Rules and Understandings about Alleles
Alleles pair together to make a gene.
They add variety to a gene as well.
Alleles are either dominant or recessive.
Dominant alleles are symbolized by uppercase letters.
Therefor, recessive alleles are symbolized with lowercase letters.
The the first letter in the dominant allele’s word is used to symbolize the trait
For example:
Eye color is a trait controlled by the “eye color gene”
The alleles for eye color are brown and blue.
The dominant allele is brown, so we will use “B” to symbolize the brown allele
The recessive allele is blue, so we will use a “b” to symbolize the blue allele
You try:
When referring to the height gene, tall is the dominant allele and short is the recessive allele.
What letter would you use to symbolize the alleles?!!
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Write the tall allele’s symbol:! !
Write the short allele’s symbol: !
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Genotype and Phenotype
Genotype is the make up of alleles on a gene
Phenotype is the allele that shows up
Example: The genotype for brown eyes is BB, the phenotype is brown eyes
Combinations of alleles:
If a child receives two dominant brown eye alleles (one from each parent) their genotype will =
BB. This is homozygous brown because both alleles are the same. Brown eyes will be the trait
that shows up. Therefor the phenotype will be brown.
Homozygous and Heterozygous
Homozygous-when the alleles of a gene are the same. Example: BB or bb
Heterozygous-when the alleles of a gene are different. Example: Bb
If a child receives two recessive blue eye alleles (one from each parent), then their genotype will
= bb. This is homozygous blue because both alleles are the same. Blue eyes will be the trait
that shows up. Blue eyes will only show up if the recessive blue allele’s pair up together.
Therefor the phenotype will be blue.
If a child receives one dominant brown eye allele from one parent and one recessive blue eye
allele from another parent, their genotype will equal=Bb. This is heterozygous because the
alleles are different. The brown allele will dominate and show up as the trait on the organism,
while hiding the recessive blue allele. Therefor the phenotype will be brown.
PUNNETT SQUARES
Two parents, Two alleles!
Both parents have pairs of alleles for each gene but YOU DON’T KNOW WHICH
ALLELE YOU WILL RECEIVE FROM EITHER PARENT...so its a toss up.
There could be a variety of outcomes!
It helps if you know the genotype of the parents’ genes.
If you only know the trait that is showing up, you may not be able to tell if the allele is
masking a recessive allele or not.
Its good to sometimes know the grandparent’s traits too!
PUNNETT SQUARES
Punnett Squares are diagrams that help predict the outcomes of offspring.
Its named after Reginald C Punnett, who devised the approach and is used by
biologists to determine the probability of an offspring’s genotype.
Its a summary of every possible combination of the alleles.
Example:
Which allele is dominant?! !
Which allele is recessive?! !
What are the two alleles?! !
Which parent is heterozygous?!
Which parent is homozygous?!
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Parent 2’s
genotype
Parent 1’s genotype
Step 1: The parent’s genotype are written across the top and side of the punnett
squares
Step 2: Add the alleles up together
Step 3: Always put the dominant allele first
How many offspring are homozygous:!
How many offspring are heterozygous: !
What is the phenotype of the offspring: !
What is the genotype of the offspring: !
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You try
Gene for height
Parent 1’s genotype is TT
Parent 2’s genotype is Tt
How many offspring are homozygous:!
How many offspring are heterozygous: !
What is the phenotype of the offspring: !
What is the genotype of the offspring: !
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Try Again:
Eye color gene
Parent 1’s genotype is bb
Parent 2’s genotype is Bb
How many offspring are homozygous:!
How many offspring are heterozygous: !
What is the phenotype of the offspring: !
What is the genotype of the offspring: !