Genetics
• Gregor Mendel-Father of Genetics
• Born-1822 in Austria
• Entered the monastery at age
21. After failing the exam to
be a teacher he went to study
at the University of Vienna.
There he studied with some
important scientists of his day.
1857-Mendel began breeding peas in the abbey gardens.
– Why was choosing peas so important?
• Traits show as “either/or”
• Had control over mating (they normally self-fertilize)
• He began with true breeding plants
• P = parent generation
• F1 = first generation (first filial)
• F2 = second generation (second filial)
• Sample cross:
–P
purple x white flowers
– F1
all purple flowers
– F2
¾ purple, ¼ white
– Mendel reasoned the white trait was not gone in the F1
but was being masked by the more dominant purple
trait
• Mendel’s law of segregation:
– The 2 alleles separate during gamete formation
– Each parent has 2 copies of every gene. When
forming the sex cells only one copy goes into
each cell.
If Mom has DD--------eggs all have D
If Dad has dd----------sperm all have d
• A test for the law of segregation:
– Purple x white
P:
F1:
F2:
PP x pp
Pp x Pp
1PP: 2Pp: 1pp
*The fact that the white appears again proves that
the alleles have to separate from each other.
• Vocabulary:
– Trait-varieties of alleles (purple or white)
– Homozygous-alleles are the same, (may be either
dominant or recessive-PP, pp, TT, tt)
– Heterozygous-alleles are different—Pp, Tt
– Phenotype-appearance, traits that are visible
– Genotype-actual genes present
• Test cross:
– Done to determine if genes are homozygous or
heterozygous dominant.
• A dominant parent can be either PP or Pp
• Cross with a plant of known genes (pp)
– If all offspring are purple then parent was PP
– If some offspring are white and some are purple then
parent was Pp
• Mendel’s Law of Independent Assortment
– From single trait crosses Mendel knew yellow
seed were dominant over green seeds and
round were dominant over wrinkled. What
would happen to theses genes when crossed
together?
• If Y and R stay together then the ratio in offspring
would be 3:1
• Actual ratio is 9:3:3:1. This means that the 2 genes
travel independently of each other to gametes
Probability and genetics
• Probability-the chance an event will occur
– An event certain to occur has a probability of 1
– An event certain not to occur has a probability of 0
– Probabilities of all outcomes must add up to 1
• Rule of multiplication:
– Use when each occurrence is a separate event
– Example: what is the chance of getting heads
on 2 coins tossed simultaneously?
• The two coins are separate events.
probability of heads on 1st coin = ½
probability of heads on 2nd coin = ½
probability of heads on both is ½ x ½ = ¼
• What is the chance of getting white flowers?
– Chance of egg having p allele is ½
– Chance of sperm having p allele is ½
–½x½=¼
• Rule of addition:
– Probability an event can occur in 2 or more
ways is the sum of each one separate
probability.
• Example: what is the probability an F2 plant will be
heterozygous from a monohybrid cross?
• Two out of 4 are heterozygous
P
p
P
PP
Pp
p
Pp
pp
¼+¼=½
Monohybrid and dihybrid crosses
• Monohybrid – one trait is crossed at a time
Punnett square-device for predicting results of
a cross
recessive trait is written with a small letter
dominant trait is written with a capital letter
• Examples:
Trait = seed shape
Genes: round = R, wrinkled = r
R
R
R
R
R
R
r
R
r
r
r
G=
P=
G=
P=
G=
P=
r
• Dihybrid –two traits crossed together
• First determine the possible combinations
of genes:
– YyRr = YR, Yr, yR, yr
– Yyrr = Yr, Yr, yr, yr
G=
P=
Yr
Yr
yr
YR
YYRr
YYRr
YyRr
Yr
YYrr
YYrr
Yyrr
Yyrr
yR
YyRr
YyRr
yyRr
yyRr
yr
Yyrr
Yyrr
yyrr
yyrr
yr
YyRr
Inheritance patterns:
1. Incomplete dominance-the appearance of
the F1 is a blend of the parents
– Example-snapdragons
–P
red x white
– F1
all pink
– F2
¼ red: ½ pink: ¼ white
• Example: sickle cell anemia
– Mutated gene for hemoglobin
– Normal genes: HbA HbA
– Sickle cell trait: HbA HbS
– Sickle cell disease: HbS HbS
A person with sickle cell trait produces both normal
and sickle cells, a person with the disease makes
only sickle cells. They rupture easily, clog arteries,
cells don’t get oxygen delivered.
2. Pleiotropy-the expression of one gene can effect many
organs or systems (pleio is Greek for more)
– Sickle cell
– Marfan syndrome-tall body, long arms, nearsighted,
weak aorta wall (President Lincoln?)
