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Genetics Key Notes
 DNA is a double stranded ladder-like shape (double helix). It contains all of the genetic information
(genome) for the whole organism. Each cell has a full set of DNA.
 The double helix is coiled up into a dense chromosome. The chromosome has sections called
genes that code for a single trait (characteristic).
 Every time we produce a new body cell we must copy all of the information in one cell, and then split
it into 2 exact copies. This is called Mitosis and is used for growth, replacement, and repair in the
body.
 Humans have 46 chromosomes in 23 homologous pairs; 1 from mum and 1 from dad. On these 23
pairs lay the 30,000 or so genes that make up a human.
 We got our genes from our parents. They produced sex cells (gametes) with a half set of information
(haploid) in a process called Meiosis.
 During meiosis genetic information is doubled, then split, then split to form a gamete. Each gamete
has one of each of the chromosomes that the parent has two of. They are divided up randomly. Each
parent can produce at least 8 million different gametes. This is important for Variation. Then the 2
gametes combine during fertilization to produce a zygote (fertilized egg) with 2 sets of
chromosomes (diploid).
 So all of us have 2 sets of information for each gene. These may be different alleles (gene forms). If
this is the case then one will be seen (dominant) and the other hidden (recessive). This is called
being Heterozygous for a trait (Rr). An individual could also be Homozygous recessive (rr), or
Homozygous dominant (RR).
 Homozygous individuals can also be said to be Pure Breeding as they do not carry recessive genes
that can show up in later generations.
 Case example: The allele for tongue rolling (R) is dominant over the non-rolling allele (r).
If 2 people who are heterozygous are crossed (breed), we can work out the expected ratio of the offspring.
This is not guaranteed to happen as it depends on
RANDOM CHANCE.
R
r
Gametes from
One tool to do this is a punnett square:
Parent 1
This can be expressed as:
¼ RR, ½ Rr, ¼ rr
Gametes from
Parent 2
RR
Rr
r
Rr
rr
this is a discretion of the genes they posses (Genotype ratio)
¾ Tongue roller, ¼ non-roller
this is a physical description (Phenotype ratio)
 Sex Determination
Gametes from
Mum
R
X
Y
X
XX
XY
X
XX
XY
Gametes from
Dad
Offspring
50% Male
50% Female
 A pedigree chart shows the family tree for one specific trait.
Offspring
Genetics Key Notes
 DNA is a double stranded ladder-like shape (double helix). It contains all of the genetic information
(genome) for the whole organism. Each cell has a full set of DNA.
 The double helix is coiled up into a dense chromosome. The chromosome has sections called
genes that code for a single trait (characteristic).
 Every time we produce a new body cell we must copy all of the information in one cell, and then split
it into 2 exact copies. This is called Mitosis and is used for growth, replacement, and repair in the
body.
 Humans have 46 chromosomes in 23 homologous pairs; 1 from mum and 1 from dad. On these 23
pairs lay the 30,000 or so genes that make up a human.
 We got our genes from our parents. They produced sex cells (gametes) with a half set of information
(haploid) in a process called Meiosis.
 During meiosis genetic information is doubled, then split, then split to form a gamete. Each gamete
has one of each of the chromosomes that the parent has two of. They are divided up randomly. Each
parent can produce at least 8 million different gametes. This is important for Variation. Then the 2
gametes combine during fertilization to produce a zygote (fertilized egg) with 2 sets of
chromosomes (diploid).
 So all of us have 2 sets of information for each gene. These may be different alleles (gene forms). If
this is the case then one will be seen (dominant) and the other hidden (recessive). This is called
being Heterozygous for a trait (Rr). An individual could also be Homozygous recessive (rr), or
Homozygous dominant (RR).
 Homozygous individuals can also be said to be Pure Breeding as they do not carry recessive genes
that can show up in later generations.
 Case example: The allele for tongue rolling (R) is dominant over the non-rolling allele (r).
If 2 people who are heterozygous are crossed (breed), we can work out the expected ratio of the offspring.
This is not guaranteed to happen as it depends on
RANDOM CHANCE.
R
r
Gametes from
One tool to do this is a punnett square:
Parent 1
This can be expressed as:
¼ RR, ½ Rr, ¼ rr
Gametes from
Parent 2
RR
Rr
r
Rr
rr
this is a discretion of the genes they posses (Genotype ratio)
¾ Tongue roller, ¼ non-roller
this is a physical description (Phenotype ratio)
 Sex Determination
Gametes from
Mum
R
X
Y
X
XX
XY
X
XX
XY
Gametes from
Dad
Offspring
50% Male
50% Female
 A pedigree chart shows the family tree for one specific trait.
Offspring