Genetics And Inheritance
One and Two trait crosses
Mendel
● He did his experiments with garden peas
● He came up with 2 laws concerning genetics, known as Mendel’s laws
● His laws are the law of segregation and the law of independent assortment
Segregation: you have 2 alleles for each trait (one from each parent) but you only pass on one
to each offspring
Independent assortment: for 2 or more traits, the alleles for each trait are passed on
independently of the other traits
Dominance (complete dominance)
● Traits do not blend. A brown eyed mother and a blue eyed father do not have children
with browny blue eyes. One trait wins
● For each trait, you receive a gene from each parent, so you have 2 genes controlling
each trait
● These genes might be for different forms of the trait (brown vs. blue eyes) – these are
referred to as alleles
● If you have different alleles, the one that is apparent is called the dominant allele and is
represented by a capital letter. The one that is hidden, is called the recessive allele and
is represented by a lowercase letter
● For the same trait, use the same letter
● Ex. can roll tongue is dominant = T. can't roll tongue is recessive = t
Eye color
● This trait is not nearly as simple as it is represented
● Brown (b) > blue (b)
, Genotype and phenotype and allele combinations
● Possible genotypes (allele combinations) for eye color:
● BB Bb bb
○ Homozygous: both alleles are the same. This could be homozygous dominant
(BB) or homozygous recessive (bb) also known as true breeding
○ Heterozygotes: your two alleles are different (Bb). note that heterozygous would
have the dominant trait. Also known as hybrids
Punnett squares
● This is a way to determine all of the possible combinations of alleles in the offspring of
two parents
● We can determine probabilities from a Punnett Square
Tracking generations
● P generation: BB x bb (p stands for parental)
● F1 generation Bb (these are the offspring of P generation)
● F2 generation (these are the offspring of the F1 generation)
The multiplication rule
● This rule states that to find the probability of two events happening together multiply their
probabilities together
● If you are asked to find the probability of two traits occurring together, find the probability
of each trait separately and multiply the two probabilities
○ Example: what is the probability that two heterozygous (Bb) have a blue eyed
daughter?
○ There are two traits here: eye color and gender. Find the probability of having a
blue-eyed child, (¼) then the probability of having a girl (½), and multiply the
probabilities together. (¼ x ½ = ⅛)
Tongue rolling
● For our purposes, there are 2 forms of the trait: can roll which is dominant (T), and cant
role which is recessive (t)
Exceptions to the rule
1. Incomplete dominance
2. Codominance
3. Multiple alleles
4. Polygenic traits
5. Linked genes
Multiple alleles
● There are more than just two alleles for a trait
● Ex. ABO blood type has 3 alleles: A, B, O
The A allele (I^A) and B allele (I^B) are codominant and both are dominant to the O allele (i)
One and Two trait crosses
Mendel
● He did his experiments with garden peas
● He came up with 2 laws concerning genetics, known as Mendel’s laws
● His laws are the law of segregation and the law of independent assortment
Segregation: you have 2 alleles for each trait (one from each parent) but you only pass on one
to each offspring
Independent assortment: for 2 or more traits, the alleles for each trait are passed on
independently of the other traits
Dominance (complete dominance)
● Traits do not blend. A brown eyed mother and a blue eyed father do not have children
with browny blue eyes. One trait wins
● For each trait, you receive a gene from each parent, so you have 2 genes controlling
each trait
● These genes might be for different forms of the trait (brown vs. blue eyes) – these are
referred to as alleles
● If you have different alleles, the one that is apparent is called the dominant allele and is
represented by a capital letter. The one that is hidden, is called the recessive allele and
is represented by a lowercase letter
● For the same trait, use the same letter
● Ex. can roll tongue is dominant = T. can't roll tongue is recessive = t
Eye color
● This trait is not nearly as simple as it is represented
● Brown (b) > blue (b)
, Genotype and phenotype and allele combinations
● Possible genotypes (allele combinations) for eye color:
● BB Bb bb
○ Homozygous: both alleles are the same. This could be homozygous dominant
(BB) or homozygous recessive (bb) also known as true breeding
○ Heterozygotes: your two alleles are different (Bb). note that heterozygous would
have the dominant trait. Also known as hybrids
Punnett squares
● This is a way to determine all of the possible combinations of alleles in the offspring of
two parents
● We can determine probabilities from a Punnett Square
Tracking generations
● P generation: BB x bb (p stands for parental)
● F1 generation Bb (these are the offspring of P generation)
● F2 generation (these are the offspring of the F1 generation)
The multiplication rule
● This rule states that to find the probability of two events happening together multiply their
probabilities together
● If you are asked to find the probability of two traits occurring together, find the probability
of each trait separately and multiply the two probabilities
○ Example: what is the probability that two heterozygous (Bb) have a blue eyed
daughter?
○ There are two traits here: eye color and gender. Find the probability of having a
blue-eyed child, (¼) then the probability of having a girl (½), and multiply the
probabilities together. (¼ x ½ = ⅛)
Tongue rolling
● For our purposes, there are 2 forms of the trait: can roll which is dominant (T), and cant
role which is recessive (t)
Exceptions to the rule
1. Incomplete dominance
2. Codominance
3. Multiple alleles
4. Polygenic traits
5. Linked genes
Multiple alleles
● There are more than just two alleles for a trait
● Ex. ABO blood type has 3 alleles: A, B, O
The A allele (I^A) and B allele (I^B) are codominant and both are dominant to the O allele (i)