– Cystic fibrosis
3. Co-dominance-both phenotypes are expressed at the same time
Example one –the four human blood types are a result of 3
genes
IA IB i
– A and B are both dominant genes
– A = IAIA or IAi
– B = IBIB or IBi
– O = ii
– AB = IAIB
Example two-roan cows
-- red and white are equal
4. Multiple alleles-three or more alleles of a gene in a
population
– Example-blood type (3 genes determine 4 blood types)
– Example-rabbit fur color
• Agouti-gray and yellow (A)
• Chinchilla-black and white (a-ch)
• Himalayan-white with black extremities (a-h)
• Albino-white (a)
5. Polygenic inheritance-many genes contribute to the trait,
creates an additive effect
– Example: human skin color
AABBCCDD is darkest, aabbccdd is lightest.
– Other examples are height, weight, eye color
6. Epistasis-one gene alters or interferes with the expression of
another.
– Example-fur color in many mammals.
In mice black hair is dominant to brown.
black – B brown – b
A second gene determines how much color is deposited in
the hair.
C—mouse will be black or brown
c—mouse will be white
*even if the mouse has BB for black hair, if the other
genes are cc for no color, the mouse will not show the
black fur trait.
• Environmental effects
• -some alleles are temperature sensitive.
Examples:
arctic foxes, Himalayan rabbits, Siamese cats
Some alleles are pH sensitive
Example:
hydrangeas
Locating genes on chromosomes:
• The first evidence that showed certain genes were located
on a specific chromosome came from Thomas Morgan.
– He chose fruit flies to work with
– Using eye color as the trait
• Females had red eyes (wild)
• Males had white (mutant)
• In his crosses he found that the white eye color was linked to
the sex of the fly.
• He determined that this meant the gene was on the sex
chromosome.
Sex linked traits
• Remember that humans have 23 pairs of chromosomes.
Of those, 22 pairs are autosomes and 1 pair are sex
chromosomes.
– Male sex chromosomes = XY
– Female sex chromosomes = XX
– Gender of the offspring is determined by the male and
is a 50/50 chance
female
(XX)
male
(XY)
eggs
sperm
X
x
Y
X
x
X
X
X
X
XX
XX
Y
XY
XY
Sex Determination in other animals
• Not all animals determine gender like humans.
– Grasshoppers have only 1 sex chromosome
• Females are XX, males are X
– Birds and some fish the female determines the sex of
offspring
• Females are ZW, males are ZZ
– Bees and ants don’t have sex chromosomes
• Females come from fertilized eggs (they are 2n)
• Males come from unfertilized eggs (they are n)
Dosage compensation
• Probably occurs to make females and males equivalent in X’s,
one X chromosome in a female becomes inactive
• Inactive X condenses into a compact unit and is pushed to
the side. It is called a Barr body. Which X becomes Barr
body is random. Females end up as a mosaic—some cells
have active X from mom and others have active X from
dad.
– Examples
• Calico cat
• Female sweat glands
Examples of sex linked traits
• X linked recessive-show up in males more often
– Hemophilia-blood clotting disorder, ran through royal
families in Europe
– Dushenne Muscular Dystrophy-muscles atrophy, are
replaced by fat tissue during ages of 2 and 10. Typically
die in early 20’s
– Red green color blindness-can’t distinguish
between those two colors
• X linked dominant-rare, few examples
– Faulty enamel trait-the hard enamel on teeth fails to
develop correctly
• Y linked dominant-few traits are on the Y
other than male traits. It is questionable if
these traits exist
Chromosome abnormalities
• Chromosome abnormalities may be caused by
a change in number or a change in the
structure of the chromosome.
Changes in chromosome number:
• Nondisjunction-homologous chromosomes do
not separate correctly during meiosis. One
gamete receives an extra copy, other receives
none. This creates:
– Polyploidy-entire sets of chromosomes may be
added
– Aneuploidy-whole chromosomes are lost or
gained
Nondisjunction in sex chromosomes
• 45 XO – Turner’s syndrome
– 1 in 5000 female births
– Short stature, barrel chest, thick neck with webbing,
normal intelligence but may have learning disabilities,
often has heart problems, no Barr bodies, sterile
• 47 XXX – triple X
– Sex- female
– Usually fertile, fairly normal
– One X will remain functional and the other two become
Barr bodies
• 47 XXY – Klinefelter’s
– Sex- male
– Unusually tall, extra X becomes Barr body, usually
sterile, may show breast development
• 45 OY - never develops
• 47 XYY – Supermale (Jacob’s syndrome)
– Unusually tall, severe acne, not well
coordinated, emotionally unstable.
Nondisjunction in autosomes
• Humans who have lost a copy of an autosome
do not survive.
• Most who inherit an extra copy also do not
survive except for 5 of the smaller
chromosomes:
13, 15, 18, 21, 22
• Trisomy 21 and 22 usually survive to
adulthood. They are usually short and have
poor muscle tone. They always have some
mental deficiency.
• Trisomy 21- Down’s syndrome
– One in 750 births
– 97% have three 21 chromosomes, 3% have 2 plus a
portion of another
– Nearly always related to mother’s age.
