1. Inheritance

2. Inheritance Organisms inherit characteristics from their parents
Characteristics are controlled by DNA
In sexual reproduction, organisms inherit DNA from both parents
The segment of DNA that controls one characteristic is called a gene
The location of the gene on a chromosome is called its locus

3. Asexual reproduction
One parent
No variation in offspring

4. Sexual reproduction
2 parents
Variation in offspring

5. DNA DNA is found in the nucleus of cells
It is organised into segments called chromosomes
Chromosomes are only visible when the cell is dividing

6. DNA controls protein synthesis

7. Why are proteins important?
Roles of proteins in the body include
Structural proteins eg collagen, keratin
Enzymes (organic catalysts) eg digestive enzymes
Transport proteins eg haemoglobin
Regulatory proteins eg hormones
Protective proteins eg antibodies, clotting factors
Therefore proteins determine what you will look like, and how your body functions

8. Genes and chromosomes
The segment of DNA that controls one characteristic is called a gene
Genes are found on structures called chromosomes
The location of the gene on a chromosome is called its locus

9. Homologous chromosomes
Chromosomes in humans come in pairs called homologous chromosomes
Organisms inherit one of each pair from each of their parents
Each chromosome of a pair has loci for the same genes
That means humans have at least 2 genes for each characteristic – one from each parent

10. Alleles Genes can come in alternative forms called alleles
Organisms can carry two identical alleles for a characteristic and be called homozygous
Organisms can carry two different alleles for a characteristic and be called heterozygous

11. Genotype and phenotype
The genotype describes the alleles that are present
The phenotype describes appearance of the organism because of those alleles

12. Environmental influences
Some inherited characteristics are influenced by the environment eg
Height is affected by diet
Body shape and weight are affected by diet and exercise
Skin colour is affected by exposure to the sun

13. Dominant – recessive inheritance
If an organism has two identical alleles, they will show the characteristics of that allele
If an individual has two different alleles, they will only show the characteristic of the dominant allele
There are 3 genotypes, but only 2 phenotypes
The allele that is expressed is called dominant
The allele that is hidden is called recessive
Examples of dominant characteristics include Huntington’s, tongue rolling, astigmatism
Examples of recessive characteristics include PKU, albinism

14. Co-dominant inheritance
If an organism has two identical alleles, they will show the characteristics of that allele
If an individual has two different alleles, they will show a characteristic that is a mixture of both alleles
There are 3 genotypes and 3 phenotypes
Examples include flower colour in snap dragons, roan cattle and horses, A & B blood groups

15. Chromosome numbers
Different species have different numbers of chromosomes
In some species the male has a different number of chromosomes to the female
The diploid number is the number of chromosomes in normal cells (2 of each homologous pair = 2n)
The haploid number is the number of chromosomes in gametes (1 of each homologous pair = n)

16. Inheritance of sex in humans
In humans, sex is determined by a pair of chromosomes called X & Y
Males have XY
Females have XX
Genes found on these chromosomes show a different pattern of inheritance to those found on the other (autosomal) chromosomes
Examples of such genes include
Duchenne muscular dystrophy
haemophilia
red-green colour blindness

17. Types of inheritance
Characteristics controlled by 1 gene locus are called monogenic
Examples include tongue rolling, haemophilia, ABO blood groups
Characteristics controlled by more than 1 gene locus are called polygenic
Examples include height, weight, intelligence, skin, hair and eye colours
Characteristics controlled by more than 2 alleles at 1 gene locus are called multiple alleles
Examples include ABO blood group, coat colour in cats, mice

18. Monogenic inheritance
This is inheritance due to genes at one locus. It shows discrete characteristics eg flower colour, pea characteristics, tongue rolling, haemophilia

19. Polygenic inheritance
This is inheritance due to genes at more than one locus. It shows continuous characteristics eg height, weight, intelligence, fingerprints, hair, skin and eye colour

20. Multiple alleles
This is inheritance due to genes at one locus, but with more than 2 alleles. It shows more than 3 discrete characteristics eg ABO blood groups, coat colour in cats & mice
Consider coat colour in mice. The presence or absence of colour is controlled by a number of alleles at one gene locus. Four alleles have been identified at this site:
C - full colour expressed
cch – chinchilla (silver points or flecks in the coat)
ch - himalayan or colour point (white coat with dark extremities)
c - albino (no pigment present - white coat with pink eyes)