• 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.

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.

• 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:

• Example 2:

• Answer:

• 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:

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

• 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.

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.

• 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

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

ABO Blood typing

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

• 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.

• 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

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

• 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.