Chapter 15 Flashcards
(56 cards)
When did the “hereditary units” become more accepted?
Around 1902, Sutton and Boveri (and others) noted parallels between chromosome behavior and the behavior of the proposed factors, and began to develop the chromosome theory of inheritance.
What was the first solid evidence of associating genes with chromosomes?
In the early 1900s from the work of Thomas Hunt Morgan. His early experiments provided convincing evidence that the chromosomes are the location of Mendel’s heritable factors.
What did Morgan study?
Drosophila melanogaster, a fruit fly.
Why did Morgan study the fruit fly?
They produce many offspring, a generation can be bred every two weeks, and they have only four pairs of chromosomes.
What is a “wild type?”
Normal phenotypes that were common in the fly populations. Traits alternative to the wild type are called mutant phenotypes.
What was the first mutant Morgan discovered?
A fly with white eyes instead of the wild type red eyes.
What was one of the experiments Morgan did?
Mated male flies with white eyes (mutant) with female flies with red eyes (wild type). The F1 generation all had red eyes, and the F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes.
What did Morgan hypothesize about the white eyes x red eyes experiment?
That the white eyed mutant allele must be located on the X chromosome. Female flies have two X chromosomes, while males only have one (X Y). Morgan’s finding supported the chromosome theory of inheritance.
Are the sex chromosomes (X Y) the only chromosomal system of sex determination?
No.
What gene on the Y chromosome is responsible for the development of testes in the embryo?
The S R Y (sex determining region on the Y).
What is a sex linked gene?
A gene that is located on either sex chromosome. Genes on the Y chromosome are called Y linked genes.
How many genes have been identified on the Y chromosome?
78 genes that code for 25 proteins.
What are X-linked genes?
Genes on the X chromosome. The human X chromosome contains about 1,100 genes.
What are the conditions a recessive X linked gene is expressed?
A female needs two copies of the allele (homozygous), a male needs only one copy of the allele (hemizygous). X linked recessive disorders are much more common in males than in females.
What are some recessive X linked disorders?
Color blindness, Duchenne muscular dystrophy, hemophilia.
What happens in utero to the X chromosomes of a female fetus?
One of the two X chromosomes in each cell is randomly inactivated during embryonic development.
What happens to the inactive X chromosome?
Condenses into a Barr body. If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character.
How is the X chromosome inactivated?
Involves modification of the DNA and proteins bound to it called histones. A part of the chromosome contains several genes involved in the inactivation process. One of the genes there becomes active only on the chromosome that will be inactivated.
What is the gene that inactivates the X chromosome?
X I S T (X-inactive specific transcript).
What are linked genes?
Genes that are located on the same chromosome that tend to be inherited together.
What is genetic recombination?
The production of offspring with combinations of traits differing from either parent.
What do the findings of Mendel and Morgan relate to?
The chromosomal basis of recombination.
What are parental types?
Offspring with a phenotype matching one of the parental (P) phenotypes.
What are recombinant/recombinant types?
Offspring with nonparental phenotypes (new combinations of traits). A 50% frequency of recombination is observed for any two genes on different chromosomes.
