Eekman
Medical Section Advisor
- Joined
- Dec 13, 2012
- Messages
- 4,611
- Reputation
- 0
- Reaction score
- 7,125
- Points
- 0
- Website
- www.everessencekd.com
Gregor Mendel developed the idea of dominant and recessive traits in the 19th century. While scientists have found many other mechanisms that affect genetic inheritance, Mendel's ideas form the basis of learning how a genetic trait is passed along to offspring and expressed.
Genetic Inheritance, How Genes Are Passed Along to Offspring
All sexually reproducing organisms, including humans, acquire one set of their genes from the mother and one from the father. These are passed down in the form of chromosomes, which reside in the nucleus of the cell. Each chromosome contains thousands of genes strung out along its DNA. Each of these genes can have different variants, called alleles. So for the hair color gene, some different alleles that exist are brown, black, red and blonde.
Heterozygous and Homozygous Individuals
Since everyone gets one allele from each parent and each parent has different genetic traits, individuals often acquire two different alleles. This is called a heterozygous individual. An example of this is someone who inherits an allele for blonde hair from the mother and an allele for brown hair from the father.
Individuals who get the same allele from both parents are called homozygous. An example of a homozygous child would have one brown hair allele from the mother and one brown hair allele from the father. In homozygous individuals, there is only one option for that genetic trait and recessive and dominant genes don't matter. The person who inherits two brown hair genes will never be a blonde unless they dye their hair.
Dominant and Recessive Genes Cause Dominant and Recessive Traits
In heterozygous individuals, recessive and dominant genes matter because they affect the visible expression of the genetic trait, the phenotype.
Dominant genes are alleles that will override any other variant of that gene. Most gene alleles are considered dominant in relation to specific other alleles. For example, brown hair is a dominant trait over blonde hair. So if someone inherits a blonde hair gene from the mother and a brown hair gene from the father, they will have brown hair. The dominant allele for brown hair will override the blonde hair allele.
A recessive trait is which requires two copies of the gene to show up. For example, blonde hair is recessive, so it will only be present in the child if both parents donate a blonde hair allele. This is sometimes called a double recessive.
It is possible for both parents to donate a recessive allele, such as a blonde allele, even if neither of them express that allele (neither of them are blonde.) This happens when both parents are heterozygous. Each parent carries one brown allele and one blonde allele, so they both appear brown. But if they each donate their blonde allele instead of their brown one, the resulting child will be blonde.
Unusual Cases, Incomplete Dominance and Multiple Alleles
This has been a fairly simple explanation of dominant and recessive traits and how genetic inheritance works. It works for simple genetic traits that follow the basic rules of recessive and dominant inheritance.
Of course, many traits are not this simple. Some genes have multiple alleles that are all dominant or recessive to each other in varying degrees. Other traits exhibit incomplete dominance, a situation where neither allele is truly dominant over the other. But for understanding basic genetic inheritance, knowing how dominant and recessive genes work is crucial.
Sources:
Alberts, Bruce. Molecular Biology of the Cell. Garland Pub., 1989
Lewin, Benjamin. Genes IV. Oxford University Press., 1990
Copyright Bridget Coila
Read more at Suite101: Understanding Dominant and Recessive Traits | Suite101 http://suite101.com/article/understanding-dominant-and-recessive-traits-a163668#ixzz2IXGMlD4K
Follow us: @suite101 on Twitter | Suite101 on Facebook
Genetic Inheritance, How Genes Are Passed Along to Offspring
All sexually reproducing organisms, including humans, acquire one set of their genes from the mother and one from the father. These are passed down in the form of chromosomes, which reside in the nucleus of the cell. Each chromosome contains thousands of genes strung out along its DNA. Each of these genes can have different variants, called alleles. So for the hair color gene, some different alleles that exist are brown, black, red and blonde.
Heterozygous and Homozygous Individuals
Since everyone gets one allele from each parent and each parent has different genetic traits, individuals often acquire two different alleles. This is called a heterozygous individual. An example of this is someone who inherits an allele for blonde hair from the mother and an allele for brown hair from the father.
Individuals who get the same allele from both parents are called homozygous. An example of a homozygous child would have one brown hair allele from the mother and one brown hair allele from the father. In homozygous individuals, there is only one option for that genetic trait and recessive and dominant genes don't matter. The person who inherits two brown hair genes will never be a blonde unless they dye their hair.
Dominant and Recessive Genes Cause Dominant and Recessive Traits
In heterozygous individuals, recessive and dominant genes matter because they affect the visible expression of the genetic trait, the phenotype.
Dominant genes are alleles that will override any other variant of that gene. Most gene alleles are considered dominant in relation to specific other alleles. For example, brown hair is a dominant trait over blonde hair. So if someone inherits a blonde hair gene from the mother and a brown hair gene from the father, they will have brown hair. The dominant allele for brown hair will override the blonde hair allele.
A recessive trait is which requires two copies of the gene to show up. For example, blonde hair is recessive, so it will only be present in the child if both parents donate a blonde hair allele. This is sometimes called a double recessive.
It is possible for both parents to donate a recessive allele, such as a blonde allele, even if neither of them express that allele (neither of them are blonde.) This happens when both parents are heterozygous. Each parent carries one brown allele and one blonde allele, so they both appear brown. But if they each donate their blonde allele instead of their brown one, the resulting child will be blonde.
Unusual Cases, Incomplete Dominance and Multiple Alleles
This has been a fairly simple explanation of dominant and recessive traits and how genetic inheritance works. It works for simple genetic traits that follow the basic rules of recessive and dominant inheritance.
Of course, many traits are not this simple. Some genes have multiple alleles that are all dominant or recessive to each other in varying degrees. Other traits exhibit incomplete dominance, a situation where neither allele is truly dominant over the other. But for understanding basic genetic inheritance, knowing how dominant and recessive genes work is crucial.
Sources:
Alberts, Bruce. Molecular Biology of the Cell. Garland Pub., 1989
Lewin, Benjamin. Genes IV. Oxford University Press., 1990
Copyright Bridget Coila
Read more at Suite101: Understanding Dominant and Recessive Traits | Suite101 http://suite101.com/article/understanding-dominant-and-recessive-traits-a163668#ixzz2IXGMlD4K
Follow us: @suite101 on Twitter | Suite101 on Facebook
