Transcript 3) Dominant and recessive traits

DOMINANT AND RECESSIVE TRAITS
January 23, 2014
1)
For about 95% of genes (on chromosomes 1-22) you have
genes from each parent, and usually if one gene is active
in 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, the
child will have black or brown hair, even though the gene
on the paternal chromosome codes for a different protein
3)
Blond and red hair are therefore “recessive”, because the protein
from the “dominant” gene is more active than the protein
from the “recessive” gene.
PATERNAL: 2 COPIES OF #19
MATERNAL: 2 COPIES OF #19
WHEN THE EMBRYO IS FORMED, IT GETS ONCE COPY
OF CHROMOSOME 19 FROM THE MOTHER, AND ONE
COPY FROM THE FATHER. THERE IS A GENE ON 19
THAT CODES FOR THE MELANOCORTIN RECEPTOR.
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-19
THE DNA SEQUENCE
OF THE GENE FOR THE
RECEPTOR MAY
BE DIFFERENT,
BETWEEN THE M COPY
AND 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 ALLOWS
YOU TO DIGEST LACTOSE: IT FORMS GLUCOSE
AND 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 METABOLIZE
LACTOSE 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, THE
CONSQUENCES CAN BE MUCH GREATER.