Biology Student’s Companion Resources

Biology Student’s Companion Resources SB015

1 I KMPK

BIOLOGY SCORE

CHAPTER 4 : GENETIC INHERITANCE

SUBTOPIC : 4.1 MENDELIAN GENETICS: MONOHYBRID AND DIHYBRID

LEARNING OUTCOMES:

a) Define terminologies used in genetic inheritance

b) State the characteristics of Mendel’s pea plants.

c) State Mendel’s First Law (Law of Segregation)

d) Construc genetic diagram on the monohybrid cross and include the genotype ratio (1:2:1) and phenotypic

ratio (3:1) of F2 generation.

e) Construct genetic diagram on Mendelian monohybrid test cross and include the genotypic ratio (1:1) and

phenotypic ratio (1:1).

f) State Mendel’s second law (Law of independent assortment).

g) Construc genetic diagram on the dihybrid cross and include the only phenotypic ratio (9:3:3:1) of F2

generation using Punnet square.

h) Construct genetic diagram on Mendelian dihybrid test cross and include phenotypic ratio (1:1:1:1) of F2

generation using Punnet square.

MAIN IDEAS

/KEY POINT EXPLANATION NOTES

Terminologies

1. Alleles : Alternative forms of a gene for a trait occupying the same locus on

homologous chromosomes.

2. Genes : A heritable unit consisting of a specific nucleotide sequence in DNA that

controls a specific characteristic or phenotype of an organism.


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3. Locus : The position on a homologous chromosome where a gene is located

4. Genotype Genetic composition of an individual; combination of alleles it

possesses/ or the alleles possessed by an individual.

5. Phenotype : The observable characteristics of an organism determined by gene.

6. Homozygous : An organism that has a pair of identical alleles for a particular

trait.

7. Heterozygous : An organism that has TWO different alleles of a gene a

particular trait.

8. Dominant : An allele which express its phenotype even in heterozygous form.


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9. Recessive : An allele that only express its phenotype when present in the

homozygous form.

10. Self cross :

SELF-POLLINATION : Transfer of the pollen grain from the anther to the

stigma of either the same or genetically similar flower.


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11. Test cross : Individual of unknown genotype with dominant phenotype is

crossed to a recessive homozygous individual.

Characteristics

of Pisum

sativum

1. Easy to grow

2. Short life cycle

3. Produce large number of offspring from each mating

4. Many easily observable characteristics

5. Have two possible traits of each characteristic

6. Pea plants can self-pollinate or cross-pollinate


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Monohybrid

cross

Breeding that involves a single characteristic determined by one gene.

T: Dominant allele for tall t: recessive allele for dwarf

Mendel's first

law/Law of

Segregation

The two alleles for a heritable character segregate during gamete formation &

end up in different gametes.

• Mendel deduced that the F1 plants must have carried two factors for the flower

color character, one for purple and one for white

• Each allele separate during gametes formation.

• Mendel‘s Law of Segregation can be explained by the homologous pairing &

segregation of chromosomes during meiosis.


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Mendel’s

Monohybrid

Cross

Mendelian

Monohybrid

Test Cross

• To determine the unknown genotype with dominant phenotype based on the

phenotype of each progeny.

• E.g. monohybrid test-cross

• If all the progenies produced are of the dominant phenotype, then the organism

being investigated is homozygous dominant

• If 50% of the progenies produced show the dominant phenotype and 50% show

the recessive phenotype, then the organism being investigated is heterozygous

• E.g: to determine the unknown genotype of purple flower:

• If all the progenies produced are purple flower, then the organism being

investigated is homozygous dominant, PP

• If 50% of the progenies produced are purple flower and 50% are white flower,

then the organism being investigated is heterozygous, Pp.


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Mendel’s

second law

(Law of

Independent

Assortment)

Each PAIR of alleles SEGREGATES INDEPENDENTLY of each other pair

of alleles during gametes formation

Which stage of meiosis creates the law of independent assortment?

Answer: Metaphase 1


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Mendel’s

Dihybrid Cross.

Allele : Y is dominant allele yellow seed

y is recessive allele for green seed

R is dominant allele for round seed

r is recessive allele for wrinkled seed

Mendel’s

Dihybrid

Testcross

How can we tell the genotype of an individual with the dominant phenotype?

