Polygenic Inheritance and Genes in Populations

7/27/2019 Polygenic Inheritance and Genes in Populations

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Inheritance


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Contents

Variation

Chromosomes and DNA

Monohybrid inheritance

Selection

Genetic engineering


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Variation

Living organisms vary in many ways.

E.g. humans all have the same general shapeand the same set of body organs, but somefeatures differ from one person to the next,such as:

height, weight, eye and hair colour, shape ofnose, language, knowledge and skills.


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Within a species there is usually a great deal ofvariation between individuals


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These snails are allthe same species

(Cepaea nemoralis)but they varyconsiderably

What variationscan you see ?


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There are variations in skin colour, hair colour,

hair curliness, eye colour and sex


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Variation

Variations may be inherited or acquired

Inherited variations result from the activity ofgenes.

They are genetically controlled

Genetically controlled variations cannot bealtered

For example, hair colour, skin colour, bloodgroup, finger prints and sex cannot bechanged naturally


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Variation

Acquired characteristics result from an individualsactivities or nutrition or from environmental conditionsduring a lifetime

Examples of acquired conditions in humans are*

language obesity

athletic skills mental skills

body building sun tan

Acquired characteristics cannot be inherited


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Two types of variations

Continuous and

Discontinuous


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Continuous variation

Characteristics that show continuous variation havedifferent features:

1. Every organism within one species shows the

characteristic, but to a different extent. Thecharacteristic can have any value within a range.

2. These characteristics are usually quantitative theycan be measured.

3. They result from several genes acting together, or from

genes and the environment.


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Examples Height

Weight

Foot length

Intelligence

Chest circumference

Body mass

Hand span


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The figures can

be made to fit a

smooth curve

because there aremany intermediates

Normal distribution curve


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Discontinuous variation

Characteristics that show discontinuous variation haveseveral features:

An organism either has the characteristic or it doesnt

have it. There is no range of these characteristicsbetween extremes.

These characteristics are usually qualitative theycannot be measured.

They are the result of genes only they are not affected

by environment.


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Examples

Your ABO blood group is either A, B, AB or O

Presence or absence of ear lobe

Gender

Eye colour

Genetic diseases such as colour blindness, albinism,sickle cell anaemia


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Blood group

The figures cannot

be made to fit asmooth curve

because there are

no intermediates


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Chromosome structure

Chromosomes are thread likestructures found inside thenucleus of a cell.

A human cell nucleus

contains 46 chromosomes. Each chromosomes contains

a long molecules of DNA


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DNA stands fordeoxyribonucleic

acid

DNA issubdivided intosegments calledgenes


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Genes

A gene is a unit of inheritance.

Genes may be copied and passed on to the next generation


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Location of genes

The position of a gene on achromosome is known asthe locus.

In sexually reproducing

organisms, most cells have ahomologous pair ofchromosomes (one fromeach parent)

Chromosomes from a

homologous pair have genesthat control the same traitat the same locus.


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Alleles Genes occupying the

same position (locus) onhomologouschromosomes are calledalleles.

Alleles are alternativeforms of a gene

Any one individual canonly have a maximum oftwo alleles for a givengene.

There may be more thantwo alleles in apopulation, e.g bloodgroups A, B, O


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Homozygous and

heterozygous Alleles are always represented by letters.

Since there are two alleles for a given gene, a gene isrepresented by a pair of letters.

E.g. AA, Aa and aa When the two letters are same, they are termed

homozygous (AA, aa)

When they are different they are termed heterozygous(Aa)


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Dominant and recessive

When an allele expresses itself both in homozygous andheterozygous conditions, it is said to be dominant.

A dominant allele is always represented by a capital

letter. When an allele is always expressed when present in

homozygous state and suppressed in the heterozygousstate it si said to be recessive.

A recessive allele is always represented by a small

letter.

E.g A is dominant, a is recessive


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Homologous

chromosomes


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Phenotype and genotype

The genetic constitution of an organism iscalled its genotype.

The external appearance of an organism isknown as its phenotype.

E.g. For a plant producing purple flowers

genotype = PP and

Phenotype = purple.


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Polygenic Inheritance and Genes in Populations