Mendelian Genetics
Probability Table*
1
Parent’s
Genotypes
(Cross)
AA x AA
2
AA x Aa
3
4
AA x aa
Aa x Aa
5
Aa x aa
6
aa x aa
Children’s
Genotype
Children’s
Phenotype
100% AA
100% Dominant
50% AA
50% Aa
100% Aa
25% AA
50% Aa
25% aa
50% Aa
50% aa
100% aa
100% dominant
100% Dominant
75% dominant
25% recessive
50% dominant
50% recessive
100% recessive
*If you don’t memorize this table, then you are a fool……
Hints for solving problems:








For every problem, you will either be given the genotype or the phenotype. Identify which one you
have.
Always, write the cross.
If given the cross, calculate the children’s genotype or phenotype.
 i.e. give the genotypes of a cross where the parents are both heterozygous (Aa x Aa).
If given the phenotypes of the parents, first find the genotypes and then calculate children’s genotype
or phenotype.
 Two individuals who have a straight hair line (straight hair line is recessive to widow’s peak) are
mated, what are the possible phenotypes of the children.
The genotype of the parents will almost always include the words homozygous or heterozygous. If it
does, write the cross first, then calculate.
Some word problems will give you the number of children resulting from a cross and ask you to
figure out the cross.
 First get a total number of children.
 Then find the percentage of each type of child (dominant or recessive).
 Then figure out what cross would yield that result.
In genetics, the numbers DO NOT have to be exact
 50% dom, 50% rec is the result of a Aa x aa cross, but if the numbers are 52% dom, 48% rec then it is
also the result of the same cross.
Only two crosses (#4, 5) result in something OTHER than 100% dominant or 100% recessive. They
are the ones most likely to be asked on a test.
Mendelian Genetics
Probability Table*
1
Parent’s
Genotypes
(Cross)
AA x AA
2
AA x Aa
3
4
AA x aa
Aa x Aa
5
Aa x aa
6
aa x aa
Children’s
Genotype
Children’s
Phenotype
100% AA
100% Dominant
50% AA
50% Aa
100% Aa
25% AA
50% Aa
25% aa
50% Aa
50% aa
100% aa
100% dominant
100% Dominant
75% dominant
25% recessive
50% dominant
50% recessive
100% recessive
*If you don’t memorize this table, then you are a fool……
Hints for solving problems:








For every problem, you will either be given the genotype or the phenotype. Identify which one you
have.
Always, write the cross.
If given the cross, calculate the children’s genotype or phenotype.
 i.e. give the genotypes of a cross where the parents are both heterozygous (Aa x Aa).
If given the phenotypes of the parents, first find the genotypes and then calculate children’s genotype
or phenotype.
 Two individuals who have a straight hair line (straight hair line is recessive to widow’s peak) are
mated, what are the possible phenotypes of the children.
The genotype of the parents will almost always include the words homozygous or heterozygous. If it
does, write the cross first, then calculate.
Some word problems will give you the number of children resulting from a cross and ask you to
figure out the cross.
 First get a total number of children.
 Then find the percentage of each type of child (dominant or recessive).
 Then figure out what cross would yield that result.
In genetics, the numbers DO NOT have to be exact
 50% dom, 50% rec is the result of a Aa x aa cross, but if the numbers are 52% dom, 48% rec then it is
also the result of the same cross.
Only two crosses (#4, 5) result in something OTHER than 100% dominant or 100% recessive. They
are the ones most likely to be asked on a test.