DL Week 5

Task 2.1A  (Reading Tx pp 20-23)

Genetics is the study of inherited characteristics called traits.

In natural populations all the individuals are genetically different—they have different traits. However, as Mendel saw, there are patterns in how these traits are inherited.

One pattern can be seen in flower colour in pea plants. Pea plants can have red flowers or they can have white flowers. Flower colour is one of the genes on an homologous pair of chromosomes (matching pair with one from father and one from mother). This gene comes in two types: known as alleles.

In the pea-plant example, the gene for flower colour has two alleles—one for the red flower trait and one for the white flower trait.

Plant breeders can create pure-breeding lines of plants—all the individuals have the same genetic information. If pollen from a pure-breeding red-flower pea plant pollinates a pure-breeding white-flower pea plant, all the flowers in the next generation will be red. The red allele is dominant to the white allele and switches it off. But the white allele has not disappeared. Each of the plants in this new generation—known as the F1 generation—'carries' the white allele within it, but the white allele is not seen in the appearance of the individual. The red allele is the dominant allele—the one that is observed in the outward appearance of the individual. In this example, the white allele is the recessive allele—the one that remains hidden.

When studying crosses and potential characteristics of offspring, geneticists use shorthand conventions. The dominant allele is represented by an upper-case letter related to the name, and it is written first. The red flower allele could be R (pronounced as ‘big r’). The recessive characteristic (white flower) is then represented by the lower-case of the same letter—r (pronounced as ‘little r’). Using R and r shows that a particular gene is being discussed.

The RR (‘big r big r’) combination of alleles will produce a red flower. Rr (‘big r little r’) will also produce a red flower, because red is dominant to (switches off) white. Only the double recessive rr (‘little r little r’) will produce a white flower. 

When an individual has two alleles the same, such as the RR and rr combinations, the individual is called homozygous for that allele. Individuals with different alleles of the gene, such as the Rr combination, are heterozygous. 

When working out the possible characteristics of offspring, it is important to remember that in meiosis the chromosomes separate into different gametes, a heterozygous parent will produce gametes with different alleles of the flower colour gene, a homozygous parent will only produce one type of gamete for that characteristic. 

Punnett squares

A Punnett square is a way of showing all the possible types of offspring that could result from a cross. It represents a statistical probability, so the exact proportions of red and white flowers might differ in reality.

In a Punnett square, the possible gametes produced by one parent are separated into two columns across the top. The gametes from the other parent are separated into two rows down the side. In each square is a possible outcome of fertilisation. 

Genotype and phenotype

RR, Rr and rr represent the pea plant genotypes—the genetic information carried by an individual. The red or white colour of the flower is the phenotype—the observable characteristics of the individual. A red rose has the phenotype ‘red colour’ but it could have a genotype of RR or Rr, because the R allele is dominant to the r allele. The white rose has the phenotype of ‘white flower’ and can only have the genotype rr.

TASK 2.2A EXTENSION 

interact with Purple People Eater PPT to complete Purple People Eater WS 

Test yourself 

• https://www.liveworksheets.com/rf1474113qq  do not use commas

• https://www.liveworksheets.com/kp1731450jm 

TASK 2.2B

Complete this quiz (have whiteboard and marker ready):

• https://www.khanacademy.org/science/biology/classical-genetics/mendelian--genetics/e/punnett-squares 

• if you're ok with this, go to 2.2D

• if this has difficulties, go to 2.2C then 2.2D

TASK 2.2C

Revision: complete STILE Lessons:

•  Decoding Life-3.1 Introduction to Inheritance https://stileapp.com/au/RC_NSW-551/subject-306846/lesson-2050615/worksheet-15521863  

• Decoding Life-3.2 Punnett Squares https://stileapp.com/au/RC_NSW-551/subject-306846/lesson-2050615/worksheet-15521963 

TASK 2.2D

Complete these Stile worksheets:

•  Decoding Life-3.1 Introduction to Inheritance https://stileapp.com/au/RC_NSW-551/subject-306846/lesson-2050615/worksheet-15521863  

• Decoding Life-3.2 Punnett Squares https://stileapp.com/au/RC_NSW-551/subject-306846/lesson-2050615/worksheet-15521963