Download Genetics

Introduction to Genetics
Mendel
Was a monk in Austria in mid-1800’s, studied math and science
As he was tending peas in the garden, he began to notice patterns in the pea characteristics ( tall
or short, white or purple flowers, etc)
Did a series of experiments over a 10 year period of time—mating, growing and counting pea
plants with differing characteristics (30,000 pea plants total) and compared them to the previous
and next generations
Concluded that some “factor” was passed between generations that could be hidden but not lost
Figure 14.2 Mendel tracked heritable characters for three generations
Mendel’s Laws
Law of segregation: homologous
chromosomes separate and are
packaged into different gametes
Law of independent assortment: each
pair of homologous chromosomes lines
up independently of each other during
meiosis
Figure 14.4 Mendel’s law of segregation
(Layer 2)
Dihybrid Cross
Dihybrid cross: crosses between individuals that differ in two traits
In Mendel’s crosses round and yellow seeds are dominant
Wrinkled and green seeds are recessive
Try a cross between plants that are true- breeding for round and yellow seeds with plants that are
true breeding for wrinkled and green seeds
Can you predict the genotypes and phenotypes of the F1 generation?
Figure 15.1 The chomosomal basis of Mendel’s laws
Vocabulary
Trait: characteristic of an organism
Gene: sequence of nucleotides that code for a certain trait
Allele: different form of a gene; symbolized by a letter
–Dominant: allele that can hide another allele; always symbolized by a capital letter; ex: B
–Recessive: allele that can be hidden by the dominant allele; always a lower case letter; ex: b
Homozygous: 2 of the same allele
–H. dominant: BB
–H. recessive: bb
Heterozygous: 2 different alleles: Bb
Genotype: the alleles an organism has: BB, Bb
Phenotype: how the trait looks; blue eyes, etc
P: parental generation
F1: their children
F2: the children of the F1
Punnett square: tool for predicting the probability of producing a certain type of offspring
Figure 14.3 Alleles, alternative versions of a gene
Inheritance patterns
Complete dominance: one allele can completely mask another; not always the most common
phenotype
6 different crosses (yes, you have to memorize them)
–TT x TT, TT x Tt, TT x tt, Tt x Tt, Tt x tt, tt x tt
–Complete all the above crosses in your journal
Figure 14.6 A testcross
Incomplete dominance
Heterozygote shows a blending of the parental traits
CRCW , RR’, RW
Co-dominance
Heterozygote shows both parental traits
MN blood type: MM has only M proteins, NN has only N proteins, MN has both M and N proteins;
Roan cows, etc
Multiple alleles
More than 2 alleles for a given trait’
ABO blood type
Pheno. Geno
Antigen
antibodies
A
A
A
–A I I or I i
A
B
B
B
–B I or I i
B
A
A
B
–AB I I
A, B
none
–O ii
none
A,B
Probability and Punnett Squares
A.
Genetics and Probability
B.
Punnett Squares
C.
Probability and Segregation
D.
Probabilities Predict Averages
Probability
Probability: branch of mathematics that predicts the chances that a certain event will occur
What are chances that a tossed coin will turn up heads?
-one out of 2, or ½
Your chance of drawing an ace from a deck of cards:
4 out of 52, or 1/13
The
chance that all four tosses will land heads up:
=1/2x1/2x1/2x1/2
=1/16 chance
Sex chromosomes and linkage
Sex chromosomes
Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes
Females are XX, males are XY (X and Y look different from each other); females give all their
eggs one X chromosome plus cytoplasm and organelles;
Males determine the sex of the child since ½ of their sperm get the X and ½ get the Y
Any information on the X chromosome will appear in males, whether recessive or dominant;
females require 2 recessive alleles to show a recessive trait
Information solely on the Y chromosome are called holandric genes (porcupine quill body hair,
hairy ear rims, SRY gene); only effect men
Barr bodies
In body cells of females one X chromosome at random is turned off early in development;
inactivated X is called a Barr body
All the cells descended from that cell have the same X turned off
If female is heterozygous she becomes a mosaic—some areas have the dominant gene
expressed, some have the recessive
Ex: calico cats, patches of colorblindness
X chromosome inactivation
Cat’s fur color
Males: XY
Females: XX
Extra X: inactivated, bar body
Spots of 2 colors: female cat
One color spots: male cat
Non-disjunction
Failure of chromosome pairs to separate during meiosis
Results in gametes with too many or too few chromosomes
Aneuploidy: abnormal # of a certain chromosome
Polyploidy: more than 2 complete chromosome sets
An embryo needs at least one X chromosome to survive
Sex linkage problems
It is now important if the offspring is male or female so the X and Y have to be used along with
superscripts to show the alleles
Examples of X-linked disorders: colorblindness, hemophilia, muscular dystrophy, eye color in fruit
flies
Hemophilia
H-normal, h-hemophiliac
Phenotype
Genotype
Normal female
Carrier female
Hemophiliac female
Normal male
Hemophiliac male
XHXH
XHXh
XhXh
XHY
XhY
Colorblindness
Colorblindness- Sex-linked disorder which causes loss of color.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if
they are recessive.
