1. Genes and alleles Chromosomes are made up of units called genes.
Each gene controls the development of a particular trait.
gene for eye colour
gene for hair colour
gene for handedness
one pair of
homologous
chromosomes

2. Genes and alleles
There are two copies of each gene – these are called alleles.
One allele comes from the egg and the other from the sperm
at fertilisation. The two alleles of a gene may be the same or
different.
eg –
If the alleles are different,
one is usually dominant
and hides the effect of the
other allele.
left
handed
right
handed
In the example above, the “right handed” allele is dominant
over the “left handed” allele. The left handed allele is said
to be recessive.

3. Genes are usually represented by letters of the alphabet.
The dominant allele is given a capital letter and the recessive
allele the same letter in lower case.
eg R = right handed r = left handed
allele allele
A person who has the alleles RR is right handed.
A person who has the alleles rr is left handed.
A person who has the alleles Rr is right handed because
the R allele is dominant over the r allele.
The particular alleles that a person has for a trait is called
their genotype.
eg – RR, Rr and rr are the genotypes for handedness.

4. The genetic mutation causes the lobster to produce an excessive amount of a particular protein, which combines with a molecule already present to form a blue complex. This gives the lobster its coloration.

5. Genes and alleles Homozygous = 2 of the same alleles RR or rr
Genotype
Phenotype RR
Right handed Rr rr
Left handed
The outward appearance of the person for a trait is called the phenotype.
Homozygous = 2 of the same alleles RR or rr
Heterozygous = 2 different alleles Rr

6. X XX Y XY Sex determination
What are the chances of having a boy or a girl???
Everyone has a pair of chromosomes that determine our sex
XX = female XY = male
The punnett square on the left shows us the probabilities that a man and woman are faced with each time they have a child.
XX = 50%
XY = 50%
If you have had two boys it does not mean you will have a girl next, each time a new zygote is formed the chances of it being male or female are 50 : 50. X XX Y XY

7. B b B b BB Bb bb Punnett Squares
A cross between 2 heterozygous black guinea pigs. B codes for black fur, b codes for white fur.
Possible fertilisations B b
Possible fertilisations B b BB Bb bb
Place parents alleles at the top and side of the punnet square

8. Possible fertilisations
Punnett Squares
The chance of the guinea pigs having a black baby is 75%.
Possible fertilisations B b BB Bb bb
The chance of the guinea pigs having a white baby is 25%.