3. Genes, Alleles & Traits

Success Criteria

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

  1. I can describe the relationship between DNA, genes, alleles and proteins.

  2. I can distinguish between the terms 'genotype' and 'phenotype'.

  3. I can define the terms 'homozygous' and heterozygous'.

  4. I can correctly use the terms 'dominant' and 'recessive' when referring to alleles.

Vocabulary

Learn these so you can communicate this concept well.

  1. Genotype: Combination of two alleles coding for a trait.

  2. Phenotype: Physical representation of the genotype.

  3. Homozygous: Two of the same alleles.

  4. Heterozygous: One dominant and one recessive allele in a genotype.

  5. Dominant allele: Allele that is always expressed, even if only one copy is present.

  6. Recessive allele: Allele that is only expressed if the dominant allele is not present.

  7. Allele: Alternative forms of a gene.

  8. Gene: Section of DNA that codes for a particular protein.

  9. Homozygous dominant: Two dominant alleles in a genotype.

  10. Homozygous recessive: Two recessive alleles in a genotype.

  11. Proteins: Molecules coded for by genes, and are the building blocks of our cells.

  12. Trait: Physical attributes

  13. Homologous chromosomes: Pairs of chromosomes that carry the same genes in the same place. One comes from mum, the other from dad.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Think back to Concept 1, and write the definition for GENE and PROTEIN.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Think back to the previous lesson, and write the definition for DOMINANT and RECESSIVE.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Genotype & Phenotype:

How is it different?

Then, write an answer for it.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Homozygous & Heterozygous:

How is it different?

Then, write an answer for it.

Alleles

The sequence of bases along a DNA molecule encodes the genetic instructions (genes) for the organism. A GENE is a specific section of this base sequence (often thousands of bases long). Each gene carries the instruction for making a specific protein, which in turn determines a particular feature (TRAIT) of the organism. Traits are simply physical attributes.

Each individual inherits two copies of the gene - one from mum, and one from dad. But the actual sequence of bases making up a particular gene can vary. These variants of the gene are called ALLELES. They are responsible for the different proteins, and so cause variations in a particular trait ('inherited' variation) between individuals

For example, a gene for pea shape in a plant species has these 2 alleles on a particular HOMOLOGOUS CHROMOSOME pair:

  • 'round' allele codes for round peas.

  • 'wrinkled' allele codes for wrinkled peas.

For example, a gene for coat colour in rabbits has these 2 alleles:

  • 'brown' allele codes for brown coat colour on a particular homologous chromosome pair.

  • 'white' allele codes for white coat colour.

There are two types of alleles:

  1. A DOMINANT ALLELE (given a capital letter) is always expressed even if only one copy is present.

  2. A RECESSIVE ALLELE (given a lower case letter) is expressed only if the dominant allele is not present. So recessive alleles can be hidden by a dominant allele.

Genotype & Phenotype

GENOTYPE is the combination of two alleles coding for a trait. The organism's genotype determines the PHENOTYPE, because phenotype is the physical representation of the genotype.

This means that the two alleles present (genotype) code for specific PROTEINS, which then determine the observable trait (phenotype). Both recessive alleles must be present for that trait to be expressed.

S1.9 (3) Genes, Alleles & Traits

There are three different genotypes possible:

  1. Two dominant alleles (e.g. RR or BB) is called HOMOZYGOUS DOMINANT.

  2. One dominant allele and one recessive allele (e.g. Rr or Bb) is called HETEROZYGOUS.

  3. Two recessive alleles (e.g. rr or bb) is called HOMOZYGOUS RECESSIVE.

S1.9 (3) Three Types of Genotypes

E.g. the plant with the alleles above will have smooth peas as one dominant S allele is present.

E.g. Possible genotypes (and the resulting phenotypes) for the two pea shape alleles shown above.

The rabbit with the alleles above will have a brown coat as the dominant B allele is present.

E.g. Possible genotypes (and the resulting phenotypes) for the two coat colour alleles shown above

Tasks & Homework

Sky Level Task - Genes, Alleles & Traits

S1.9 (3) Genes, Alleles & Traits (2).pdf

Sun Level Reading - What are Dominant and Recessive?

sciPad Workbook

  • Page 185 - DNA to Protein

  • Page 186 - Genes and Alleles

  • Page 195 - Genotype and Phenotype

Education Perfect HOMEWORK

Work through the Education Perfect task called "S1.9 Concept 3: Genes, Alleles & Traits"

Grass Level Task - Wordwalls!