Download Inheritance Intro

What is DNA?
Chromosomes and their genes are made of a molecule
called DNA.
DNA stands for
deoxyribosenucleic
acid.
Each chromosome
is a very long molecule
of tightly coiled DNA.
DNA molecules carry the code that controls what your cells
are made of and what they do.
Structure of DNA
DNA and base pairs
Base pairs hold the two strands of the DNA helix together.
The rules for base pairing are…
A always pairs with T
C always pairs with G
There are millions of base pairs in a DNA molecule that
always follow these rules.
Amazingly, it is the sequence of bases along a DNA
molecule that forms the genetic code - it’s that simple!
What is mitosis?
Mitosis begins with a single cell.
How many chromosomes does
this cell contain?
original
cell
First the cell makes a copy
of each chromosome…
…then it divides.
cell
division
Each new cell has a full set
of chromosomes and is
identical to the original cell.
2 new cells
What is mitosis?
Each new cell can keep on
dividing by mitosis.
Mitosis makes new cells for
growth and repair in all living
things. That’s how you get
from one cell to 50 billion!
Mitosis is also called
copying division.
What does this mean?
Mitosis animation
What is meiosis?
Meiosis begins with a single cell.
original
cell
First the cell makes a copy
of each chromosome.
Now, there are two stages
of cell division…
How many chromosomes
do the new cells have
compared to the original?
first
division
second
division
4 new cells
What is meiosis?
Meiosis produces four new
sex cells with half the original
number of chromosomes.
In animals, meiosis
produces ova in females
and sperm cells in males.
In plants the gametes
produced by meiosis are
pollen grains and ovules.
Mitosis is also called
reduction division.
What does this mean?
Meiosis animation
How many chromosomes?
Human sex cells have a set of 23 unpaired chromosomes
- that’s half the usual number.
Sex cells in animals and plants follow the same rules.
 They have half the usual number of chromosomes.
 They have a complete set of single chromosomes.
Why is this important for fertilization and reproduction?
Sex cells and cell division
Sex cells have half the usual number of chromosomes.
Why aren’t new sex cells produced by mitosis?
Sex cells have to be produced by a special type of cell
division. This is why there are two types of cell division.
Sex cells are produced by meiosis.
In which organs does meiosis take place?
Inherited characteristics
What makes this baby human? What determines its gender?
In all living things, characteristics are passed on in the
chromosomes that offspring inherit from their parents.
So all human characteristics, including gender, must be
something to do with chromosomes.
Where are chromosomes found in cells?
What are chromosomes?
Chromosomes are long strands of genetic information
located in the nuclei of cells.
Chromosomes are most visible
during cell division when they
replicate and look like this…
You will see chromosomes represented both ways.
Homologous chromosomes
In most cells chromosomes are matched in pairs based on
their size and shape.
chromosome from
female parent
chromosome from
male parent
homologous
chromosomes
Matching pairs of chromosomes are called homologous
chromosomes.
Where do homologous chromosomes come from?
Each pair of homologous chromosomes contains one
chromosome that has been inherited from each parent.
Human chromosomes
In human body cells there are a total
of 46 chromosomes.
How many pairs of homologous
chromosomes are there in
human body cells?
23 pairs of chromosomes
You inherit half your chromosomes from
your mother and half from your father.
How many chromosomes do you inherit from each parent?
23 unpaired chromosomes
What is meiosis?
Meiosis begins with a single cell.
First the cell makes a copy of each chromosome.
Meiosis produces four new sex cells with half the
original number of chromosomes.
In animals, meiosis produces ova in females and sperm
cells in males.
In plants the gametes produced by meiosis are pollen
grains and ovules.
Meiosis is also called reduction division.
What does this mean?
Mitosis or meiosis?
X and Y chromosomes
There are two types of sex chromosome with names
that are very easy to remember!
X chromosome
Y chromosome
Females have two X chromosomes,
one from each parent.
XX
Males have one X chromosome
and one Y chromosome.
Which parent does a baby boy
inherit its Y chromosome from?
XY
Homologous chromosomes
In all living things, characteristics are passed on in the
chromosomes that offspring inherit from their parents.
chromosome from
female parent
chromosome from
male parent
Chromosomes are matched in pairs that contain
one chromosome inherited from each parent.
So are the genes in a matching pair of chromosomes
exactly the same?
© Boardworks Ltd 2004
Different versions of genes
The chromosomes in a matching pair contain the same type
of genes that code for the same characteristics.
gene for
petal colour
gene for
petal colour
version for
red petals
version for
yellow petals
Each chromosome may have a different version of a gene.
Different versions of a gene, that code for different versions
of a characteristic, are called alleles.
Pairs of alleles – homozygous
If the alleles in a matching pair are the same,
they are called homozygous alleles.
allele for
yellow petals
allele for
yellow petals
allele for
red petals
allele for
red petals
What colour are the flowers with these
homozygous pairs of alleles?
(Click twice on each bud
to reveal the flower;
click again to close them.)
Pairs of alleles – heterozygous
If the alleles in a matching pair are different,
they are called heterozygous alleles.
allele for
red petals
(Click twice on the bud
to reveal the flower;
click again to close it.)
allele for
yellow petals
Which characteristic is expressed if alleles are different?
Some alleles are dominant to other forms of a gene
and will always be expressed.
Which is the dominant allele in this heterozygous pair?
Which is the recessive allele in this heterozygous pair?
Representing alleles
Letters are used to represent different alleles.
A dominant allele is always a capital letter.
allele for
red petals
= R
A recessive allele is always the corresponding small letter.
allele for
yellow petals
=
r
The allele pair for each characteristic is called the genotype.
What colour are flowers with the genotype Rr?
Genotypes and phenotypes
The allele pair for each characteristic is called the genotype.
The physical expression of an allele pair is the phenotype.
What are the phenotypes of these genotypes?
genotype:
phenotype:
(Click twice on each bud
to reveal the flowers;
click again to close them.)
RR
rr
Rr
Genetic jargon
1. Section of DNA that codes for a particular
trait or characteristic.
2. A different form of a gene that codes for a
different version of a characteristic.
3. A description of the pair of alleles
present for a characteristic.
4. The physical expression of the alleles.
Genetic jargon
5. Pair of alleles that produce a
characteristic are the same, e.g. HH.
6. Pair of alleles that produce a
characteristic are different, e.g. Hh.
7. An allele that will always be expressed even
when there is only one of these alleles present,
represented by a capital letter.
8. An allele that will only be expressed
when both alleles are of this type e.g. hh.
Genetic jargon
What do these genetic terms mean?
gene
1. Section of DNA that codes for a particular
trait or characteristic.
allele
2. A different form of a gene that codes for a
different version of a characteristic.
genotype
3. A description of the pair of alleles
present for a characteristic.
phenotype 4. The physical expression of the alleles.
Genetic jargon
What do these genetic terms mean?
homozygous 5. Pair of alleles that produce a
characteristic are the same, e.g. HH.
heterozygous 6. Pair of alleles that produce a
characteristic are different, e.g. Hh.
dominant
7. An allele that will always be expressed even
when there is only one of these alleles present,
represented by a capital letter.
recessive
8. An allele that will only be expressed
when both alleles are of this type e.g. hh.