• At age 20 it is a 1 in 1700 chance
• At age 45 it is a 1 in 16 chance
– Symptoms include: short stature, heart defects,
mental deficiency, shorter lifespan, most are
sterile, slanted eyes, hand fold, large tongue
1
2
3
6
7
8
13
14
15
19
20
4
9
5
10
11
12
16
17
18
21
22
23
• Trisomy 18, 13 and 15 all cause severe
developmental defects and infants will die
within a few months.
Changes in structure:
• Deletion-section of chromosome broken off and lost
– Example: Cri-du-Chat “cat cry syndrome”
• Deletion on chromosome 5
• Mental deficiency, cry like a cat, usually die in early
infancy
– Example: Prader-Willi syndrome
• Deletion on chromosome 15 from Dad
• Mental deficiency, obesity, short
– Example: Angelman syndrome
• Deletion on chromosome 15 from mom
• Jerky movements, uncontrollable laughter
• Duplication- occurs at crossing over, part of
a chromosome is copied twice
• Translocation-part of a chromosome breaks
off and reattaches to another chromosome
• Fragile X-tip of X chromosome hangs by a thin
thread of DNA
–
–
–
–
Males- 1 in 1500 births
Females- 1 in 2500 births
Most common form of mental deficiency
Children are often hyperactive or autistic, adults have
protruding ears, short stature, long face and prominent
jaw
Autosomal recessive disorders
• PKU- phenylketonuria
– Missing an enzyme to break down phenylalanine,
corrected by diet, causes deficency.
– Newborns automatically screened at birth
• Cystic fibrosis
– Body cells secrete excess mucus that clogs the lungs,
currently they may live into 20’s.
– 1 in 20 in U.S. are carriers
• Tay Sachs
– Blindness, mental deficiency, death usually occurs by age 5
– Gene common among Ashkenazi Jews
– Caused by a nonfunctional enzyme needed to break down
lipids.
Autosomal dominant disorders
• Huntington’s
– chromosome #4 has too many
repetitions of CAG on the end
(11-34 is normal)
– symptoms begin around age 40
– Uncontrollable muscle spasms, personality changes,
insanity
– Affects 1 in 20,000 people
• Progeria
– Rapid aging (7-8 times normal rate)
– Don’t live to reproduce (spontaneous mutation)
• Achondroplasia
– dwarfism
• polydactyly
Diagnosis and counseling
• The likelihood of passing on recessive genes
can be determined by using a pedigree chart.
• High risk pregnancies often occur when the
mother is more than 35 years old.
• Amniocentesis-permits prenatal diagnosis
– Withdrawing fluid from around baby to test
cells. There is a small danger to offspring.
• Ultrasound
– Using sound waves to produce a picture of baby
– Cannot see chromosomes but can see if major
abnormalities are present, the sex of the child and
developmental age by its size
• CVS-less invasive of a procedure
– Takes a sample from the chorion which is a
membrane that surrounds the placenta
– It can be done earlier in the pregnancy and gives
rapid results.
• Pedigree chart
– Shows genetic connections
– Follows a trait through a family
Gene mapping, linkage, crossing over
• Crossing over
– Occurs during prophase I
– Two homologous chromosomes break and
exchange pieces.
– The outcome is genetic variation
• Crossing over can be used to create a genetic
map which measures the distance between
genes based on the frequency of
recombination.
• The distance between any two linked genes is
expressed in map units.
– One map unit corresponds to an expected crossover
frequency of 1%
– So, 20 map units has a crossover frequency of 20 %
A little genetics humor…..
• Inbred cat
Picture credits
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http://www.beekeeping.com/biove/bee.jpg
http://www.ento.vt.edu/~idlab/ornimages/grasshopper/grasshopper.jpg
http://www.mayoclinic.org/marfan/images/symptom1.jpg
http://medimages.healthopedia.com/large/marfans-syndrome.jpg
http://www.blc.arizona.edu/courses/181summer/graphics/graphics%20lect11/Life7e-Fig-10-130%20incomplete%20dominance.jpg
http://www.ob-ultrasound.net/images/transducer_abdomen.jpg
http://fig.cox.miami.edu/Faculty/Dana/amniocentesis.jpg
http://imgen.bcm.tmc.edu/IPIF/9181.jpg
http://www.accessexcellence.org/RC/VL/GG/images/polydactyly.gif
http://www.mun.ca/biology/scarr/Human_Achondroplasia.gif
http://www.ndpteachers.org/perit/mendel.GIF
http://www.anselm.edu/homepage/jpitocch/genbio/peachar.JPG
http://www.umuc.edu/ade/images/colorblind_compare2.jpg
http://www.perret-optic.ch/optometrie/Vision_des_couleurs/Vis_couleur_images/Color6.jpg
http://www.bbc.co.uk/schools/gcsebitesize/biology/variationandinheritance/0dnaandgenesrev5.sht
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