Linkage Part 1 (Hard)

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Questions and Answers

What is the main reason for the high frequency of non-parental combinations in Morgan's three-factor cross experiment?

The three genes are located on different chromosomes

The genes are closely linked and tend to be transmitted together

The genes are unlinked and assort independently

Crossing over occurs between the homologous chromosomes (correct)

What is the term for the exchange of DNA segments between non-sister chromatids of homologous chromosomes during meiosis?

Crossing over (correct)

Genetic linkage

Independent assortment

Recombination

Which of the following statements about recombinant offspring is true?

They are always homozygous for a particular trait

They have the same combination of alleles as one of their parents

They have a novel combination of genetic material due to crossing over (correct)

They are produced by the fusion of gametes with identical alleles

What is the primary reason for the linkage of genes in Morgan's experiment?

The genes are closely linked and tend to be transmitted together Signup and view all the answers

What is the result of crossing over during meiosis?

Increased frequency of non-parental combinations Signup and view all the answers

What is the term for the phenomenon where genes that are close together on the same chromosome tend to be inherited together?

Genetic linkage Signup and view all the answers

Why did Morgan's experiment involving three traits show a significant number of non-parental combinations?

Crossing over occurred between the homologous chromosomes Signup and view all the answers

What is the significance of Morgan's three-factor cross experiment?

It showed that genes can be linked and tend to be transmitted together Signup and view all the answers

What is the main reason why genes close to each other on the same chromosome tend to be transmitted as a unit?

Because of the physical linkage between genes on the same chromosome Signup and view all the answers

What is the term for two or more genes being located on the same chromosome and being physically linked?

Synteny Signup and view all the answers

What is the main difference between a two-factor cross and a three-factor cross?

The number of genes being studied Signup and view all the answers

Why did Bateson and Punnett's cross involving two traits not yield the expected 9:3:3:1 phenotypic ratio?

Because of genetic linkage Signup and view all the answers

What is the term for a group of genes that are linked together on the same chromosome?

Linkage group Signup and view all the answers

What is the primary reason why Janssens proposed that crossing over involves a physical exchange between homologous chromosomes?

He microscopically observed chiasmata Signup and view all the answers

Why do genes that are far apart on the same chromosome independently assort from each other?

Because of crossing over Signup and view all the answers

According to Morgan's hypotheses, what is the reason for the alleles of the three genes being inherited together?

Because they are located on the same chromosome Signup and view all the answers

What is the relationship between the number of linkage groups and the number of types of chromosomes in a species?

The number of linkage groups is equal to the number of types of chromosomes Signup and view all the answers

What is the effect of crossing over on the alleles of the three genes, according to Morgan's hypotheses?

It creates new combinations of alleles Signup and view all the answers

What is the main implication of genetic linkage on inheritance patterns?

It influences the transmission of genes as a unit Signup and view all the answers

What is the relationship between the distance between two genes and the likelihood of crossing over, according to Morgan's hypotheses?

Crossing over is more likely to occur between genes that are far apart Signup and view all the answers

Why did Morgan assume that crossing over did not occur between the X and Y chromosomes?

Because the genes for body color, eye color, and wing length are not found on the Y chromosome Signup and view all the answers

What is the underlying mechanism that leads to genetic linkage, according to Morgan's hypotheses?

The location of genes on the same chromosome Signup and view all the answers

Study Notes

Meiosis and Crossing Over

Crossing over occurs during Prophase of Meiosis I, where non-sister chromatids of homologous chromosomes exchange DNA segments.
Replicated sister chromatids (homologues) associate as bivalents.
Crossing over can produce recombinant genotypes.

Recombinant Cells and Offspring

Haploid cells containing a combination of alleles not found in the original chromosomes are called non-parental or recombinant cells.
Recombinant offspring are produced by the exchange of DNA between two homologous chromosomes during meiosis in one or both parents.
Recombinant offspring have a novel combination of genetic material due to a crossover event during gamete formation in (at least) one of their parents.

Morgan's Experiments

Thomas Hunt Morgan's experiments provided the first direct evidence of linkage in Drosophila.
He investigated three traits: body color, eye color, and wing length, which showed a non-parental combination of traits.
Morgan observed a higher proportion of trait combinations found in the parental generation, suggesting that the genes are located on the X chromosome and tend to be transmitted together as a unit.

Linkage and Synteny

Linkage refers to the phenomenon where genes close together on a chromosome tend to be transmitted as a unit, influencing inheritance patterns.
Synteny means two or more genes are located on the same chromosome and are physically linked.
Chromosomes are called linkage groups, containing a group of genes that are linked together.
The number of linkage groups is equal to the number of types of chromosomes in a species.

Bateson and Punnett's Cross

Bateson and Punnett's cross involving two traits (flower color and pollen shape) did not yield the expected 9:3:3:1 phenotypic ratio.
This suggests that independent assortment does not always occur.

Altering Linkage

Janssens proposed that crossing over involves a physical exchange between homologous chromosomes, which Morgan realized was consistent with his data.
Morgan assumed crossing over did not occur between the X and Y chromosomes.
The three genes studied by Morgan were not found on the Y chromosome.

Morgan's Hypotheses

Morgan hypothesized that the genes for body color, eye color, and wing length are located on the X-chromosome.
He proposed that alleles will tend to be inherited together due to crossing over.
The likelihood of crossing over depends on the distance between the two genes, with crossing over more likely to occur between genes that are far apart.

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Description

Understanding the process of crossing over during meiosis, including its effects on DNA segments and genotypes.