Lesson 7 — Chromosomal Theory and Sex-linked Inheritance


Lesson Summary

By recognizing that genes are located on chromosomes, you can go far beyond what Mendel was able to do by crossing peas and analyzing outcomes. You should now be able to explain X-linked inheritance because you know that females have two X chromosomes and that males have only one.

Understanding that genes occur on chromosomes and understanding that males have only one X chromosome while females have two helps us to explain why some traits occur more frequently in males than they do in females. Remember that the Y chromosome is virtually "blank", and that any allele on the X will express itself. For females to express an X-linked trait, the trait must be homozygous recessive, and they have a one in four chance of that happening.

That genes occur on chromosomes has implications far greater than just male baldness. At this time, you should review your meiosis notes because the understanding of how chromosomes move or interact affects the modes of inheritance to be studied next.

During this lesson, you were to consider two focusing questions:

  • Why do some traits appear more frequently in one sex rather than in the other?
  • How did the work of Thomas Hunt Morgan provide 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. Because males receive only one X chromosome, they have a different rate of inheritance than females do for all traits whose alleles are located on the X chromosome. Most genetic diseases are recessive traits. That means, for any recessive traits or diseases appearing on the X chromosome, males have a higher incidence of that trait or disease than females have. Males express the allele they receive on the X chromosome because the Y has nothing to mask over it . This results in males having a 50 : 50 chance of expressing the trait. Females often have at least one normal, dominant allele to cover 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, his work generated experimental evidence that supported the theory. This evidence indicated that genes are located on chromosomes. For this work, Morgan received the Nobel Prize in 1933.


Assignment

Complete the Lesson 7 set of questions in Assignment 6C and Assignment 6D