Download PATTERNS OF INHERITANCE Gene - sequence of DNA that codes

PATTERNS OF INHERITANCE
Gene - sequence of DNA that codes for, or specifies the
production of a particular protein.
Alleles - A different form of the same gene (affects same trait but in different ways).
Locus - The physical site along a chromosome that is occupied by a gene.
Gene product - The protein that is made based on a gene sequence.
Homologous - Chromosomes that are similar in physical appearance and which carry the same
genes in the same order (may have different alleles at a given locus).
gene pair - Genes on homologous chromosomes at a specific locus.
homozygous - An organism with the same alleles present on each member of a homologous gene
pair.
heterozygous - An organism with different alleles present on each member of an homologous
gene pair.
If an allele produces an effect, it is said to be expressed. If one allele is expressed or produces an
effect that masks or hides the effect of the other gene, that gene is called dominant and the gene
that is not apparently affecting the trait is called recessive.
The sum total of all the genes in an individual constitute the genotype.
The outward expression of the genes as influenced by the environment as the organism develops
is called the phenotype.
Genotype : genetic constitution
Phenotype : apperance of a trait in a given environment
phenotype = genotype x environment.
(signifies an interaction between environment and genotype.)
Segregation and recombination of genes occurs during gamete formation and reproduction.
B
b
Example : Gene for hair color in guinea pigs.
2 alleles (2 different forms of the same gene)
B - allele for black hair ; b - allele for brown hair
BB
B
Bb
Bb - heterozygous - get black hair (B dominant to b)
BB - homozygous - get black hair
bb - homozygous - get brown hair
b
Bb
bb
In simple dominant recessive relationsip, 2 heterozygous individuals produces
3 : 1 pheneotype
1 : 2 : 1 genotype.
Test cross
If you cross a homozygous recessive individual (for a given trait), with another individual, you
can figure out if the second individual is homozygous or heterozygous. You can do this based on
predicted expectations knowing how dominance works. The probability is independent for each
event.
e.g.
B
?
b
Bb
Bb
b
Bb
Bb
B
?
b
Bb
bb
b
Bb
bb
The phenotypes resultant from the above cross would be all black in the first case, thus indicating
the unknown allele (?) had to be a “B,” whereas in the second case, because half of the offspring
are black and half are brown, the unknown allele (?) had to be “b.”
Autosomal Recessive Inheritance
Recessive allele exists on an autosomal chromosome. Since the trait is recessive, if a dominant
allele is present, as in a heterozygous individual, the trait is not expressed. However, in a
homozygous recessive individual, it is expressed.
E.g. Galactosemia - individuals with this trait cannot digest milk sugar. As infants, if not
diagnosed, they exhibit malnutrition, diarrhea and vomiting. Usually they die in infancy or
childhood, if the condition is undetected. If a heterozygous male marries a heterozygous female,
neither adult exhibits galactosemia, but their children have a 1 in 4 chance that they will.
G
G
g
GG
Gg
genotype of offspring - 1GG:2Gg:1gg
g
Gg
gg
phenotype of offspring - 3 normal:1 galactosemic
Autosomal Dominant Inheritance
Huntington's disorder - causes progressive degeneration of the nervous sytem. Onset is usually
after the child-bearing years, in forties or so. In this case, the trait is caused by a dominant allele.
Expression leads to brain and nerve deterioration and death. An individual only has to have 1
allele for this trait to be expressed. So either a homozygous dominant or a heterozygous
individual will be afflicted with the disorder.
Thinking point:
If the trait is dominant and leads to death, why isn't it removed from the population? If all people
that have it die, why does it still exist?
Because they don't die until after they have had a chance to pass that gene along.
Incomplete Dominance
Phenotype is intermediate between two parents. E.g., you cross a red-flowered snapdragon with
a white one, and you get pink-flowered offspring. Has to do with the amount of red pigment
required to make a red flower. If you have both alleles coding for pigment-producing proteins,
then you can make enough pigment to make red flowers.
Codominance
Each allele can produce it's own phenotype, and they can exist in combination. E.g., with human
blood groups.
Blood types are A, B, AB or O.
You have genes that produce proteins that are located on the red blood cells.
ALLELES
IAIA, IAIO
IBIB, I BIO
IAIB
IOIO
BLOOD TYPE
A
B
AB
O
Rh System
If you have Rh + blood, on the surface of your RBC's you have a protein called antigen D.
If you have Rh - blood you don't make antigen D, but if you are exposed to antigen D (by
transfusion, e.g.) you will make antibodies to it.
If a man is Rh + and marries a woman who is Rh - the baby may be Rh + inside a Rh - mother.
During birth, some of the baby's blood may be forced back through the placenta and enter the
mother.
