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Genetics

the study of hereditary variation

© 2007 Paul Billiet ODWS

Phenotype

Organisms have characteristic appearances

These appearances may vary from one individual to another

The characteristics shown by an organism is called its phenotype

(From the Greek phainein = to show and typos = type)


Variations

Variations between organisms may be: interspecific – variations between

different species (e.g. tigers have stripes and leopards have spots)

intraspecific – variations within a species (e.g. blood type or height in humans)

It is intraspecific variations that concern us here.


Nature or nurture?

Variations may be influenced by what is inherited from the parents (the genotype)

Variations may be influenced by the environment the organism encounters as it grows and develops.

Genotype(nature)

Environment(nurture)

Phenotype© 2007 Paul Billiet ODWS

Studying heredity

The Neolithic revolution

Breeding from the animals or plants

Gregor Mendel

http://history.nih.gov/exhibits/nirenberg/popup_htm/01_mendel.htm


Pea plants for genetics Peas have many

recognisable characteristics (e.g. seed shape)

They are easy to cultivate Their life cycle is reasonably

short so results can be obtained quickly

Peas produce a large number of offspring (seeds), which makes results easier to verify

http://www.ppdl.purdue.edu/PPDL/images/pisum-sativum.jpg


Pea plants for genetics

Peas have hermaphroditic flowers

Self fertilisation is possible.

The male parts can be pulled out to emasculate the flowers, preventing self fertilisation

jeantosti.com/fleurs4/pois.htm © 2007 Paul Billiet ODWS

Mendel’s breeding experiments

Taking one character only as an example, seed colourParents (P)

First generation (F1)

Female sex cellsfrom a yellow-seeded plant

Male sex cells in pollen from a

green-seeded plant

Cross fertilised (crossed)

All seeds produced turned out yellow


The reciprocal cross

Mendel tried the cross the other way round

Green seed female plant x yellow seed male plant

The same results were obtained


Dominant and recessive traits

The green seed coloured trait had disappeared but it reappeared in later generations as though it were hidden

Traits that disappear and reappear (e.g. green seed colour in peas) are called recessive

Those that hide them are called dominant traits (e.g. yellow seed colour in peas)


Selfing

Mendel produced a second generation of plants using the first generation.

He brushed the male pollen grains onto the female parts of the same flower

This is called self pollination and it leads to self fertilisation or selfing


SelfingYellow seed

producing plants

Selfed

Yellow seeds 6022

Green seeds 2001

About 75% show the dominant trait

About 25% show the recessive trait

First generation (F1)

Second generation (F2)


Selfing

All green seedsPure

breeding

SelfedSelfed

66% produce a mixture of yellow &

green seeds

33% produce yellow seeds

onlyPure breeding

Yellow

seeds 6022

Green

seeds 2001

Second generation (F2)

Third generation (F3)


Pure breeding

Those plants that only produce one type are called pure breeding (or true breeding)


The particulate theory of inheritance: Genes

Mendel concluded from this and other similar experiments, that characters are controlled by factors (later called genes)

These genes like separate particles, passed on from generation to generation

They are not changed or diluted to give intermediates

The gene is the unit of hereditary information


Genes and variation A character is controlled by a gene that may come in

different types called allelomorphs (meaning “other forms”) or alleles

These different alleles produce the different traits in a character

In the above example Pea seed colour is controlled by the seed colour

gene There are two alleles of this gene (dialleleic), the

yellow allele and the green allele. The yellow allele is dominant and the green allele is

recessive.About 30% of human genes are thought to be diallelelic© 2007 Paul Billiet ODWS

Symbols for genes

Dominant alleles are given CAPITAL CASE LETTERS

Recessive alleles are given small case letters

Use letters that look different when written as small case and capital case (e.g. avoid C, O, P, S, U)


Monohybrid inheritance

The behaviour of the alleles controlling different traits revealed patterns in the way they are inherited

These patters always seemed to be the same so they are considered as a scientific law

If we only consider the inheritance of one character (e.g. seed colour) and ignore all the others (such as flower colour, seed shape etc) this is called monohybrid inheritance


The Law of Segregation

Organisms seem to possess two genes for each character (they are diploid). This pair of genes segregate (separate) when gametes are made (they are haploid)

Pairs of genes are reformed when the gametes fuse and they recombine in definite proportions (e.g. 75% to 25%)


Mendel and meiosis

Mendel worked this out in 1866 Though he no doubt understood

fertilisation, meiosis was not observed until 30 years later


Meiosis 1: Anaphase 1 Maternal and paternal

chromosomes segregate (pulled separate on the spindle)

They move to opposite poles

Meiosis & Mendel


The Law of Segregation revisited

Organisms seem to possess two genes for each character (they are diploid). This pair of genes segregate (separate) when gametes are made (they are haploid)

= meiosis Pairs of genes are reformed when the

gametes fuse and they recombine in definite proportions (e.g. 75% to 25%)

= fertilisation


Combinations of genes

The combination of alleles in an individual is called the genotype

If the two alleles are the same it is homozygous

If the two alleles are different it is heterozygous


In human genetics heterozygotes who have a dominant and a recessive allele are called carriers

They are carrying a recessive allele without expressing it

Many genetic diseases are caused by recessive alleles

Genotypes Phenotypes

YY Homozygous Yellow Pure breeding

Yy Heterozygous Yellow

yy Homozygous Green Pure breeding


P Phenotypes

Yellow seed X Green seed

Genotypes

YY yy

Gametes Y Y y y

F1 Phenotypes

Yy

Genotypes

Yellow

Proportions

100%

Genetic diagrams


F1 Phenotypes

Yy

Genotypes Yellow

Proportions

100%

Gametes Y y

Genetic diagrams

Where there are several possible gametes a Punnett square should be used

F2 Genotypes:

Y y

Y YY Yy

y Yy yy

Phenotypes:

Yellow Green

Proportions:

75% 25%

Selfed


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