biology student’s companion resources
TRANSCRIPT
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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/KEY POINT EXPLANATION NOTES
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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MAIN IDEAS
/KEY POINT EXPLANATION NOTES
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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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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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
Biology Student’s Companion Resources SB015
11 I KMPK
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
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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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IDEAS /KEY
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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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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
/KEY POINT EXPLANATION NOTES
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).