1. Characteristics & Inheritance
Unit 1.3
Year 10
Human Biology

2. Genetics
Gregor Mendel was a monk who carried out experiments on pea plants.
Although he did not know about DNA and genes, he was able to develop theories that have formed the basis of modern genetics.

4. wrinkled
round
Punnett square

5. Dominant / Recessive inheritance
In our DNA there are section of nucleotides that code for our characteristics.
These sections are called genes.
Genes occur in pairs, and are inherited from our parents.
There are different forms of these genes – and these are called alleles.

6. Dominant / Recessive inheritance
The allele that is expressed or observed in an individual is the dominant allele.
The characteristics that is hidden is the recessive allele.
As genes occur in pairs an individual can have both a recessive and dominant allele.

7. Allele combinations Homozygous – having the same alleles for a gene.
2 dominant alleles
2 recessive alleles
Heterozygous – having two different alleles for a gene
1 dominant and 1 recessive allele

8. Genetics rules
To represent the dominant allele you use CAPITAL letters.
ie: B, A, E, H, D, R, T
Lowercase letters are used to represent the recessive allele.
ie: b, a, e, h, d, r, t
Putting the two together you can show the alleles for one characteristic that a person has. This is their GENOTYPE
ie: Bb, Tt, AA, ee

9. Genetics rules
Punnet squares are used to determine the probability of a characteristic being shown in the offspring (progeny)
You can determine the genotype (the alleles) and then determine the PHENOTYPE (the expressed characteristic)
Percentages & Ratios are used to show the probabilities of the genotypes and phenotypes
Practise the Dominant & Recessive problems

10. Incomplete dominance
There are some alleles for genes that do not follow the dominant / recessive pattern.
These alleles show an incomplete dominance, meaning both alleles can be expressed in the phenotype of the individual.
You can determine a THIRD phenotype from incompletely dominant alleles.
This new phenotype usually a blending of two alleles

11. Incomplete Dominance rules
As the two alleles are equally dominant, you must use two different CAPITAL letters
ie: RW,
Example: Red and White flowers are incompletely dominant in snapdragon plants.
RR = Red WW = White RW= Pink
Practise the Incomplete Dominance questions

12. Sex determination Humans have 23 pairs of chromosomes.
22 pairs of these chromosomes determine body characteristics and are termed SOMATIC chromosomes.
1 pair determine the sex of a human
XX – Female
XY - Male

13. Sex linkage
There are a few genes that are located on the X and Y Chromosomes.
These genes are inherited differently as they rely on the sex of an individual.
Examples
Red-green colour blindness
Haemophillia
Duchenne’s muscular dystrophy

14. Sex linkage rules
As these alleles are found on the sex chromosomes you MUST show them on there!
ie: XNXN = Normal female
XNXn = Carrier female
XnXn = Colour blind female
XNY = Normal male
XnY = Colour blind male
Practise the Sex Linkage questions

15. Chromosome abnormalities
A mutation is defined as a spontaneous change in the DNA of an individual
They can occur within a chromosome, affecting one or many genes
A chromosomal mutation occurs when there is a problem separating chromosomes during mitosis or meiosis
These mutations can change the genotype and phenotype of an individual