Physical Anthropology / M. Waters
Mendelian Genetics Terminology
Allele:
the alternate form of a gene (each parent may pass on a
different allele)
Genotype:
the genetic makeup of an individual
Phenotype:
the physical appearance of a genetic trait
Homozygous: when both alleles in the pair are the same
Heterozygous: when both alleles in the pair are different
Dominant:
an allele that overrides the effect of the other allele in the pair
(a dominant allele prevents the expression of a recessive allele
in a heterozygous genotype)
Recessive:
the allele that is masked by the other allele in the pair; the trait
that is not expressed in a heterozygous genotype
EXAMPLE: Albinism in humans
Albinism is a recessive disorder. People who are albinos, have no pigment in
their hair, skin and eyes (the hair is white, skin is very fair, and eyes appear
pink).
A = the allele that results in pigment in the hair, skin & eyes (non-albinism)
a = the allele that results in no pigment in the hair, skin & eyes (albinism)
GENOTYPES
PHENOTYPES
AA (homozygous dominant)
pigment / no albinism
Aa (heterozygous)
pigment / no albinism
aa (homozygous recessive)
no pigment / albinism
Punnett Squares
A useful tool in doing genetics problems is the Punnett square. It shows the different
ways alleles can be combined, and shows (and predicts) the probabilities of phenotypes
and genotypes that can result from a specific mating (or couple).
For example, below is a cross between two people:
the mother is homozygous recessive (albino) - aa
the father is heterozygous (non-albino) - Aa
The parents’ alleles must always be written along the left side of the box, and the top of
the box. Each of the four squares within the box represent a possible allele combination
in an offspring. Each offspring receives one allele from each parent.
From the information produced by the Punnett square, we can determine the
proportions of phenotypes and genotypes that would result from this cross. Since
albinism is recessive, we know that individuals who are AA or Aa would not have
albinism, but that someone who is aa would be an albino.
The probability of this couple having a homozygous recessive (albino) child would be
50% (each of the four squares represents a probability of 25%; since two of the four
squares are aa, this comes out to be 50%). *Note that although the father is not
albino, he is a carrier of the albino allele (heterozygous genotype), and can pass this on
to his children. When combined with one of the mother’s alleles (both of which are
recessive) this would result in an albino child with a homozygous recessive genotype. In
other words, an albino parent is not automatically going to have albino children - only if
one of his/her alleles is paired up with another recessive allele (from the other parent).
The probability that this couple would have a heterozygous (non-albino) child would
also be 50%. It’s not possible for this couple to have children with a homozygous
dominant genotype.