biology 2b inheritance. organisms inherit characteristics from their parents characteristics are...
TRANSCRIPT
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)