Genetics
and Heredity Notes
I. Background
A. It
was known for 1000s of years that traits were inherited but scientists were
unsure about the laws that governed this inheritance.
B. Gregor Mendel (1822-1884) was an Austrian monk who
experimented with garden peas and developed the foundation of modern genetics.
He noticed that peas had several traits and always showed only one of a pair
rather than a blend which was previously believed. He crossed plants with different
traits to see what the offspring would look like.
C. Mendel
found that no matter what combinations he tried, one trait always dominated and
masked the other. It didn’t matter if the trait came from the male or female
parent. The traits were controlled by factors which were later known as genes.
D. Mendel’s
Laws of Heredity
1. Inherited
traits are controlled by genes that occur in pairs. These two versions are
called alleles. For example, the gene that controls the color of the flowers in
Mendel’s peas has two alleles - purple and white.
2. An
organism inherits an allele for each trait from each parent (2 alleles for each
trait total)
3. One
allele masks the presence of the other. Called the principle
of dominance. Dominant (R) vs recessive (r)
4. Alleles separate during
E. Vocabulary
1. Homozygous
- both alleles for a trait are the same
2. Heterozygous
- the alleles for trait are different
3. Genotype
- the actual genetic makeup for a trait
4. Phenotype
- the way in which the genotype is expressed
II. Monohybrid
Cross
A. Mendel
found a 3:1 ratio in F2
e.g., round
seed wrinkled
P RR x rr
gametes R,
R r,
r
F1 Rr (all round)
use two F1 individuals as new Parents
P Rr
x Rr
gametes R,
r R,
r
F2 RR,
Rr, Rr,
rr (round, round, round, wrinkled; 3:1)
B. A
Punnett square can be used to show genotype,
phenotype, and probability.
e.g., heterozygous purple (Pp) x white (pp)
|
|
p |
p |
|
P |
Pp |
Pp |
|
p |
pp |
pp |
F1 1 purple: 1 white
e.g., two heterozygous tall plants (Tt)
|
|
T |
t |
|
T |
TT |
Tt |
|
t |
Tt |
tt |
F1 3 tall: 1 short
C. Test
cross
1. Imagine
that you have an organism showing a dominant phenotype. Is the individual
homozygous or heterozygous? To be able to say for certain, a test cross is
performed.
2. The
unknown individual is crossed with a homozygous recessive individual.
3. The
genotype of the unknown parent can be deduced from the appearance of the
offspring.
III. Dihybrid Cross
A. Mendel
wondered if traits always travelled together or if
they were inherited separately. e.g., if he crossed a yellow, round
plant with a green, wrinkled plant would all the offspring be yellow, round or
green, wrinkled or would some be yellow, wrinkled and
some green, round
B. He
found a 9:3:3:1 ratio in the F2
C. This
showed that traits are inherited independently.
e.g., YyRr x YyRrr
|
|
YR |
Yr |
yR |
yr |
|
YR |
YYRR |
YYRr |
YyRR |
YyRr |
|
Yr |
YYRr |
YYrr |
YyRR |
Yyrr |
|
yR |
YyRR |
YyRr |
yyRR |
yyRr |
|
yr |
YyRr |
Yyrr |
yyRr |
yyrr |
F1 9 yellow, round: 3 yellow,
wrinkled: 3 green, round; 1 green, wrinkled
IV. The
probability scale ranges from 0 to 1, where 0 means there is no chance the
event will occur and 1 means the event will occur every time. Probability can
be calculated using the equation:
P
= #correct outcomes
# total
outcomes
A. Independent
events - the outcome of previous events does not affect the outcome of future
events. e.g., the chance of getting heads in a coin toss is ˝; the
chance of getting heads a second time is ˝
B. Rule
of Multiplication - The chance of two events occurring together is P1
x P2
C. Rule
of Addition - The chance of either one of two possible outcomes occurring is
the sum of the two individual probabilities.
V. Cases
of Non-Simple Dominance
A. Incomplete
Dominance
1. So
far, offspring have showed the phenotype of one parent or the other. In some
traits, the offspring have a phenotype which seems to be a blend of the two
parents.
2. This
means that heterozygotes will have a phenotype
different from that of the two homozygous genotypes.
3. A
1:2:1 is characteristic of incomplete dominance.
B. Codominance
1. A
case in which two alleles are expressed at the same time.
2. The
heterozygote phenotype appears to be a blend of the two homozygous phenotypes.
3. An
example is roan cattle. A cross between a red bull and a white cow yields roan
calves. They calves appear reddish in color but on closer inspection, they have
both red and white hairs. In other words, BOTH alleles are expressed..
4. A
1:2:1 is characteristic of codominance.
C. Multiple
alleles
1. Many
genes actually have more than two alleles.
2. Remember
that, although more than two alleles exist in the population, each individual
only possesses two - one inherited from each parent.
