4. Punnett Squares

Success Criteria

Your learning has been successful if you can do the following:

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.

Vocabulary

Learn these so you can communicate this concept well.

Cross: Mating
Fertilisation: When sperm and egg fuse / combine.
Genotype: Combination of two alleles coding for a trait.
Inheritance: The process in which genetic information is passed down from parent to child.
Phenotype: Physical representation of the genotype.
Punnett square: A grid that shows all possible gamete combinations in a cross of parents, giving the expected genotype and phenotype ratios of offspring.
Random fertilisation: The egg and sperm that end up fusing together are randomly selected from a larger pool of gametes.
Trait: Physical attributes

Hei Mahi (Do Now)

Do Now: Draw and finish this table in your OneNote/Notebook.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook.

Explain the difference between "expected" and "observed" phenotype ratios.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Punnett Squares:

What can you infer?

Then, write an answer for it.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Random Fertilisation:

What is the consequence?

Then, write an answer for it.

Monohybrid Inheritance

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). The combination of these two alleles is called the GENOTYPE, and the physysical expression of this genotype is called the PHENOTYPE.

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

Punnett Squares

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.
Genotype ratio: Count the number of each genotype and write it as a ratio or percentage.
1. - 1 BB : 2 Bb : 1 bb
  - 25% BB, 50% Bb and 25% bb
Phenotype ratio: Use genotypes of offspring to determine phenotype ratio.
1. - 3 brown coat offspring : 1 white coat offspring
  - 75% of offspring brown coat, 25% of offspring white coat.

However, remember that these % and ratio are just showing statistical probability… NOT what will definitely happen. Each FERTILISATION is a separate, random event that is not affected by previous fertilisations!

S1.9 (4) Punnett Squares

S1.9 (4) Genotype & Phenotype Ratios

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.

Sex Determination

In females the sex chromosomes are identical and they are called the X chromosomes. Females have two X chromosomes (XX). In males the sex chromosomes are not identical. The larger chromosome is the X chromosome which is exactly the same as the females X chromosome. The other chromosome males have is the Y chromosome and it is much smaller than the X chromosome. So males have and X and a Y chromosome (XY).

Meiosis produces gametes. As a result of meiosis each gamete only gets one sex chromosome. Since females only have X chromosomes, all a females eggs will carry an X.

Male body cells carry both an X and a Y so the sperm that is produced by meiosis can carry an X or a Y chromosome. 50% of the sperm will carry an X chromosome and 50% will carry a Y chromosome. The gender is therefore decided by which sperm fertilises the egg. A sperm carrying an X will produce a female, a sperm carrying a Y will produce a male.

S1.9 (4) Sex Determination