Mendelian Patterns of Inheritance
The history of a particular trait is shown across the generations
Genotype of individuals in the pedigree can be deduced by following the patterns of inheritance.
Autosomal dominant
Autosomal recessive
Sex linked
Autosomal Dominant Traits are expressed even if only one copy of the gene is inherited.
Autosomal Recessive Traits are only expressed in individuals who have two copies of the gene. In addition, they are more frequently expressed in isolated groups; with inbreeding.
Sex-linked traits are associated with genes on the X chromosome and with genes on the Y chromosome
Symptoms start later in life. Dominant inheritance is shown clearly when the F2 generation is all affected; both males and females are affected therefore, indicating that it is not sex-linked.
If either parent carries a mutant HD allele, each of their children, regardless of sex have a 50% chance of inheriting the mutant allele and hence, being affected. The disease is caused by an active protein.
The active protein is activated by its amino acid, glutamic acid; glutamic acid is repeatedly added to the protein, activating the gene and leading to
50% of gametes have mutated allele during fertilization; the same goes for the father.
Genes Likely to be Effected in Autosomal Dominant Inheritance:
Genetic traits in which the mutant gene makes a harmful protein (“gain of function”).
The mutant gene has the overall control
The mutant gene can cause the protein to be more active by repeatedly adding an amino acid to the protein’s polypeptide chain.
Genes involved in enzymatic activity are more prone to be effected in autosomal dominant inheritance. Proteins that are deficient (poor/incomplete) and are in short supply even in health are prone to being effected in autosomal dominant inheritance; both alleles are needed to make the proteins.
mutations reduce the activity in this case
most alleles are inactive
specific activities of the proteins are inactivated
Only homozygous recessive individuals. Carriers of the disorder who are phenotypically normal but may transmit the recessive allele to their offspring.
Lethal genetic disease
Effects children at a very young age; affected children can die before their fifth birthday
The inheritance of two recessive alleles
Effect of the homozygous recessive gene: chloride channels in certain cell membranes that function to transport chloride ions between cells and the extracellular fluid are defective. The consequence is an abnormally high concentration of extracellular chloride, which causes the mucus that coats certain cells to become thicker and stickier than normal. This mucus thickening, builds up in the pancreas, lungs, digestive tract, and other organs, leading to multiple (pleiotropic) effects, including: poor absorption of nutrients from the intestines, chronic bronchitis, foul stools, and recurrent bacterial infections, especially with Pseudomonas.
Caused by the substitution of a single amino acid in the haemoglobin protein of red blood cells
When the oxygen content is low in the blood of an affected individual the sickle-cell haemoglobin molecules change their shape into long rods that deform the red cells into a sickle shape. Now the sickled cells may clump and clog small blood vessels, leading to other symptoms throughout the body, including, physical weakness, pain, organ damage, and even paralysis
This disease is found in areas where malaria is common like in Africa because the resistance to malaria accompanies the sickle-cell trait, explaining why the recessive allele for the sickle-cell disease remains in high frequency in these areas
Disorders due to dominant alleles. Achondroplasia is a form of dwarfism that is due to a single copy of a mutant allele. The recessive allele in achondroplasia is much more prevalent than the corresponding dominant allele. The genotype is heterozygous for those who have the disease. Normal individuals are homozygous recessive. Lethal alleles develop from mutations (changes to DNA) in a sperm or egg. It does not matter if the allele is dominant or recessive; the mutations will occur equally often.
If a lethal dominant allele causes the death of offspring before they mature and reproduce then, the dominant allele will not be passed on to future generations, it will stop.
A lethal recessive allele can continue to be passed on from generation to generation by heterozygous carriers who have normal phenotypes. The heterozygous carriers can reproduce and pass on the recessive allele.
The lethal dominant allele can however, act like a lethal recessive meaning, it can escape elimination if it causes death at an advanced age. Causing death at an advanced age also signifies that the lethal dominant allele may have already been transmitted to their children, for example, Huntington’s disease is a disease that does not show its phenotypic effect until later on in the individual’s life.
A is the dominant allele causing the disease
Aa * aa = a 50% chance of the offspring inheriting the allele
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