Download Biology 12

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
Epigenetics
Epigenetics 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 methylation
2. 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 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 the characteristic of one of the
alleles
This is called Dominant - recessive inheritance
The allele that is expressed is called Dominant
The allele that is hidden is called recessive
Examples 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
SBSB
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
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 inheritance
Males and females have different chromosomes
Males 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 alleles
Show 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)