Concept 4: Punnett Squares
Use pedigree charts to determine the genotype of offspring.
Use Punnett squares to determine the expected genotype and phenotype ratios of offspring.
Use Punnett squares to determine the expected sex of offspring.
Monohybrid inheritance involves the inheritance of a single characteristic (trait) that is controlled by one gene with two alleles.
E.g. The inheritance of pea shape from a gene in pea plants that has the following two alleles: 'Round' allele R, 'Wrinkled' allele r. The 'round' allele is dominant over the recessive 'wrinkled' allele.
E.g. The inheritance of coat colour from a gene in rabbits that has the following two alleles: 'Brown' allele B, 'White' allele b. The 'brown' allele is dominant over the recessive 'white' allele.
An individual inherits two of these alleles from its parents at fertilisation (when egg and sperm combine).
E.g. A plant inherits these two alleles for pea shape - Genotype: rr (homozygous recessive), Phenotype: wrinkled peas
E.g. A rabbit inherits these two alleles for coat colour - Genotype: Bb (heterozygous), Phenotype: brown coat
I can use Punnett squares to determine the expected genotype and phenotype ratios of offspring.
I can use Punnett squares to determine the expected sex of offspring.
A Punnett square shows all possible gamete combinations in a cross of parents, giving the expected genotype and phenotype ratios of offspring.
E.g. In rabbits, the allele for a brown coat (B) is dominant over the allele for a white coat (b). Two heterozygous (Bb) rabbits are crossed.
Parent genotypes: Identify the genotype of the mother and father.
Gamete genotypes: Identify the possible alleles in the father's sperm and the mother's eggs.
Offspring genotypes: Combine different pairs of gametes to give all possible genotypes of the offspring.
Phenotype ratio: Use genotypes of offspring to determine phenotype ratio. 75% of offspring brown coat, 25% of offspring white coat.
Random fertilisations of eggs by sperm means that the number of offspring showing each phenotype will not always match the expected phenotype ratio. This is particularly true for small numbers of offspring.
Crossing different parent genotypes from those shown above, will give different expected phenotype ratios of offspring.
E.g. Homozygous dominant x homozygous recessive = 100% of offspring have brown coat.
E.g. Homozygous dominant x heterozygous = 100% of offspring have brown coat.
E.g. Heterozygous x homozygous recessive = 50% brown coat, 50% white coat.
Concept 5: Pedigree Charts & Test Crosses
Draw and describe a labelled diagram of a DNA molecule.
Use the complementary base-pairing rule to help explain the process of DNA replication.
Look at the concepts in the tree of knowledge for the 1.9 Genetics topic.
Write down the concepts that have been covered in class. E.g. Concepts 1 + 2.
(This will help me plan our future lessons!)
I can interpret a pedigree chart.
A pedigree chart shows the inheritance of a single genetic trait over several generations. Males are shown with a square, females with a circle.
A mating couple is connected by a horizontal line. Their offspring are placed on branches below. Colouring indicates a specific trait exhibited by an individual.
For example, the pedigree chart below shows the inheritance of a hair length gene in cats over 3 generations. Cats are either short-haired or long-haired.
Dominant or recessive?
A pedigree chart can be used to determine which allele is dominant.
For example, 6 and 7 (who both have short hair) produce offspring with long hair. If the short hair was recessive, the parents would not be able to produce offspring with long hair, so short hair allele must be dominant.
What's their genotype?
A pedigree chart can also be used to determine the genotype of individuals.
For example, the genotype of individual 6 must be heterozygous. Since 6 has short hair, it must have at least one dominant allele. Individuals 6 and 7 have offspring with long hair. This means that 6 and 7 must have passed on a long hair allele, because for their kittens to have long hair, they must have two recessive alleles, one from each parent.
Task 1: Solve the Mysteries
Click on this link.
For each trait, read the description of the family. Use this information to solve the family's pedigree chart.
Task 2: Worksheet
Complete the worksheet assignment called "1.9 Concept 5: Pedigree Charts".
Task 3: SciPAD
Page 208-209 - Pedigree Charts
Page 210 - Cystic Fibrosis
Page 211 - Pedigree Rats
Task 4: Education Perfect
Complete the task called "Concept 5: Pedigree Charts & Test Crosses".
Read the question from the Lemonade-Ed Instagram post.
Write and answer the question in your Science note book.
I can use a test cross to determine the likely genotype of a phenotypically dominant individual.
A test cross is used to determine whether an individual with an unknown genotype is homozygous dominant or heterozygous for a particular trait.
For example, in wild rabbits, the brown coat allele is dominant over white coat allele. The brown rabbit to the right could be BB (homozygous dominant) or Bb (heterozygous) as both result in brown coats.
In a test cross, the unknown individual is crossed with a homozygous recessive individual (a pure breeder).
The phenotypes of the offspring help determine the unknown genotype.
If any of the offspring have white coats, then the unknown individual above must be Bb (heterozygous).
The hidden recessive allele in the individual has been paired up with another recessive allele.
When Bb is crossed with bb, expect 50% white coat rabbit offspring.
If none of the large number of offspring are white, then the individual above is most probably BB (homozygous dominant).
A large number of offspring is required to be sure that the white allele is not present in the 'unknown' individual.
When BB is crossed with bb, expect 100% brown rabbit offspring.