Biology 2B

Inheritance

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

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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 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)

Autosomal inheritanceBoth males and females have 2 alleles for the characteristicHomozygous individuals have 2 alleles the same and produce

gametes with only 1 type of alleleHeterozygous individuals have 2 different alleles and produce

two types of gametes with each alleleAt fertilisation gametes combine so the new individual has 2 of

each allele – one from each parentWe can show the probabilities of allele combinations from

different crosses by using a Punnett square

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 allelesThis 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

Co-dominant 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 a characteristic that is a mixture of both allelesThis is called co-dominance, incomplete dominance or blendingExamples include flower colour in snap dragons, A & B blood

groups, roan colour in cows and horses

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)You need to show alleles on the X chromosome (Y

chromosomes don’t carry an allele)