OverviewA) Monohybrid InheritanceB) Dihybrid InheritanceC) The test crossD) Autosomal dominant and autosomal recessive

pedigree chartsE) LinkageF) Sex determination

G) Alleles and their interactions – multiple alleles, incomplete dominance, codominance

H) Gene interactions – polygenic inheritance, epistasis

Mendel’s model of inheritance assumes that:each trait is controlled by a single geneeach gene has only 2 allelesthere is a clear dominant-recessive

relationship between the alleles

Most genes do not meet these criteria.

Extensions to Mendel

Single genes with two alleles result in two phenotypes:

Tongue rolling Ear attachment

Hairline

dominant

dominant

dominant

Free earlobe

Attachedearlobe

MULTIPLE ALLELES

Multiple Alleleswhen more than two different alleles exist

for the same traite.g.

coat colour in rabbits is determined by: 1 gene with 4 alleles : C, c, ch, cch

Multiple Alleles

exist in a population

How many alleles can a diploid

individual carry at a particular locus?

Multiple Alleles e.g.

eye colour in mice blood groups in humans

ABO blood group system– 3 alleles: IA, IB, IO

Multiple alleles increase the number of possible phenotypes

Question: [MAY, 2006]

Suggest explanations for the following observation.A single gene may often have two or more alleles. How do such alleles arise? (3)

Alleles arise through mutations. Changes in the base sequence occur either naturally or are induced artificially. Base deletion, substitution, insertion, duplication and inversion are possible.

BLOOD GROUP(phenotype)

GENOTYPE

A IA IA or IA IO

B IB IB or IB IO

AB IA IB O IO IO

Human blood group phenotypes and genotypes

the gene locus is represented by the symbol I (stands for isohaemoagglutinogen)

NOTE:in table allele for Type O is io

in syllabus it is IO

IA was the original allele: Io and IB resulted by mutation

[This is background

information.]

What is the probability for these parents to produce a homozygous

child?

Parental phenotypes: Type A x Type BParental genotypes: IAIO x IBIO

Gametes:

F1 genotypes:

IA IO IB IOx

IAIB IAIO IBIO IOIO

25%

Question: [MAY, 2006]

Suggest explanations for the following observation.Blood group in humans is determined by a single gene that produces four phenotypes.

(2)Blood groups in humans is a case of multiple alleles and is determined by 3 alleles, IA, IB and IO. IO is recessive to the other two, while IA and IB are codominant. The four phenotypes are blood group A, B, AB and O.

Question: [MAY, 2007]

Use your knowledge of biology to explain the evolutionary significance of polymorphism in human blood types. [5 marks] (MAY, 2007)

[unfair question: NOT in syllabus]

There are many ways to classify blood types in humans, e.g. ABO system and rhesus factor. Different blood groups result due to the presence of different glycolipids and glycoproteins on the surface of red blood cells. The carbohydrate part of these substances can act as receptors for pathogens.

There is evidence that the susceptibility to infectious pathogens is affected by their blood group. Examples include increased susceptibility to Helicobacter pylori induced peptic ulceration in O individuals, to Candida species in Lea + individuals; the red cell P-antigen is a receptor for human parvovirus B19, whereas the Duffy red cell antigen is a receptor for Plasmodium vivax which causes malaria. Thus in some malarial regions persons with Duffy-negative blood type have a distinct survival advantage as they are protected against malaria. The high level of red blood group polymorphism could thus be the result of a tug of war between host and pathogen.

Question: Getting Started

1a) The diagram below shows a family tree in which the blood group phenotypes are shown for some individuals.

Using the symbols IA, IB and IO to represent the alleles, indicate the genotype of the following people.1: 2: 4: 5: 6:

IO IO IA IB

IA IO IB IO

Using the symbols IA, IB and IO to represent the alleles, indicate the genotype of the following people.1: 2: 4: 5: 6:

IB IO

b) State the possible blood groups of person 3. Explain your answer. (5)

Blood group A or B.Person 3 inherits IO allele from parent 1 and either IA or IB from parent 2.The genotype IA IO results in blood group A while IB IO, in blood group B.

