Chapt 05

GENERAL BIOLOGY

SCHOOL OF MLTFACULTY OF HEALTH SCIENCE

PREPARED BY:MANEGA

HDL 121GENETICS MENDELS LAW

GENETICS MENDELS LAW

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Learning Outcomes

After completing this lecture, students will be able to:

(a) Know some terminologies used in genetics

(b) Know the Mendel’s law

(c) Recall the Mendel’s experiments

(d) Describe the hybrid not in accordance to

Mendel’s law

(e) Explain genetic mapping

Topics© 2010 Cosmopoint


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Topic Outlines

1.1. Terminologies

1.2. Mendel’s experiment1.2.1 Garden pea plant1.2.2 Monohybrid cross1.2.3 Dyhybrid Cross1.2.4 Result & conclusion from the experiment

1.3. Hybrid not in accordance to Mendel’s law1.3.1 Codominance1.3.2 Imcomplete dominance1.3.3 Multiple alleles

1.4. Genetic Mapping

© 2010 Cosmopoint


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Introduction

Allele: is one member of pair or series of different forms of a gene found on the same locus on homologous chromosome

Gamete: Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans)

Gene: part of DNA molecule found in the chromosome that determines a polypeptide through which an inheritable trait is expressed

Genotype: The genetic constitution of an organism (only one or two genes are considered at one time). Genotype can be homozygous or heterozygous

Dominant allele: A gene is said to be dominant if it expresses its phenotype even in the presence of a recessive gene.

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1.1. Terminologies


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Phenotype: Observable trait / traits of an individual; arises from interactions between genes, & between genes & the environment. Phenotype determines individual structure, physiology & behaviour that include followings:

(a) character that can be observed.eg. Colour

(b) character that can be felt. eg. Texture of the hair

(c) character that can be tested serologically. eg. Blood group

(d) quantitative character that can be measured including

intelligence using IQ test.

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1.1. Terminologies


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Heterozygote: a person possessing two different forms of a particular gene, one inherited from each parent. A heterozygote is also called a carrier (Eg. Pp, Tt)

Homozygous: genotype of an individual that has any of a pair or more of alleles considered are identical eg. AA, aa, AABB, Aabb, aaBB or aabb

Homozygote: diploid individual with two identical alleles at a given locus.

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1.1. Terminologies


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Mendel’s Experiment

Crossed garden peas in his monastery garden & analysed the offsprings of these mating

Reasons:

(a) could be grown easily in large numbers

(b) had a short life cycle

(c) their pollination could be controlled

(d) their reproduction could be manipulated

(e) had easily observable characteristics.

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1.2. Mendel’s experiment


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Pea plants

Have both male & female reproductive organsCan either self pollinate / cross-pollinate with another plant

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1.2.1 Garden pea plant


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Monohydrid cross

Established pure-breed stock for tall plants & a pure-breed stock for short plants

Studied the inheritance of one trait, eg. Plant’s height

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1.2.2 Monohybrid cross


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Monohydrid cross

Gene – some DNA molecules that controls

Trait – Height (short @ tall)

Genotype

Homozygote (TT) Heterozygote (Tt) Homozygote (tt)

Phenotype – Tall Tall Short

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Cross-pollinated tall pea plants (TT) with each other

Parental generation (Genotype) TT XTT

Gamete T T

F1 TT

Phenotype All tall

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Cross-pollinated short pea plants (tt) with each other

Parental generation (genotype) tt x tt

Gamete t t

F1 tt

Phenotype All short

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Crossed tall plants with short plants

Parental generation (phenotype) tall plant short plant

Parental generation (genotype) TT X tt

Gamete T t

F1 Tt

Phenotype All tall plants

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Allowed plants in the F1 generation to self-pollinate (Self-cross)

F1 Tt X Tt

Gamete T t T t

F2 TT Tt Tt tt

Phenotype Tall Tall Tall short

Ratio 3 : 1

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Monohydrid cross

Height of plant must have been determined by certain factors

Factors occur in pairs, because the offsprings of the F2 generation were both tall & short

F1 generation must contain both tall & shorts factors

Two types of factor

(a) dominant: tall hides the effect of short

(b) recessive: short is recessive to being tall; hidden by the dominant factor

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Mendel’s Laws of Inheritance

Mendel first law of inheritance (The law of segregation)

(a) states that from only one parent only one factor (allele) is passes from the parent to the offspring through the gamete.

