Chapter 5 Genetics

Thomas Hunt Morgan

A pioneering geneticist known for his work with fruit flies, which provided key evidence for the chromosome theory of inheritance. He was the first to demonstrate that genes are located on chromosomes and established the concept of genetic linkage.

Genetic Linkage

The phenomenon where genes located close to each other on the same chromosome tend to be inherited together during meiosis, affecting the inheritance patterns of traits.

Recombination

The process by which genetic material is physically mixed during meiosis, leading to new combinations of alleles in offspring.

The greater the recombination frequency means…

The greater the distance between the genes

Linked genes do not sort independently, why?

Because they are located close together on the same chromosome, which reduces the likelihood of recombination occurring between them during meiosis. They are inherited together as a unit.

Syntenic genes

Genes that are located on the same chromosome and tend to be inherited together due to their proximity.

We have 4 genes, A and B and X and Y. If A and B have a recombination frequency of 5%, and X and Y have a recombination frequency of 1%, what does that mean?

This means that genes A and B are more likely to recombine and assort independently than genes X and Y, which are closer together on the chromosome and less likely to undergo recombination.

Recombinant Chromosomes

Chromosomes that have undergone recombination during meiosis, resulting in a new combination of alleles.

We have parent 1 with AaBb, and parent 2 with aabb. If a crossover occurs between the A and B loci, this could produce recombinant chromosomes…

With the genotypes Ab and aB, resulting in gametes that carry combinations of alleles different from those of the parents.

We have parent 1 with AaBb, and parent 2 with aabb. If no crossover occurs between the A and B loci, this could produce non-recombinant chromosomes…

With the genotypes AB and ab, resulting in gametes that carry the same combinations of alleles as the parents.

Linked gene are always syntenic and always located near one another

True

Crossing over is less likely to occur between closely linked genes than between those farther apart on a chromosome

True

A dihybrid AaBb, involving unlinked genes A and B, will undergo independent assortment and produce four different gamete combinations with equal frequency. If the A and B genes are linked, what will the parental combinations be? How about the recombinant combinations?

The parental combinations will be AB and ab, while the recombinant combinations will be Ab and aB.

Parental Combination

The combinations of alleles that are inherited together without crossover, typically represented as AB and ab in linked genes.

Recombinant Combination

They are the result of crossing over between linked genes, producing new allele combinations that differ from the parental types. These combinations include Ab and aB.

You cross an organism with the genotype AB/ab (heterozygous for both genes) with an organism with the genotype ab/ab (homozygous recessive for both genes).

1. What are the expected frequencies of the different gametes produced in this cross?

2. What will be the genotypes of the offspring, and in what proportions?

1. 80% parental gametes (40% for AB and 40% for ab) and 20% recombinant gametes (10% for Ab and 10% for aB).

2. The expected genotypes of the offspring will be 1:1:1:1 ratio of AB, Ab, aB, and ab.

What is vg?

what is pr?

Morgan crossed flies with purple eyes and vestigial (short) wings to a wild type and obtained wild type F1. F1 females were then crossed to males that had purple eyes and vestigial wings. What would we see as the F2 progeny?

The F2 progeny would produce a 1:1:1:1 ratio. The F2 progeny would exhibit a phenotypic ratio of 1:1:1:1 for purple vestigial, purple wild type, red vestigial, and red wild type.