IO IO IA IB

IA IO IB IO

4. The ABO blood group is governed by a set of 3 multiple alleles, IA, IB and IO. IA and IB are codominant while IO is recessive. Another blood group system is known as the MN system. The MN blood groups are governed by a pair of codominant alleles giving three possible blood groups, MM, MN and NN.

a) A man of blood group B married a woman of unknown ABO blood group. They had three children. One of the children had blood group A, one had group AB and one had blood group O.State the genotypes of the parents and give an explanation for your answer. (5)

a) A man of blood group B married a woman of unknown ABO blood group. They had three children. One of the children had blood group A, one had group AB and one had blood group O.State the genotypes of the parents and give an explanation for your answer. (5)

Blood Group B x

AIO IO

AB

OIA IB

IB IOIA IO

Man of blood group B: IB IO

Woman of unknown ABO blood group: IA IO

Alternative genotype for man is IB IB and would not lead to a child of blood group O. Genotype of this child is IO IO.Woman must have IO allele to produce a blood group O child and IA allele for a blood group A child.

Draw a genetic diagram to show the inheritance of ABO blood groups in this family. (2)Parental phenotypes: Parental genotypes: IB IO x IA IO

F1 genotypes : IA IB IB IO IA IO IO IO

F1 phenotypes:

Gametes:

group O

group A

group AB

group B

IB IA IOIO x

group B group Ax

Incomplete Dominance

A condition when neither allele is dominant over the other.

Incomplete dominance

Incomplete Dominance

Is recognised by the heterozygotes expressing an intermediate phenotype relative to the parental phenotypes.

A ratio of 1:2:1 (in F2) is characteristic of

INCOMPLETE DOMINANCE

Codominance

Codominant alleles

the heterozygotes express both homozygous phenotypes

VC VC

VP VP

VP VC

Question:A cross between two white clover plants, each with the mixed pattern, was carried out.State the expected ratio of the phenotypes in the offspring of this cross. (1)

VP VC

VP VC

1 patch :

VP VC

X

VP VC

VP

VP VC

VC

VP VC

2 mixed : 1 chevron

Examples of Codominant alleles

in human ABO blood types: alleles IA and IB are codominant

persons of blood group MN [other blood groups are MM and NN]

OverviewA) Monohybrid InheritanceB) Dihybrid InheritanceC) The test crossD) Autosomal dominant and autosomal recessive

pedigree chartsE) LinkageF) Sex determinationG) Alleles and their interactions – multiple alleles,

incomplete dominance, codominance

H) Gene interactions – polygenic inheritance, epistasis

Gene interactions

sometimes several genes act additively, so that the phenotype can be predicted by how many of these genes are active

To complicate things further:

the physical environment may interact with the genetic constitution of an individual in determining the phenotype:

e.g.

e.g. Height / weight of persons: genes + nutrition

Genes + Environmental Effect

Soil pH determines flower colour in Hydrangea

An acidic soil (pH below 6):Blue flowers

An alkaline soil (pH above 6):Pink flowers

The environment affects gene action

- coat colour in animals

Light, temperature & diet affect phenotype

Flamingos get their colour from carotenoids in diet (shrimps)

Carrots cause canary feathers to become orange

… and even human skin

Seasonal coat colour change in Arctic foxes

Brown in summer (+ melanin)

White in winter (- melanin)

Himalayan rabbits & Siamese cats

Extremities – tips of ears, nose, paws – are much darker than the rest of the animal .

How?

They have an allele of the tyrosinase gene which produces an enzyme

Enzyme is inactivated at normal body temperature

The extremities of the body are slightly cooler: tyrosinase enzyme is active = melanin = dark tips.

In the warmer parts of the body: tyrosinase is denatured = no melanin = white hair .

