Download Chapter 8: Fundamentals of Genetics

Chapter 8: Fundamentals of Genetics
1. Early concept in genetics
a. Trait: any characteristic that can be passed from parent to
offspring
b. Earliest experiments centered on the domestication of wolves
i. Seems certain that ancient people chose to live with dogs
that were less wild than wolves they descended from
ii. By choosing one trait over another, ancient people began
the process that changed wolves into domestic dogs
iii. In early civilization, people observed that certain traits
were inherited in domestic plants and animals
1. Americans developed 300 varieties of corn from a
type of wild plant called TEOSINITE
c. 2300 years ago, Aristotle explained inheritance as:
i. pangenes in the blood contained a memory of each
structure in a person’s body
ii. blood carried pangenes to reproductive organs
iii. information then passed to the offspring
1. blood relative/bloodline
iv. today we realize that blood is not involved in inheritance
d. 1600’s – both parents contributed traits to each offspring
through sexual reproduction
i. method not clearly known
ii. most believed parental traits blended – offspring got
intermediate between traits of the two parents
e. Gregor Mendel
i. Genetics: scientific study of heredity
ii. 1860 – modern genetics began
iii. Austrian monk and scientist
iv. Used garden peas to study how traits were passed
v. Studied biology, Mathematics and Physics
vi. Took mathematical approach to experimentation
vii. His math observations did not support blending
hypothesis
viii. Because most biologists at the time lacked math training,
the value of Mendel’s work was not fully recognized until
about 50 years later
ix. Mendel investigated with garden peas for 8 years
1. produced hybrids
a. hybrid: an organism that receives different
genetic information for a trait from each
parent
2. he published his results in 1865
3. his goal was to gather data through several
generations of breeding experiments
4. at the end, he developed a hypothesis to explain
his results
x. Mendel’s experiments
1. used garden peas
a. because of enclosed structure of pea
flowers, pea plants self fertilize in nature
b. pollen from stamens fertilize eggs in pistil
of same plant
c. he cross-bred plants by transferring pollen
by hand
2. he studied 7 traits: seed shape, seed color, seed
coat color, pod shape, pod color, flower position
and flower height
a. he chose these traits because each
appeared in 2 distinct forms
3. peas were easy to work with because:
a. seeds were easy to obtain
b. new seed were produced every 90 days
c. peas have short reproductive cycle – got
quick results
4. Pure bred – referring to a group of organisms that
produce only offspring with a given trait when
allowed to self fertilize
5. Mendel crossed pure bred tall with pure bred
short
a. Pure breds called parental generation (P)
i. Offsprings were hybrids – received
information from 1 tall parent and 1
short parent
b. Called offspring 1st filial (F1)
c. Mendel let hybrids (F1) self fertilize
d. Called the second generation F2
6. Mendel’s observations:
a. Blending hypothesis suggest F1’s should be
medium height
b. But all F1’s were tall
c. When F1’s self fertilized, F2’s showed short
traits 3 tall, 1 short)
d. Repeated experiment for other 6 traits
showed that all F1’s resembled 1 parent; but,
all F2’s showed both traits in 3:1 ratio
7. Terms:
a. Dominant trait: trait that showed in hybrid
b. Recessive trait: trait that did not show in a
hybrid
8. Mendel’s Hypothesis – his greatest contribution
was his mathematical analysis of data
a. Each trait controlled by a “factor
i. Gene: a genetic factor that controls a
trait
b. Many factor have 2 or more forms
c. One form of each factor is dominant over
the other
d. 2 factors separate when gametes form.
