Genetics Study Guide
I.
Every trait consists of two alleles, one from each parent
a. Allele – one form of a gene
II. Genotype – genetic make up of a cell or the specific alleles of an individual
(letters)
a. Homozygous – two of the same letter form either two capital letters or
lower case letters (AA, aa)
i. Homozygous dominant – two dominant alleles shown with capital
letters (AA)
ii. Homozygous recessive – two recessive alleles shown with lower
case letters (aa)
1. is the only time a recessive trait is expressed or seen in a
population
b. Heterozygous – two different alleles, one capital and one lower case (Aa)
III. Phenotype – physical expression of the trait, what the organism looks like (hair
color, eye color)
a. The dominant allele is always expressed when it is present
i. Heterozygous or homozygous dominant
b. the recessive allele is only shown when there are two recessive alleles, one
from BOTH parents
i. homozygous recessive
IV. Sex linked traits – when the allele for a trait is located on one of the sex
chromosomes (X or Y)
a. Almost always located on the X chromosome (X’)
b. More common in males, because only have one X
i. If male is (X’Y) they have the trait
c. Females can be carriers – only carry the trait on one of the X alleles
i. Will not show the trait in their phenotype (X’X)
ii. If the female has two of the alleles then they will have the trait
(X’X’)
d. common sex linked traits
i. colorblindness
ii. hemophilia
V. Multiple alleles – more than one allele for a trait is dominant
a. ABO blood groups
i. Type A and B are both dominant
1. Type A (AA, AO)
2. Type B (BB, BO)
3. Type AB
ii. Type O is recessive – must have two OO alleles in order to have type
O blood (OO)
VI. Punnett square – tool used to predict the probability of heredity
a. Can be used to predict % of offspring that will have a particular genotype
or phenotype
i. Can be 0% (no squares), 25% (one square), 50% (two squares ), 75%
(three squares), or 100% (4 squares) depending on how many
squares show the genotype or phenotype in question
1. Example, what is the probability of a heterozygous
genotype?
AA
Aa
AA
Aa
Probability 50%
b. Can be used to predict ratio of genotype / phenotype in offspring
i. Compares the number of squares genotype or phenotype to the rest
of the squares
1. Example, what is the ratio of a heterozygous genotype?
AA
Aa
AA
Aa
Ratio 2:2 or 1:1
VII. Test cross – used to determine the genotype of a dominant individual
a. In order to determine if an individual is heterozygous (Aa) or homozygous
dominant (AA) for a particular trait it MUST be crossed with a homozygous
recessive (aa) individual
VIII. Pedigree – a chart used to show the occurrence of a particular trait with in a
family tree
a. Male – square
i.
(normal male)
(male with trait)
b. Female – circle
i.
(normal female)
(female with trait)
c. vertical lines – marriage
d. horizontal lines – offspring
e. Can be used to determine genotype of individuals