Question

Crossing over mixes up a. chromosomes c. zygotes b. alleles d. gametes

Short answer

Crossing over mixes up alleles.

Step-by-step solution

Understanding Crossing Over

Crossing over occurs during prophase I of meiosis. It involves the exchange of genetic material between homologous chromosomes. This process results in new combinations of alleles on each chromosome.

Analyzing the Options

Let's analyze the options: (a) Chromosomes are the structures that contain DNA. (b) Alleles are different versions of a gene found on homologous chromosomes. (c) Zygotes are formed after fertilization, not during meiosis. (d) Gametes are the reproductive cells (sperm and eggs) formed at the end of meiosis.

Selecting the Correct Answer

During crossing over, genetic material is exchanged between homologous chromosomes but specifically involves alleles, which are the alternative versions of a gene. As crossing over changes the combination of alleles on each chromosome, the correct answer is (b) alleles.

Key concepts

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the formation of gametes, which are sperm and egg cells. This process is critical for sexual reproduction because it ensures that offspring have the normal number of chromosomes.
During meiosis, a single cell goes through two rounds of division, known as meiosis I and meiosis II. Before meiosis begins, the chromosomes are duplicated during a phase called interphase. The first round of division, meiosis I, is where the crucial event of crossing over occurs.
Crossing over happens during prophase I of meiosis, where homologous chromosomes pair up and exchange pieces of genetic material. This exchange increases genetic diversity by shuffling the alleles between chromosomes, which results in new genetic combinations in the resulting gametes.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes in a diploid cell that hold the same genes at identical loci. Each pair consists of one chromosome from the mother and one from the father. They are similar in size, shape, and genetic content but can contain different alleles of the same gene.
During meiosis, these pairs play a vital role, especially in the process of crossing over. In prophase I, homologous chromosomes line up closely together in a formation known as synapsis. This proximity allows them to exchange genetic material, a key aspect of crossing over.

• Each homologous chromosome consists of two sister chromatids.
• They bear genes that impact traits such as eye color, hair color, and more.
• Variations between them arise due to the different alleles they might carry.

This exchange of genetic material results in new combinations of alleles, increasing the genetic variation within a population.

Alleles

Alleles are different versions of a gene that exist at the same locus on homologous chromosomes. These variations can lead to different traits within an organism. For example, a gene for eye color may have alleles for brown, blue, or green.
Understanding alleles is essential because they are the fundamental units of genetic variation. In the context of meiosis, alleles are shuffled during crossing over, which mixes up the alleles on each chromosome. This mixing is vital because it contributes to the genetic uniqueness of each gamete.

• Alleles can be dominant or recessive, impacting how traits are expressed.
• The combination of alleles an organism inherits determines its phenotype.
• Crossing over and the subsequent formation of unique gametes lead to diverse offspring.

This variation is crucial for the evolution and adaptation of species, as it provides a genetic basis for selection to act upon.