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Theoretical Genetics 1

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  • Theoretical Genetics 1

Genetics Definitions and Using Punnet Squares

In this lesson students will be shown the link between chromosomes and genetics. A powerpoint intro to allele and genotypes in a simple case of inheritance of flower colour illustrates the link and demonstrates new terminology. Working with flashcards and a card sort activity students build their confidence in these terms before using them in some genetics problems involving Punnet squares and phenotype ratios. The square punnet of strawberries in the image is the wrong type of punnet!

Lesson Description

Guiding Questions

This penguin inherits genes from both parents.  How many from each parent?

Can we predict the Emperor penguin chick's characteristics from the characteristics of the parents?

What additional information do we need?

Activity 1 - Genetics Introduction - an example of Mendel or another

These slides are useful for introducing the concept of inheritance and how genes are passed on from parent to offspring by the transfer of chromosomes during meiosis, and sexual reproduction.

  

Activity 2 - Quizlet Flashcard Revision of the terms (including meiosis)

It is worth spending some time becoming familiar with the vocabulary of genetics.
Definitions of genotype, phenotype, dominant allele, recessive allele, co-dominant alleles, locus, homozygous, heterozygous, carrier and test cross are listed in the IB guide.
These  Quizlet flashcards for this lesson and all the genetics theory in this topic will help. Start by reading all the cards, then try to do the Scatter Game, in the fastest time possible.

Activity 3 - Definitions Card Sort Activity

Complete the activity on the Theoretical genetics definitions students' worksheet below. Cut and stick the definitions into the correct place on the table.
You can check the answers after using the flashcards above.

Activity 4 - Improving Answers in IB Exam Punnet Square Questions

Using these facts in IB exams under exam conditions is not always easy.

There is a technique to theoretical genetics questions and using punnet squares. Work through this IB style question with your teacher, then compare your answers to the model answer beneath the questions.

At the end of this activity there is a sheet of additional problems to solve.

Genetics question

In ten minutes, write answers to the questions below.  Then compare your results with the model answers.

1. A genetic cross was made between pure-breeding (homozygous) pea plants with smooth seeds and pure-breeding pea plants with wrinkly seeds. The cross produced F1 offspring that had only smooth seeds.

When the F1 plants were self-pollinated, the resulting offspring (F2 generation) had some pea plants with smooth seeds, and some pea plants with wrinkly seeds

(i) Determine the allele which is dominant, and the allele which is recessive.

..........................................................................................................................(1)


(ii) Deduce the genotype of the F1 plants.

..........................................................................................................................(1)


(iii) Construct a genetic diagram, with a Punnett grid to show the cross between two F1 plants.
Be sure to include parental genotypes, gametes, and both offspring genotype and phenotype.

Parental genotypes    ......................................................................................

Gametes ..........................................................................................................

Punnet square

.

F2 Offspring genotype(s)............................................................................................

F2 Offspring phenotype(s)............................................................................................(5)


(iv) Deduce the proportion (ratio or percentage) of the different phenotypes of the F2 offspring.
.
..........................................................................................................................(1)

This is a model answer. Compare it to your answer. There is more than one way to do this question.

Model answer

1. A genetic cross was made between pure-breeding pea plants with smooth seed and pure-breeding pea plants with wrinkly seeds. The cross produced F1 offspring that had only smooth seeds.

When the F1 plants were self-pollinated, the resulting offspring (F2 generation) had some pea plants with smooth seeds, and some pea plants with wrinkly seeds

(i) Determine the allele which is dominant, and the allele which is recessive.
.........................Smooth seeds is dominant, wrinkled seeds is recessive.....................(1)


(ii) Deduce the genotype of the F1 plants.
................Ss where S is allele for Smooth and s the allele for wrinkled ....................(1)


(iii) Construct a genetic diagram, with a Punnett grid to show the cross between two F1 plants.
Be sure to include parental genotypes, gametes, and both offspring genotype and phenotype.

Parental genotypes...............Ss ............................................................................

Gametes.................S and s ................................................................

Punnet Square

F2 Offspring genotype(s)...... SS, Ss (also sS which is the same as Ss) and ss ....

F2 Offspring phenotype(s).....Smooth seed producing and wrinkled seed producing ............(5)


(iv) Deduce the proportion (ratio or percentage) of the different phenotypes of the F2 offspring.

.............75% Smooth seed and 25 % wrinkled seed producing ................
........... 3 Smooth seed producing to 1 wrinkled seed producing pea plants ...(1)

Homework activity

Practice this new technique with this sheet of genetics questions. Theoretical genetics - homework punnet square questions

Extra Reading:

Simple inheritance of characteristics controlled by single genes really does occur. One example is cystic fibrosis. However as we learn more about genetics, and epigenetics simple inheritance of this kind is becoming a less favoured explanation of many characteristics. Is this the beginning of a paradigm shift in Mendelian genetics?

A quick reference to inherited human traits, which outlines some recent discoveries which cast doubt on single gene inheritance of nearly all visible human traits.

Information and a video from the Council for Responsible Genetics, incisive, succinct articles by leading scientists disputing the exaggerations and misrepresentations of the power of genes that circulate in both popular media and scholarly journals.

More detail and some references to explain what the latest thinking is about the genetics of human traits in a website called the Myths of Human Genetics

Teachers notes

This lesson introduces the basic ideas of genetics and then illustrates them with the example of sickle cell disease.

There are four sections and a homework task which is a lot for one activity, but actually the two flashcard activities might be done as preparation to the lesson.

  • Activity 1 introduces the basic concepts - designed for the teacher to explain the slides.
  • Activity 2 gives students themselves an opportunity to think about the key terms for this topic using some flashcards
  • Activity 3 is a puzzle to test understanding of the genetics terms in the flashcards. The content is the same as activity two. It's an alternative, if you prefer this worksheet to the flashcards.
  • Activity 4 is important to give students an understanding of how to structure answers to genetics questions.  Teachers could ask the students to attempt these questions first, or dive straight into correcting the sample student response.
  • The Homework activity gives students a chance to practice answering some genetics questions on their own.
    Model answers for the 'Further punnet square questions can be found here: Futher punnet square qns model answers.

The two worksheets are especially useful to help students to prepare for exams.

There is a great linking activity also available which mentions Stargardt disease. This is an interesting condition affecting the retina of the eye.  It is mentioned as an application in the IB diploma guide.