Section 2

Genetics Since Mendel

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.

Four o’clock plants

Incomplete Dominance

when the offspring of two homozygous parents show an

intermediate phenotype.

Incomplete Dominance

Inc

om

plet

e D

om

ina

nc

e

Inc

om

plet

e D

om

ina

nc

e

Mendel studied traits in peas that were controlled by just

2 alleles.

Many traits are controlled by

more than 2 alleles.

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.

Multiple Alleles

Multiple Alleles

N D Q

N NN DN QN

D ND DD QD

Q NQ DQ QQ

X

X X

6 possible outcomes

Multiple Alleles

Blood types is an example of a trait that

has multiple alleles.

The alleles are A, B, O.

Blood types

A B O

A AA AB AO

B AB BB BO

O AO BO OO

A and B are dominant

O is recessive

Genotype: AB Phenotype: AB

Genotype: AA or AO Phenotype: A

Genotype: BB or BO Phenotype: B

Genotype: OO Phenotype: O

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.

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.

polygenic inheritance

Other examples of polygenic inheritance are: ~grain colour in wheat

~milk production in cows~egg production in chickens

polygenetic inheritance

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.

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.

CancerSome people have genes that make them at risk

for developing certain cancers. External environmental factors can influence whether they

get cancer.

Impact of the Environment

Impact of the EnvironmentHimalayan 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 coloured. Himalayan Rabbit

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.

CHromosomal Disorders

Incorrect number of chromosomes is inherited

Mistake in meiosis

Example: Down syndrome

CHromosomal Disorders

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

Cystic Fibrosis

Gender Determination

Gender for humans is determined by the

chromosomes in the 23 pair.

XX results in a female

XY results in a male

Gender Determination

X X

X XX XX

Y XY XY

egg MOM

DAD

sperm

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.

Sex-Linked Disorders

Red-green colour blindness is a recessive sex-linked disorder.

It occurs on the X chromosome.

What Do they see??

Let’s take the Test

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.

Pedigrees Trace Traits

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.

Pedigrees are also used when breeding animals and plants.

Pedigrees