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Sexual reproduction and chromosomal inheritance generate genetic variation because, as we learned in meiosis, the combination of chromosomes inherited from each parent is different for each gamete and offspring. Over 50 years after Mendel’s laws were formulated, the role of DNA in heredity was discovered, and yet these principles still hold true when it comes to understanding the basis of genetic diversity. Three main principles explain the production of genetically diverse offspring: segregation, independent assortment, and fertilization.
Segregation
Segregation refers to the separation of alleles of a given gene during meiosis when gametes are formed. Organisms have two alleles for each gene – one from each parent. This ties back to Mendel’s Law of Segregation which we discussed earlier. As you will recall from our discussion on meiosis, during gamete production, these alleles are separated, and each gamete only gets one per gene. For example, if an organism has allele A and allele a, its offspring will only get either A or a, but not both. This produces genetic variation because it limits the amount of genetic material passed on from a parent to an offspring and creates new combinations of alleles within offspring.
Independent Assortment
During meiosis, homologous chromosomes line up at the center of the cell, but which one ends up in which gamete is random and not connected to what is happening to the other homologous chromosomes. This principle, that alleles for different genes are inherited independently of one another (as long as they are on different chromosomes), is called independent assortment. This feature is what facilitates Mendel’s Law of Independent Assortment. This contribution to genetic diversity is enhanced when we consider the crossing-over process that occurs during meiosis I.
Random Fertilization
Random fertilization is the term used to describe the idea that the combination of one gamete with another gamete is up to chance. Fertilization involves the fusion of two haploid gametes, creating a zygote that has a new combination of genes and alleles. This creates genetic diversity because offspring from the same two parents will have different combinations of alleles depending on which two gametes fused together.
Human Genetic Disorders
The chromosomal basis of inheritance explains how genes are transmitted from parent to offspring. It also provides an understanding for how single affected or mutated alleles or specific chromosomal changes can be inherited. A few examples of this are:
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Sickle cell anemia: This disease follows an autosomal recessive pattern. A copy of the mutated hemoglobin gene must be inherited from both parents. When it is inherited from both parents, the red blood cells in the individual become crescent shaped which can be painful as they move through the body and can cause damage to organs.
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Tay-Sachs disease: This disease follows an autosomal recessive pattern. A copy of the mutated gene must be inherited from both parents in order for the offspring to develop the disease. This disease is caused by the lack of an enzyme needed to break down a fatty substance in nerve cells. The build up of this substance can cause damage to the nervous system.
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Huntington’s disease: This disease follows an autosomal dominant inheritance pattern. When a copy of the mutated gene is inherited from the parent, the offspring will eventually develop the disease and produce a form of the huntingtin protein which causes nerve damage. Symptoms typically start presenting themselves in mid-adulthood.