Both parents contribute to offspring’s
inheritance
e. so each organism must have 2 factors for
each trait
i. during fertilization, single factors join
to make a pair
f. Mendel noted that some traits disappear for
a generation and then reappear unchanged in
next generation
i. He reasoned that factors must remain
separate and distinct in offspring –
factors do no blend
2. Modern Genetics: Mendel’s Laws – basic rules of inheritance
a. Ideas to remember:
i. Diploid organisms have pairs of chromosomes. One from
mom’s egg and one from dad’s sperm
ii. Chromosomes made up of matching sequences of genes
iii. Diploid organisms have 2 copies of a gene for a given
trait
iv. Alleles are different versions of a gene for the same
trait
b. Mendel’s Laws
i. Law of Segregation: “each pair of alleles segregates or
separates during meiosis
1. half the organisms gametes have one allele from
each pair, and half of the gametes have the other
allele
2. when homologous pairs of chromosomes separate
during meiosis, the genes encoded on that
chromosomes must separate also
ii. Law of Dominance: “when 2 alleles in a gene pair are
different, as in a hybrid, one allele can control the trait
and the other is hidden
1. dominant allele is expressed; recessive allele is
hidden
2. recessive allele can be expressed only when
organism has no copy of corresponding dominant
allele for gene
iii. Law of Independent Assortment
1. Mendel studied 2 traits at the same time
2. crossed yellow, round (both dominant traits) with
green wrinkled (both recessive)
a. F1 ‘s all yellow, round
b. F1’s allowed to self fertilize
c. F2’s all possible combinations of color and
shape show
d. Factors are inherited separately
3. “Gene pairs segregate into gametes randomly and
independently of each other”
a. results from behavior of chromosomes
during meiosis
b. separation of chromosome pairs occurs
randomly and produces many different
combinations of chromosomes
i. since genes are on chromosomes,
random separation of chromosome
pairs also randomly separates gene
pairs
c. Language of Genetics
i. Uppercase letter – used to indicate dominate traits
ii. Lowercase letters – used to indicate recessive traits
iii. Same letter used for alleles for same trait – since alleles
are different versions of the same gene
iv. Phenotype – the form of the trait that is observed (tall,
not tall)
v. Genotype – actual genetic make-up of an organism; the
gene pair; shows both alleles (expressed or not), (TT, Tt,
tt)
vi. Purebred – organism having same alleles for a trait (TT,
tt)
vii. Hybrid – organism having two different alleles for a trait
(Tt)
viii. Homozygous – an organism in which two alleles for a trait
are identical
1. TT homozygous dominant
2. tt homozygous recessive
ix. Heterozygous – organism in which 2 alleles for a trait are
not identical (Tt)
d. Probability and Genetics
i. Probability – use of fractions or ratios to predict the
likelihood of an events occurrence
1. symbolized by P
2. used to predict phenotypes and genotypes
a. for example: what is the probability that 2
independent events will both happen – the
chance of the first event is ½; the chance of
the 2nd event is ½; the chance of both
happening is ½ X ½ or ¼ or 25%
e. Punnett Square: a grid for organizing genetic information
i. Makes it easier to predict results of genetic cross
ii. Shows probabilities – not results!
iii. “rules”
1. determine alleles in gametes of parents
2. place alleles of gametes of one parent along the
top of the grid (MOM) and those of the other
parent along left side (DAD)
3. combine alleles inside boxes
4. determine genotype and phenotype s inside the
boxes
iv. Monohybrid Crosses
1. consider only 1 trait
2. G = green; g = not green
v. Dihybrid Cross:
1. studies 2 traits at 1 time
vi. Test Cross
1. used to determine/distinguish between
homozygous dominant and heterozygous organisms
2. bred an individual whose genotype is unknown with
a homozygous recessive individual
a. is unknown is heterozygous, offspring will be
½ dominant phenotype
b. if F1’s are all dominant phenotype – then P
was homozygous dominant
c. if F1’s show both phenotypes, then P was
heterozygous
f. Other patterns of Heredity
i. Intermediate inheritance: some genes do not have
dominant and recessive alleles
1. shows and intermediate inheritance
2. in intermediate inheritance, a heterozygous
individual has a phenotype that is not exactly like
either homozygous phenotypes – both alleles show
ii. Incomplete Dominance: a heterozygote shows a
phenotype that is intermediate between 2 homozygous
phenotypes – neither allele is expressed fully
1. in incomplete dominance only phenotype is
intermediate
2. since there are no dominant alleles – upper and
lower case letters are not used
a. use uppercase with superscripts
iii. co dominance: both alleles in heterozygote express
themselves fully
1. blood type in humans
2. 3 alleles: Ia, Ib, I
iv. Polygenic traits: traits controlled by 2 or more gene pairs
1. human eye pigment, tone, amount, distribution –
each controlled by different gene pair
2. human skin color is polygenic
v. Multiple alleles: many genes have more than 2 alleles
1. human blood types
g. gene expression can be influenced by the activity of other
genes
h. Genes can be affected by the environment inside and outside
the organism