Linkage and Recombination

Complete linkage

When genes are so closely associated that they are always inherited together

Thomas Hunt Morgan

He pointed out that genes were arranged in a linear order along the length of the chromosome and linkage was an indication of their relative proximity to each other

Incomplete linkage

Linkage is not complete and gene pairs assort at least partially independent of each other

Crossing-over

- The process of exchanges of parts between the homologous chromosomes leading to the formation of recombinants

Recombinants

The results of non-sister chromatid crossing over

Cis form

- The dominant alleles are on one chromosome and the recessive alleles are on its homolog

- Coupling phase

Trans form

- The linked genes being located on separate chromosomes, enter the zygote separately

- Repulsion phase

Sex determination

It is basically a genetically regulated process. Some are direct consequences of the genotypes while others are influenced by other factors in addition to the genotype

heterozygosis; homozygosis

In insects like the Hymenopterans, it was suggested that there are nine alleles of the sex gene. The femaleness was an expression of _______________, and maleness was due to ________________

Haplo-diploidy

Sex determination wherein the unfertilized eggs develop into males or drones (no father) and females develop from fertilized eggs

Bonellia viridis

The individuals that remain free swimming throughout their entire larval stages become female. The larvae that attach themselves to the bodies of mature female becomes males because of the masculinizing hormones they absorb from such females. (Environmental sex determination)

Labroides dimidiatus

They live in groups consisting of one male and several females. When the male dies, the most dominant female repels any new males that will approach the group. If she succeeds, she will start courting the females and behave just like a male. In about two week, there will be a complete sex reversal and she (now he) produces fertile sperms. (Environmental sex determination)

XX-XO Mechanism

The male is XO and forms two kinds of gametes, with and without the X chromosome. The male is heterogametic sex, while the female is XX and homogametic since only X-carrying gametes are produced

XX-XY Mechanism

This type of mechanism is found in most mammals including humans and insects. In this system, the female has two X chromosomes while the male has the XY chromosome composition

ZW-ZZ

Sex determination mechanism in bird, moths, and butterflies

Homozygous or heterozygous; hemizygous

In X-linked inheritance, because the female has two X chromosomes, she may either be __________ or __________. She becomes a carrier of the recessive allele when she is heterozygous. On the other hand, a male having only one X-chromosome is said to be ___________

X-linked inheritance

Y-linked inheritance

Two types of sex-linked inheritance

1. Genetic sex determination

2. Environmental sex determination

3. Chromosomal sex determination

Types of sex determination:

Y-linked inheritance

In most species, the Y chromosomes carry very few genes other than those associated with sec determination

Webbed toes

Hypertrichosis of the ears

Examples of Y-linked genes in humans:

Webbed toes

This is characterized by a web-like connection between the second and third toes

Hypertrichosis of the ears

A conspicuous growth of hair on the outer rim of the ear is transmitted only from father to son