Definitions:
Trait - Characteristic
Gene - Segment of DNA controlling a trait
Alleles - Different forms of a gene
“Flower color” is a gene
“Purple” & “White” are alleles
Lessons
Traits are controlled by gene pairs
Pairs separate during meiosis ⇒
Law of Segregation
Each gamete has only one allele
Law of Independent Assortment
Which allele gets into which gamete is random
Dominant allele masks recessive allele
Same alleles = “true breeding”/homozygous
Different alleles = “hybrids”/heterozygous
An individual’s looks (phenotype) does not always represent its alleles (genotype)
Genetic Symbolism
Use the first letter of dominant allele
Capital = dominant
Lower case = recessive
If purple is dominant to white:
“P” = allele for purple
, “p” = allele for white
Making a Punnett Square: Monohybrid Cross
Test Cross (backcross)
An individual with the dominant phenotype (brown rabbit) and an individual
with the recessive phenotype (white rabbit)
Reveals the genotype of the dominant individual
Incomplete Dominance
A heterozygous phenotype is intermediate between 2 homozygous phenotypes
Dihybrid Crosses
Trait - Characteristic
Gene - Segment of DNA controlling a trait
Alleles - Different forms of a gene
“Flower color” is a gene
“Purple” & “White” are alleles
Lessons
Traits are controlled by gene pairs
Pairs separate during meiosis ⇒
Law of Segregation
Each gamete has only one allele
Law of Independent Assortment
Which allele gets into which gamete is random
Dominant allele masks recessive allele
Same alleles = “true breeding”/homozygous
Different alleles = “hybrids”/heterozygous
An individual’s looks (phenotype) does not always represent its alleles (genotype)
Genetic Symbolism
Use the first letter of dominant allele
Capital = dominant
Lower case = recessive
If purple is dominant to white:
“P” = allele for purple
, “p” = allele for white
Making a Punnett Square: Monohybrid Cross
Test Cross (backcross)
An individual with the dominant phenotype (brown rabbit) and an individual
with the recessive phenotype (white rabbit)
Reveals the genotype of the dominant individual
Incomplete Dominance
A heterozygous phenotype is intermediate between 2 homozygous phenotypes
Dihybrid Crosses