When a heterozygous F1 pea plants were crossed with the homozygous

recessive parental plants, the cross yielded a phenotypic ratio of 1:1:1:1


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BIOLOGY SCORE

CHAPTER 4 : GENETIC INHERITANCE SUBTOPIC : 4.2 Deviations From The Mendelian Inheritance

LEARNING OUTCOMES:

a) Explain briefly types of inheritance that deviate from Mendelian:

i) Codominant alleles.

o Construct genetic diagram to show codominant alleles using human MN blood

group and include phenotypic ratio and genotypic ratio (1:2:1)

ii) Incomplete dominant alleles

o Construct genetic diagram to show incomplete dominant alleles and include

phenotypic ratio (1:2:1)and genotypic ratio by using Antirrhinum sp.

(snapdragon) flower colour.

iii) Multiple alleles

o Construct genetic diagram to show multiple allele using human ABO blood

group.

MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

Codominant

alleles

Both alleles of a pair are fully expressed in a heterozygote

Eg. Human MN blood group.

Incomplete

dominant

alleles

A form of intermediate inheritance in which one allele for a specific

trait is not completely dominant over the other allele.

Heterozygous progeny show intermediate characteristic of both parent

Eg. Colour of snapdragon flower ( Antirrhinum majus)


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IDEAS /KEY

POINT

EXPLANATION NOTES

F2 genotypic ratio : 1 CRCR : 2 CRCW :1 CWCW

F2 phenotypic ratio : 1 Red : 2 Pink : 1 White

Multiple

alleles

MORE THAN TWO alleles control for particular trait OF A

CHARACTER

But each individual has only two alleles that can occupy gene loci on

homologous chromosome.

Eg : ABO blood group alleles in humans

o 4 blood types : A, B, AB and O


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BIOLOGY SCORE

CHAPTER 4 : GENETIC INHERITANCE SUBTOPIC : 4.2 Deviations From The Mendelian Inheritance

LEARNING OUTCOMES:

a) Explain briefly types of inheritance that deviate from Mendelian:

iv) Codominant alleles.

o Illustrate the effect of linked genes with crossing over on the dihybrid

test cross ratio.

v) Linked gene

o Construct genetic diagram to show sex-linked gene using

haemophilia.

vi) Explain polygenes/ polygenic inheritance.

o Explain polygenes/ polygenic inheritance using human skin colour.

MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

Linked gene

Each chromosome carries a number of genes in particular locus

The genes that are closely together on the same chromosome not assort

independently but inherited together in the same gamate..

Crossing over during meiosis, homologous chromosomes can exchange

pieces of chromosomes and produce recombinant phenotypes


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IDEAS /KEY

POINT

EXPLANATION NOTES

Crossing over depends on the distance between two genes.

Genes are close: few recombinant offspring.

Genes are far apart: many recombinant offspring.

In linked gene, without crossing over can produce two types of gametes.

In linked gene , with crossing over can produce four types of gametes.

Dihybrid test cross ratio is 9:3:3:1.

Linked gene with crossing over test cross ratio is not 3:1, will produce two

parental phenotypes and two recombinant phenotypes.

2 parental phenotypes in large number and new trait phenotypes in small

number.

Eg:

Genotype GL/gl gl/gl Gl/gl gL/gl

No. observed 965 944 206 185

Phenotypic ratio Not 1:1:1:1

Phenotype Parental

phenotypes

Recombinant

phenotypes


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MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

Sex-linked

genes

Genes that are carried by either sex chromosomes.

Humans have 23 pairs of chromosomes, consist of 22 pairs autosome

and 1pair sex chromosomes.

Most sex-linked trait are controlled by gene on the X chromosome

Eg; haemophilia


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IDEAS /KEY

POINT

EXPLANATION NOTES

Eg; Colour blindness.

Polygenes

/polygenic

inheritance

An additive effect of two or more genes on a single phenotypic

character.

It shows no dominance.

Quantitative traits.

Each gene located on different chromosomes

Eg: height, weight, skin colour


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CHAPTER 4 : GENETIC INHERITANCE SUBTOPIC : 4.3 Genetic Mapping

LEARNING OUTCOMES :

a) Define genetic mapping.

b) Calculate the genetic distance (map unit) between genes using the given

recombination data.

c) Identify the position or order of genes along a chromosome based on

recombination data.

MAIN IDEAS


Genetic

mapping

Genetic map = plan to show the position of a gene on a

chromosome of an organism

Genetic mapping is a process of locating the relative position

and sequence of gene on chromosome.

Can be carried out when the genes are linked and crossing

over has taken place.

Formula

Recombination frequency or cross over value (COV)

= number of recombinants offspring x 100

Total number of offspring


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Recombination

frequency

Percentage of recombinant offspring is correlated with the

distance between the two genes.

A measure of distance between gene.

The lower the value, the closer the genes.

Unit is map unit or centimorgan (cM).

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Biology Student’s Companion Resources