5. Pedigree Charts & Test Crosses

Success Criteria

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

I can use pedigree charts to determine the genotype of offspring.
I can use a test cross to determine the likely genotype of a phenotypically dominant individual.

Vocabulary

Learn these so you can communicate this concept well.

Heterozygous: One dominant and one recessive allele in a genotype.
Homozygous dominant: Two dominant alleles in a genotype.
Homozygous recessive: Two recessive alleles in a genotype.
Pedigree chart: A chart that shows how one phenotype is passed on from one generation to the next.
Pure bred: An individual which is homozygous (either recessive or dominant).
Test cross: A cross used to determine whether a phenotypically dominant individual with an unknown genotype is homozygous dominant or heterozygous for a particular trait.

Hei Mahi (Do Now)

Do Now: In your OneNote/Notebook, write the correct answer and justify why.

Hei Mahi (Do Now)

Do Now: In your OneNote/Notebook, write the correct answer and justify why.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Pedigree Charts:

What are the parts?

Then, write an answer for it.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Pure Breeding and Test Crosses:

How does this relate to...?

Then, write an answer for it.

Pedigree Charts

In Science we can use family trees or PEDIGREE CHARTS to look at inheritable traits and how they have been passed on from parents to offspring, over several generations. They can also help us determine the probability that an offspring will inherit a genetic disorder.

S1.9 (5) Pedigree Charts

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.

Purebreeding

An individual which is HOMOZYGOUS (either homozygous recessive – gg, or homozygous dominant - GG) is called PURE BREEDING.

When two pure breeding individuals breed you can be 100% sure of the offspring produced.

(15) Pure Breeding

Test Crosses

S1.9 (5) Test Crosses

A TEST CROSS is used to determine whether a phenotypically dominant 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 BRED individual).

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.

How could an animal or plant breeder know if an individual is purebreeding?

To ensure that an individual is purebreeding, the breeder could cross the individuals with homozygous recessive individuals. If any of the offspring are phenotypically recessive, the original individual was not purebreeding. If all the offspring are phenotypically dominant, the individual is a purebred.

If two parents are able to have a homozygous recessive offspring, both parents must have at least one recessive allele each to be able to pass on a recessive allele to the offspring. In this way, the hidden allele can be passed on from the parents to show up in the offspring.

Many offspring are needed to e confident of the genotype, but this cannot be proven.

How do we find out the genotype of a phenotypically dominant individual?

If all the actual offspring were phenotypically dominant, the most likely genotype for this is homozygous dominant.

However, these Punnett squares only show the probability of an event occurring. The heterozygous/homozygous recessive cross can also produce phenotypically dominant offspring. It may just be chance that phenotypically recessive individuals were not produced.

To be certain of the genotype of a phenotypically dominant individual, you would have to carry out many more crosses with a phenotypically recessive parent.

If, after a LARGE number of crosses there were no phenotypically recessive offspring, you would have confidence that the individual was homozygous dominant, but only one phenotypically recessive offspring will prove a heterozygous individual.