This document discusses several genetics concepts discovered after Mendel's experiments, including incomplete dominance when offspring show intermediate phenotypes, multiple alleles where traits are controlled by more than two alleles like blood types, and polygenic inheritance where many gene pairs act together to produce a wide range of traits like height. It also examines environmental impacts on gene expression, sex determination by chromosomes, genetic disorders, and using pedigrees to understand trait inheritance in families.

1. Section 2
Genetics Since Mendel

2. In the early 1900’s scientists repeated
Mendel’s experiments. For some plants
Mendel’s expected results proved true. In
other plant, though, the results were not the
same.

3. Incomplete Dominance
Four o’clock
plants

4. Incomplete
Dominance
when the offspring of two
homozygous parents show an
intermediate
phenotype.

5. Incomplete
Dominance

6. Incomplete
Dominance

7. Multiple Alleles
Mendel studied traits in peas
that were controlled by just
2 alleles.
Many traits are controlled by
more than
2 alleles.

8. Multiple Alleles
Imagine that there were only 3 types of coins
- nickels, dimes and quarters.
If every person could only have 2, then there
would be 6 possible outcomes.

9. Multiple Alleles
N D Q
N X
NN DN QN
D ND DD QD X
Q X
NQ DQ QQ
6 possible outcomes

10. Multiple Alleles
Blood types
is an example of a trait that
has multiple alleles.
The alleles are A, B, O.

11. Blood types
A and B are dominant
A B O
O is recessive
A AA AB AO Genotype: AB
Phenotype: AB
B AB BB BO
Genotype: AA or AO
Phenotype: A
O AO BO OO
Genotype: BB or BO
Phenotype: B
Genotype: OO
Phenotype: O

12. polygenic inheritance
Polygenic Inheritance
occurs when a group of gene
pairs acts together to
produce a trait.
The effects of many alleles
produces a wide range of
phenotypes.

13. polygenic inheritance
Examples: Eye colour, height, skin colour
3 to 6 gene pairs control your skin colour,
and even more control eye and hair colour.
WOW!!!!!!!! that’s amazing =)
It is therefore, hard to classify all the different
shades of eye/skin/hair colour.

14. polygenetic inheritance
Other examples of polygenic inheritance are:
~grain colour in wheat
~milk production in cows
~egg production in chickens

15. Impact of the Environment
Environment also plays a role in
the expression of traits.
Environment plays a role in
how some of your genes are
expressed.
Environmental influences can be
internal or external.

16. Impact of the Environment
Most male birds are more brightly
coloured than female.
Chemicals in their bodies determine whether the
gene for brightly coloured feathers is expressed.
Cancer
Some people have genes that make them at risk
for developing certain cancers. External
environmental factors can influence whether they
get cancer.

17. Impact of the Environment
Himalayan rabbits have
alleles for dark-coloured
fur, which is only
displayed at lower
temperatures.
The fur in areas further
from the rabbit’s body
heat i.e. nose, ears, feet,
are therefore darkly
Himalayan Rabbit coloured.

18. Human Genes and Mutations
Errors can occur in DNA when it is
copied inside of a cell.
Certain chemicals can cause
mutations in plants and animals.
X rays and radioactive substances
are other causes of some mutations.
Mutations are any permanent
change in the gene.

19. CHromosomal Disorders
Incorrect number of
chromosomes is inherited
Mistake in meiosis
Example: Down syndrome

20. CHromosomal Disorders

21. Recessive Genetic Disorders
Disorders caused by recessive genes. You must have
both recessive alleles in order to have the disorder.

22. Cystic Fibrosis

24. Gender Determination
Gender for humans is
determined by the
chromosomes in the
23 pair.
XX results in a female
XY results in a male

25. Gender Determination
egg
MOM
X X
s
pD X XX XX
eA
rD Y XY XY
m

26. Sex-Linked Disorders
Sex-linked gene
an allele that is on a sex
chromosome.
Some conditions that result from
inheriting a sex-linked gene are
called
sex-linked disorders.

27. Sex-Linked Disorders
Red-green colour blindness is a
recessive sex-linked disorder.
It occurs on the X chromosome.
Let’s take the Test
What Do they see??

29. Sex-Linked Disorders
Dominant sex-linked disorders are rare
and result when a person inherits at least
one dominant sex-linked allele.
Example: Vitamin D-resistant rickets. The
person cannot absorb adequate amounts
of phosphorus, therefore they have low
blood phosphorus levels, soft bones, and
poor teeth formation.

30. Pedigrees
Trace Traits

31. Using Pedigrees
Useful tool for geneticists to help
understand who has had a trait in a family
over several generations to determine its
pattern of inheritance.
They can determine if the trait is
dominant, recessive, sex-linked, or has
some other pattern of inheritance.
They can use this to determine the
probability of a child inheriting a disorder.

32. Pedigrees are also used when
breeding animals and plants.

33. Pedigrees