1. the study of hereditary variation
Genetics
the study of hereditary variation
© 2016 Paul Billiet ODWS

2. 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).
© 2016 Paul Billiet ODWS

3. 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.
© 2016 Paul Billiet ODWS

4. 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
© 2016 Paul Billiet ODWS

5. Studying heredity The Neolithic revolution led to folk knowledge
Breeding from the animals or plants
Gregor Mendel.
© 2016 Paul Billiet ODWS

6. 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.
© 2016 Paul Billiet ODWS

7. 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
© 2016 Paul Billiet ODWS

8. Mendel’s breeding experiments
Taking one character only as an example, seed colour
Parents (P)
First generation (F1)
Female sex cells
from a yellow-seeded plant
Male sex cells in pollen from a green-seeded plant
Cross fertilised (crossed)
All seeds produced turned out yellow
© 2016 Paul Billiet ODWS

9. 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.
© 2016 Paul Billiet ODWS

10. 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).
© 2016 Paul Billiet ODWS

11. 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.
© 2016 Paul Billiet ODWS

12. Selfing Yellow seed producing plants Selfed Yellow seeds Green seeds
6022
Green seeds
2001
About 75% show the dominant trait
About 25% show the recessive trait
First generation (F1)
Second generation (F2)
© 2016 Paul Billiet ODWS

13. Selfing Selfed 33% produce yellow seeds only
All green seeds
Pure breeding
Selfed
66% produce a mixture of yellow & green seeds
33% produce yellow seeds only
Yellow seeds 6022
Green seeds 2001
Second generation (F2)
Third generation (F3)
© 2016 Paul Billiet ODWS

14. Pure breeding
Those plants that only produce one type are called pure breeding (or true breeding).
© 2016 Paul Billiet ODWS

15. 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 nor diluted to give intermediates.
The gene is the unit of hereditary information
© 2016 Paul Billiet ODWS

16. Genes and variation
A character is controlled by a gene that may come in different types called allelomorphs (meaning “other forms”) or alleles
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.
© 2016 Paul Billiet ODWS

17. 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).
© 2016 Paul Billiet ODWS

18. Monohybrid inheritance
Consistent patterns revealed in the behaviour of alleles
So they are predictable
The predictions can be tested by breeding experiments
Therefore 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.
© 2016 Paul Billiet ODWS

19. 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%)
© 2016 Paul Billiet ODWS

20. Mendel and meiosis Mendel worked this out in 1866
Though he no doubt understood fertilisation, meiosis was not observed until 30 years later.
© 2016 Paul Billiet ODWS

21. Meiosis & Mendel Meiosis 1: Anaphase 1
Maternal and paternal chromosomes segregate (pulled separate on the spindle)
They move to opposite poles.
© 2016 Paul Billiet ODWS

22. 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
© 2016 Paul Billiet ODWS

23. 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.
© 2016 Paul Billiet ODWS

24. In human genetics heterozygotes are called carriers
Genotypes
Phenotypes YY
Homozygous
Yellow
Pure breeding Yy
Heterozygous yy
Green
In human genetics heterozygotes are called carriers
They are carrying a recessive allele without expressing it
Many genetic diseases are caused by recessive alleles.
© 2016 Paul Billiet ODWS

25. Genetic diagrams P Phenotypes Yellow seed X Green seed Genotypes YY yy
Gametes Y y F1 Yy
Yellow
Proportions
100%
© 2016 Paul Billiet ODWS

26. Genetic diagrams F1 Genotypes Yy Phenotypes Yellow Proportions 100%
Gametes Y y
Where there are several possible gametes a Punnett square should be used
Selfed F2
Genotypes: Y y YY Yy yy
Phenotypes:
Yellow
Green
Proportions:
75%
25%
© 2016 Paul Billiet ODWS