Download Genetics Notes

Warmup 11/28
• What is probability?
• In a deck of regular playing cards, what is the chance
that you will pull out a red card?
• What is the chance that you will pull out a 4?
• What is the chance that you will pull out the 8 of
spades?
Genetics
Key Terms
Terms
• Allele: different form of the same gene
• Dominant: trait that is always
expressed, written with a capital letter
“A”
• Recessive: trait that is only expressed
when a dominant trait is not present,
written as a lower case letter “a”
Terms
• Homozygous: both alleles are the same
AA or aa
• Also known as a purebred
• Heterozygous: the alleles are different
Aa
• Also known as a hybrid
• Genotype: combination of dominant and
recessive alleles (Tt)
• Phenotype: what you look like on the outside
(tall)
• Test Cross: a cross between a dominant
individual of unknown genotype with a recessive
individual to determine the genotype of the
dominant individual
• P Generation: the 2 individuals that
are used as parents in the original cross
• F1 Generation: The offspring of the P
generation
• F2 Generation: The offspring of two
plants crossed from the F1 generation
Warmup 11/29
• What is the F2 generation?
• Write out an example of a homozygous
dominant genotype.
• What is a test cross used for?
Mendelian Genetics
• Carried out the first quantitative studies of inheritance.
• Used garden peas. Reasons:
• Can easily produce hybrid peas-knew he could observe segregation
of traits.
• A large number of true-breeding varieties were available.
• Pea plants small and easy to grow with a relatively short generation
time.
• Pea plants contain both male and female reproductive organs-easy
to control self or cross pollination.
Mendel's Experimental Design:
• Usually contained 3 stages:
• Allowed pea plants of a given variety to produce progeny
for several generations through self-fertilization allowing
him to be assured that some traits were pure breeding.
• Performed crosses b/t varieties, exhibiting alternative
forms. To do this he has to remove male parts of the
flower.
• Mendel permitted hybrid offspring from cross pollination
to self-fertilize for several generations and counted the
number of each generation exhibiting each trait.
7 Characteristics
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Dominant Form
Purple flowers
Yellow seeds
Round seeds
Green pods
Inflated pods
Axial Flowers
Tall plants
Recessive Form
White Flowers
Green seeds
Wrinkled seeds
Yellow pods
Constricted pods
Terminal Flowers
Dwarf plants
•Mendel discovered a 1:2:1 ratio
• ¼ pure breeding dominant (ex BB)
• ½ not pure-breeding dominant (ex.
Bb)
• ¼ pure-breeding recessive (ex bb)
Warmup 12/01
• What is a dihybrid cross used for?
• Two traits are crossed in dogs. Dog fur color can
be brown (dominant) or white. Dog noses can be
wet (dominant) or dry. What would be the
genotype and phenotype ratios if a dog that was
heterozygous for both traits was crossed with a
dog that was recessive for both traits?
Warmup 12/02
• What form of inheritance is shown when you have
the following phenotypes? Tall rose plant, short
rose plant, medium rose plant
• A woman has blood type A, her baby has blood
type O. What is the genotype of the father?
Mendel's Model of Genetics:
• Parents do not transmit physiological traits,
but instead factors that later act to produce
these traits in offspring.
• Each individual receives 2 factors that may
code for the same version of a trait or a
different one.
Mendel's Model of Genetics:
• Not all factors are identical.
• The 2 alleles do not influence each other in
any way.
• The presence of an allele does not ensure
its expression.
Mendel’s Laws
• Law of Segregation: Each parent can only pass on
one gene for each trait. Half of the sperm or eggs
created will get one of each of that parents
available genes.
• Law of Independent Assortment: Different traits
segregate independently and remain distinct (ex.
brown hair and brown eyes are not a "package"
deal.)
How do dominant traits mask recessive traits?
• The dominant allele codes for a product where the recessive
allele does not. Ex. Albinism is caused by a homozygous
recessive gene that lacks the necessary enzyme to produce
melanin. Having one dominant gene allows a person to
produce enough melanin to appear normal.
