Biology 2B 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 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 Chromosomes 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

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 with homologous pairs (ie found in normal cells) The haploid number is the number of chromosomes with only 1 of each homologous pair (ie found in gametes)

Inheritance 2 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 The genotype describes the alleles that are present The phenotype describes the characteristic that the organism shows A Punnett square is a tool used in genetics A monohybrid cross is a cross of individuals looking at a characteristic inherited at one gene locus A test cross is crossing an individual back to a homozygous recessive individual in order to determine whether it is a carrier b B b b Bb bb Bb bb

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

Environmental sex determination In some species sex is determined by the environment or other factors In many reptiles, sex is determined by egg temperature -males are produced when the eggs are incubated at higher temperatures and females are produced when eggs are incubated at lower temperatures In many species of fish, sex can change – fish start as males, then become females

Inheritance of sex in mammals In 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

Inheritance of sex in birds In birds, sex is determined by a pair of chromosomes called Z & W Males have ZZ Females have ZW Genes found on these chromosomes show a different pattern of inheritance to those found on the other (autosomal) chromosomes

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

Inheritance of sex in insects In many species there are two types of sex chromosome – X & Y eg flies In some species the male is haploid and the female is diploid – eg grasshoppers, moths

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)

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 We can show the probabilities of allele combinations from different crosses by using a Punnett square

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

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

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) You need to show alleles on the X chromosome (Y chromosomes don’t carry an allele)