Inheritance

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

Asexual reproduction

• One parent• No variation in offspring

Sexual reproduction

• 2 parents• Variation in offspring

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

DNA controls protein synthesis

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

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

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

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

Genotype and phenotype

• The genotype describes the alleles that are present

• The phenotype describes appearance of the organism because of those alleles

Environmental influences

Some inherited characteristics are influenced by the environment eg

Height is affected by dietBody shape and weight are affected by diet and

exerciseSkin colour is affected by exposure to the sun

Dominant – recessive inheritanceIf 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, astigmatismExamples of recessive characteristics include PKU, albinism

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 phenotypesExamples include flower colour in snap dragons,

roan cattle and horses, A & B blood groups

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)

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 dystrophyhaemophilia red-green colour blindness

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

Monogenic inheritance

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

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

Multiple allelesThis 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 expressedcch – 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)