1865 - Gregor Mendel - performed a series of genetic experiments with the edible pea plant. Several factors worked in his favor for this type of study.

1. Peas usually self-pollinate.

3. Peas can be cross-pollinated by hand, so two true-breeding plants could be crossed.

Original plants that are cross-pollinated are called the Parental Generation, and are abbreviated, P. First generation offspring are called F1, for 'first filial.' Second generation offspring are F2.

Each trait is determined by discrete units (genes). Each organism has 2 genes for each trait.

Pairs of genes separate during gamete formation and each gamete receives only one of an organism's pair of genes. This is called the Law of Segregation.

The specific gene that is included in a gamete is determined by chance.

There may be 2 or more alternative forms for a gene. These forms are called alleles. One allele may be dominant (and is designated with a capital letter) and another recessive (designated with a lower case letter).

True breeding organisms have two of the same allele for a given trait, e.g. PP, or pp. This is called Homozygous. Having two different alleles are called Heterozygous.

Test crosses. Once you understand how a monohybrid cross works, and how to set up a Punnett square, you can perform a test cross. Cross-fertilization of a phenotypically dominant individual with a homozygous recessive individual is called a test cross, because it can be used to test whether the dominant parent is homozygous or heterozygous.. By setting up Punnett squares of a test cross between a purple flowered plant and a white flowered plant, could you tell what the genotype of the purple parent is based on the offspring of that test cross?

1. There are two alleles of each gene in each organism; likewise, there are two homologous chromosomes in each diploid cell.
2. Only one of each gene is passed on to the offspring in each sperm or egg; during meiosis only one of each pair of homologues ends up in each haploid daughter cell.
3. Different genes sort out independently of each other; the different types of chromosomes also appear to assort independently. (Mendel's Law of Independent Assortment)

Once Mendel understood and could predict the outcomes of mono-hybrid and test crosses, he turned to more complicated crosses, dihybrid crosses.

 

Sex Linkage/Gene Linkage

Genes that are found on one sex chromosome (X or Y) but not on the other are called sex-linked genes. The X chromosome might carry genes that are not found on the Y chromosome, and vice versa. The Y chromosome carries relatively few genes other than those determining maleness, but the X chromosome bears many genes. In fruit flies, for example, the gene controlling eye-color is found on the X chromosome although it has nothing to do with female traits. Because this gene is found on the X chromosome, it is X-linked. There are only a few Y-linked genes.

 

 

 

 

 

Females can be homozygous or heterozygous for an X-linked trait and this would be designated, X^R X^R or X^R X^r, respectively. Whatever gene a male carries on his single X chromosome is the only copy of that genes that he has. That gene cannot be masked by a corresponding gene on a second X chromosome as they can in females. The result of X-linked traits, such as color-blindness or hemophilia, is that the trait is much more prevalent in the male population than the female population.

 

 

Red eyed female x white eyed male.

What proportions of offspring would you predict from a cross between a white eyed male and a heterozygous red eyed female?

 

 

 

X linkage is a special case of the general phenomenon of gene linkage, in which two or more genes tend to be inherited together because they are on the same chromosome.

1. Genes located on different chromosomes assort independently during meiosis. (Mendel studied traits whose genes were on different chromosomes.)
2. Genes located on the same chromosome usually will not assort independently. The exception to this is if there is a cross-over during meiosis.
3. F2 generation of a dihybrid cross has a 3:1 ratio (not 9:3:3:1) if the genes are linked.

So, there are modifications to Mendel's Law of Independent Assortment: Genes on different chromosomes assort independently, BUT genes on the same chromosome tend to be inherited together (e.g., flower color and pollen grain shape)

Mendel's situation was unique in that the dominant traits he studied always overpowered or masked the recessive allele. Often however, a heterozygous individual may appear to have a 'blended' phenotype, somewhere between the homozygous dominant and homozygous recessive traits. This is the case with flower color in sweet pea flowers. In order to have a deep red color, there must be two copies of the dominant allele (RR). Heterozygotes (Rr) will only have one 'copy of the red gene so the flowers appear pink. Homozygous recessive flowers (rr) will be white.

While an individual may only have up to two alleles for a trait (because of their two homologous chromosomes), some traits have more than two alleles. Often, several alleles may show dominance, as in the ABO blood groups. These are called multiple alleles. The blood groups arise from three separate alleles for a glycoprotein on the surface of red blood cells, and individual genes are designated I^A I^B or i, which is the O blood type (absence of A or B glycoprotein gene). Following is the breakdown of the different genotypes and phenotypes that humans may exhibit.

Some traits are influenced by more than one genes. This is polygenic inheritance. Wheat kernal color and skin pigmentation are good examples of this. Each gene contributes one 'dose' of pigment.

Sometimes there are genes whose actions are required for other genes to be expressed. This is called epistasis. This has an effect on mammalian hair color. The dominant allele of this gene allows pigment to be produced, while the recessive allele does not. A second gene controls the distribution of the pigment in the hair.

Coat color in Labrador Retrievers

BB or Bb-----------> Black

bb-------------------->Chocolate

Where do Yellow Labs come from?

Yellow vs. Dark (Black or Chocolate) is controlled by the Extension Gene (E)

EE or Ee--------->dark color

ee------------------>yellow (regardless of BB or bb)

BbEe X BbEe Set up this cross and determine the ratios of the offspring?

Sometimes one gene will have multiple effects. Examples include albinism and sickle-cell disease.

Factors such as temperature may effect gene expression. Temperature sensitive alleles are found in the Himalayan rabbit and Siamese cats

Normal body temperatures - no pigments

Lower temperatures (as found on extremities) results in darker pigment production.