Biology Chapter 12: Chromosomal Basis of Inheritance

What is the physical basis of Mendelian inheritance?

The behavior of chromosomes during meiosis.

What are sex-linked genes?

Genes that exhibit unique patterns of inheritance based on their location on sex chromosomes.

Why do linked genes tend to be inherited together?

Because they are located near each other on the same chromosome.

What can cause genetic disorders?

Alterations of chromosome number or structure.

What does the chromosome theory of inheritance state?

Genes are located at specific positions (loci) on chromosomes, and chromosome behavior during meiosis illustrates Mendel's laws.

Who provided the first evidence linking a specific gene to a specific chromosome?

Thomas Hunt Morgan.

What organism did Thomas Hunt Morgan use for his genetic studies?

Drosophila melanogaster (fruit fly).

List three reasons why fruit flies are excellent subjects for genetic studies.

1. Single mating produces hundreds of offspring. 2. A new generation can be bred every two weeks. 3. They have only four pairs of distinguishable chromosomes.

What is meant by the wild type in genetics?

Wild-type flies have traits that are most commonly observed in natural populations.

What is the alternative to the wild type phenotype?

Mutant phenotypes, which are due to changes or mutations in the wild-type allele.

How is a homozygous red-eyed female fruit fly notated?

Xw+Xw+.

How is a heterozygous red-eyed female fruit fly notated?

Xw+Xw.

How is a homozygous white-eyed female fruit fly notated?

XwXw.

What was the phenotypic result of Morgan's cross between a white-eyed male and a red-eyed female in the F1 generation?

All offspring had red eyes.

What unusual result was seen in the F2 generation of Morgan's cross?

A 3:1 Mendelian ratio of dominant (red) to recessive (white), with all recessive phenotypes being male.

What does the result in the F2 generation suggest about the inheritance of red eye color?

It suggests that the red eye color trait is located on the X chromosome.

What Mendelian law explains the formation of gametes with one long and one short chromosome?

The law of segregation.

What Mendelian law explains that each resultant gamete has one of each homologous chromosome pair?

The law of independent assortment.

When do the law of segregation and the law of independent assortment occur during meiosis?

In anaphase I of meiosis.

What is the significance of Figure 12.4 in relation to Mendel's laws?

It ties together Mendel's laws with the chromosome theory of inheritance.

What is the importance of active learning in genetics?

It involves practicing problems and exercises to better understand concepts.

What should you do to prepare for labeling diagrams related to genes and chromosomes?

Test yourself by trying to add all the labels before looking at the text.

What does it mean if homologous chromosomes have different alleles?

It means the alleles for a particular gene could be different depending on the maternal and paternal genotype.

What is the role of meiosis in genetics?

Meiosis is responsible for the segregation and assortment of chromosomes, leading to genetic variation.

What is the expected outcome of a cross if wild type is dominant?

All offspring in the F1 generation would exhibit the wild type phenotype.

What are the four different ways chromosomes might be arranged during metaphase I of meiosis?

The arrangements are not specified in the notes, but they typically involve different combinations of maternal and paternal chromosomes.

How do the X and Y chromosomes differ?

The X chromosome is much larger than the Y chromosome and they are not homologous except for short segments on the ends.

What is the SRY gene?

The SRY (Sex-determining Region of Y) gene is located on the Y chromosome and is required for the development of testes.

What is a sex-linked gene?

A gene located on either sex chromosome, specifically referring to genes on the X chromosome that have no homologous portion on the Y chromosome.

Name a human sex-linked disorder caused by a recessive allele.

Duchenne muscular dystrophy, characterized by progressive weakening and loss of muscle tissue.

What is hemophilia?

A genetic disease caused by a sex-linked recessive allele, resulting in the absence of blood-clotting proteins and excessive bleeding.

What is color blindness?

A mild disorder almost always inherited as an X-linked allele.

What does the notation 'XnY' represent?

A male with color-blindness.

What does the notation 'XNXn' represent?

A female carrier of color-blindness.

What is a Barr body?

A dense object representing a highly condensed, inactivated X chromosome in female mammal cells.

Why do human females show a Barr body in their cells?

