Genetic Inheritance

 

Chromosomal theory of inheritance

Genes are located on chromosomes. Chromosomes are in pairs and genes, or their alleles, are located on each of these pairs.

When the cell divides in half, each chromosome ends up in a different cell. This is seen during meiosis in formation of egg cells of sperms. The genes also split into two halves. These are called alleles. Meiotic products have one of each homologous chromosome but not both. Meiosis is a series of cell divisions that creates haploid cells with half of the total number of chromosomes.

Once the egg and sperm meet, the pairs are restored but now the genetic combination of the pair is altered. One of the alleles thus comes from the mother and another from the father. This is how a defective gene causing a genetic disorder is also inherited by offspring.

Gene Linkage on Chromosomes

As per the Mendelian principles of inheritance genes need to be inherited independently of each other. However, there are far more genes than chromosome pairs. It is found that all of the genes on a chromosome are physically inherited together as a single linked group. Only genes that are located on different chromosomes have independent assortment during meiosis.

Crossing-Over and Recombination

According to the chromosomal theory 25% would resemble one parent, 25%, the second parent, 25% would have one trait from one parent and one from the other parent – and 25% would have also have the "other" traits from each parent.

Since each chromosome is a diploid, or occurs in pairs, genes on different chromosomes assort independently during sexual reproduction, recombining to form new combinations of genes. Genes on the same chromosome would theoretically never recombine.

However, genes do undergo cross over. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. This process of chromosomal crossover generally occurs during meiosis.

The probability of crossover occurring between two given points on the chromosome is related to the distance between the points. If the distance is long there is a higher chance of a crossover. For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage that means that the alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear linkage map.

Autosomal recessive inheritance

Most genes have a second working copy and one of them may not be actively functional at all. When this is inherited it is known as autosomal recessive inheritance. With recessive genes, it is only if someone inherits two altered copies of the same gene from each of their parents, they are likely to be expressed.

For example, inheritance of thalassemia and cystic fibrosis occurs in an autosomal recessive pattern. If both parents carry the faulty gene on one of their alleles they are likely pass on their defective gene in only 25% of their offspring. The baby of this couple has a 75% chance of not developing the condition. There are also other inherited characteristics that are inherited in this way such as blue eyes or red hair.