Natural Selection and Genetic Modification – 1

Year 9 Topic 3: Natural Selection and Genetic Modification

Learning Opportunity 1: Alleles

This Learning Opportunity has 7 tasks.

Target Time: 90 minutes

By the end of this learning opportunity you will be able to:

Describe differences in the inherited characteristics of organisms in terms of genotype and phenotype.  

Draw Punnet squares to illustrate inheritance of sex chromosomes.

Explain these key words: chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote. 

Key words: Chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote.

Task 1: Write answers to the following questions:

  1. Can you list 5 ways in which humans have different features? 

2. What features are caused by inheritance (from DNA from parents) or from the environment? 

Task 2: Draw a table similar to the one shown below to help you make notes on this reading task.

KeywordDefinition and notes
Chromosome
Gene
Alleles
Homozygous
Heterozygous
Click to read the information below and make notes by filling in the ‘definition and notes’ sections of your table.

DNA is formed into chromosomes. These contain instructions for characteristics / features (e.g. hair colour) called genes. The variations of these characteristics are called alleles (e.g. brown hair and blonde hair are different alleles).

There are two copies of every chromosome in each nucleus of body cells. These copies might have different alleles which leads to genetic variation.  

If both alleles are the same (e.g. one allele is blonde hair and the other is also blonde hair) these are homozygous genes.  

If the alleles are different (e.g. one allele is brown hair and the other is blonde hair) these are heterozygous gene.  

Answers are available at the end of this learning opportunity.

Task 3: Watch the video and answer the questions:

  • What are the definitions of ‘dominant alleles’ and ‘recessive alleles’?
  • The alleles the organism has is called its genotype. The visible characteristic it has is the phenotype. Using the video, give the 3 different versions of the cat’s genotype and the phenotypes these give.

Answers are available at the end of this learning opportunity.

Task 4: Label the chromosome diagram with the labels below:

  • different genes
  • same gene and same allele in both copies
  • same gene with different allele copies

Answers are available at the end of this learning opportunity.

Task 5: Use this information to answer the question below:

  • We can show possible alleles of offspring using a punnett square. ​
  • For example, sperm and egg cells (gametes) only contain one copy of each chromosome. When they combine there is a complete set of chromosomes. ​
  • Two chromosomes determine what sex you have; sex chromosome X and Y. If you are male you have sex chromosomes X and Y. If you are female you have the sex chromosomes X and X. ​Male = XY Female = XX.
  • So female egg cells can only contain X sex chromosomes. Male sperm cells have a 50% chance of having either X or Y sex chromosomes. We can predict the possible gender of offspring by doing a Punnett Square.

Question: Using the Punnett square; explain why during fertilisation there is always a 50/50 chance of the offspring being male or female. 

Answers are available at the end of this learning opportunity.

Task 6: Complete the Punnett Square.

  • Punnet squares are also used to predict the characteristics of offspring. ​
  • Below is an example of a parent with only brown hair alleles (B) and a parent with only blonde hair alleles (b)​
  • Complete the Punnett square to predict what chance their offspring will have of: 1. only having brown hair alleles (BB), 2. only having blonde hair alleles (bb), or 3. having both alleles (Bb).
  • Also state what their phenotypes will be. (Blonde hair or brown hair).
Task 2 Definitions
KeywordDefinition
ChromosomeA strand of DNA made up of genes.
GeneA section of DNA which codes for a particular protein.
AllelesDifferent versions of the same gene.
HomozygousWhen both alleles of a gene are the same.
HeterozygousWhen both alleles of a gene are different.
Task 3 Answers
  • Dominant alleles: Versions of a gene that will be expressed over the recessive version. They are expressed as capital letters.
  • Recessive alleles: Versions of a gene that will only be expressed if there is no dominant version present.
  • Genotype 1: FF gives phenotype: orange cat.
  • Geneotype 2: ff gives phenotype: grey cat.
  • Geneotype 3: Ff gives phenotype: orange cat.
Task 4 Answers

Different genes: lines drawn to blue and grey line or blue and red line.

Same gene and same allele in both copies: lines drawn to both blue lines.

Same gene with different allele copies: lines drawn to grey and red lines.

Task 5 Answers

During fertilisation there is a 50% chance that a sperm containing an X chromosome will fuse with the egg. This would result in a female offspring – XX. There is also a 50% chance that a sperm containing a Y chromosome will fuse with the egg. This would result in a male offspring – XY.

Task 6 Complete Punnett Square
B​B​
b​Bb​Bb​
b​Bb​Bb​

Task 6 Answer

During fertilisation there is a 50% chance that a sperm containing an X chromosome will fuse with the egg. This would result in a female offspring – XX. There is also a 50% chance that a sperm containing a Y chromosome will fuse with the egg. This would result in a male offspring – XY.

Task 7: Complete the exam questions.

Q1. 
   

Eye colour is controlled by genes. 

The allele for brown eyes, B, is dominant to the allele for blue eyes, b. 

(a)  A female with blue eyes and a male with brown eyes are about to have a child. 

Complete the Punnett square to determine the phenotype of the child. 

(2) 

 
Phenotype of child ………………………………………………………………………………………………………………………………… 

 
(b)  A scientist recorded the eye colour of 30 people. 

The results are shown in Figure 1. 

Figure 1 

(i)  Complete the tally chart, in Figure 2, for this data. 

(2) 

Figure 2 

(ii)  Give another appropriate method of displaying this information. 

(1) 

 ……………………………………………………………………………………………………………………………  

 ……………………………………………………………………………………………………………………………  

(Total for question = 5 marks) 

Mark Scheme – Mark your answers

Q1. 
  

If you would like to learn more about the topics covered in this learning opportunity then follow the link below!

https://www.bbc.co.uk/bitesize/guides/z2rm3k7/revision/4

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