Transcript 3-HumanGen Linkage

Human
Genetics
Part 3
11.3 HUMAN GENETICS
• Linkage:
Genes that are on the same
chromosome are said to be
LINKED ,
• and the general phenomenon
in which genes occur on the
same chromosome is called
LINKAGE.
• Genes may be linked other than just on the sex
chromosomes. Linked genes do not assort
INDEPENDENTLY, and do not result in the
predicted Mendelian ratios for two or more
genes .
• Linkage was first observed by WILLIAM
BATESON and RC PUNNETT in the early
1900s in pea plants.
• Bateson and Punnett crossed true breeding
plants with purple flowers and long pollen
grains (genotype PPLL) to red flowers and
round pollen grains ( ppll).
• The F1 plants (PpLl) were then crossed
among each other.
• Mendel’s law would predict a 9:3:3:1
ratio for an independently assorting,
dihybrid cross, The F2 generation did
not adhere to this ratio:
• Batson and Punnett noticed that
– two types of F2 peas ( P_L_ ) and (ppll) were more
abundant
– and the other two types of peas ( P_ll ) and (ppL_)
were less abundant.
• Bateson and Punnett hypothesized that
the more prevalent allele combinations
could reflect genes that are transmitted
on the same chromosome and do not
separate during meiosis.
Crossing-Over:
• Thomas H. Morgan noticed
that sometimes not all alleles in
a linkage group were inherited
together.
• When he crossed pairs of flies he
did not get the expected results.
• He could explained that his
outcomes were a result of
crossing over- the exchange of
alleles between homologous
chromosomes.
LINKAGE MAPS
• The correlation between crossover
frequency and the distance between
genes is used to construct LINKAGE
MAPS (Chromosome Maps).
• Crossing-over occurs more often between
some alleles than between others.
– Since most alleles occupy a fixed place on a
chromosome.
– The smaller the distance
between two alleles, the greater
the likelihood that these alleles
will not be separated by crossing
over.
– Data from crossing over was
used by Alfred Sturtevant to
construct a LINKAGE
MAP(chromosome map).
– This was a diagram of allele
positions on a particular
chromosome.
• To construct a chromosome map researchers
compare FREQUENCY of crossover for three
characteristics such as eye color, body color and
wing shape.
Two alleles that are
separated by crossing
over 1 percent of the
time is only 1MAP UNIT
apart.
– That means that two
alleles that are separated
by crossing over 10
percent of the time is
10 MAP UNITS apart.
Studying Humans
• Scientist cannot use the same techniques used to
study organisms such as Drosophila to study
human inheritance.
• There are three tools that geneticists use in studies
human genetics.
• 1. Population Sampling: the
technique of selecting a small
number of individuals to represent
larger population.
• 2.Twin Studies: The study of twins
(especially twins raise in different
environments) to distinguish
between genetic and environmental
influences.
• What is the difference between
identical and fraternal twins? How do
Siamese or conjoined twins occur?
(FYI, not in notes!)
• 3. Pedigree Studies: Geneticists use
diagrams to trace a family record.
– These diagrams often reveals a
CARRIER, someone who is heterozygous
for a trait.
– These types of flow charts reveal those
individuals that are carriers.
– Families are the primary tool of the human
geneticist and the larger the family the better.
– The more children in a generation , the easier
it is to discern patterns or modes of
inheritance.
– Family relationships and phenotypes are
displayed in a standard chart called a
PEDIGREE. A pedigree is built of shapes
connected by lines.
First Pedigree
• In 1845 a genealogist named Pliny
Earle constructed a pedigree of a family
with colorblindness, using MUSICAL
NOTATION– half notes for unaffected females,
– quarter notes for colorblind females,
– and filled-in and squared-off notes to
represent the many colorblind males.
A Historical Pedigree:
• This pedigree
is of Egypt’s
Ptolemy
dynasty and
shows only
genealogy, not
traits.
It appears almost ladderlike
because of the extensive inbreeding.
From 323 B.C. to Cleopatra’s death in 30
B.C. , the family experienced five brother-
sister pairings, plus an uncle-niece
relationship.
•
Cleopatra married her brother,
Ptolemy XIII, when he was ten years
old. These type of marriages were an
attempt to preserve the royal blood.
How do we use pedigrees today?:
• Pedigrees are used to help families
identify the risk of transmitting an
inherited illness and as starting
points for gene searches.
Huntington’s
• Researchers looked for the gene that causes
Huntington disease took researchers to a remote
village in Venezuela and a large family pedigree.
• The gene was eventually traced to a Portuguese
sailor believed to have introduced the mutant
gene in a fishing village.
• Through genetic studies geneticists have
identified several patterns of inheritance.
* Single Allele Traits: traits controlled by a
single allele, i.e., Sickle Cell Anemia.
* Polygenic Traits: traits that controlled by two or
more genes.
– Example… skin color in which four to seven genes
control this trait. Each gene has an additive effect.
• * Multiple Allele Traits: some traits are
controlled by three or more alleles. One
example is human blood type.
• IA , IB , IO , are alleles that code for blood
type.
– The alleles IA and IB are co-dominant and both are
dominant to IO.
• * Sex linked (X-linked) traits: Human traits that
appear only on the X chromosome.
• Males have only one X chromosome, any
recessive allele on the X chromosome will be
expressed because it cannot be masked by a
dominant allele.
• One example is color blindness.
*Another sex linked trait is Hemophilia
•
Nondisjunction: A disorder in which
there is a failure of chromatids to
separate during cell division.
• When this occurs the sperm or the
egg will receive an extra copy or a
missing chromosome.
• If a zygote has a condition in which
there are three of a particular
chromosome, the condition is called
trisomy.
•
• Two examples are
Down’s Syndrome and
Klinefelter’s Syndrome.
(XXY, jot this down!)
Detecting Genetic Disorders
When there is a history of genetic disorders an
individual may desire to undergo genetic screening.
This is a process of examining a persons genetic
makeup.
Karyotyping
A karyotype is one method of genetic
screening. This is a process of taking a
picture of an individual’s
chromosomes.
• Another tool of genetic screening is
amniocentesis.
• This is the removal of a small amount of
amniotic fluid from the amnion, the sac that
surround the fetus.
• Physicians use the fluid to make karyotypes.
• Another method of sampling fetal
chromosomes is through chorion villus
sampling. The chorion villa grows
between the mother’s uterus and the
placenta and has the same genetic
makeup as the fetus.