DOMINANT AND RECESSIVE TRAITS January 23, 2014

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) Blond and red hair are therefore “recessive”, because the proteinfrom the “dominant” gene is more active than the proteinfrom the “recessive” gene.

WHEN THE EMBRYO IS FORMED, IT GETS ONCE COPYOF CHROMOSOME 19 FROM THE MOTHER, AND ONECOPY FROM THE FATHER. THERE IS A GENE ON 19THAT CODES FOR THE MELANOCORTIN RECEPTOR.

PATERNAL: 2 COPIES OF #19 MATERNAL: 2 COPIES OF #19

WHAT DOES THIS MEAN? THE CELLS THAT MAKE PIGMENT (LIKE MOST CELLS) HAVE ONE COPY OF CHROMOSOME 19 THAT CONTAINS GENES FROM THE MOTHER, AND ANOTHER COPY OF CHROMOSOME 19 THAT CONTAINS GENES FROM THE FATHER.

THROUGHOUT THE CLASS, FOR CHROMOSOMES 1-22, WE WILL REFER TO THE MATERNAL COPY AND THE PATERNAL COPY OF EACH CHROMOSOME.

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 19.

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, the right gene for the receptor protein has to be obtained from each parent, to have red hair.

M-19 P-19THE DNA SEQUENCEOF THE GENE FOR THE RECEPTOR MAYBE DIFFERENT,BETWEEN THE M COPYAND THE P COPY

IF THE MELANOCORTIN RECEPTOR IS ACTIVE, IT TRIGGERS THE PRODUCTION OF TYROSINASE, WHICH CONVERTS THE AMINO ACID TYROSINE TO EUMELANIN. WITH THE ENZYME TYROSINASE, THE PRODUCT IS PHEOMELANIN.

IN NUTRITION, A VERY FAMOUS EXAMPLE IS THE GENE FOR LACTASE, ON CHROMOSOME 2.LACTASE IS A GI-TRACT ENZYME THAT ALLOWSYOU TO DIGEST LACTOSE: IT FORMS GLUCOSEAND GALACTOSE, WHICH ARE ABSORBED.

WE EACH HAVE TWO GENES FOR LACTASE.BOTH GENES USUALLY ARE ACTIVE THROUGH ABOUT THE AGE OF 5, SO A SMALL CHILD CAN USUALLY DIGEST LACTOSE.

SOME GENE VARIANTS LOSE ACTIVITY AFTER CHILDHOOD, AND STOP MAKING LACTASE PROTEIN.

IF YOU RECEIVE AN “ADULT-ACTIVE” FROM EITHER(OR BOTH) PARENTS, YOU CAN METABOLIZELACTOSE INTO ADULTHOOD.

LACTASE ENZYME IN THE INTESTINAL MUCOSA CARRIES OUT THE DIGESTION OF LACTOSE.

IN CHILDREN, BOTH COPIES OF THE GENE ARE USUALLY ACTIVE AND MAKING LACTASE.

IN ADULTHOOD, ONE OR BOTH COPIES MAY BE IN THE NON-ACTIVE STATE, AND LACTASE IS NOT MADE.

LACK OF LACTASE IS A MODERATE INCONVENIENCE:SOME DAIRY PRODUCTS JUST HAVE TO BE AVOIDED.

LACK OF LACTOSE IS DEFINED AS A RECESSIVE TRAIT, SINCE ONE FUNCTIONING GENE USUALLY ALLOWS A PERSON TO DIGEST LACTOSE.

FOR THE LACK OF OTHER FUNCTIONAL GENES, THECONSQUENCES CAN BE MUCH GREATER.