1. Mendelian Genetics
Revising the Basics.

2. Glossary Gene Recessive Dominant Phenotype Genotype Alleles Homozygous
Heterozygous
Patrial generation
First filial generation
Second filial generation
Inheritance
Punnet square
trait

3. Mendel’s Law of Segregation
The two members of each gene pair must separate when they form gametes, with only one allele going into each gamete.
At fertilisation there is random uniting of gametes resulting in ne pairing combinations.

4. Mendel’s Law of Independent Assortment
Traits are inherited independently. Alleles maintain their identity and are not changed by their association with other alleles.

5. Monohybrid Inheritance
This genetic cross considers only one characteristic.
E.g. Harry Potter Genetics
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6. Test or Back Cross
This is done by crossing an organism showing a dominant trait with a homozygous recessive organism.
If all offspring show the dominanst trait then one of the parents is homozygous dominant.
If you get a 1:1 ratio then the parent was heterozygous.

7. Incomplete Dominance
One allele is not completely dominant over another.
E.g. in snapdragons a red flower crossed with a white flower gives a heterozygous pink flower in the F1 generation

8. Codominance
This is the same as incomplete dominance except the heterozygote shows both the other traits.
E.g. a black spotted cat crossed with an orange spotted cat gives a cat with black and orange spots.

9. Overdominance
The phenotype of the heterozygote does not always have to be in the range of the phenotype of the 2 homozygotes.
E.g. if one homozygote is tall and the other is short, the heterozygote is even taller than the homozygote.

10. Lethal Genes
In this example the homozygous dominant is lethal to the organism, so it dies as an embryo.
E.g. A roan horse (brown with white spots) is crossed with another roan horse to give 2 roan offspring and 1 normal coloured horse. A homozygous dominant foetus is miscarried.

11. Multiple Alleles
This is where there are more than 2 alleles that can fit at a locus on a chromosome. Where there are 3 or more alleles for a single trait, this is called multiple alleles.
E.g. the ABO blood typing
IA; IB; Io also known as i

12. Dihybrid Inheritance
This is a cross between organisms with 2 different characteristics.
Mendel did a cross with homozygous smooth (S), yellow (Y) seeds and homozygous green (s), wrinkled (y) seeds
9:3:3:1 ratio

13. Dihybrid Test or Back Cross.
The genotype of the organism showing the dominant trait can be determined by crossing it with an organism which os recessive for both characteristics.
E.g. AaBb x aabb
E.g.AABb x aabb

14. Practise Questions
Pg 162 Designs of Life
Qu 1-3

15. Gene-gene Interactions
Epistasis
Most traits are controlled by several genes.
e.g. a biochemical pathway with 2 steps controlled by several genes.
A B C
Gene 1
Gene 2
Enzyme 1
Enzyme 2

16. Epistasis
This is the type of gene interaction where the expression of the gene on one locus is affected by the alleles present at another gene locus.
There are different forms of epistasis
Collaboration
Supplementary genes
Complementary genes

17. Collaboration
This is where one characteristic is controlled by 2 or more pairs of alleles. i.e. 2 genes interact to produce a novel phenotype.
E.g. comb shape in poultry.
there are 4 possibilities: Pea, Walnut, Rose and single. Pg 163 txt or 109 wkbk

18. Supplementary Genes
This refers to the masking of a characteristic determined by 1 pair of alleles, by the action of another pair of alleles.
Pg 163/164 txt or 110 wkbk

19. Fur Colour in Mice
E.g. the gene for mouse fur colour. One gene controls the laying-down of melanin in the skin. The dominant form of the gene results in lots of melanin giving black and the recessive results in less melanin giving brown colour. Another gene allows the deposition of melanin when present in its dominant form but not its recessive.

20. Complementary Genes
2 dominant alleles from 2 different genes are needed to get any colour i.e. they are complementary.
Pg 164 txt or pg 110 wkbk

21. Modifier Genes
It is probable that no inherited characteristic is controlled completely by just one pair of genes; their expression is usually modified in some way by another gene at a different loci.
E.g. eye colour in humans. Pg 164 txt

22. To Do
Pg 111/112 work book – self check.