Question

What is it called when two homologous chromosomes code for different alleles?

Answer 1

A chromosome is a long DNA molecule that contains part or all of an organism's genetic material. Most eukaryotic chromosomes contain histones, which, with the help of chaperone proteins, bind to and condense the DNA molecule to keep it intact. The three-dimensional structure of these chromosomes plays an important role in transcriptional regulation.

Complete answer:
Homologous chromosomes are paired chromosomes in biology. They have nearly identical gene sequences, loci (gene positions), centromere positions, and chromosomal lengths. They may have the same genetic sequence and loci, but their alleles may differ. A paternal chromosome and a maternal chromosome make up the homologous pair. These chromosomes pair up during meiosis but not during mitosis.
A variant form of a gene is known as an allele. Some genes exist in several different forms, all of which are found at the same genetic locus on a chromosome. Humans are referred to as diploid organisms because they have two alleles at each genetic locus, one from each parent.
In the nucleus of a somatic cell in humans, there are 46 chromosomes in total. The father inherits half of them (22 autosomes + X or Y chromosome) and the mother inherits the other half (22 autosomes + X chromosome). During meiosis, each maternal chromosome pairs up with a corresponding paternal chromosome.A female usually has 23 homologous chromosomes, while a male usually has 22. This is due to the fact that males' sex chromosomes, X and Y, are not homologous. During meiosis, homologous chromosomes must be paired up in order to promote genetic variation.
Heterozygous for a gene refers to when a homologous pair of chromosomes has different alleles of the same gene. A gene pair's two alleles are found on homologous chromosomes. Alleles are a type of gene that differs from others. A homologous chromosome is a pair of chromosomes that come from mother and father and are found in each cell.

Thus, when two homologous chromosomes code for different alleles are called heterozygous.

Note:
The term homologous comes from the Greek homologos, which means "to agree" or "to be of one mind". It's a combination of homos, which means "same", and logos, which means "relationship". Wilhelm von Waldeyer-Hartz, a German anatomist, coined the term chromosome in 1889. It was named after the Greek word khroma, which means "color", because it is stained easily with basic dyes.