1.4 Mendelian Genetics and Inherited Traits

Who is Gregor Mendel?

For thousands of years farmers and herders have been selectively breeding plants and animals to produce offspring who have inherited the best traits from the parents. It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown. Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half. By the 1890's, the invention of better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction. The focus of genetic research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children. A number of hypotheses were suggested to explain heredity, but Gregor Mendel , a little known Central European monk, was the only one who got it more or less right. His ideas had been published in 1866 but largely went unrecognized until 1900, which was long after his death. His early adult life was spent in relative obscurity doing basic genetics research using pea plants. He came to three important conclusions from his experimental results:

1. that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged (these units are now called genes).
2. that an individual inherits one such unit from each parent for each trait.
3. that a trait may not show up in an individual but can still be passed on to the next generation.

'Click here' to read about Gregor Mendel and his research into heredity using pea plants.

Gametes and Chromosome Number Review

Recall from Science 9 that meiosis occurring in the gametes or sex cells of the parent organism contain half the number of chromosomes as all other body cells.

Monohybrid Crosses

Watch the video which introduces you to Mendelian Genetics using a 'Monohybrid Cross' and a 'Punnett Square'.

'Click here' to download a worksheet where you can practice monohybrid crosses using a punnett square

Dihybrid Crosses

This video introduces you to a 'Dihybrid Cross' using a Punnett Square.

'Click here' to download a worksheet where you can practice dihybrid crosses using a pungent square.

Spirit Bears: Rainforest Guardians

"The raven left every tenth bear white, just to remind us of when the earth was covered in glacier," says Marven Robinson of the Gitga'at First Nation in British Columbia, Canada. The white bears are known as Moksgm'ol to the people of the First Nations, and as Kermode bears to scientists. They aren't polar bears or albino bears—rather, they're black bears born with a double recessive gene that turns their fur white.

Punnett Square Vocabulary Review

'Click here'  to review the vocabulary introduced with the monohybrid and dihybrid crosses you just learned . A key is provided for your reference.

Punnett Square Practice Questions

'Click here'  for Punnett Square practice questions.  The solutions are provided at the end of the worksheet so you can check your work.

Pedigree's and Inheritance

A pedigree in genetics is a diagram showing the lineage or genealogy of an individual and all the direct ancestors, usually to analyze or follow the inheritance of trait.

Hemophilia has been called a "royal disease". This is because the hemophilia gene was passed from Queen Victoria, who became Queen of England in 1837, to the ruling families of Russia, Spain, and Germany. Queen Victoria's gene for hemophilia was caused by spontaneous mutation.

Hemophilia is inherited in a sex-linked recessive pattern. The genes associated with these conditions are located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, it is very rare for females to have hemophilia. A characteristic of an altered gene carried on the X chromosome is that fathers cannot pass this sex-linked trait to their sons.