We have found some genetic diseases in families but it is not affecting all the generations . Some generations are free from this diseases and some times it appears after two or three generations , What will be the scientific reason behind this?
Our genome have 23 pairs of chromosomes , half from mother and half from father. Twenty-two of the chromosomes are numbered 1 to 22 (called autosomes) and the twenty-third pair determines sex– XX for females and XY for males. Within our chromosomes we have genes, which tell our body how to work and develop. Sometimes there are changes, also known as mutations, in the genes which cause them not to work properly.
Autosomal dominant is one of several ways that a trait or disorder can be passed down (inherited) through families. In an autosomal dominant disease, if you inherit the abnormal gene from only one parent, you can get the disease. Often, one of the parents may also have the disease.
Autosomal dominant conditions occur equally in males and females. These are the conditions that you will find in multiple generations of a family since each person that has an autosomal dominant condition has a 50% chance with each pregnancy to pass the dominant gene mutation to their children. Conditions include Marfan syndrome, achondroplasia are examples.
Autosomal recessive is one of several ways that a trait, disorder, or disease can be passed down through families. An autosomal recessive disorder means two copies of an abnormal gene must be present in order for the disease or trait to develop. Genes come in pairs. One gene in each pair comes from the mother, and the other gene comes from the father. Recessive inheritance means both genes in a pair must be abnormal to cause disease. People with only one defective gene in the pair are called carriers. These people are most often not affected with the condition. However, they can pass the abnormal gene to their children.
If you are born to parents who carry the same autosomal recessive change (mutation), you have a 1 in 4 chance of inheriting the abnormal gene from both parents and developing the disease. You have a 50% (1 in 2) chance of inheriting one abnormal gene. This would make you a carrier. Autosomal recessive conditions also occur equally in males and females. These are the conditions that aren’t seen in every generation. That is because when both parents are carriers of mutations in the same gene, they have a 25% chance with each pregnancy to have a child with the disorder. Cystic fibrosis (CF) is an example of an autosomal recessive disorder.
Traits that are determined by alleles carried on the X chromosome are referred to as X-linked. X-linked alleles require a specific notation: Xc or X+ where the “+” represents the dominant allele and the lowercase letter the recessive allele. Females will have two X-linked alleles (because females are XX), whereas males will only have one X-linked allele (because males are XY). Most X-linked traits in humans are recessive so females do not usually have symptoms. Males with a mutation on their X chromosome will have the condition since they only have one X. Having a normal Y chromosome does not prevent symptoms of an X-linked condition
Males with X-linked conditions will never pass the condition to their sons (since they give their sons their Y chromosome) and all of their daughters will be carriers of the condition (since they give their daughters their X chromosome). These daughters will have a 50% chance that their sons will have the disorder (and once again, none of her daughters will have the condition but 50% will be carriers).
One example of an X-linked trait is red-green colorblindness.