why a chance of abnormal children with brother/sister as parents?

Question

Why is there a higher chance of having an abnormal child if the parents are brothers and sisters?

Answer 1

Because birth defects often appear in "recessive" genes. If one parent has a defective recessive gene but the other has a normal dominant gene, the dominant gene is more likely to prevail so the birth defect doesn't appear.

When the parents are brother and sister, both are likely to have the same defective recessive gene, so there is no normal dominant gene and the child ends up with the recessive defect.

Answer 2

Many of the abnormal conditions that are genetic in nature are caused by recessive genes, meaning that in order for the disorder to manifest itself both of the genes have to be the recessive one, meaning that each parent contributed a recessive gene. Assuming that the brother and sister came from the same parents (half- brother's and sister's to a degree as well), they would have greater chance to both be carrying the recessive gene, and that would increase the risks of having a child with the disorder greatly (1 in 4 or 25%, I believe).

Answer 3

Here's a thirty year old memory from school.

A lot of biological things are made up of two
genes. One of these genes may determine the
phenotype (the expressed trait).

So unrelated Mom and Dad are

Mom = AB
Dad = CD


Bro and Sis are

Bro = AD
SIS = AC

If bro and SIs have a child, the child
could be AA, which if A were not so good, would
be a terrible thing.

If the parents are unrelated the chances of getting
a child with a repeat bad gene are lower.

Thats the simple story.

Answer 4

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Answer 5

Because if there is a recessive, deleterious gene carried by the members of the family, then a child of two members of this family would show the trait.

Answer 6

It's a pretty gross IDEA, but not really. They might pick up some traits that run in the family. (Bad eyesight, etc.) The cases you see where people are mutants are cases where it's generation after generation of inbreeding.

Answer 7

see when chromosones fuse from males and females not from the same family they make a 50 50 gene line.thus making it normal.

however when chromosones fuse from males and females from the same family they make a 100 same gene line.thus making it normal.

good question.