Chromosomal Basis of Inheritance I Flashcards
In 1883, what did August Weismann do?
His germ plasm theory separates germ cells from somatic cells and claims that only the first are involved in inheritance.
What idea did Theodor Boveri propose?
He proposed the idea that chromosomes are the physical basis of inheritance.
What led to the chromosomal theory of heredity in 1902?
The parallels between Mendelian inheritance and chromosome behavior were well noted and gave rise to the theory.
What is the chromosomal theory of heredity?
Mendel’s hereditary factors (genes) have specific loci (positions) on chromosomes, and it is the chromosomes that undergo segregation and independent assortment.
The first solid evidence associating a specific gene with a specific chromosome came from which scientist?
The embryologist Thomas Hunt Morgan.
What did Morgan’s experiments show?
His experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors.
What characteristics make fruit flies a convenient organism for genetic studies?
- They breed at a high rate.
- A generation can be bred every two weeks.
- They have only four pairs of chromosomes.
How did Morgan create mutant phenotypes in flies?
First, he noted wild type, or normal, phenotypes that were common in the fly population. He then started searching for traits alternative to the wild type; these are called mutant phenotypes.
What was the P generation of Morgan’s fly experiments?
He mated male flies with white eyes (mutant) with female flies with red eyes (wild type).
Describe the F1 generation of Morgan’s fly experiments.
The F1 generation all had red eyes.
Describe the F2 generation of Morgan’s fly experiments.
The F2 generation showed the 3:1 red : white eye ratio, but only males had white eyes.
What did Morgan determine from his experiments with fly eye color?
He determined that the white-eye mutant allele must be located on the X chromosome. This finding supported the chromosome theory of inheritance.
How does the Z-W system of sex determination work?
Similar to the X-Y system, but the female has a Z and a W and the male has two Zs. The presence of the W makes the female female.
How does the XO system of sex determination work?
There is only one type of chromosome: X. It is a matter of how many X chromosomes the individual has that determines the sex. Two Xs = female and one X = male.
How does the haploid-diploid system of sex determination work?
There are no sex chromosomes. The females develop from fertilized eggs and are diploid and the males develop via parthenogenesis from unfertilized eggs and are haploid.
What must happen for a recessive X-linked trait to be expressed?
- A female needs two copies of the allele (homozygous)
- A male needs only one copy of the allele (hemizygous)
What must happen for a female to phenotypically express a recessive X-linked trait?
The female must be a carrier for the trait and the male must phenotypically have the trait. This explains why X-linked recessive disorders are much more common in males than in females.
Give three examples of X-linked recessive disorders.
- Color blindness (mostly X-linked)
- Duchenne muscular dystrophy
- Hemophilia
