Gametes and Inheritance

7.2 Gametes and Inheritance

Gametes

• Sperm
• Ovum

Learning Objectives

• Define a gamete.
• Compare and contrast male and female gametes.
• Describe fertilization.
• Explain sex determination in humans.

Keywords

• Egg cell
• Fertilisation
• Gametes
• Inheritance
• Sex chromosomes
• Sex inheritance
• Sperm cell
• X chromosomes
• Y chromosomes
• Zygote

Getting Started

• All cells share common features but are often specialized for specific functions.

Gametes

• Most human cells contain 46 chromosomes.
• Chromosomes come in pairs, one from each parent.
• Each chromosome pair has the same genes in the same order but with slight DNA sequence variations.

Genetic Information

• A complete set of genetic information is present in each cell.

Chromosomes

• Are inherited from both father and mother.

Autosomes

• Non-sex chromosomes.

Genes

• Units of heredity.

Sex Chromosomes

• Determine sex (XX for female, XY for male).

Fertilization

• Every human begins as a single cell formed by the fusion of a sperm cell and an egg cell.
• Sperm and egg cells are specialized cells called gametes.
• A sperm cell is a male gamete.
• An egg cell is a female gamete.
• The process of sperm and egg fusion is called fertilization, resulting in a zygote.

Sperm Cell

• Diagram of a human sperm cell showing:
  • Tail for swimming movements
  • Nucleus containing 23 chromosomes
  • Small amount of cytoplasm
  • Cell surface membrane
• Sperm cells are small and use their tails to swim.

Egg Cell

• Diagram of a human egg cell showing:
  • Cell surface membrane
  • Cytoplasm containing food reserves
  • Nucleus containing 23 chromosomes
• Egg cells are much larger than sperm cells.

Sperm vs. Egg

Fertilisation

• Human cells have 46 chromosomes, in two sets of 23.
• Gametes have only one set of 23 chromosomes.
• Fusion of sperm and egg restores the two sets (46 chromosomes).
• The joining of a sperm cell with an egg cell is called fertilisation.
• During fertilisation the head of the sperm cell enters the egg cell.
• The nucleus of the sperm cell and the nucleus of the egg cell fuse together.

Zygote

• The new cell formed by the fusion of sperm and egg nuclei is called a zygote.
• The zygote contains two sets of chromosomes (46 total).
• A zygote divides repeatedly to form all the cells in a human body.

Boy or Girl?

• Chromosomes are inherited from both parents.
• Sex is determined by sex chromosomes.
• Two X chromosomes (XX) result in a female.
• One X and one Y chromosome (XY) result in a male.

Sex Chromosomes

• Chromosomes that determine sex.
  • X Chromosome
  • Y Chromosome

Sex Determination

• Egg cells contain one X chromosome.
• Sperm cells contain either one X chromosome or one Y chromosome.

Fertilization Outcomes

• X-containing sperm + egg → XX (female).
• Y-containing sperm + egg → XY (male).
• The chance of either outcome is approximately equal, leading to roughly equal numbers of boys and girls being born.

Inheritance

• Inheritance is the passing of DNA (chromosomes containing genes) from parents to offspring.
• Sex inheritance refers to the inheritance of X or Y chromosomes, determining a baby’s sex.
• All organisms inherit features from their parents.

Key Points

• All eggs contain an X chromosome.
• Half of all sperm cells contain an X chromosome, and half contain a Y chromosome.
• If a sperm cell containing an X chromosome fertilises an egg, the zygote has chromosomes XX, and the XX zygote will develop into a girl.
• If a sperm cell containing a Y chromosome fertilises an egg, the zygote has chromosomes XY, and the XY zygote will develop into a boy.