You and Your You and Your GenesGenes

Revision Lesson 1Revision Lesson 1

What are Genes?What are Genes?

• All living organisms are made of cellsAll living organisms are made of cells• Most cells have a nucleusMost cells have a nucleus• In the nucleus are chromosomes, In the nucleus are chromosomes,

made from DNA.made from DNA.

• To write:To write: DNA makes up chromosomes in the DNA makes up chromosomes in the

nucleus of cells.nucleus of cells.

• Chromosomes contain thousands of Chromosomes contain thousands of genesgenes

• Genes carry the information that controls Genes carry the information that controls what an organism is like, with each gene what an organism is like, with each gene determining one characteristic.determining one characteristic.

• Each gene codes for a protein e.g. Each gene codes for a protein e.g. structural proteins and enzymes.structural proteins and enzymes.

• To write:To write: Sections of chromosomes are called Sections of chromosomes are called

genes, which control cells and code for genes, which control cells and code for proteins.proteins.

Why do we look similar but not Why do we look similar but not the same as our parents?the same as our parents?

• Human cells, except gametes, contain Human cells, except gametes, contain 46 chromosomes in 23 pairs.46 chromosomes in 23 pairs.

• One chromosome in each pair came One chromosome in each pair came from the father, and one from the from the father, and one from the mother.mother.

• In each chromosome, the same genes In each chromosome, the same genes are in the same place.are in the same place.

• Different versions of the same gene Different versions of the same gene are called allelesare called alleles

• For each gene, a person has 2 For each gene, a person has 2 identical or 2 different alleles. They identical or 2 different alleles. They have inherited one from the mother have inherited one from the mother and one from the father.and one from the father.

• We are therefore a mix of alleles We are therefore a mix of alleles from both of our parents, so we are from both of our parents, so we are not the same. Siblings have a not the same. Siblings have a different mix (apart from identical different mix (apart from identical twins).twins).

• Some characteristics are determined Some characteristics are determined by one gene but most depend on by one gene but most depend on many genesmany genes

• Characteristics are also determined Characteristics are also determined by environmental factors.by environmental factors.

• To write:To write:

Most human cells have 46 chromosomes Most human cells have 46 chromosomes (23 pairs – one from each parent).(23 pairs – one from each parent).

Different versions of genes are called Different versions of genes are called alleles.alleles.

Our mix of alleles determines what we Our mix of alleles determines what we are like.are like.

Most characteristics are due to a number Most characteristics are due to a number of genes. Some are due to only one of genes. Some are due to only one gene, and some are affected by gene, and some are affected by environment.environment.

What makes us male or female?What makes us male or female? • Humans have one pair of sex Humans have one pair of sex

chromosomes in each body cell. chromosomes in each body cell. Females have XX, males XY.Females have XX, males XY.

• Sex cells (sperm or egg) contain only Sex cells (sperm or egg) contain only 23 chromosomes. Every egg has an X, 23 chromosomes. Every egg has an X, half of sperm have X, half Y. The Y half of sperm have X, half Y. The Y chromosome has the instructions to chromosome has the instructions to make a male.make a male.

• At fertilisation, if a Y sperm fertilises an At fertilisation, if a Y sperm fertilises an egg, a male will develop. If an X sperm egg, a male will develop. If an X sperm fertilises an egg a female will develop. fertilises an egg a female will develop.

To write:To write:

• There are 1 pair of sex chromosomes There are 1 pair of sex chromosomes in each human cell, XX or XY.in each human cell, XX or XY.

• Sex cells contain half the number of Sex cells contain half the number of chromosomes (23)chromosomes (23)

• A pair of XX alleles makes a female. A pair of XX alleles makes a female. XY makes a male.XY makes a male.