Download You and Your Genes Revision Lesson 1

You and Your
Genes
Revision Lesson 1
What are Genes?
• All living organisms are made of cells
• Most cells have a nucleus
• In the nucleus are chromosomes, made
from DNA.
• To write:
DNA makes up chromosomes in the
nucleus of cells.
• Chromosomes contain thousands of genes
• Genes carry the information that controls
what an organism is like, with each gene
determining one characteristic.
• Each gene codes for a protein e.g.
structural proteins and enzymes.
• To write:
Sections of chromosomes are called
genes, which control cells and code for
proteins.
Why do we look similar but not the
same as our parents?
• Human cells, except gametes, contain 46
chromosomes in 23 pairs.
• One chromosome in each pair came from
the father, and one from the mother.
• In each chromosome, the same genes are
in the same place.
• Different versions of the same gene are
called alleles
• For each gene, a person has 2 identical or
2 different alleles. They have inherited one
from the mother and one from the father.
• We are therefore a mix of alleles from
both of our parents, so we are not the
same. Siblings have a different mix (apart
from identical twins).
• Some characteristics are determined by
one gene but most depend on many
genes
• Characteristics are also determined by
environmental factors.
• To write:
Most human cells have 46 chromosomes
(23 pairs – one from each parent).
Different versions of genes are called
alleles.
Our mix of alleles determines what we are
like.
Most characteristics are due to a number
of genes. Some are due to only one gene,
and some are affected by environment.
What makes us male or female?
• Humans have one pair of sex
chromosomes in each body cell. Females
have XX, males XY.
• Sex cells (sperm or egg) contain only 23
chromosomes. Every egg has an X, half of
sperm have X, half Y. The Y chromosome
has the instructions to make a male.
• At fertilisation, if a Y sperm fertilises an
egg, a male will develop. If an X sperm
fertilises an egg a female will develop.
To write:
• There are 1 pair of sex chromosomes in
each human cell, XX or XY.
• Sex cells contain half the number of
chromosomes (23)
• A pair of XX alleles makes a female. XY
makes a male.