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

Figure 14.2 Mendel tracked heritable characters for three generations

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)

Figure 14.3 Alleles, alternative versions of a gene

Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

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

Figure 14.6 A testcross

Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

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.

Figure 14.8 Segregation of alleles and fertilization as chance events

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.

Figure 14.9 Incomplete dominance in snapdragon color

Figure 14.9x Incomplete dominance in carnations

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

Figure 14.10 Multiple alleles for the ABO blood groups

Figure 14.10x ABO blood types

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.

Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote

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.

Figure 14.12 A simplified model for polygenic inheritance of skin color

Figure 14.13 The effect of environment of phenotype

Hydrangeas and pH of the soil.

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.

Identifying Carriers

Fetal Testing through ultrasound

anatomical abnormalities