Download Quick Unit Summary A gene is a stretch of DNA that codes for a

Quick Unit Summary
A gene is a stretch of DNA that codes for a specific characteristic or physical trait. Organisms
always have 2 copies of each gene- the one they inherited from their biological mother and the
one they inherited from their biological father. Genes have varying levels of expression, referred
to as dominant or recessive. Dominant genes affect the physical appearance/trait of the organism.
The effect of a recessive gene is hidden when a dominant gene is
present.
Strand of DNA
Genes are often represented by using single letters. Capital letters represent dominant genes and
lower-case letters represent recessive genes. For instance, the dominant gene for allowing an
individual to roll his/her tongue could be represented by a capital 'R." The recessive version of the
tongue-rolling gene would be represented by a lower-case 'r.'
A person's genotype refers to the combination of genes he/she inherited. For instance, if a person
has one dominant and one recessive tongue-rolling gene, their genotype could be written as 'Rr.' If
a person inherited two recessive tongue-rolling genes, their genotype would be written as 'rr.'
There are 3 categories of genotypes as shown below: heterozygous, homozygous dominant, and
homozygous recessive.
RR = homozygous dominant (the individual has 2 dominant versions of the gene)
Rr= heterozygous (the individual has 1 dominant version and 1 recessive version of the gene)
rr= homozygous recessive (the individual has 2 recessive versions of the gene)
A person's phenotype refers to the physical manifestation of his/her genes. For instance, a person
with a 'Rr' genotype would have a tongue-rolling phenotype since the presence of a dominant gene
allows for the dominant expression of that trait. All of the possible genotype combinations for this
gene are:
RR= tongue roller
Rr= tongue roller
rr= NON-tongue roller
A Punnett Square is a diagram used to determine the possible genetic combinations of the child
resulting from the cross of two individuals. By convention, the male's genotype is written on the
top and the female's genotype is written on the side.
The Punnet Square above shows the possible genetic combinations for the child of a father with a
'Bb' genotype and a mother with a 'bb' genotype. Each box represents 1 possible outcome for the
child resulting from the cross. If the gene represented by the letter 'b' controls hair color and
brown hair is dominant to blonde hair, then the results of this Punnett Square can be summarized
as follows:
Since
Bb= Brown hair and
bb= blonde hair,
then there is a 2/4, or 50%, chance that the child resulting from this cross would have brown hair
and
a 2/4, or 50%, chance that the child resulting from this cross would have blonde hair.