Genetics
• Genetics is the study of heredity, which is the passing of traits from
parents to offspring
• Mendel was successful in determining the basic laws of heredity
because of his experimental method
•
Mendel`s approach was mathematical, using many repeated trials
to find the likely outcome from a large sample size
•
Mendel choose to study garden peas, Pisum sativa, which has
many useful features
– many traits with 2 distinct forms (no intermediates)
– mating can be controlled, using either self-fertilization or crossfertilization
– small, grows easily, matures quickly, produces many offspring
Mendel observed that traits are
expressed in simple ratios
• 1. Monohybrid crosses involve one pair of
contrasting traits
Step 1: use self-fertilization for several generations
to ensure that parental generations are truebreeding
Step 2: cross-fertilize two contrasting P generation
plants, with offspring known as the F1 generation
(1st filial generation)
Step 3: allow F1 generation to self-fertilize,
producing the F2 generation (2nd filial generation)
• 2. Mendel`s results
• F1 generation always shows one of the two
contrasting traits, known as the dominant trait
(recessive not seen)
• F2 generation always shows both traits, but in a
3:1 ratio of dominant to recessive
• 3:1 ratio of dominant:recessive was constant for
seven different pairs of contrasting traits studied by
Mendel
Mendel's Work Became a Theory of
Heredity
• Mendel's Hypotheses
– For each inherited trait, an individual has two copies of the gene,
one from each parent
– There are alternative versions of genes, known as alleles
– When two different alleles occur together, one of them may be
completely expressed, while the other may have no observable
effect on the organism's appearance
• 1) dominant trait expressed
• 2) recessive trait not expressed
When gametes are formed, the alleles for each gene in an
individual separate independently of one another. Thus,
gametes carry only one allele for each inherited trait. When
gametes unite during fertilization, each gamete contributes one
allele.
• 2. Mendel`s findings in modern terms a. homozygous
refers to two of the same alleles of a particular gene in
an individual
• homozygous dominant AA
• homozygous recessive aa
• heterozygous refers to two different alleles of a particular gene in
an individual: Aa
– genotype refers to the set of alleles in an individual
– phenotype refers to the physical appearance of a trait in an
individual
Mendel's Ideas Gave Rise to the
Laws of Heredity
1. law of segregation: the two alleles for a
trait segregate when gametes are formed
during meiosis
2. law of independent assortment: the
alleles of different genes separate
independently of one another during
gamete formation
The Human Genome
• Defn.: the entire collection of genetic material in
each typical cell of the human body
• Human genome includes 46 individual nuclear
chromosomes and one mitochondirial
chromosome
• Contains ~30,000 genes (about 200 are
bacterial in origin)
• Analysis of genome called genomics
Study of Protein
• Proteomics: analysis of protein codes in
order to understand the role of each
individual one.
• Proteome: entire protein group in human
genome
Ideogram
• Cartoon of a chromosome
• A banding pattern is the characteristic pattern of
dark and light bands that appears when a
chromosome is stained with a chemical solution
and then viewed under a microscope. These
bands are used to describe the location of genes
on each chromosome.
• Short arm called the p-arm
• Longer arm is the q-arm
Meiosis
• Remember meiosis causes
gametogenesis
• Each gamete is a haploid (Mendels
principle os segregation)
• Make sure you review steps of meiosis
Independent assortment
• Each chromosome separates itself
independently in meiosis
• Maternal and paternal chromosomes get
mixed up and redistributed
• Each gamete becomes genetically unique:
called genetic variation
• Gene linkage: genes on an individual
chromosome stay together
• Exception---crossing over
– During meiosis homologous chromosomes
line up along the equator and exchange
genes
Karyotype
• A picture of homologous chromosomes
• Allows to see
– Trisomy (extra chromosome)
– Monosomy (missing chromosome)
– Broken chromosomes
Punnett Squares Can Predict the
Expected Results in Crosses
a. write the parental genotypes (e.g., Aa x Aa)
b. write the possible gametes from one parent along the top of the
square (using the law of segregation: Aa --> A, a)
c. write the possible gametes from the other parent along the left
side of the square (using the law of segregation: Aa --> A, a)
d. write the combination of two alleles produced during
fertilization by combining the allele along the top of the box
with the allele along the side of the box
e. calculate the genotypic ratio
f. calculate the phenotypic ratio
• 2. Dihybrid crosses: crosses the involve
two traits
• 3. Test crosses: an individual whose
phenotype is dominant, but whose
genotype is unknown, is crossed with a
homozygous recessive individual
Probabilities Can Also Predict the
Expected Results of Crosses
• 1. Probability is the likelihood that a specific
event will occur, expressed as a decimal,
fraction or percentage
• 2. Probability of a specific allele in a gamete a. if
heterozygous, 50% b. if homozygous, 100%
• 3. Probability of the outcome of a cross: the
probability that a combination of two
independent events will occur is the product of
the two independent probabilities
Family Pedigrees Can Be Used to
Study How Traits Are Inherited
•
•
1. A pedigree is a family history that shows how a trait is inherited over
generations, especially used for genetic disorders
2. Autosomal or sex-linked?
– a. if autosomal, the trait appears in both sexes equally
– b. if sex-linked, the trait is on the X chromosome, and appears more often
in males, with only one X chromosome, than females, with 2 X
chromosomes
•
3. Dominant or recessive?
– a. if autosomal dominant, the an individual with the trait must have a
parent with the trait
– b. if autosomal recessive, the individual with the trait might have both
parents without the trait, if both are heterozygous carriers
•
4. Heterozygous or homozygous?
– a. for autosomal traits, the dominant phenotype will be the same for
homozygous dominant and heterozygous individuals
– b. for autosomal traits, the recessive phenotype will only be seen in
homozygous recessive individuals
Most Traits are not Controlled by
Simple Dominant-Recessive Alleles
•
1. Polygenic traits are produced by several genes influencing a trait
together
– a. crossing over and independent assortment produce many combinations
– b. thus, many intermediate conditions (hair color, eye color, skin color)
•
2. Incomplete dominance is produced when neither allele is dominant,
producing a trait intermediate between the two alleles (curly x
straight=wavy)
•
3. Codominance involves two dominant alleles that are both expressed
in the heterozygous genotype (type AB blood)
•
4. Multiple alleles involve genes with three or more alleles (A, B, AB, O
blood)
•
5. Traits influenced by the environment
– a. human height influenced by nutrition
– b. arctic fox coat color influenced by temperature
– c. hydrangea flower color influenced by soil acidity d. human skin color
influenced by exposure to sun
Some Traits are Caused by
Mutations
• 1. Sickle cell anemia is an autosomal
recessive disorder producing defective
hemoglobin
a. usually fatal in a homozygous genotype, thus
selected against
b. for heterozygotes, protects from malaria, thus
selected for
c. combination of two selective pressures
balance each other
• 2. Hemophilia is a sex-linked recessive disorder
impairing blood clotting
• 3. Huntington's disease (HD) is an autosomal
dominant disorder producing progressive
degeneration of some nervous system functions
• 4. Detecting and treating genetic disorders
– a. genetic counseling
– b. gene technology, such as gene therapy