• The recessive allele produces less of a product so it is
masked by the dominant allele that makes more product.
• The recessive allele produces a fully functional enzyme that
is masked by a dominant allele.
Other dominant relationships:
• Lethal recessive-homozygous recessive organisms do not
have the same life expectancy as a dominant individual.
(ex. tay sachs and cystic fibrosis)
• Incomplete dominance-When the heterozygous individuals
will be intermediate to the two homozygotes.
• Ex. Snapdragons-RR(red)XWW(white) =RW=pink
• Palomino horses (light gold color) are heterozgotes produced
from this inhertiance pattern.
Other dominant relationships:
• Codominance-When the heterozygote shows both
homozygous traits. Ex. black rooster + white hen=black and
white bird
• Multiple Alleles-When there are more than two alleles for a
trait, such as blood type. IA and IB are both dominant.
• IAIA or IAi=A blood
• IBIB or IBi=B blood
• IAIB=AB blood
• ii=O blood
Gene interactions and other modified
Mendelian ratios:
• Epistasis-When one gene interferes with the
expression of another gene in a dihybrid cross.
• Ex. Labrador coat color
• eebb=yellow fur; brown nose, lips and eye rims
(Yellow lab)
• eeB_=Yellow fur; black nose, lips and eye rims (Yellow
lab)
• E_bb=Brown fur; nose, lips and eye rim (chocolate lab)
• E_B_=Black fur; nose, lips, eye rims (black)
Gene interactions and other modified
Mendelian ratios:
• Pleiotropy-When one gene can affect more
than one characteristic. Ex. Cats with white
fur also usually have blue eyes and are deaf.
Nature vs. Nurture
• Your phenotype is determined not just by your genes, but
also by your environment.
• Ex. Nutrition influences height, exercise effects build,
tanning effect pigmentation.
• Some alleles are heat sensitve. Arctic foxes make fur
pigments only when the weather is warm (white in winter,
reddish brown in summer). Himalayan rabbits and siamese
cats have darker fur on cooler regions of their body such as
ears and tails. This is due to an enzyme that is only triggered
at 33 degrees Celsius.
Warmup 12/05
• Provide an example of epistasis.
• What is pleitropy?
• How can the environment effect phenotype?
Gene interactions and other modified
Mendelian ratios:
• Incomplete Penetrance -Occurs when a person
has an abnormal genotype without showing it. Ex.
polydactly-a dominant trait where people have
more than 5 digits on a hand or foot. In some
people they may have the gene with no extra
digits, or only have it on one foot, hand etc.
When they have children the offspring do have a
higher chance of expressing the trait.
Gene interactions and other modified
Mendelian ratios:
• Sex-limited and sex-influenced traits-A trait
that is limited to or affects one gender
more often than the other. Ex. a dominant
gene causes a rare type of uterine cancer
would not affect men, a dominant trait for
baldness doesn't affect women.
Gene interactions and other modified
Mendelian ratios:
• Variable age of onset-Some traits do not
appear till later in life.
• Ex. Muscular dystrophy has different onset
ages, even for related individuals.
• Huntington's disease, a dominant condition,
does not usually appear till after the age of 40.
Sex-Linked Traits
• In 1910 Thomas Hunt Morgan began working with fruit flies
and was the first to determine linkage of traits.
• He did this by realizing that there was a higher incidence of
white eyed males than females determining that the gene
for white eyes was linked to the X chromosome. Since the
only chromosome that does not have a homologous
chromosome in the body is the male sex chromosomes he
was able to trace the cause. The X chromosome carries
thousands of genes where the Y only carries a few.
Human Sex-Linked Traits
• The Y chromosomes carries the gene SRY which
triggers the development of male reproductive
structures. The X chromosome carries the DAX
gene that makes female reproductive structures.
• Several recessive traits are also carried on the X
chromosome; many that may cause
abnormalities.
