Biology 12

Inheritance Organisms inherit characteristics from their parents Characteristics are controlled by DNA In sexual reproduction, organisms inherit DNA from both parents The segment of DNA that controls one characteristic is called a gene The location of the gene on a chromosome is called its locus

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

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, they will only show the characteristic of the dominant allele There are 3 genotypes, but only 2 phenotypes The allele that is expressed is called dominant The allele that is hidden is called recessive Examples include tongue rolling, Huntington’s, astigmatism, 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, they will show a characteristic that is a mixture of both alleles There are 3 genotypes and 3 phenotypes Examples include flower colour in snap dragons, roan cattle and horses, A & B blood groups

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 in normal cells (2 of each homologous pair = 2n) The haploid number is the number of chromosomes in gametes (1 of each homologous pair = n)

Inherited sex determination In many species sex is inherited Chromosomes that determine sex are called In many species there are two types of sex chromosome – X & Y (or W & Z) eg mammals, birds In some species the male is haploid and the female is diploid - eg grasshoppers, moths

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

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)

Genetic problems 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 A Punnett square is a tool used in genetics Genotype refers to the alleles present in an individual Phenotype refers to the characteristic shown by the individual b b B Bb Bb b bb bb

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

Autosomal dominant/recessive BB 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

Autosomal dominant recessive crosses Crossing a homozygous dominant individual with a homozygous recessive individual leads to offspring who are all heterozygous and show the dominant trait Crossing two heterozygous individuals leads to 1 homozygous dominant individual, showing the dominant trait : 2 heterozygous individuals, showing the dominant trait :1 homozygous recessive individual, showing the recessive trait bb BB Bb Bb B b BB Bb B b Bb bb

Example – dominant recessive problem A heterozygous black male mouse mates with a homozygous brown female mouse. Black fur is dominant over brown fur. What is the probability of having: a) a homozygous black offspring? 0% b) a heterozygous black offspring? 50% c) a homozygous brown offspring? 50% bb Bb b b B Bb Bb b bb bb

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

Autosomal co-dominant crosses SBSB SWSW Crossing an individual homozygous for one allele with an individual homozygous with the second allele leads to offspring showing a mixture of the two traits Crossing two heterozygous individuals leads to 1 homozygous individual showing the first trait : 2 heterozygous individuals showing the mixed trait :1 homozygous individual showing the second trait SBSW SBSW SB SW SB SBSB SBSW SW SBSW SWSW

Example – co-dominance problem Two heterozygous grey sheep are mated. Black wool is co-dominant to white wool. What is the probability of having: a) a black offspring? 25% b) a grey offspring? 50% c) a white offspring? 25% SBSW SBSW SB SW SB SBSB SBSW SW SBSW SWSW

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)

Example – sex linked recessive problem XR = normal In humans, red-green colour blindness is a relatively common condition that is inherited as an X-linked recessive trait. a) A woman with normal vision whose father was red-green colour-blind marries a man with normal vision. i) What proportion of her sons would you expect to be colour-blind? 50% ii) What proportion of her daughters would you expect to be colour-blind? 0% b) If she married a man who was red-green colour-blind, i) what proportion of her sons would you expect to be colour-blind? 50% ii) what proportion of her daughters would you expect to be colour-blind? 50% Xr = red-green colour blind Y = male chromosome XR Y XR XRXR XRY Xr XRXr XrY Xr Y XR XRXr XRY Xr XrXr XrY

Genetic problems H = short, h = long 3. In rabbits, long hair is recessive to short hair. What are the expected genotypes and phenotypes for the following crosses: a) 2 pure breeding short haired rabbits? HH x HH = HH ( all short) b) 2 pure breeding long haired rabbits? hh x hh = hh (all long) c) a pure breeding short haired rabbit and a pure breeding long haired rabbit? HH x hh = Hh (all short) d) two of the offspring of the cross in (c) above? Hh x Hh = 1HH (short) : 2 Hh (short) : 1 hh (long) 4. Two bald parents have four children, two bald and two with normal hair. Assuming that it is governed by a single pair of alleles, is this kind of baldness best explained as an example of dominant or recessive inheritance? Baldness is dominant as normal hair has skipped a generation H h H HH Hh Hh hh h

Genetic problems 2 HYHY = yellow 5. In guinea pigs, a yellow coated animal, crossed with a white coated animal, produces a litter all of which are cream coated. Crossing two yellow coated guinea pigs results in offspring which are all yellow, while crossing two white coated guinea pigs results in offspring which are all white. a) What is the pattern of inheritance? Co-dominance b) What offspring would you expect from a cross between a cream guinea pig and a white guinea pig? HWHY x HWHW = ½ HWHY (cream), ½ HWHW (white) yellow guinea pig? HWHY x HYHY = ½ HWHY (cream), ½ HYHY (yellow) cream guinea pig? HWHY x HWHY = ¼ HYHY (yellow) ½ HWHY (cream), ¼ HWHW (white) HWHW = white HWHY = cream HW HY HW HWHWY HWHY HW HWHW HWHY HW HY HY HWHY HYHY HY HWHY HYHY HW HY HW HWHW HWHY HY HWHY HYHY