QuestionSuggest why Siamese kittens have white fur when they are born and do not develop their characteristic markings until some days after their birth. (3)

Question

During development, embryos are at a constant temperature in the uterus and the enzyme is inactive leading to lack of colour. After birth, the ears, face and tail become cooler than the rest of the body and the enzyme becomes active leading to colour development.

HW Correction: GENETICS MATSEC

Numbers 5, 8, 9

5. Individual plants belonging to a particular species of angiosperm vary considerably in appearance. Much of the variation is due to the colour of the petals and to the distribution of chlorophyll in the leaves. Petal colour may be blue or white, with no intermediate, while chlorophyll may be present throughout the leaves (giving green leaves), only present in patches (giving variegated leaves) or altogether absent. A researcher is investigating the inheritance of these characteristics by crossing plants with blue petals and green leaves, known to be pure breeding for both characteristics, with plants having white petals and variegated leaves. The F1 generation from this cross was made up of plants with blue petals and green leaves and plants with blue petals and variegated leaves in a 1:1 ratio.

MAY, 2003

5.1 Construct a genetic diagram explaining how this cross produced the F1 generation that was observed. (3)

Let: A represent the allele for blue petals a represent the allele for white petals B represent the allele for green leaves W represent the allele for white leavesBW – variegated leavesParental genotypes: AABB x aaBWGametes:

F1 genotypes: F1 phenotypes:

AaBBBlue petal, green leaf

AaBWBlue petal, variegated leaf

AB aB aWX

The F1 plants with blue petals and variegated leaves were self-pollinated and 1300 seeds sown. All the seeds that were sown germinated successfully. Phenotype patterns in the F2 plants were noted three months after germination. The following results were recorded:

Phenotype NumberPlants with blue petals and green leaves 248Plants with blue petals and variegated leaves 502Plants with white petals and green leaves 80Plants with white petals and variegated leaves 164

5.2 Construct a genetic diagram explaining how this cross produced the F2 generation that was observed. (6)

F1 Genotypes AaBW x AaBWGametes:

F2 genotypes:

AB AW aB aWAB AABB AABW AaBB AaBWAW AABW AAWW AaBW AaWWaB AaBB AaBW aaBB aaBWaW AaBW AaWW aaBW aaWW

AB AW aB aW X AB AW aB aW

F2 Phenotypes produced:6 blue petal and variegated (A_BW)3 blue petal and green leaf (A_BB) 1 white petal and green leaf (aaBB) 2 white petals and variegated leaves (aaBW) 3 blue petals and white leaves (A_WW) 1 white petal and white (colourless leaves) (aaWW)

AB AW aB aWAB AABB AABW AaBB AaBW AW AABW AAWW AaBW AaWW aB AaBB AaBW aaBB aaBW aW AaBW AaWW aaBW aaWW

A - blue petals a - white petals B - green leaves W - white leavesBW - variegated

F2 Phenotypes produced:3 blue petal and green leaf (A_BB) [248]6 blue petal and variegated (A_BW) [502]1 white petal and green leaf (aaBB) [80]2 white petals and variegated leaves (aaBW) [164]3 blue petals and white leaves (A_WW) 1 white petal and white (colourless leaves) (aaWW)

5.3 How are the F2 genotypes related to the F2 phenotypes observed? (2)

Those plants lacking chlorophyll failed to thrive as they could not photosynthesise and hence, when their food reserves finished the plants died.

Phenotype NumberPlants with blue petals and green leaves 248Plants with blue petals and variegated leaves 502Plants with white petals and green leaves 80Plants with white petals and variegated leaves 164

dead

8. The diagram in the figure below shows the pedigree of two families in which there are a number of albino individuals. Square symbols represent males and circles represent females. Solid (black) symbols represent affected individuals whilst clear (white) symbols represent unaffected individuals. Albinism is a condition in which melanin is not formed and is caused by homozygozity of a recessive allele.