(b) This law can be explained by meiosis. In garden pea that is diploid, a heterozygous yellow seed (Yy) can only transmit one of the alleles to each of its offspring. (Y is a dominant allele for yellow seed coat whereas y is a recessive allele for green seed coat)

Parent (P1): Yy (yellow)

Gametes: Y y

(c) Each gamete can only obtain one allele from the parent because meiosis reduces a diploid gamete mother cell to haploid gamete

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(d) Mendel used garden pea plants for his experiments. One of the characters was seed colour. He started by crossing two pure breeding strains; eg. One had yellow & the other had green seeds. He then allowed the offsprings (F1 generation of first filial generation) to self-fertilise (selfing) & the results are always the same as follows:

P1: YY X yy

Phenotype: yellow green

Gametes: Y y

F1: Yy

Phenotypes: yellow

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(e) The F2 generation (second filial generation obtained by random crossing or selfing of the F1 generation) has a ratio of ¾ of one character and ¼ of the contrasting character, the classical Mendelian ration is 3:1

P2 (selfing): Yy X Yy

Phenotype: yellow yellow

Gamete: Y y Y y

F2: YY Yy Yy yy

Phenotype: yellow yellow yellow green

Genotypic ratio= ¼ YY : ½ Yy : ¼ yy

Phenotypic ratio= ¾ yellow : ¼ green

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Dihybrid cross

(f) He repeated his experiment using several contrasting characteristics, which include tall & dwarf plants, round & wrinkled seeds, inflated & constricted pods, red & white flower. Therefore, he concluded that each plant carried two factors through only one factor was exhibited in F1. When selfed, the F1 would segregate the factors & produced the characteristic ratio.

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1.2.3.Dihybrid Cross


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Mendel second law of inheritance

(a) Dihybrid cross

(b) Mendel crossed pea plants that differed in 2 contrasting traits (pure breeding plants)

(c) He crossed a yellow plant (Y) with round seed (R) with a green plant (y) with wrinkled seed (r)

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Mendel’s Second Law

Law of Independent AssortmentDuring gamete formation, segregation of the alleles of

one allelic pair is independent of the segregation of the alleles of another allelic pair.

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F2: 9 yellow, round: 3 yellow, wrinkled: 3 green, round: 1 green, wrinkled

(9:3:3:1)

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Dihybrid Punnet Square

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Mendel confirmed the results of his second law by performing a back cross where he crossed an F1 dihybrid with a recessive parent.

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1.2.4 Result & conclusion from the experiment

Conclusion on his 2nd experiment

(a) During gamete formation, segregation of the alleles of 1 allelic pair is independent of the segregation of the alleles of another allelic pair

(b) Genes that are on different chromosomes assort independently

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1.2.4 Result & conclusion from the experiment

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Hybrib not accordance to Mendel Law

Codominance: when both alleles are fully expressed in the heterozygous form.

Eg. Human MN blood typing2 antigens, M & N, which are determined by a gene with 2

alleles, LM & LN

Individual with genotype LM LM will have only M antigen in their RBC

LN LN: N antigen onlyLM LN: M & N antigens

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1.3. Hybrid not in accordance to Mendel’s law


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Cross between LM LM and LN LN

P LM LM X LN LN

Gametes LM LN

F1 LM LN

(individual produces both antigens)

P LM LN X LM LN

Gametes LM LN LM LN

F2 LM LM LM LN LM LN LN LN

1 : 2 : 1

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1.3. Hybrid not in accordance to Mendel’s law


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1.3.2 Imcomplete dominance

Incomplete dominance: a blending of traits, condition when neither allele is dominant over the other

Recognised by the heterozygote expressing an intermediate phenotype relative to the parental phenotypes

Eg. Red flowered plant is crossed with a white flowered plant, all progeny will be pink.

