Chapter 15: The Chromosomal Basis of Inheritance Flashcards
Homologous chromosomes may have alleles that are the same or they may have alleles that are different. Explain how this can be possible.
Since the two chromosomes in a pair of homologous chromosomes come from two parents, the alleles for a particular gene could be different or could be the same depending on the alleles present in the maternal and paternal genotype.
What is the chromosome theory of inheritance, and how does it provide a physical explanation for Mendelian inheritance?
The chromosome theory of inheritance is a basic principle in biology stating that genes are
located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis illustrates Mendel’s laws of segregation and independent assortment.
The first evidence that a specific gene was associated on a specific chromosome came
from the work of Thomas Hunt Morgan in the early 1900s. He selected Drosophila melanogaster as his experimental organism. List at least three reasons why the fruit fly is an excellent subject for genetics studies.
- Single mating will produce hundreds of offspring.
- A new generation can be bred every two weeks.
- The fruit fly has only four pairs of chromosomes, which are easily distinguishable with a light microscope. (3 pair of autosomes and 1 pair of sex chromosomes)
What is meant by the wild type?
Wild type flies have traits that are most commonly observed in natural populations.
What is the alternative to the wild type phenotype?
Mutant phenotypes are due to alleles assumed to have originated as changes or mutations
in the wild-type allele.
The notation developed for wild type and mutant traits follows some accepted conventions.
Notate the following genotypes for a female fruit fly:
a. a fly homozygous for red eyes
b. a fly heterozygous for red eyes Xw+Xw
c. a fly homozygous for white eyes XwXw
a. Xw+ Xw+
b. Xw+ Xw
c. XwXw
When Thomas Hunt Morgan mated his first white-eyed male fly with a red-eyed female
fly, he came to the startling conclusion that the trait for eye color was located on the chromosome that determines sex. What was the phenotypic result of this cross in the F1 and F2 generations?
F1: XwY x Xw+Xw+ = all red eyes
F2: Xw+Y x Xw+Xw
2 red eye females - Xw+Xw+, Xw+Xw
1 red eye male - Xw+Y
1 white eye male - XwY
Morgan made a major breakthrough in thinking about inheritance. He showed
that genes are located on specific chromosomes.
What would Mendelian genetics predict for the results of the F1 cross described in
question 5?
Assuming that wild type (red eyes) is dominant, all offspring in the F1 would have red
eyes.
What unusual result of phenotypes was seen in the F2 generation of that cross (Morgans Fruit Flies)?
What does this suggest?
The F2 generation demonstrated a typical 3:1 Mendian ratio of dominant (red) to recessive (white) however, all of the recessive phenotype, white, were male.
The inheritance of the red eye color is on the X or sex chromosome. It could not be on
the Y chromosome because females show the trait.
Note that Morgan made a major breakthrough in thinking about inheritance. He had now
shown that genes are located on specific chromosomes.
Law of Segregation
During the formation of gamete, each gene separates from each other so that each gamete carries only one allele for each gene
alleles segregate/separate in anaphase I
Law of Independent Assortment
The alleles of genes on nonhomologous chromosomes assort independently of one another.
The alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.
Alleles are assorted in anaphase I
Humans and other mammals have two types of sex chromosomes, designated X and Y.
Describe how they differ from each other.
The X chromosome is much larger than the Y chromosome. They are not homologous except for short segments on the ends of the Y chromosome that are homologous for regions on the X chromosome.
What is the SRY gene? Where is it found, and what does it do?
The Sex-determining Region of Y, a gene found on the Y chromosome that is required for the development of testes. This gene codes for a protein that regulates other genes - triggering a cascade of biochemical, physiological, and anatomical features.
The male programming gene
What is the definition of a sex-linked gene?
A gene located on either sex chromosome.
In humans, the term sex-linked gene refers to genes located on the portion of the X chromosome that has no homologous portion on the Y chromosome. The genes are actually X-linked genes
Name and describe three human sex-linked disorders.
- Duchenne muscular dystrophy: A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue.
- Hemophilia: A human genetic disease caused by a sex-linked recessive allele, resulting
in the absence of one or more blood-clotting proteins; characterized by excessive
bleeding following injury. - Color Blindness: A mild disorder almost always inherited as an X-linked allele.
deletion
A deficiency in a chromosome resulting from the loss of a fragment through
breakage.
duplication
An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated.
inversion
An aberration in chromosome structure resulting from reattachment of a chromosomal fragment in a reverse orientation to the chromosome from which it originated.
translocation
An aberoration in chromosome structure resulting from attachment of a
chromosomal fragment to a nonhomologous chromosome
Complete the following
