Genetics Vocabulary
Allele
Different versions of one gene.
An allele is usually represented by a
capital or lower case letter.
Examples: Pea plants have two alleles
for height. One allele is for tall plants (T)
the other allele is for short plants (t).
Remember, half your chromosomes came
from your mom and half from dad. This is
true for any organism that reproduces
sexually. This is why one can have more
than one allele for a gene.
Also there can be many alleles in a
population for one gene. Blood type for
example. A B O
Called Multiple Alleles. Having more than
two alleles for a genetic trait.
Dominant
The allele that is expressed if different
alleles occur together. By expressed I
mean seen or used or better yet it is the
gene that is transcribed into protein.
For example: In humans freckles is the
dominant allele, so if one has the
dominant allele (F) and the recessive
allele (f), they will have freckles.
Recessive
The allele that is not expressed if the
dominant allele is present. In order to be
expressed, both alleles need to be
recessive. For example in height of peas
the recessive allele is for height is a short
plant (t). In order for a plant to be short
it needs to have both alleles for shortness.
(t t)
Homozygous and Heterozygous
Homozygous: the two alleles are the
same. TT or tt
Heterozygous: the two alleles are
different.
Tt
Let’s Put Some of these Terms
to Use
A pea plant is homozygous recessive for
height. Its two alleles would be _____
A pea plant is heterozygous for height.
Its two alleles would be ____.
A pea plant is homozygous dominant for
height. Its two alleles would be _____.
Genotype and Phenotype
Genotype: Actual alleles or genes
present in the individual. (Tt) is the
genotype. (the actual genes / letters)
Phenotype: The genotype above would
be a tall plant. Tall is the phenotype.
Phenotype is the physical result of the
genes. Another way of explaining
phenotype, it is the protein that results
from the genotype and its (the protein’s)
resulting effect on the body to produce a
trait.
P generation
P stands for parents. The p generation
are the first two individuals crossed in a
breeding experiment.
These parents are true-breeding or pure
breeding, thus they are homozygous.
Ex: A tall pea plant crossed with itself
always produces tall pea plants.
A dachshund crossed with a dachshund
always has dachshund.
F1 Generation and F2
Generation
F1 Generation is the offspring (the first
generation) from the p generation.
F2 Generation comes from the F1
Generation being self pollinated with
itself.
See Figs. 11-4 and 11-5 pp. 311-312
Mendel’s Law of Segregation
The first law states that the two alleles for
a trait segregate when gametes are
formed. Fig. 11-5 and 11-7
What does this mean? During meiosis
when the chromosome number is reduced
in half, only 1 allele goes into each sex
cell. Fig. 11-15
Mendel’s Law of Independent
Assortment
The alleles of different genes separate
independently of one another during
gamete formation. P. 317
What does this mean? A gene for height
separates independently from one for
color. During metaphase in meiosis 1 the
chromosomes line up randomly.
We now know this applies to genes on
different chromosomes or genes that are
far apart on the same chromosome.
Figs. 11-9 and 11-10 on p. 317
Incomplete Dominance
Occurs when an individual displays a
trait that is intermediate between two
allele types.
Tall x short = intermediate/medium
height
Example of Incomplete
Dominance
In snapdragons, a pure-bred red flower is
crossed with a white flower, and the
offspring are pink flowers. That is
because neither red or white allele is
completely dominant. Due to this, less
red pigment is produced and the flowers
appear pink.
Codominance
Two dominant alleles are expressed at
the same time. This is different from
incomplete dominance. The reason is in
codominance both dominant traits are
displayed.
Example of Codominance
Roan coat in horses.
A cross between homozygous red and
homozygous white coats produces a roan
coat, a heterozygous offspring that
produces both red and white hair. The
mixed color is roan.
Another example
Blood Type: There is type A and type B if
you have both alleles you will produce
both type of blood groups.
Autosomal and Sex
Chromosomes
Autosomal: refers to any chromosome
besides the sex chromosomes. In human
these are chromosomes 1-22.
Sex chromosomes: chromosomes that
determine whether an organism is female
or male.
In humans: XX = females XY = males
Sex-Linked Traits
A trait whose allele is located on the
X chromosome.
Sex-linked traits are recessive.
Males and Sex Linked
Since the alleles are found on the X
chromosome, males usually show sex
linked traits much more frequently than
females.
Males only have one X chromosome, so if
they receive an X chromosome with the
trait, they will exhibit the trait.
Females are carriers
Females have two X chromosomes, thus
to show the sex linked trait they need to
be homozygous recessive. Meaning both
of their X chromosomes carry the gene.
Females are carriers because they can be
heterozygous dominant and still have the
recessive allele, which they can pass on
to their children.
Sex linked traits happen in other
organisms also, not just humans.
Sex linked traits are not related just to
male or female (sex) characteristics.
Example hemophilia is a sex-linked trait,
but involves clotting factors in blood.