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1.3.2 Imcomplete dominance

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Multiple allele: genes may exist in more than 2 allelic forms Eg. ABO blood type Three different alleles for blood type:

(a) IA (Type A)

(b) IB (Type B)

(c) IO (Type O)

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1.3.3 Multiple alleles


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Only two of these alleles are presented in an individual They combine to form genotypes that result from codominance.

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An individual with blood type O mates with an individual with blood type A.

P: IO IO X IA IA

Gametes: IO IA

F1: IA IO (blood type A)

Individual with IA IO genotype mate

F2: IA IA IA IO IO IO

1 : 2 : 1

Phenotype 3 blood group A : 1 blood group O

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Genetic Mapping

= chromosome mapping Determination of the position of a gene on a chromosome by the

means of recombination frequencies The percentage of recombinant phenotypes can be used to map the

chromosomes Why? – Direct relationship btw frequency of crossing-over & the

percentage of recombinant phenotypes

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If we want to determine the order of any three genes on a chromosome, we can perform crosses that will provide us the map distance between the three pairs of alleles

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Introduction

Population genetics is the study of genes in a population

i.e. the study of Mendelian inheritance mathematically in a population

The population in this context is a Mendelian population, consisting of only one species of diploid organisms, which reproduce sexually within a certain geographical border

The study of population is important for the understanding of evolution. Evolution is not the change of one individual but that of a population over a long period of time

The study of population genetics reconciles the fact of Darwin theory of evolution with that of Mendelian genetics

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1.1. Hardy-Weinberg Law


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Darwin theory of natural selection is based on variation created by mutation in the form of different genes / alleles. Individuals with certain combination of alleles survive over the years bringing about changes in a population

The change in allelic frequency caused by environmental forces is evolution

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For easy calculation, a concept based on one gene locus is treated at one time. So, gene pool is diagrammatically represented as

A gene pool of A & a alleles

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A A a

A a a a

A a A

a A a A

1.2. Principle


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Darwin theory of natural selection is based on variation created by mutation in the form of different genes / alleles. Individuals with certain combination of alleles survive over the years bringing about changes in a population

The change in allelic frequency caused by environmental forces is evolution

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1.2. Principle


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Only one gene or locus is considered. That locus consists of dominant & recessive alleles,

i.e. A & a allelesThe frequencies of alleles A & a depend on the

genotypic frequencies of AA, Aa & aa. Hence, if the frequency of AA is very high, the frequency of A would be high too.

From the frequencies of the alleles, the frequencies of the genotypes of the next generation can be calculated if we assumed that random fertilisation of the gametes occurs.

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1.2. Principle


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Concept of a Gene Pool

A gene pool is an aggregate of genes/gametes of a Mendelian population from which the next generation is produced

It can be considered as the total genetic information possessed by reproductive members in a population of sexually reproducing organisms.

Genes in the pool have dynamic relationships with one another & with the environment around where the organisms live

Environmental factors such as selection can alter allelic frequencies & cause evolutionary changes in the population

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1.2. Principle


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Hardy-Weinberg Law

States that after one generation of random mating, a population will become in equilibrium

i.e. the allelic & genotypic frequencies will not change from one generation to the other

However, equilibrium is only achieved depending on conditions / assumptions as follows:

(a) the population must be large

(b) the mating must be random or panmitic

(c) there must not be any selection

(d) there must not be any migration

(e) there must not be any mutation

(f) meiosis must be normal

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1.2. Principle


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Uses of the law / its formula

To study the changes of gene frequencies in a wild population so that the direction & rate of evolution can be determined.

To study the changes of gene frequencies in an artificial population such as that of a herd of cattle / a plantation of crop.

To plan for breeding programme so that a large population of animals or plants can be manipulated to produce more quantitatively and/or qualitatively

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1.2. Principle


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Chapt 05

Education

genetics mendels law1

genetics mendels lawprepared

mendels law c

mendels law e

genetics b

mendels experiments

plant8 slide

tall short f1 generation