Human Sex-Linked Traits
• Colorblindness-Most common type is red/green
colorblindness. There are 3 types of cones in the
eye red, green and blue. A person who is
colorblind generally lacks 1 of the 3 cones. 10% of
all males are colorblind and only 0.4% of females.
To be affected a man only needs to receive one
recessive gene from his mother since the Y
chromosome does not carry the same genes
therefore cannot mask it. Affected women must
receive the recessive gene from both parents.
Human Sex-Linked Traits
• Hemophilia: a recessive mutation that
prevents the blood from clotting properly.
It is caused by a traits the results in a
missing clotting protein.
Human Sex-Linked Traits
• Duchenne's muscular dystrophy-includes
progressive muscle weakening, loss of
coordination and hemophilia. 1 of every 3500
males in the US is affected.
X inactivation in females
• Females are XX but in each cell one X chromosome is mostly
inactivated and is a barr body. Small regions of the
chromosome remain active but most of the genes are not
expressed. Barr bodies are "reactivated" in the cells that
undergo meiosis to form gametes.
• The selection of which X chromosome occurs randomly and
independently in each of the embryonic cells present at the
time of X inactivation. Therefore the female consists of a
mosaic of 2 types of cells, those with X derived from mother
and those with X derived from father.
Warmup 12/07
• What is does a trait that is sex-limited
mean? What is an example?
• What does an X-linked trait mean?
• What does incomplete penetrance mean?
Groups for the presentations
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Lyndsay , Curtis
Victoria , Tamia
Teana , Kyasia
Caroline , Michael
Hilda , Vanessa
Ruth , Sarah
Audrey , Rachel
Justine , Christian
Warmup 12/12
• What form of inheritance is caused by a missing X
chromosome?
• What type of inheritance is Duschene’s Muscular
Dystrophy?
• What disorder effects only men and includes
increased female characteristics in its symptoms?
• Polygenic Inheritance: multiple genes controlling 1 trait.
This results in multiple phenotypes.
• Ex: skin, eye, hair color; height; weight to a degree
Alterations to the Chromosomes
• Nondisjunction-Failure of chromosomes to seperate
properly. Results in an individual with too many or too
few chromosomes. Ex. Down's syndrome, Klinefelter's
and Turner's syndrome
• Polyploidy-More than two complete chromosome sets
such as triploidy=3n or tetraploidy=4n. This is common in
plant kingdom.
• Alterations of chromosome structure-The breakage of a
chromosome. Ex. Cri du chat and Down's syndrome.
• Genomic Imprinting-Geneticists have identified traits that
seem to depend on which parent passed along the allele.
• Ex. 2 conditions caused by deletion of a segment of chromosome
15.
• Prader-Willi occurs when chromosome comes from father.
Symptoms: mental retardation, obesity, short height, small
hands and feet.
• Angelman syndrome occurs when chromosome comes from
mother. Symptoms: spontaneous laughter, jerky
movements, and other motor and mental symptoms.
Prenatal Detection of Disorders
• Amniocentesis-tested between 14th and 16th week-uses a needle to
withdraw amniotic fluid and then fetal cells are grown and used to
prepare a karyotype.
• CVS-tested b/t 8th and 10th week-Chorionic villus sampling,
removes a small sample of chorion (outermost membrane
surrounding fetus) and fetal cells are examined for abnormalities.
• Ultrasound-throughout pregnancy-uses soundwaves to get an image
of the baby.
• Newborn screening-some genetic anomalies are detected at birth.
• Human genetic anomalies can be caused in the following
ways:
• Autosomal dominant-one gene causes the anomaly (polydactyly)
• Autosomal recessive-2 genes cause the anomaly (PKU, sickle cell
anemia)
• X-linked recessive-found on the X chromosome (color blindness,
muscular dystrophy)
• Chromosomal Anomalies-trisomy 21, turner's syndrome
• Multifactorial-combinations of genes and environment, both
factors must be present (cleft lip, spina bifida)