Pedigree of two families with albinism

8.1 Give the genotypes of the following family members using the symbols M and m to represent the dominant and recessive alleles respectively of the gene controlling production of melanin. (6)

M

m

M – unaffected: MM, Mmm – affected: mm

M

m

M – unaffected: MM, Mmm – affected: mmPerson Genotype

Male IV1 mmFemale III3 mmMale II1 MmFemale II2 MmMale II3 MmFemale II4 Mm

mm

mm

Mm MmMm

Mm

8.2 What is the probability of Male III1 being a carrier for albinism? Show your reasoning. (1)

Probability of male III1 being a carrier : 2/3 x ½ = 1/3

2/3 – offspring of Male II1 and Female II2 are : 1 Mm : 2 Mm : 1 mm. Male III1 cannot be mm as symbol is white, so chance of being a carrier is 2 out of 3.Chance of being male: ½

Mm x Mm

MM Mm Mm mm

2/3

8.3 What are the possible genotypes of Male I1 and Female I2? Show your reasoning. (2)

Both Mm. Since they are unaffected, they carry M. They must have passed on m to Male II1 as this person had an affected child. OR One parent MM and the other Mm, as this combination also produces a heterozygous individual which is Male II1.

MM, Mm

mm

8.4 Is albinism a sex-linked trait? Explain your answer. (1)

Not sex-linked. Ratio of affected males and females is equal. More males than females would have been affected, if condition was sex-linked.

[SEP, 2006]

9. Flower colour in roses is controlled by two allelomorphic pairs of genes Rr and Yy. If at least one dominant gene from each allelomorphic pair is present, the flowers exhibit a violet phenotype. All other genotypes give rise to a white phenotype. If two roses with violet flowers, each having the genotype RrYy are crossed, what will the phenotypic ratio of the offspring be? You may use the space in the box below for your working. (10)

RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

Phenotypic ratios: 9 violet: 7 white [MAY, 2010]

violet: R_Y_

POLYGENES

occurs when one characteristic is controlled by two or more genes

e.g. eye colour, height, weight, skin colour

Where are the polygenes affecting a particular quantitative

character located?

on many different chromosomes

Each gene on its own produces only a very small effect

The additive effects of the alleles of many genes produces a wide range of genotypes and phenotypes i.e. variation

BUT

Discontinuous variation: single-gene inheritance traits occur in distinct categories

Two types of variation:

Continuous variation:

due to polygenes

distribution of phenotypes in the

population varies along a

continuum

individuals differ by small degrees

Human skin colouration is explained by three genes (A, B and C)

2n = 23 = 8

F1

F2

Three genes are involved in human skin colour

The dominant alleles (A, B, and C) each contribute one "unit" of pigment to the individual (additive effect), so that individuals with more of these alleles will be darker than those with fewer alleles.

The recessive alleles (a, b, and c) do not contribute any units of pigment.

Therefore, skin color is related to the number of dominant alleles present in each individual's genotype.

Essay: SEP, 2003

Variations among organisms and between individuals of the same species is purely the result of genetic differences. Discuss this statement.Variations due to:- Different genes- Meiosis creates variation- The environment affects expression of genes

EPISTASIS

Epistasis occurs when

the phenotypic expression of one gene is affected by another

gene

In epistasis

two genes interact to control a single phenotype– not producing new phenotypes– one modifies or masks the expression

of the other– The gene that masks another is

epistatic– The gene that is masked is hypostatic

Epistasis : Coat color in mice

Yello w tip

B lack

the epistatic gene: 1) controls synthesis of melanin2) has two alleles: coloured (M - dominant) albino (m - recessive)

the hypostatic gene:1) controls distribution of pigment in the hair2) its alleles are:agouti (A - grey, dominant) black (a - recessive)

