2.2 - Heredity and Genetics

DNA and the Genetic Code:

The nucleus of the cell contains DNA which has the geneticinformation that is required for cells to divide.

DNA is packaged in the form of chromosomes.

Unit 1 - Life Science 1 / 15

2.2 - Heredity and Genetics

DNA and the Genetic Code:

The nucleus of the cell contains DNA which has the geneticinformation that is required for cells to divide.

DNA is packaged in the form of chromosomes.

Unit 1 - Life Science 1 / 15

What is DNA?

Deoxyribonucleic acid (DNA) is a molecule that encodes the geneticinstructions used in the development and functioning of all livingorganisms.

DNA is modelled in the shape of a double helix (a ladder that has beentwisted in a spiral). The steps of the ladder vary in composition.

Unit 1 - Life Science 2 / 15

How can the blueprints for so many different organisms be passedon by such a small molecule?

DNA is encoded using the arrangement offour chemicals:

Guanine & cytosine

Adenine & thymine

Guanine can only pair with cytosine andadenine can only pair with thymine.

This arrangement of G, C, A, and T forms acode that cells can “read”.

Unit 1 - Life Science 3 / 15

The genetic code is based on arranging the four chemicals intoinstructions that describe how to make an organism.

A gene is an uninterrupted segment of DNA, which contains codedinstructions for a specific characteristic.

Note: Genes are responsible for the inheritance of anorganism’s characteristics.

Each possible version of a gene is called an allele.

Unit 1 - Life Science 4 / 15

A useful analogy:

Recall, DNA is packaged in chromosomes.

A chromosome is like a single volume of a set of encyclopedias.

A cell’s genetic information is organized into many volumes ofencyclopedias.

The information in the encyclopedias is made up of letters (GCAT).

A gene is the words that make up a section of the encyclopedia.

A set of chromosomes is the complete set of encyclopedias.

Note: A complete set of human DNA is made up of 46 chromosomes.

Unit 1 - Life Science 5 / 15

2.2 - Review

1 Which of the following contain DNA? Explain your answer.

chromosomenucleusgene

2 What four chemicals make up the genetic code? Describe how thesechemicals are arranged in a DNA molecule.

3 Define the following terms: DNA, chromosome, gene, allele.

4 Explain why chromosomes are considered to be the “source ofdiversity” in organisms.

5 How do alleles fit into the analogy of chromosomes as encyclopedias?Explain.

Unit 1 - Life Science 6 / 15

Patterns of Inheritance and GeneticsSelective breeding has been practiced for much of human history. Forexample, to produce ideal or desirable offspring only plants or animals withthe most desirable traits are allowed to reproduce.

(a) Selective breeding accounts for muchdiversity in dogs today.

(b) Scientists have madethe perfect-lookingtomato through selectivebreeding.

Unit 1 - Life Science 7 / 15

After many generations of selective breeding, most of our plants no longerresemble the original species from which they were bred.

Example: Corn was bred by Indigenous peoples from species of grass calledTeosinte. A comparison of teosinte and modern corn shows the difference in seednumber and cob size.

Unit 1 - Life Science 8 / 15

Dominant Traits:

Example 1. Consider the following crossbreeding:

A cross between a purebred albino female cat and a purebred black male catproduces black kittens 100% of the time.

Unit 1 - Life Science 9 / 15

Summary of Example 1:

The kittens are hybrids, but they show no outward sign that theirmother is a purebred albino cat.

They’ve inherited an allele for black-coat fur and an allele forwhite-coat.

Thus, black-coat fur is a dominant allele and the black-coat colouris a dominant trait.

A dominant trait is a trait that will appear in the offspring if one of theparents contributes it.

Unit 1 - Life Science 10 / 15

Recessive Traits:

Example 2. Consider the following crossbreeding:

A cross breeding between two hybrid cats. Why is a white kitten produced 25%of the time in this breeding?

Unit 1 - Life Science 11 / 15

Summary of Example 2:

Two hybrid parents can produce a kitten with four possiblecombinations of alleles. Let B = black-coat, b = white-coat.

B + B (black-coat allele + black-coat allele) = BB (black-coat)

B + b (black-coat allele + albino-coat allele) = Bb (black-coat)

b + B (albino-coat allele + black-coat allele) = Bb (black-coat)

b + b (albino-coat allele + albino-coat allele) = bb (albino-coat)

Thus the albino condition is a recessive allele and white fur is arecessive trait.

A recessive trait is a trait that must be contributed by both parents inorder to appear in the offspring.

Unit 1 - Life Science 12 / 15

Punnett SquaresA Punnet Square is a diagram that is used to predict an outcome of aparticular breeding experiment.

Example 3. Pure brown eyed parent + blue eyed parent

There is a 100% chance that theoffspring will have brown eyes.

Unit 1 - Life Science 13 / 15

Example 4. Hybrid brown eyed parent + hybrid brown eyed parent

There is a 75% chance that theoffspring will have brown eyes and25% chance the offspring will haveblue eyes.

Unit 1 - Life Science 14 / 15

2.2 - Review continued...

6 Explain how dominant and recessive traits differ from each other.

7 How does a purebred individual differ from a hybrid individual?

8 How could two black cats produce an albino kitten? Use a Punnettsquare to explain your answer.

9 Is it possible to produce black kittens from two albino parents?Explain using a Punnett square.

10 If you need to be certain that a trait is going to appear in theoffspring of the plants or animals you are breeding, what must youfind out about the parents? Explain your answer.

Unit 1 - Life Science 15 / 15