D. Epistasis
1. A
gene at one locus alters the phenotypic expression of a gene at another locus.
E. Polygenic
inheritance
1. For
some traits, an either-or result for phenotype does not exist. Rather, the
phenotype differs along a continuum.
2. Examples
are human height and skin color.
VI. Human
Genetic Disorders
A. Recessively
Inherited
1. Remember
that genes code for proteins. An allele that causes a genetic disorder codes
for a non-functional protein. Homozygous dominant (AA) and heterozygous (Aa) individuals are normal in
phenotype because the one copy of the normal allele produces a sufficient
quantity of the protein to prevent the disorder.
2. A
homozygous recessive (aa)
individual is unable to produce any of the protein in question.
3. Heterozygous
individuals are said to be carriers - they have the recessive allele but do not
show the recessive phenotype.
4. If
the disorder is lethal before reproductive age, no homozygous recessives will
reproduce.
5. In
general populations, it is unlikely that two carriers of the same disorder will
meet and mate. This probability increases, however, in matings
between close relatives such as siblings or relatives. In these so called
“inbred” matings, there is an increased risk that
offspring will have a recessive genetic disorder.
B. Dominantly
Inherited Disorders
1. A
lethal dominant allele is more rare because even heterozygotes
are affected (i.e., die). If the disorder is lethal before reproductive
age, the allele will not be passed on. The allele can be perpetuated in the
population if it is late-acting.
C. Genetic
counselling
1. Carrier
recognition
a. Because
most children with genetic disorders are born to parents of normal phenotypes,
it is important to identify parents who might be carriers before they
reproduce.
2. Fetal
testing - testing the fetus to determine the presence of any genetic disorders.
Fetal testing gives parents the option of
a. Amniocentesis
- Beginning around the 14th week of pregnancy, amniotic fluid is
withdrawn from the uterus. Some disorders can be detected by the presence of
certain chemicals in the fluid while fetal cells present in the fluid can be
grown to be used for karyotyping to identify
chromosomal abnormalities.
b. CVS
- chorionic villus sampling
- A small amount of fetal tissue is removed from the placenta and the cells are
used for karyotyping. The cells are growing quickly
so results are available in 24h as opposed to several weeks as with
amniocentesis. CVS can also be performed as early as the 8th wekk of pregnancy.
c. Ultrasound
and fetoscopy - these techniques are used to produce
and image of the fetus
3. New-born
screening - some disorders can be detected at birth by testing the new-born
baby.
Chromosomal
Theory
VII. Main
Points - it was noticed that the behaviour of
chromosomes and that of genes were related.
A. Chromosomes
carry genes, the unit of hereditary. Both chromosomes and genes are both
present in pairs in diploid cells.
B. Homologous
chromosomes separate and alleles segregate independently during meiosis.
C. Fertilization
restores chromosomes and genes to pairs.
VIII. Morgan’s
Work
A. Morgan
was the first to work out that genes are located on chromosomes.
B. He
developed a different notation for genetic symbols. For example, the allele for
the white eye mutation is symbolized by w, while the normal allele (called the
wild type) is symbolized by w+.
C. If
the mutation is recessive, a lower case letter is used. Upper case is used for
dominant mutant alleles. For example, curly wings is
caused by a dominant allele and is symbolized by Cy, while normal wings is Cy+.
D. Sex-linkage
1. Morgan
noticed a male with white eyes among many red-eyed males.
2. He
crossed the white-eyed male with a red-eyed female and the F1 were
all red, as expected. The F2 was 3 red:1
white. Only males, however, had white eyes and, among males, he noticed there
was 1 red:1 white eyes.
3. A
karyotype showed that males had a different chromosome
from the females - the sex chromosome. Morgan reasoned that the gene for eye
color must be located on the sex chromosomes and called the trait sex-linked.
4. Remember
that a male always inherits a sex-linked trait from the female parent because
the father always supplies the y chromosome.
5. X-inactivation
- During early embryonic development, one X chromosome in each cell randomly
becomes inactive. The chromosome condenses to a small spot near the nuclear
membrane and is called the Barr body.
E. Linked
genes and Chromosome Mapping
1. The
number of genes in a cell is far greater than the number of chromosomes so it
stands to reason that each chromosome must carry many genes. These genes would
tend to be inherited together and are called linked.
2. In
some dihybrid crosses, Morgan found that most
offspring had the same phenotypes as the parents, but other phenotypes were
also observed. How could these other phenotypes arise?
3. Crossing
over would occur more often between two genes as the distance between those two
genes increased. If this relationship is linear, the frequency of crossing over
could be used to determine the distance between two genes on the same
chromosome.
4. One
“map unit” is defined as 1% crossing over frequency. What is the maximum
frequency for crossing over? If two genes are further apart than this value,
how can they be mapped?