B lack

Epistasis : Coat color in mice

Yello w tip

B lack

Agouti Black Albino

B lack

Two different genes (M & A) affect coat colour in mice

MM or Mm

mm

AA or Aa

BLACK

AGOUTI

aaColour forms

Albino

Epistatic gene (M):Melanin production Hypostatic gene (A):

Melanin distribution

Phenotypes &Genotypes

MMaaMmaa

Black Agouti Albino

mmaammAAmmAa

MMAAMMAaMmAAMmAa

Epistasis: unusual phenotypic ratios are obtained

A - agouti fur (dominant)a - black fur (recessive)M - coloured fur (dominant)m - albino fur (recessive)

Parental phenotypes: agouti x albinoParental genotypes: AaMm x Aamm

♂

♀

AM Am aM am

Am AAMm AAmm AaMm Aamm am AaMm Aamm aaMm aamm

Offspring phenotypes: 3 agouti [A_M_] 4 albino [_ _mm]1 black [aaM_]

♂♀

AM Am aM am

Am AAMm AAmm AaMm Aamm

am AaMm Aamm aaMm aamm

A - agouti fur (dominant)a - black fur (recessive)M - coloured fur (dominant)m - albino fur (recessive)

Question: [SEP, 2005]

Write brief notes on each of the following terms associated with genetics, giving an example of the phenotypic expression of each.

[2 marks each]Multiple alleles.Epistasis.Polygenic inheritance.Sex linkage

Question: [SEP, 2005]

Occurs when the phenotypic expression of one gene is affected by another gene.Epistatic genes are sometimes called inhibiting genes because of their effect on other genes which are described as hypostatic.Coat colour in mice is controlled by a pair of genes occupying different loci.The epistatic gene controls synthesis of melanin while the hypostatic gene controls distribution of pigment in the hair.

Epistasis in summer squash (zuccini)

Inheritance of fruit color in summer squash: two loci together control color a dominant allele (A) at one locus can mask the

expression of the alleles at the second locus (B)

A_ _ _ white

aaB_ yellowaabb green

F2 from mating: AaBb x AaBb

Dominant Epistasis – A negates the dominance of B for color expression

A_ B _A_ bb

aabb

aaB _

GENE COMPLEX

a group of genes of an individual or of a potentially interbreeding group that constitute an interacting functional unit

What is a ‘Gene Complex’?

e.g. inheritance of the shape of the comb of domestic fowl

there are genes at two loci situated on different chromosomes which interact and give rise to four distinct phenotypes:

In the inheritance of comb shape of domestic fowl:

Comb shape is determined by two different genes :

– R (rose comb) is dominant to r– P (pea comb) is dominant to p – R and P are codominant (walnut comb)– rrpp produces single comb

Phenotype Possible genotypesPeaRoseWalnutSingle

PPrr, PprrRRpp, RrppPPRR, PpRR, PPRr, PpRrpprr

What will the F2 genotypes and phenotypic ratios resulting from crossing a pure-breeding pea-comb hen with a pure-breeding rose-comb cock be?

P - presence of pea comb p - absence of pea combR - presence of rose combr - represent absence of rose comb

Pr Rp

Parental phenotypes: pea comb x rose combParental genotypes: PPrr x RRpp

F1 genotypes:

xGametes:

F1 phenotypes:

PpRr100% w alnut comb

Get the F2 generation

PR Pr pR prPR PPRR PPRr PpRR PpRrPr PPRr PPrr PpRr PprrpR PpRR PpRr ppRR ppRrpr PpRr Pprr ppRr pprr

F1 Genotypes: PpRr x PpRr

Gametes:

F2 genotypes:

PR Pr pR pr PR Pr pR prX

F2 phenotypes

PR Pr pR prPR PPRR PPRr PpRR PpRr Pr PPRr PPrr PpRr Pprr pR PpRR PpRr ppRR ppRr pr PpRr Pprr ppRr pprr

9 Walnut [P_R_]3 Pea [P_rr]3 Rose [ppR_]1 Single [pprr]