Inheritance

HBS3A

Inheritance

• Organisms inherit characteristics from their parents

• Characteristics are controlled by DNA

• In asexual reproduction, organisms inherit DNA from 1 parent

• In sexual reproduction, organisms inherit DNA from both parents

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

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

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

Epigenetics

Epigenetics is

These changes may occur spontaneously; in response to environmental factors; or in response to the presence of a particular allele for another characteristic.

Two main mechanisms are

1. __________________

2. __________________

These act to turn on or turn off genes.

Many of these are activated by the environment.

Some examples of diseases that are epigenetic

EpigeneticsEpigenetics is the study of epigenetic inheritance, a set of reversible

inheritable changes in gene function or other cell phenotype that occur without any changes in DNA sequence (genotype).

These changes may occur spontaneously; in response to environmental factors; or in response to the presence of a particular allele for another characteristic.

Two main mechanisms are 1. DNA methylation2. modifications of nucleosomal histones. (acetylation). These act to turn on or turn off genes. Many of these are activated by

the environment.

Some examples of diseases that are epigenetic include many of the cancers, heart disease and diabetes. Some examples of factors that can trigger epigenetic changes include diet (including diet of mother during pregnancy), drugs, disease (particularly viruses) and stress.

Epigenetics

Environmental influences

Some inherited characteristics are influenced by the environment eg height and weight are affected by diet & exercise, skin colour is affected by exposure to the sun

Sexual reproduction• 2 parents• Variation in offspring• Advantages – variation gives better

chance of species survival if change occurs

• Disadvantages – more complex, takes longer to produce offspring

Homologous chromosomes• Chromosomes in diploid

organisms 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 organisms have at least 2 genes for each characteristic – one from each parent

Alleles

• Genes can come in alternative forms called a• 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 the characteristic that the organism shows

Autosomal inheritance

Both males and females have 2 alleles for the characteristic

Homozygous individuals have 2 alleles the same and produce gametes with only 1 type of allele

Heterozygous individuals have 2 different alleles and produce two types of gametes with each allele

At fertilisation gametes combine so the new individual has 2 of each allele – one from each parent

Dominant – recessive inheritanceIf an organism has two identical alleles, they will

show the characteristics of that alleleIf an individual has two different alleles, sometimes

they will only show the characteristic of one of the alleles

This is called Dominant - recessive inheritanceThe allele that is expressed is called DominantThe allele that is hidden is called recessiveExamples include Tongue rolling, Huntington’s chorea, Purple flower colour in peas

Autosomal dominant/recessive

Individuals with two dominant alleles show the dominant phenotype

Individuals with two recessive alleles show the recessive phenotype

Individuals with one of each allele show the dominant phenotype

BB

bb

Bb

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, sometimes they will only show a characteristic that is a mixture of both alleles

This is called co-dominance, incomplete dominance or blending

Examples include flower colour in snap dragons, A & B blood groups, roan colour in cows and horses

Autosomal co-dominance

Individuals with two of the 1st allele show the first trait

Individuals with two of the 2nd allele show the second trait

Individuals with one of each allele show a mixture of both traits

SBSB

SWSW

SBSW

Inheritance of sex in humans• In all mammals, 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

haemophilia, red-green colour blindness

Sex linked inheritanceMales and females have different chromosomesMales can only show 2 phenotypes (ie males

can not be carriers)Females can show 3 phenotypes (if

codominant) or 2 phenotypes (if dominant recessive, with a carrier)

In X-linked inheritance alleles are found on the X chromosome and Y chromosomes don’t carry an allele

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

Shows discrete characteristics eg flower colour, pea characteristics, tongue rolling, haemophilia

Polygenic inheritance

Shows continuous characteristics eg height, weight, intelligence, fingerprints, hair, skin and eye colour

Multiple allelesShow 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)