Chapter 15: The Chromosomal Basis of Inheritance

What is the chromosome theory of inheritance state?

Around 1902, Sutton and Boveri developed the chromosome theory of inheritance, stating that genes are located on chromosomes

Distinguish between wild type and mutant types fruit flies.

Comparing their phenotypes, where wild type flies exhibit the most common trait, while mutant types display traits alternative to the normal

What sex chromosomes (in humans) determine that a fetus will be male? Female?

The presence of a Y chromosome indicates that a fetus will be male, while the absence of a Y chromosome, with two X chromosomes, indicates a female.

What are a sex-linked genes?

It is a gene that is located on either sex chromosome

Distinguish Y-linked genes

• These genes contain 78 genes, coding for about 25 proteins

• Contains SRY (Sex-determining Region on the Y chromosome)— responsible of the testes in an embryo

Distnguish X-linked genes

• These genes contain about 1,100 genes. X chromosomes have genes for many characters unrelated to sex

• Lacks SRY

What is the actual gene that determines gender and where is it located?

A gene on the short arm of the Y chromosome called SRY (Sex-determining Region on the Y chromosome) is responsible for the development of the testes in an embryo. The primary gene responsible for determining male sex in humans

Why are males considered hemizygous with respect to the sex chromosomes? What is the implication of this condition for males with respect to sex-linked genes?

Males are considered hemizygous for sex-linked genes because they inherit only one copy of the X chromosome, while females inherit two. This has the implication that any recessive X-linked trait will be expressed in males regardless of whether there is a dominant allele present. Females, with two X chromosomes, can have a dominant allele to mask a recessive one.

Explain what a Barr body is and why it is only found in females. Describe the function of the XIST gene. Why is this gene important?

In mammalian females (XX), one of the two X chromosomes in each cell is randomly inactivated during embryonic development. A gene called XIST (X-Inactive Specific Transcript) is only active on one of the X chromosomes. The XIST gene which will condense the X chromosome into a Barr body, inactivating the X chromosome. The genes on the Barr body are not expressed. Without X-inactivation, females would have a double dose of X-linked genes, which could disrupt cellular function and development

Provide an example of the effects of Barr bodies in cats.

Mosaicism results in the patchy coloration of a tortoiseshell cat.

What is a linked gene? Why are linked genes often inherited together with little to no genetic recombination?

Genes that are located on the same chromosome tend to be inherited together and are called linked genes. Linked genes are often inherited together with little to no genetic recombination because they are located close together on the same chromosome. This proximity decreases the likelihood of a crossing-over event during meiosis, where homologous chromosomes exchange genetic material.

What is a recombinant?

Offspring with nonparental phenotypes (new combinations of traits)

What process results in the recombination of linked genes?

That mechanism was the crossing over of homologous chromosomes. Recombinant chromosomes bring alleles together in new combinations in gametes

Explain how Morgan’s experiments with Drosophila melanogaster lead to his discovery of sex-linked genes.

His experiments led to revealing a pattern of inheritance where a trait (white eye color) appeared more frequently in males than in females, suggesting a connection to the sex chromosomes. (Morgan’s discovery of a trait that correlated with the sex of flies was key to the development of the Chromosome Theory of Inheritance)

What is a genetic map?

It is an ordered list of the genetic loci along a particular chromosome (shows the relative distances between genes based on recombination frequency)

Explain this statement: Recombination frequencies can be used to create a linkage map. 

It’s measured as the percentage of offspring displaying new combinations of traits due to genetic exchange during meiosis, are used to create linkage maps. A linkage map visually represents the relative positions of genes on a chromosome, with the distance between genes indicated by their recombination frequency.

Explain what nondisjunction is?

It is pairs of homologous chromosomes that do not separate normally during meiosis. As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy.

How can nondisjunction lead to aneuploidy or polyploidy?

Aneuploidy results from the fertilization of gametes in which nondisjunction occurred.

• Offspring with this condition have an abnormal number of a particular chromosome

Polyploidy is a condition in which an organism has more than two complete sets of chromosomes

Differentiate between monoploidy and triploidy.

Monoploidy refers to the state of having only one set of chromosomes in a cell or organism, typically a haploid number, represented as 1n. Triploidy (3n) is three sets of chromosomes.

Down Syndrome

• an aneuploid condition that results from three copies of chromosome 21

• It affects about one out of every 830 children born in the United States

• The frequency of this genetic condition increases with the age of the mother, a correlation that has not been explained

Kleinfelter Syndrome

• The result of an extra chromosome in a male, producing XXY individuals (1 in 500-1,000 males born)

• May express reduced primary and secondary sex features, typically infertile

Turner Syndrome

• Monosomy X, produces X0 females; (1 in 2,000-4,000 females born)

• Only known viable monosomy in humans

• Females are infertile

Trisomy X

• XXX females--occur with a frequency of about one in 1,000

• No unusual physical features, though they are at risk for learning disabilities

• genetic condition where females have an extra X chromosome, leading to a total of three X chromosomes

List the 4 types of chromosome structure alterations

Deletion, Inversion, Duplication, and Translocation

Delection

A type of chromosome structure alteration where a segment of a chromosome is lost or removed, leading to the absence of genetic material.

Inversion

A type of chromosome structure alteration where a segment of a chromosome is reversed end to end, potentially altering gene expression and function.

Duplication

A type of chromosome structure alteration in which a segment of a chromosome is copied, resulting in multiple copies of that segment, which can lead to gene dosage effects.

Translocation

Moves a segment from one chromosome to a nonhomologous chromosome, which can disrupt gene function and lead to genetic disorders.

Differentiate between the types of mutations that lead to genetic disorders such as Cri du chat and CML.

The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5.

• A child born with this syndrome is severely intellectually disabled and has a catlike cry. Individuals born with this usually die in infancy or early childhood.

Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes in 9 and 22