Duchenne Muscular Dystrophy
Sex-linked disorder that results in progressive weakening and loss of skeletal muscle.
In the U.S. 1/3000 males
Defective muscle protein
Chromosome
46, XX
46, XY
45, XO
47, XXY

Name of Syndrome
Normal
Normal
Turner's Syndrome
Klinefelter's Syndrome
Sex
Female
Male
Female
Male
Frequency in Population
0.511*
0.489*
1/5,000
1/700
The Family Pedigree
A Family Tree
To understand how traits are passed on from generation to generation, a pedigree, or a diagram
that shows the relationships within a family, is used. In a pedigree, a circle represents a female,
and a square represents a male. A filled-in circle or square shows that the individual has the trait
being studied. The horizontal line that connects a circle and a square represents a marriage. The
vertical line(s) and brackets below that line show the child(ren) of that couple.
1.
This pedigree shows the inheritance of attached ear lobes. Which parent has attached ear
lobes?
2.
How many children do the parents have?
Which child has attached ear lobes?
Which child is married? Does this child’s spouse have attached ear lobes? Do any of this
child’s children have attached ear lobes?
3.
Section 14-1
A circle
represents a
female.
A square
represents a
male.
A horizontal line
connecting a male and
female represents a
marriage.
A half-shaded circle
or square indicates
that a person is a
carrier of the trait.
A completely
shaded circle or
square indicates
that a person
expresses the trait.
A circle or square
that is not shaded
indicates that a
person neither
expresses the trait
nor is a carrier of
the trait.
Genetics
Human Genetic Disorders
Chromosome Abnormalities
Normal development requires 46 chromosomes; any other number results in an abnormality
Chromosome abnormalities are seen in less than 1% of live births
Most fetuses lost through spontaneous abortion have major chromosomal abnormalities
These disorders are a result of non-disjunction—the failure of chromosomes to separate in
meiosis 1
Types of disorders
Turner’s syndrome: XO; 1/5000 births; results in short, sterile females with some degree of
retardation
Kleinfelter’s syndrome: XXY; 1/2000 births; results in tall, slender, sterile males with some
mental retardation, have some female characteristics
More disorders
Down’s syndrome: trisomy 21; 3/2000 births
more common in births to older women
1/50 at age 45
results in short stature, round heads, protruding tongues, prone to respiratory and heart
disease, varying degrees of retardation
And lastly..
Supermale: XYY; 1/2000 births; results in tall stature, subnormal intelligence, many have
personality disorders and a tendency towards violence
Allele disorders
Recessive alleles cause these diseases
Cystic Fibrosis: most common disorder in caucasians
–Codes for a malfunctioning cell membrane protein which results in thick sticky mucous lining the
lungs and digestive tract
–Treatment: gene therapy—inhalation of good gene, but must be repeated
More disorders
Tay-sachs: most common in eastern european Jews; can be tested for the allele
–Enzyme deficiency causes build-up of harmful substances in the brain
–Babies begin showing symptoms by 6 months old—lose their eyesight, hearing, become
paralyzed
–Most die by age 4
And more..
Phenylketonuria: also more common in caucasians
–Lack of enzyme that breaks down phenylalanine (found in nutrasweet)
–Can cause mental retardation and convulsions
–In US babies are tested at birth and treated with a low protein diet until their brains are fully
developed
And one more…
Sickle cell anemia: most common disorder in African-americans
–Only 1 base different causes the abnormal protein in red blood cells (hemoglobin) to crystallize
under low oxygen conditions
–Causes severe pain, strokes, tissue damage, etc
–Heterozygous individuals are resistant to malaria
Dominant allele disorders
Only 1 dominant allele can cause these disorder
Neurofibromatosis: causes tumors on the nerves, skin, bones, vital organs
–Can vary in degree from small brown bumps to large disfiguring tumors
And finally..
Huntington’s disease
–Causes degeneration of the nerve cells in the brain, causes loss of motor control and eventually
paralysis
–Symptoms are not normally seen until after age 40