DOMINANT AND RECESSIVE TRAITS

January 22, 2015

These terms occur very often in the study ofgenetics. This lecture will deal these topics togive you an overview.

We will discuss two kinds of recessive traits:

-the first type, tends to refer to harmless traits (blue eyescompared to brown eyes, curly hair, etc).

-the second type can be of great medical importance(lack of ability to metabolize phenylalanine)

When you read about recessive traits, make sure youknow which class of trait is being discussed.

We will discuss two recessive traits in this lecture:

-blond or red hair: linked to genes on chromosome 16,

-phenylketonuria (PKU): linked to genes on chromosome 12.

PKU Hair color

HAIR COLOR: BROWN/BLACK COMPARED TO RED/BLOND

1) For about 95% of genes (on chromosomes 1-22) you havegenes from each parent, and usually if one gene is activein the cell, the other gene is also active. Each gene is producing mRNA, which is then translated to make protein.

2) Genes for melanin production have a very strong effect: if the maternal gene codes for a lot of melanin in the hair, thechild will have black or brown hair, even though the geneon the paternal chromosome codes for a different protein

3) Red and blond hair are therefore “recessive”, because the proteinfrom the “dominant” gene is more active than the proteinfrom the “recessive” gene.

The MC1R gene provides instructions for making a protein called the melanocortin 1 receptor. This receptor plays an important role in normal pigmentation. The receptor is primarily located on the surface of melanocytes, which are specialized cells that produce a pigment called melanin. Melanin is the substance that gives skin, hair, and eyes their color. THE GENE FOR THE RECEPTOR IS ON CHROMOSOME 16.

Melanocytes make two forms of melanin, eumelanin and pheomelanin. The relative amounts of these two pigments help determine the color of a person's hair and skin. People who produce mostly eumelanin tend to have brown or black hair and dark skin that tans easily. People who produce mostly pheomelanin tend to have red (or sometimes blond) hair, freckles, and light-colored skin that tans poorly.

Pheomelanin is a compound that gives hair its red color.

The melanocortin 1 receptor controls which type of melanin is produced by melanocytes. When the receptor is activated, it triggers a series of chemical reactions inside melanocytes that stimulate these cells to make eumelanin. If the receptor is not activated or is blocked, melanocytes make pheomelanin instead of eumelanin.

Common variations (polymorphisms) in the MC1R gene are associated with normal differences in skin and hair color. Certain genetic variations are most common in people with red hair, fair skin, freckles, and an increased sensitivity to sun exposure. These MC1R polymorphisms reduce the ability of the melanocortin 1 receptor to stimulate eumelanin production, causing melanocytes to make mostly pheomelanin.

If only ½ of the receptors on the melanocyte activate eumelanin production, enough eumelanin is made to produce black or brown hair. So red hair is recessive, a certain gene for the receptor protein has to be obtained from each parent, to have red hair.

M-16 P-16THE DNA SEQUENCEOF THE GENE FOR THE RECEPTOR MAYBE DIFFERENT,BETWEEN THE M COPYAND THE P COPY

Phenylketonuria: caused by a recessive gene that can have harmful effects

1) The enzyme Phenylalanine Hydroxylase (see next slides)converts much of our phenylalanine to tyrosine.

2) We have two copies of that gene, on each Chromosome 12.We get one copy from the maternal gene, and one fromthe paternal gene. In a normal person, usually some proteinis being made from each copy of the gene.

3) Suppose one copy is defective, and does not make normalPhenylalanine Hydroxylase? There is no apparent harm,because the GOOD copy makes enough enzyme to preventany problems.

4) But if you get a BAD copy of the gene from both parents,now you have a problem, and must restrict dietaryintake of phenylalanine.

OH

Normalmetabolism

Homework assignment: look up and draw the structuresof the compounds, that appear in the urine during PKU

M-12 P-12

The gene that codes for the protein Phenylalanine Hydroxylaseis located on the long arm of Chromosome 12.

If the DNA sequence is mutated on both the Maternal and Paternalversions, so that neither gene produces functional enzyme, thenthe child with have the disorder PKU. But if ONE gene works andmakes active enzyme protein, the child will be healthy.

NEXT WEEK IN CLASS, WE WILL DISCUSSINHERITANCE, AND HOW IT’S POSSIBLE TO GET A DEFECTIVE COPY OF A GENE FROM BOTH PARENTS.

THAT CAN BE A REAL PROBLEM.