To ensure that females and males have the same effective dose of most X-linked genes.

How is the X chromosome inactivated?

Through random selection in embryonic cells, involving methylation of DNA and associated histone proteins.

Why can you say that all tortoiseshell cats are females?

The tortoiseshell gene is on the X chromosome and requires two different alleles, which only females can have.

What are linked genes?

Genes located close together on a chromosome that tend to be inherited together and do not sort independently.

What is genetic recombination?

The process during which linked genes become unlinked, often due to crossing over during meiosis.

What is a linkage map?

A genetic map based on the frequencies of recombination between markers during crossing over.

What is a map unit?

A unit of measurement of the distance between genes, equivalent to a 1% recombination frequency.

When does crossing over occur during meiosis?

During Prophase I.

What is the expected phenotypic ratio of a testcross between a wild-type dihybrid female fly and a homozygous recessive fly if the genes are not linked?

The expected ratio would typically be 1:1:1:1 for the four possible phenotypes.

What is the difference between a homozygous and heterozygous individual?

A homozygous individual has two identical alleles for a trait, while a heterozygous individual has two different alleles.

Why can the parental female produce only one type of gamete?

The parental female is homozygous for both traits, producing gametes with only those genes.

What is the gamete production of a dihybrid F1 female?

The F1 female is heterozygous for both genes and can produce four different types of gametes.

How is recombination frequency calculated?

Recombination frequency is calculated by dividing the number of recombinant offspring by the total number of offspring and multiplying by 100.

What is a map unit in genetics?

A map unit is a measure of the distance between genes, equivalent to a 1% recombination frequency.

How many map units apart are genes M and n based on the given offspring results?

5.6 map units apart.

What is nondisjunction?

Nondisjunction is an error in meiosis or mitosis where homologous chromosomes or sister chromatids fail to separate properly.

Define aneuploidy.

Aneuploidy is a chromosomal aberration where one or more chromosomes are present in extra copies or are deficient.

What is monosomy?

Monosomy refers to a diploid cell that has only one copy of a particular chromosome instead of the normal two.

What is trisomy?

Trisomy refers to a diploid cell that has three copies of a particular chromosome instead of the normal two.

What is polyploidy?

Polyploidy is a chromosomal alteration where an organism possesses more than two complete chromosome sets.

What causes Down syndrome?

Down syndrome is usually caused by an extra chromosome 21, resulting in a total of 47 chromosomes.

List four characteristics of Down syndrome.

Characteristic facial features, short stature, correctable heart defects, and developmental delays.

What are the physical traits of Klinefelter syndrome (XXY)?

Male sex organs, abnormally small/sterile testes, and some breast enlargement.

What are the physical traits of Trisomy X (XXX)?

Female with no unusual physical features, slightly taller than average, and at risk for learning disabilities.

What are the physical traits of Turner syndrome (XO)?

Phenotypically female, sterile due to lack of maturation in sex organs, normal intelligence.

What are the physical traits of XYY syndrome?

Normal sexual development and taller-than-average stature.

What is deletion in chromosome structure?

Deletion is a deficiency in a chromosome resulting from the loss of a fragment through breakage.

What is duplication in chromosome structure?

Duplication is an aberration where a portion of a chromosome is duplicated due to fusion with a fragment from a homologous chromosome.

What is inversion in chromosome structure?

Inversion is an aberration resulting from reattachment of a chromosomal fragment in reverse orientation.

What is translocation in chromosome structure?

Translocation is an aberration resulting from the attachment of a chromosomal fragment to a nonhomologous chromosome.

What is the significance of studying recombination frequency?

It allows for mapping the location of genes on a chromosome based on the frequency of crossing over.

What is the relationship between crossing over frequency and gene mapping?

Higher crossing over frequency indicates greater distance between genes on a chromosome.

What is the expected outcome of a testcross?

A testcross reveals the genotype of an individual by crossing it with a homozygous recessive individual.

What is the importance of the Chi-Square Test in genetics?

It helps determine if observed genetic ratios differ significantly from expected ratios.

What is the expected ratio for a dihybrid cross?

The expected ratio is typically 9:3:3:1 for phenotypes in a dihybrid cross.