Genetics

Classical Genetics.

Vocabulary:

• Genetics: The scientific study of heredity
• Allele: Alternate forms of a gene/factor.
• Genotype: combination of alleles an organism has.
• Phenotype: How an organism appears.
• Dominant: An allele which is expressed (masks the other).
• Recessive: An allele which is present but remains unexpressed (masked)
• Homozygous: Both alleles for a trait are the same.
• Heterozygous: The organism's alleles for a trait are different.

History:

• Principles of genetics were developed in the mid 19^th century by Gregor Mendel an Austrian Monk
• Developed these principles without ANY scientific equipment - only his mind.
• Experimented with pea plants, by crossing various strains and observing the characteristics of their offspring.
• Studied the following characteristics:
• Pea color (Green, yellow)
• Pea shape (round, wrinkled)
• Flower color (purple, white)
• Plant height (tall, short)

• Made the following observations (example given is pea shape)
• When he crossed a round pea and wrinkled pea, the offspring (F1 gen.) always had round peas.
• When he crossed these F1 plants, however, he would get offspring which produced round and wrinkled peas in a 3:1 ratio.

• From this he developed 2 laws of inheritance:
• Law of Segregation: When gametes (sperm egg etc…) are formed each gamete will receive one allele or the other.
• Law of independent assortment: Two or more alleles will separate independently of each other when gametes are formed.

Using a Punnett square:

• Genetic problems can be easily solved using a tool called a punnett square.

Punnett square

• Using this is a several step process, look at the following example

• Tallness (T) is dominant over shortness (t) in pea plants. A Homozygous tall plant (TT) is crossed with a short plant (tt). What is the genotypic makeup of the offspring? The genotypic makeup?

• First, take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.

Step 1

• Second, write the letter for each allele across each column or down each row. The resultant mix is the genotype for the offspring. In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.

Step 2

• Lets take this a step further and cross these F1 offspring (Tt) to see what genotypes and phenotypes we get.
• Since each parent can contribute a T and a t to the offspring, the punnett square should look like this….

• Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses.

Dihybrid crosses

• Dihybrid crosses are made when phenotypes and genotypes composed of 2 independent alleles are analyzed.
• Process is very similar to monohybrid crosses.
• Example:

• 2 traits are being analyzed
• Plant height (Tt) with tall being dominant to short,
• Flower color (Ww) with Purple flowers being dominant to white.

• The cross with a pure-breeding (homozygous) Tall,Purple plant with a pure-breeding Short, white plant should look like this.

f₁ generation

• Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f₁ generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this.

f₂ generation

• Note that there is a 9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits.
• Also note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortment.

Probability & Genetics

• The products of a genetic cross can also be predicted using laws of proobability.
• There are two basic laws which apply

• The Product law applies when two or more events occur independently of each other.

• Each individual probability is multiplied to obtain an overall probability of an event occurring

• Example: What is the probability of a coin toss resulting in 2 consecutive heads?

• Answer:

The probability for each toss individually resulting in heads is 1/2.
Thus: to obtain the probability of two successively one multiplies their individual probabilities. (1/2) X (1/2) = 1/4

• Example 2:

If both parents are Bb what is the probability of the offspring being bb?

• Answer:

The offspring must receive a b gamete from both parents.
The probability of a b sperm is 1/2
The probability of a b ovum is also 1/2
Thus the probability of a bb offspring is the product of the two individual probabilities :(1/2)X(1/2)=1/4

• The Sum Law Predicts the probability of mutually exclusive events:

• This is done by adding the individual probabilities of events

• Example: What is the probability if we flip a coin twice, that it will come up with heads one time, and tails the next if we do not specify the order in which they come?

• Answer:

There are 2 mutually exclusive ways this can occur:

1. Heads the first time, tails the second.
2. Tails the first time heads the second.

The probability for each event (H/T) is 1/2
Thus the probability of situation 1 can be derived using the product law:
(1/2) X (1/2) = 1/4
The probability of situation 2 can thus be the same:
(1/2) X (1/2) = 1/4
Since the individual events are mutually exclusive, the total probability of either occurring can be derived using the sum law:
(1/4) + (1/4) = 1/2

Chromosomes and Classical Genetics

• Walter Sutton in 1902 proposed that chromosomes were the physical carriers of Mendel's alleles
• Problems arose however regarding the following question:
• Why are the number of alleles which undergo independent assortment greater than the number of chromosomes of an organism?

• This was explained understanding of 2 additional factors; Sex Linkage and crossing over:

Sex Linkage:

• All chromosomes are homologous except on sex chromosomes.
• Sex chromosomes are either X or Y.
• If an organism is XX, it is a female, if XY it is male.
• If a recessive allele exists on the X chromosome. It will not have a corresponding allele on the Y chromosome, and will therefor always be expressed.

Example:

• Recessive gene for white eye color located on the X^w chromosome of Drosophila.
• All Males which receive this gene during fertilization (50%) will express this.
• If a female receives the X^w chromosome. It will usually not be expressed since she carries an X chromosome with the normal gene.

Crossing Over / Recombination

As seen before, genetic recombination can occur during synapsis of meiosis. This can explain much of the variability which challenged the principles of independent assortment through chromosome migration.

Gene Mapping

• Since generally the farther genes are apart from each other on a chromosome the more frequent their recombination during meiosis.

• Given the frequency of recombination of genes, we can construct a gene map which roughly estimates the physical locus of that gene on the chromosome. This distance is expressed as a Map Unit.

Example Gene Map

Other Factors Effecting Phenotypic Expression:

Multiple alleles:

• Phenotypes are controlled by more than 1 allele. Eg. Blood types are regulated by 3 separate genes.

ABO Blood typing

• Humans have multiple types of surface antigens on RBC's
• The nature of these surface proteins determines a person's Blood Type.
• There are 3 alleles which determine blood type I^A, I^B, or I^O. This is referred to as having multiple alleles
• Human blood types are designated as A, B or O.
• Type A denotes having the A surface antigen, and is denoted by I^A
• Type B denotes having the B surface antigen, and is denoted by I^B
• Type O denotes having neither A or B surface antigen, and is denoted by I^O

• There are several blood type combinations possible
• A
• B
• AB (Universal recipient)
• O (Universal donor)

• A person can receive blood only when the donor's blood type does not contain any surface antigen the recipient does not. This is because the recipient has antibodies which will attack any foreign surface protein.
• Thus, Type AB can accept any blood types because it will not attack A or B surface antigens. However, a type AB person could only donate blood to another AB person.
• Also, Type O persons are Universal donors because they have NO surface antigens that recipients' immune systems can attack. Type O persons can ONLY receive blood from other type O persons.
• There is another blood type factor known as Rh.
• People are either Rh⁺ or Rh^- based on a basic dominant/recessive mechanism.
• Not usually a problem except with pregnancy.
• It is possible that an Rh^- mother can carry an Rh⁺ fetus and develop antibodies which will attack & destroy the fetal blood
• This usually occurs with 2^nd or 3^rd pregnancies, and is detectable and treatable.

Gene interaction:

• Many biological pathways are governed by multiple enzymes, involving multiple steps. If any one of these steps are altered. The end product of the pathway may be disrupted.

Continuous variation:

• Many traits may have a wide range of continuous values. Eg. Human height can vary considerably. There are not just "tall" or "short" humans.

Continuous Variation of Human Height

Environmental effects:

• Sometimes genes will not be fully expressed owing to external factors. Example: Human height may not be fully expressed if individuals experience poor nutrition.

Incomplete Dominance.

• Some alleles for a gene are not completely dominant over the others. This results in partially masked phenotypes which are intermediate to the two extremes.