INSpiREzine Discovering DNA | Page 38

Because alleles provide at least two sets of instructions for each gene , the body has to figure out how to follow the instructions , or in other words , which traits are expressed . An allele can be dominant ( A ) or recessive ( a ).

● Dominant alleles express a trait even if there is only one copy ( AA , Aa ).

● Recessive alleles can only express themselves if there are two copies ( aa ) - one from each parent .

Depending on whether the alleles are present on the numbered chromosomes or the sex chromosomes , traits can be referred to as either autosomal dominant , autosomal recessive ,

X-linked dominant , or X-linked recessive . Though rarer , traits can also be expressed through Y-linked , codominant , or mitochondrial inheritance patterns .

The specific ways that alleles are paired together are known as inheritance patterns . For example , when two people have a child , one with version A and the other with version a of a gene , there are several possibilities :

Parent Parent

Parent Child Child

Parent Child Child

Both parents are AA : All of their children will also be AA ( homozygous for AA ). 100 % dominant .

Both parents are aa : All of their children will also be aa ( homozygous for aa ). 100 % recessive .

One parent is AA and the other is aa : All of their children will be Aa ( heterozygous for Aa ). 100 % dominant .

Both parents are Aa : Their child has a 25 % chance of being AA ( homozygous ), a 50 % chance of being Aa ( heterozygous ), and a 25 % chance of being aa ( homozygous ). 75 % dominant and 25 % recessive .