chapter 14: mendel and the gene thing points of emphasis know: 1.all the bold-faced terms 2.the...
TRANSCRIPT
Chapter 14: Mendel and The Gene Thing
Points of Emphasis
Know:
1. all the bold-faced terms
2. the basic crosses, especially a test cross
3. Know something about each genetic disorder
4. Terms you are expected to know: true-breeding, P generation, F1 generation, F2 generation, alleles, dominant and recessive alleles, homozygous, heterozygous, phenotype and genotype, monohybrid and dihybrid
Law of Segregation
There was no “blending” or mixing of the traits that were being studied by Mendel. Pale purple flowers were not produced in the F1
And the trait for white flowers was not lost since it reappeared in the F2. So the trait was not “diluted.”
Mendel was fortunate to pick traits that showed dominance and recessiveness and were determined by one allelic.
Law of Segregation, cont’d
1. Alternative forms of genes are called alleles; genes reside at a particular locus on a particular chromosome. And yes, if we have different forms of a gene, the DNA is different at these loci.
2. Each parent contributes one allele to its offspring, therefore the offspring inherits two alleles.
3. Dominant alleles and recessive alleles exist.
4. Law of Segregation: each of the alleles segregates or separates during the formation of sperm or egg cells. So one sperm cell or egg cell has one of the alleles and the other allele is located in another sperm or egg cell.
Mendel’s Basic Ideas From His Experimentation
(with a little help from modern terminology)
The Testcross
Purpose: to determine the genotype of the dominant allele expressing offspring.
It is always a cross with heterozygous individual for that same trait.
Example: P_ x pp will produce all offspring that are expressing the dominant phenotype if the genotype in question is PP.
P_ x pp will produce offspring that are both expressing dominant and recessive phenotype if the genotype in question is Pp
Therefore you know if the genotype if PP or Pp
Law of Independent Assortment
Applies to different traits, therefore 2 different sets of alleles
The dihybrid cross is the typical example.
Law of Independent Assortment: Alleles of different traits will separate and assort themselves independently of each other. The alleles are put into the gametes in all possible combinations as long as a gamete has one allele for each gene. All of “mom’s” alleles don’t have to segregate together. In a sperm or egg cell you have some of mom’s and some of dad’s and each sperm and egg differs in which of mom’s and which of dad’s alleles it got. That’s why siblings may look really alike (lots of similar allelic combinations), kind of alike (some similar combos) or not very much alike at all (not many similar allelic combinations)
Rules of Probability
Rule of Multiplication
1. First make sure the two events are independent of each other. For example, two tosses of a coin or two alleles from different parents.
2. Compute the probability for each independent event
a) ½ for getting a “heads” and ½ for getting a “tails.”
b) ½ for getting the dominant form, P from the parent; and ½ for getting the recessive form p.
3. Multiply the probabilities.
a) So if we cross Pp x Pp, what is the chance of the offspring being PP? pp?
Rules of Probability (cont’d)
Rule of Addition
1. So this applies if there is more than one way to get a specific outcome, like getting Pp or pP
2. Then we add the separate probabilities.
So: Pp is ½ x ½ or ¼
pP is ½ x ½ or ¼
Then Pp is likely ¼ + ¼ or ½.
Incomplete Dominance
The offspring have an appearance in between the two parents.
This is not “blending.”
Classic example: Snapdragons
Tay-Sachs Disease: accumulation of lipid in the brain cells because of a lack of an enzyme. The heterozygote produces an intermediate level of this enzyme to prevent lipid accumulation. Heterozygotes lack the disease even though at the molecular level they do produce some dysfunctional enzymes.
What is a Dominant Allele?
“Dominance” does not mean it overcomes another allele.
The two forms of a gene really don’t interact. It is just that the dominant gene codes for an enzyme producing a certain trait and the recessive allele does not.
Two dominant alleles will form more enzyme, let’s say, than a heterozygote.
A dominant allele does not mean it is more frequent in the population.
The allele for polydactyly, extra fingers or toes, is a dominant allele but 399 out of 400 people are recessive homozygotes so they show no polydactyly
Dominant traits can be demonstrated as completely dominant, incomplete dominance or codominance.
Codominance
Definition: where both alleles are observed phenotypically to some degree in the heterozygote condition. So the heterozygote is distinctly different from either of the parents but possesses characteristics of each.
For codominant alleles, all uppercase base symbols are used with different superscripts.
LM LM LM LN LN LN
Codominant Example
M-N Blood Groups: M represents an M protein on the RBC and therefore anti-M serum will interact with this protein and give a positive or (+) reaction.
Genotype Anti-M Anti-N Blood Group
LM LM + O M
LM LN + + MN
LN LN O O N
Pleiotropy
All of the phenotypic manifestations of a single gene are described as a Pleiotropic gene effect.
Many of the biochemical pathways are interconnected and interdependent so the phenotypic expression of one gene can effect more than one trait by influencing a multitude of pathways.
Some of the traits are “major”; some have secondary effects that may be less evident. A number of these related changes could be called a syndrome.
Sickle-cell anemia is due to abnormal Hb. This is the primary effect of a mutant gene. Other effects include the clumping of RBCs and the clogging of blood vessels in heart, kidney, spleen and brain. Defective RBCs are destroyed by the body causes anemia.
Epistasis
Epistasis is the interaction between genes that causes the masking of one of the genes effects.
This is the masking effect of one gene locus on another.
This is not dominance which involves intra-allelic gene suppression but epistasis is interallelic gene suppression.
The typical 9 : 3 : 3 : 1 ratio for a dihybrid cross can become modified by epistasis into ratios that are various combinations of
the 9 : 3 : 3 : 1 grouping.
Figure 14.11 An example of epistasis
B represents coat color;
C represents the deposition of the pigment.
Polygenic Inheritance
Many characteristics in a population can be found as a continuum or gradation.
This is due to the additive effect of two or more genes on a single phenotype.
Skin pigmentation, height.
Human Disorders
Recessively Inherited Disorders
Albinism, cystic fibrosis, Tay-Sachs, Sickle-cell
Shows up in homozygous recessive individuals
Heterozygote is the carrier
Human Disorders
Dominantly Inherited Disorders
Achondroplasia
1/25,000
all races, sexes
normal torso; arms and legs are dwarfed in size
the rate at which the cartilage turns into bones is affected in the long bones.
Human Disorders
Dominantly Inherited Disorders
Huntington’s Disease
lethal dominant
symptoms set in later in life so the allele has been passed on to offspring.
Identifying Carriers
Carrier Recognition: identify the heterozygotes
Fetal Testing through Amniocentesis
done at about 4th month
amniotic fluid is removed and lost cells are karyotyped.
Fetal Testing through Chorionic Villus Sampling (CVS)
small sample of the placenta is removed by insertion of a suction tube through cervix and into uterus.
Karyotype is done on these rapidly dividing cells and so results are seen sooner than the amnio test (within 24 hours)
CVS can be performed as early as 2nd month.