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 inheritanceIf 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 phenotypesExamples 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 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)

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

b

Bb

bb

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

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

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

BB Bb

Bb bb

B b

B

b

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

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

SBSB

SWSW

SBSW

Autosomal co-dominant crosses

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

SBSB SWSW

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

SBSB SBSW

SBSW SWSW

SB SW

SB

SW

Sex linked inheritance

Males 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)

Example – sex linked recessive problem

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 = normalXr = red-green colour blindY = male chromosome

XRXR

XRXr

XRY

XrY

XR

XR

Xr

Y

XrXr

XRXr XRY

XrY

XR

YXr

Xr

Genetic problems3. 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 = short, h = long

H h

H

h

HH Hh

Hh hh

Genetic problems 25. 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

i) white guinea pig? HWHY x HWHW = ½ HWHY (cream), ½ HWHW (white)

ii) yellow guinea pig? HWHY x HYHY = ½ HWHY (cream), ½ HYHY (yellow)

iii) cream guinea pig? HWHY x HWHY = ¼ HYHY

(yellow) ½ HWHY (cream), ¼ HWHW (white)

HYHY = yellowHWHW = white

HWHY = cream

HW HY

HWHWY HWHYHW

HWHWHW HWHY

HW HY

HY HWHY HYHY

HY HWHY HYHY

HW HY

HWHW

HWHY

HW

HY

HWHY

HYHY