26:1 Genetics, How and Why

• Objectives:• 1. Compare the number to chromosomes in

sex cells and in body cells.• 2. Distinguish between dominant and

recessive genes.• 3. Describe how different gene combinations

result from fertilization and how traits are passed to offspring

Chromosomes

• Genetics is the study of how traits are passed from parent to offspring

• Traits are characteristics that an organism has, such as hair or eye color, height, or color

• Chromosomes are located in the nucleus.• The nucleus's job is to direct the functions of

the cell and to allow the cell to reproduce.

• There are two types of cells in the body• The body cells contains pairs of

chromosomes are make up most of the tissue and organs.

• These chromosomes are in pairs.• Sex cells are responsible for reproduction.• They can be either a sperm cell or an egg

cell.• They contain only half the number of

chromosomes.

Genes and Chromosomes

• A gene is a small section of chromosome that determines a specific trait of an organism.

• There are thousands of different traits.• Each chromosome has different kinds of genes that

control each trait.• Since chromosomes are paired each one has a

partner trait on the opposite chromosome

Passing Traits to Offspring

• Traits are passed on because one set of chromosomes comes from the mother and one set comes from the father.

• This occurs when the sperm and the egg join during fertilization.

• Remember, during meiosis chromosome duplication does not occur, so each cell has only half the required chromosomes.

Dominant and Recessive Genes

• Dominant genes are genes that keep other genes from showing their traits

• Recessive genes are genes that do not show their traits when dominant genes are present

• Pure dominant is when an organism has two dominant genes for a trait

• Pure recessive is when an organism has two recessive genes

• Pure means that both genes are the same.

• Heterozygous is an organism that has one dominant and one recessive gene

• Even though the organism has one of each gene, only the dominant one shows.

• The recessive gene does not show.

When Both Parents are Heterozygous

• A mother or father can pass on different types of genes to her offspring, because of how the chromosomes split.

• One of each pair will go to different new cells.

• There is a 3:1 ratio that a dominant trait will be passed on.