Principles of Inheritance and Variations

7/28/2019 Principles of Inheritance and Variations

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Biology Project Work

TOPIC Principles Of Inheritance And Variations
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Topics Mendel and the process of science

Mendels principles of inheritance Crosses: Parental, F1, F2, and test

Chromosome mapping

Variations on Mendelian inheritance

In-breeding and out-breeding


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Breeders knew of hybrids at the time that Mendel started his

experiments (in 1856), but only knew that progeny resembledeach parent, and that some resembled grandparents. No oneknew why.

Mendel developed a systematic way of understandingheredity, using very good scientific technique.

He applied the scientific method combined with mathematical

analysis to demonstrate:1. Unit characteristics2. Segregation of those characteristics

3. Dominance of some characteristics

4. Independent assortment of characteristics

What Did Mendel Show?
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Mendel Carefully Chose His Organism Pisum sativum: the garden pea

1. Very productive: produces many peas (large N: good statistics)

2. Short life cycle: produce many generations in a short time3. Typically self-pollinating: good for inbreeding4. Easily cross-pollinated due to flower structure

Has 7 distinct phenotypic characteristics:1. Yellow versus green seeds

2. Round versus wrinkled seeds3. Green versus yellow pods4. Tall versus short plants5. Fat versus tight pods

6. White versus grey seed coats7.

Flowers: end o stem versus alon the len th o stem


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The Pea Flower The flower is well-suited to this study, which required

controlled pollination

Protects the reproductive apparatus Male (anther: makes pollen)

Female (ovary: makes ova)

Stigma arises from the ovary

Good for inbreeding and artificial breeding

Can bag flowers, easily cut off the anthers toprevent further pollination
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One of Mendels Crosses Mendel knew nothing of the chemical basis

for inheritance

He had to work very carefully andexamine ONLY the outward appearance ofthe plants

He compared what we call today thephenotype,which is the outwardexpression of the genes


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A Parental Cross Homozygous Tall (TT) X

Homozygous Short (tt)

All of the F1generation aretall.

Demonstrates the concept ofdominance


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Cross of F1 Cross the F1 generationat randomto

itself.

1/4 of F2 (the progeny of theF1 cross)

are short Mendel showed that the short plant bred

true, so it was aparental type.

That a parental phenotype disappeared

and then reappeared destroys the conceptof blending.

It was masked in the F1 Instead, it supports the concept of unit

characteristicsgenes, as we nowunderstand them


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Segregation of Characteristics Mendel recognized that the only way to achieve the masking of

the characteristics was to segregate (separate) them somehow inthe gametes.

The characters must have been separated during formation of thepollen (male) and the ova (female).

This is the concept of segregation.


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Chromosomal Basis of Segregation Segregation occurs in meiosis

at Metaphase I and then at

Metaphase IIAs a result, the ova or sperm

contribute different homologouschromosomes to the progeny


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Mendels Concepts & Modern Terminology Mendel worked in a strictly conceptual framework he had NO IDEA of

the nature of the genetic material

Today we know his characters are the products of genes. The collection ofgenes is the genotype, or genome.

A gene is positioned at a given locus(loci, plural).

Position is very important, it can define the effect of a gene

Regulation of genes is position-dependent.


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Alleles Different varieties of genes are calledalleles.

Since diploid (2n) organisms have 2 sets of chromosomes, thereare two possible alleles for each gene locus in a 2n organism.

If those alleles are the same, the organism is homozygousat thatlocation

If different, it is heterozygous


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Homozygotes & Heterozygotes We indicate the genotype by the letters:

Dominant alleles are represented by

capital letters Recessive alleles are represented by lowercase letters

Examples:

TT is homozygous dominant

Phenotype is dominant

tt is homozygous recessive

Phenotype is recessive

Tt is heterozygous

Phenotype is dominant


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Homologs & Alleles 2n organisms have a chromosome received from

each parent.

Called homologous chromosomes These have alleles of A, B, C & D.

A & a, B & b, C & c, D & d

C is NOT an allele to D; nor is A

or B A locusis a physical location of DNA that

encodes for a protein product; i.e. agene

A, B, C & D are at specific loci


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Diploid cells


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Homologous and Non homologous Chromosomes: Definitions


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A Test Cross What if you only know that

you have a black and a

brown guinea pig (i.e. youdont know the genotype)?

Do a test cross

Cross with a known true-

breeding recessive brownguinea pig


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Test Crosses: Mechanism On inspection, heterozygotes are

indistinguishable from homozygotedominants.

Phenotypes can be tested for genotype withtest crosses

Test crosses are easily checked w/ Punnettsquares.

Always cross the unknown into homozygous

recessive to reveal the dominant genotype. The distribution of alleles follows the

product law and the sum law.

A

A

a a

Aa

Aa

Aa

Aa

A

a

a

Aa

aa

Aa

aa

a

All A phenotype: Homozygous A

A phenotype; a phenotype


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Mapping Chromosomes If there is 8% crossover

between A & C, 5%crossover between A & B,and 3% between B and C,we can arrange them inthis manner (a):

If the crossover between Aand C was 2% instead of8%, we could rearrange as

in (b).


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Incomplete Dominance


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Inbreeding Inbreeding was useful to Mendel for producing true-breeders

But it can concentrate undesirable attributes, which are often recessive.

Human inbreeding tends to increase the frequency of rare geneticdisorders, although the Tamil of India extensively intermarry with littleor no ill effect.

Australian sheepdogs must be euthanized if they are white puppies: they

become blind, deaf, and have many skeletal muscle problems The English bulldog has significant bone structure problems many

members of this breed can barely mate and bear puppies, and many havedifficulty breathing


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Outbreeding Outbreedingmeans breeding of different strains or stocks of the same organism

Breeding to different strains tends to produce more robust individuals, referred to as hybridvigor

Produces individuals that are multiply heterozygous at many loci Masks recessives in many different traits

In many cases, multiple heterozygosis seems to confer additional advantage beyond the maskingof recessive traits. This is referred to as over dominanceor the Heterozygote Advantage.

Most modern grain crops are multiple hybrids: they are more resistant to disease, tolerate

changes in weather, etc. A simple example: Sickle Cell Anemia

People heterozygous for sickle-cell anemia have a distinct advantage in Africa, where thetrait improves resistance to malaria by interfering with the parasite life cycle.

The moderate illness due to the heterozygote sickle cell condition is not severely disabling.


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