Erythroblastosis fetalis:
Being exposed to Rh + blood with antigen D, she makes antibodies against Rh + blood. If she
gets pregnant again with a baby with Rh + blood, her antibodies will attack the baby's RBC's and
can kill the baby.
But if they know the parent's blood types are a potential problem, the woman receives an injection
of RhoGam within 72 hours of delivery to block her production of antibodies, so the next baby
will be OK.
The RhoGam injection contains modified antibodies. These antibodies find the baby’s red blood
cells in the mother and bind to the Rh surface protein that serves as an antigen. Then the
mother’s immune cells never see the antigen since it’s not exposed and therefore they don’t make
antibodies.
Genetic Determination of Sex
22 pairs of autosomes, 1 pair of sex chromosomes.
Females - 2X chromosomes
Males - XY chromosomes
Father always determines sex (gender) of child.
Female contributes only X's. Male can contribute X or Y.
X
X
X
XX
XX
Y
XY
XY
The X chromosome has more genes on it than the Y chromosome. The Y has genes for
"maleness".
Since the male has only 1 X chromosome, and recessive genes present will be expressed since
there is no dominant allele to mask it's expression. A female may have a recessive gene, but if
have a dominant allele on the other X, won't see effect of recessive allele. Will only see effect if
she is homozygous for the recessive trait. A male that has a gene expressed from the X
chromosome is said to be hemizygous for that trait (neither homozygous nor heterozygous since
don't have 2). Trait is called sex-linked because inheritance of trait is linked to gender of child.
Permanent Gene Inactivation
Normally, one of the female's X chromosomes is in activated in each cell. It becomes a dense,
dark-staining body in the cell's nucleus called the Barr body. One can use the presence of the
Barr body to sex type a person. To sex type is to determine the gender.
Gene inactivation by condensation of chromosome leads to formation of gentic mosaics in
women. Exhibited by women with anhidrotic ectodermal dysplasia (absence of sweat glands).
X-Linked Recessive Inheritance
One sex-linked trait is color-blindness. More frequent in males than females, because it is a
recessive trait. Females are carriers of the gene, but it is usually expressed in the male offspring.
Hemophilia is another sex-linked trait.
Female is "carrier", has gene but not expressed.
If mate with normal male could produce:
(1) daughter with color vision
(2) daughter who is carrier
(3) son with normal vision
(4) son with color-blindness
X
XC
X
XX
XXC
Y
XY
XCY
If color-blind male, marries a "carrier" female, can produce:
(1) daughter "carrier"
(2) color-blind daughter
(3) color-blind son
(4) color-vision son
If color-blind male marries color-blind female,
all children will be color-blind.
X
XC
X
XXC
XCXC
Y
XY
XCY
GENE LINKAGE
Just as a trait can be linked to the sex of an individual by being physically located on a sex
chromosome (X or Y), traits can be linked by being located along one autosome. They would not
segregate independently because they were physically located along the same DNA molecule.
But remember that homologous chromosomes or chromatids can cross-over (physically exchange
parts). And remember that crossing over is very common.
It is easy to imagine that if two genes are far apart, there is a greater chance that crossing over
will occur between them than if they are very close. By observing the frequency with which 2
alleles are separating by crossing over, you can get an estimate of how far apart the 2 genes are.
This is the basis for gene mapping. The map units are based on cross over frequencies. You can
thus get a relative ordering of the genes, and can tell which gene is next to which gene.
POLYGENIC INHERITANCE
Not all traits are controlled by simple Mendelian patterns of inheritance. Some traits exhibit
continuous variation and are called quantitative traits. They are traits that are controlled by a
number of genes interacting, thus are polygenic traits, e.g. intelligence, looks, height, skin color,
eye color, etc. We can measure variability and predict inheritance of patterns using mathematical
formulas.
POPULATION GENETICS
If some traits are dominant over others, why don’t the recessive alleles disappear? R. C. Punnett
and his mathematician friend G. H. Hardy addressed this question, as did a German physician
named Wilhelm Weinberg, and an American named W. E. Castle. They derived a mathematical
formula that demonstrated that allele frequencies will remain the same generation after generation
if certain conditions are met.
The assumptions are:
1. You start with a very large population.
2. There is no immigration (in) or emmigration (out).
3. No mutations occur.
4. Mating is random.
5. All alleles are equally viable.
If these conditions are met, then allelic frequencies will remain unchanged. The extent to which
allele frequencies change is a reflection of how many of the assumptions are not met and to what
extent.
MUTATIONS
Point Mutations - changes in a single nucleotide.
Chromosomal Mutations
Deletions - portion of a chromosome is lost.
Inversions - portion of a chromosome is detached and re-attached in reverse orientation.
Duplications - portion of a chromosome carries the same alleles as another portion.
(Usually the duplicated portion comes from another chromosome that
had a deletion.)