Page 6 and 7 Notes: Test Crossing and Probability
- test crossing: the mating between an individual of unknown genotype and an individual of a homozygous recessive genotype
- test crossing is used to determine the unknown genotype
In Mendel's experiments: a purple flower with an unknown genotype 'B_' (unknown second allele) and a white flower with a homozygous recessive genotype 'bb' are bred. If the genotype of the purple flower is homozygous dominant or 'BB', all offspring will be purple. If the genotype of the purple flower is heterozygous dominant or 'Bb',
some of the offspring will be white.
- probability: what is the chance that the offspring will exhibit a particular genotype of phenotype?
- What is the chance that two parents will have two girls? There is a 50% chance that each baby is female because the father can give the child either an X (the child is female) or Y chromosome (the child is male). Multiply 50% (or 1/2) by 50% (1/2) to get 25%. There is a 25% that both children will be girls.
UP pg 55-57: Punnett Squares and 1-factor crosses
- Punnett Squares are used to find the different possibilities of phenotypes and genotypes in the offspring of two parents.
If two heterozygous dominant purple flower (Bb) are bred, the offspring possible phenotypes and genotypes include:
Phenotype: purple or white
Genotype: BB (homozygous dominant), Bb (heterozygous dominant) and bb (homozygous recessive)
The Punnett Square for this situation looks like this:
Phenotype: 75% of the offspring will be purple and 25% of the offspring will be white. The ratio of purple to white flowers is 3:1.
Genotype: 25% of the offspring have the genotype BB, 50% have Bb and 25% have bb. The ratio of BB to Bb to bb is 1
This is a 1 factor cross because the we are finding possibilities for just one gene.
With certain genes, an offspring with a heterozygous genotype will exhibit a mixed phenotype. This is called a blending of genes. For example, one parent has curly hair and a homozygous dominant genotype HH. The other parent has straight has and a homozygous recessive genotype hh. The offspring will have the genotype Hh but in this case the dominant gene doesn't completely mask the recessive gene. Instead, the genes "blend" and the child has wavy hair.
Homework: UP pg. 45-49 and 55-57 and work ahead if you are ready to.
next scribe: Kiran