Module 6
1. Module 6
1.31. Page 4
Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation
Lesson Summary
During this lesson you were to examine the following focusing questions:
- Why do some traits appear more frequently in one gender than the other?
- How did Thomas Hunt Morgan’s work create experimental support for the chromosomal theory of inheritance?
Genes occur on chromosomes. This is true for all chromosomes, including the X chromosome. However, the Y chromosome has few genes. Since males receive only one X chromosome, they will have a different rate of inheritance than females for all traits whose alleles are located on the X chromosome. Most genetic diseases are recessive traits. So, for any recessive traits or diseases appearing on the X chromosome, males will have a higher incidence of that trait or disease than will females. Males will always express whichever allele they receive on the X chromosome. Therefore, males have a 50 : 50 chance of expressing the trait. Females often have at least one normal, dominant allele to “cover up” the disease, as in the homozygous dominant and in the heterozygous condition. To express the recessive X-linked trait, a female must be homozygous recessive.
Thomas Hunt Morgan and his team worked with fruit flies to try to disprove the chromosomal theory of inheritance. However, Morgan's work proved to generate experimental evidence that actually supported the theory. This evidence indicated that genes were in fact located on chromosomes. For this work he received the Nobel Prize in 1933.
Lesson Glossary
Consult the glossary in the textbook for other definitions that you may need to complete your work.
chromosomal theory of inheritance: the theory that chromosomes are strings of genes and that each gene has a specific location on a specific chromosome
hemophilia: an X-linked recessive condition; caused by the recessive allele of the Factor VIII gene on the X chromosome; characterized by inability to clot blood
progeny: offspring
sex-linked: a trait that is found in one gender more than another; contrary to autosomal traits that are found equally in both genders
X-linked: a trait that is found in one gender more than in the other gender, and so is sex-linked; contrairy to autosomal traits, which are found equally in both genders