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Powerpoint Presentation: Genetics

 

Genetics Index

Chromosomes
Meiosis
Meiosis: Crossing over
Mitosis and Meiosis
Test Cross
Codominance
Multiple Alleles
Pedigree Charts
Twin Studies
The dihybrid cross
Dihybrid Cross : Test Cross
Autosomal Linkage
The Genetic Diagram for Linked Genes
Calculating the cross over value using a test cross
Sex determination and sex linkage
Sex linkage
Genetic diagram for sex linked genes
Blood Clotting and Haemophilia
The Retina and Daltonism
Genetic Modification
Cloning Animals
Cloning Plants

Topic Chapters Index

 

Studying heredity

Mankind probably had an idea about the influence of heredity since the Neolithic revolution.

Humans started to domesticate animals and plants for agriculture from about 10000 years ago.

Breeding from the animals or plants with the best characters is common practice amongst farmers.

The scientific study of genetics began in 1866 with the work of Gregor Mendel (who was brought up in a region of Europe actively experimenting with new ways in agriculture).

Gregor Mendel

© Wellcome Library, London

Mendel became interested in the inheritance of variations in pea plants (Pisum sativum). These plants became the first in a long line of organisms used for genetic research.

Characteristics which make pea plants useful for genetics research

  • Peas have many recognisable characteristics (e.g. flower colour, seed shape, seed colour).
  • 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.

In addition to these characters:
Peas have hermaphroditic flowers (both male and female sex cells are produced).

  • Self fertilisation is possible.
  • The male parts can be pulled out to emasculate the flowers, preventing self fertilisation.

 

GENETICS

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Introduction to Mendelian Genetics

Genetics is the study of hereditary variations

Organisms have characteristic appearances. These appearances may vary from one individual to another.

The characteristics shown by an organisms are called its phenotype (From the Greek phainein = to show and typos = type)

Variations between organisms may be:

  • interspecific - variations between different species (eg tigers have stripes and leopards have spots)

  • intraspecific - variations within a species (eg 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) or by the environment the organism encounters as it grows and develops.

nature or nurture?

The way we grow and develop could be because of the genes we inherit from our parents or because of the environment we experience (e.g. diet, climate, education) as we grow.

It could be a mixture of the two.You could grow to 1m90 because you inherited genes for tallness from your parents but you will not reach 1m90 unless you are well nourished as you grow.

 

Mendel's breeding experiments

Taking one character only as an example, seed colour, Mendel observed the following:

mendel seed colour

 

The reciprocal cross

Mendel tried the cross the other way round (green seed female plant x yellow seed male plant) and got the same results.

 

Dominant and recessive traits

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

Traits which 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 (remember pea plants are hermaphrodites).

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

selfing

The particulate theory of inheritance

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

 

Alleles

A character is controlled by a gene which may come in different types called allelomorphs (meaning "other forms") or alleles. These different alleles produce the different traits in a character.

In the example above pea seed colour is controlled by the seed colour gene. There are two alleles of this gene, the yellow allele and the green allele. The yellow allele is dominant and the green allele is recessive.

 

Symbols

It becomes a bit tedious referring to "the recessive green pea seed colour allele" so it is convenient to use symbols for alleles.

Dominant alleles are given CAPITAL CASE LETTERS and recessive alleles are given small case letters.

It does not matter what letter you choose but it is useful if it has some connection with the name of the allele. It is also very useful if the alleles look different when written as small case and capital case (e.g. avoid C, O, P, S, U)

Thus: Yellow seed colour allele = Y and green seed colour allele = y (Note not "g", use the same letter for the alleles of a gene).

There is another notation which tends to used by geneticists using animals such as fruit flies.

  • Dominant alleles which are present in the wild population (called wild type) carry a + symbol

  • Recessive alleles caused by a change in a gene (a mutation) do not carry a + symbol.

Both letters are in small case.

Thus: w+ and w would represent two alleles for a gene where w+ is the dominant wild type allele and w is the recessive mutant allele.

Another convention exists for what are called co-dominant alleles.

 

The Law of Segregation

Mendel observed that 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 (eg seed colour) and ignore all the others (such as flower colour, seed shape etc) this is called monohybrid inheritance.

monohybrid

 

Mendel worked this out in 1866. Though he no doubt understood fertilisation, meiosis was not observed until 30 years later

 

Homozygotes and heterozygotes

  • 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.

The zygote is the fertilised egg cell. So homozygous indicates that this individual received two identical alleles from their parents at fertilisation.

In our example:

homo and heterozygous

In human genetics heterozygotes who have a dominant and a recessive alleles are called carriers. They are carrying a recessive allele without expressing it. Many genetic diseases are caused by recessive alleles.

 

Genetic diagrams

When explaining a cross between two individuals we use a genetic diagram. There are certain conventions which you must use.

genetic diagram

genetic proportions

 

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