fibrosis
Duchenne muscular
dystrophy
•
•
•
•
•
•
Sickle Cell Anemia
Trisomy 18 (Edwards)
Tay-Sachs disease
Color blindness
PKU (Phenylketonuria)
Fragile X syndrome
Thalassemia
Marfan syndrome
Breast Cancer
Hemophilia
Cri du Chat
Polydactyly
Maple Syrup urine disease
Turner Syndrome
Klinefelter's syndrome,
(XXY syndrome)
Super males (XYY)
Patau syndrome
Albinism
•See me about other topics