Junior Science

Chromosomes and Genes

Lesson Objectives

• Understand the terms – Chromosome, DNA, Genes and Allele.

• Explain the difference between Genotype and Phenotype.

• Give examples of dominant and recessive traits.

• Complete a simple Punnett square genetic cross.

• Chromosomes are thread-like structures located inside the nucleus of animal and plant cells.

• Each chromosome is made up of the protein histone and a single molecule of deoxyribonucleic acid (DNA).

• Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

• Every cell in our body except the sex cells (Sperm and Egg) contain 23 pairs of chromosomes that is 46 chromosomes in total

What are Chromosomes?

Karyotype

• A karyotype is simply a

picture of a person's

chromosomes.

• The chromosomes are

isolated, stained, and

examined under the

microscope.

• The chromosomes in the

white blood cells work

best.

• A picture is then taken

through the microscope.

DNA

• Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and functioning of living organisms.

• The main role of DNA in the cell is the long-term storage of information.

• The genome is often compared to a set of blueprints.

• The DNA segments that carry this genetic information are called genes.

Genes

• The instructions necessary to form a new individual are carried in chromosomes in a series of chemicals called GENES.

What are Genes?

• Genes are the working subunits of DNA.

• Each gene contains a particular set of instructions, usually coding for a particular protein or function for making some part of a cell.

Heredity and Genes

• The traits are passed on through genes.

• There are Dominant and Recessive genes.

• Dominant genes overshadow recessive genes and thus can determine the given characteristic.

• The term ‘genotype’ is used for the genetic makeup of an organism i.e. the pair of genes controlling the characteristic.

• The ‘phenotype’ is the actual appearance of that trait.

Alleles

• An Allele is a alternate form of a gene.

• There is a tendency for more people to have Dark hair (D) than Blonde hair (d).

• Thus the Allele for dark hair is dominant over the Allele for blonde hair.

• Dominant Alleles tend to appear more often that recessive genes.

Heterozygous and Homozygous

• An organism can be homozygous dominant, if it carries two copies of the same dominant allele (DD)

• An organism is homozygous recessive, if it carries two copies of the same recessive allele (dd)

• Heterozygous means that an organism has two different alleles of a gene (Dd)

Phenotype - Dark Hair

Genotype - Dd

Phenotype - Blonde Hair

Genotype - dd

Genotype and PhenotypeDark hair (D) is dominant over Blonde hair (d)

d d

D

d

Dd Dd

dddd

The Punnett Square

The Punnett squareis a diagram that is used to predict an outcome of a particular cross.

Make a punnet square for a cross between a homozygous Brown eyed

mother (BB) and a homozygous blue eyed father (bb).

Heredity – Example (2)

b b

B

B

Bb Bb

BbBb

Summary• Chromosomes are thread-like structures found inside the nucleus of animal and

plant cells.

• They are made up of the protein histone and deoxyribonucleic acid (DNA).

• Every cell in our body except the sex cells (Sperm and Egg) contain 23 pairs of chromosomes that is 46 chromosomes.

• The instructions necessary to form a new individual are carried in chromosomes in a series of chemicals called genes.

• There are Dominant and Recessive genes. Dominant genes overshadow recessive genes and thus can determine the given characteristic.

• An Allele is a alternate form of a gene.

• The term ‘genotype’ is used for the genetic makeup of an organism.

• Homozygous means that an organism has two copies of the same allele (DD)

• Heterozygous means that an organism has two different alleles of a gene (Dd)

• The ‘phenotype’ is the actual appearance of that trait.

• The Punnett square is a diagram that is used to predict an outcome of a particular cross.