HEREDITARY PHYSICAL CHARACTERISTICS
Genotype and Phenotype
Genotype means the type of genes a person has, or their genetic make-up.
Those genes that affect the same trait are called alleles.
A dominant allele is given a capital letter, and a recessive allele is given the same letter in
lower case. For instance, having an earlobe that is unattached to the face is a dominant trait,
so we can call it E. An attached earlobe would then be called e.
Alleles occur in pairs; just as one pair of each type of chromosome is inherited from each
parent, so too each pair of alleles inherited from each parent.
The allele which is traditionally indicated by an uppercase (capital) letter is the
dominant trait. The allele which is traditionally indicated by a lowercase (small)
letter is the recessive trait.
If a sperm cell has e and the egg cell has e, the offspring must have ee. That is called
homozygous (pure) recessive. That means the person would have an attached earlobe.
If a sperm cell has E and the egg cell has E, the offspring must have EE. This is called
homozygous (pure) dominant. That means the person would have an unattached
earlobe.
The term for “pure” is homo. It refers to something being the same. The old days, you
had to shake up milk because the cream would rise to the top. Nowadays, people want
less fat, so the cream is removed before you get it; this is called homogenized milk. A
homogenized mixture is one that is the same throughout, and requires no periodic mixing.
Therefore, when the allele pairs are either EE or ee, they are homozygous.
The opposite of homo is “hetero”, so an allele pair that is “Ee” is heterozygous.
If one of the sex cells has E and the other sex cell has e, what will the offspring have? Ee.
What type of earlobe will they have? Attached. Why? Because the dominant trait is
stronger, so if it is present at all, it will manifest.
The physical appearance of a person is called the phenotype.
A person with Ee will therefore be called a heterozygous genotype, with an unattached
earlobe phenotype.
What earlobe alleles will a person have who is homozygous recessive? ee
What earlobe alleles will a person have who is homozygous dominant? EE
What earlobe alleles will a person have who is heterozygous? Ee
Figuring the Odds
If one of the parents is homozygous dominant (EE), the chances of their having a child
with unattached earlobes is 100 %, because this parent has only a dominant allele (E) to
pass on to the offspring. On the other hand, if both parents are homozygous recessive
(cc), there is a 100% chance that each of their children will have attached earlobes.
However, if both parents are heterozygous, then what are the chances that their child will
have unattached or attached earlobes? To solve a problem of this type, it is customary
first make a table of the genotype of the parents and their possible gametes.
E e
E EE Ee
e Ee ee
That means that when Harry meets Sally, their child has a 25% chance (1:3) of being ee,
and 25% chance of being EE, and 50% chance (1:1) of being Ee.
But that’s just the genotype. What about the phenotype (what will the child look like)?
There is a 75% chance (3:1) of having an attached earlobe (ee).