A = dominant allele for albinisma = recessive allele for fewer pigments/albinism

What is the probability of the offspring developing albinism?

Punnet squares

Steps to remember…

1.Choose a letter2.Identify parents genotypes3.Identify gametes parents could produce4.Draw punnet square5.Calculate chances

These are all mono-hybrid crosses. They show one trait.

Multiple alleles

With two alleles, three different genotypes are possible producing two different phenotypes. This is the simplified version.

In blood types, there are three alleles for the same gene – ABO

There are four possible Human blood types – A, B, AB, O. These can produce six genotypes

The gene for blood type is I (or Ii) The four blood types are written IA, IB, i. IAB

A and B are co-dominant

Phenotype Group A Group BX

Genotype IAIA IBIB

Gametes IA IB

IAIBF1 genotype

F1 phenotype Group AB

Phenotype Group A X

Genotype IAi

Gametes IA IB

F1 genotype?

F1 phenotype?

Group B

IBi

i i

IB i

IA IAIB IAi

i IBi ii

The six genotypes are:

IAIA, IAi = blood type A

IBIB,IBi = blood type B

IAIB = blood type AB

ii = blood type O

How to test for blood types?

All three genes (A, B and O) cause the production of glycoprotein (inside the membrane of RBC)

IA adds acetyl-galactosamine to the glycoprotein. Only people with A blood have this protein, therefore if someone who does not have IA is exposed to the altered glycoprotein, they produce anti-A antibodies.

IB adds galactose, Only people with B blood have this protein. therefore if someone who does not have IB is exposed to the altered glycoprotein, they produce anti-B antibodies.

IAIB causes both proteins to be produced. As a results neither antibodies are produced when someone with IAIB blood is exposed to either protein (their bodies recognise the proteins).

i is recessive, basic glycoprotein is produced.

Sex chromosomes

XX or XY

X is longer than Y. All female eggs contain one X chromosome. In sperm, 50% contain X, 50% contain

Y.

Alleles which have their loci on the X or Y chromosomes are sex linked. E.g. colour blindness &

haemophilia

What are the main effects of these two conditions?

Alleles & genotypesXb – recessive colour blindness

XB – colour vision

Xh - haemophilia

XH – blood clotting

Construct a monohybrid cross for eachAssume each genotype is heterozygous

XBXB – phenotype, non-affected female

XBXb – phenotype, carrier female

XbXb – phenotype, affected female

XBY – phenotype, non-affect male

XbY – phenotype, affected male

Xb alleles are rare in human population, the likely hood of having two recessive alleles is very small.

This is why colour blindness mainly affects males.

Females can carry the allele – XBXb

Males cannot be carriers – XbY

This is the same for haemophillia – it affects the X chromosome

Dihybrid crosses & gene linkage

Mendel (yes again!) experimented with these traits in his pea experiments:

- Seed shape- Seed colour

True breeding = homozygous for desired traits.

R = round peasr = wrinkled peasY = yellow peasy = green peas

Complete the following table to show the genotypes:Parents phenotypes Round yellow Green wrinkled

Parents genotypes RRYY rryy

Parent gametes RY ry

F1 genotypes RrYy

F1 phenotype Round yellow

F1 = first generation

What would the offspring of the F1

generation look like?

RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

Round yellow peas x9

Round green peas x3

Wrinkled yellow peas x3

Wrinkled green peas x1

Mendels conclusions from his pea experiments were very close to these theoretical

probabilities.

This is unusual.

Statisticians are convinced that Mendel’s results are too close to exact ratios to be genuine. We shall never know how this came

about, but it offers an opportunity to discuss the need for scientists to be truthful about their results, whether it is right to discard results that do not fit a theory as Louis Pasteur is known

to have done, and the danger of publishing results only when they show statistically significant differences.

TOK: Reasons for Mendel’s theories not being accepted by the scientific community for a long time could be considered. Other cases of paradigm shifts taking a long time to be accepted could be considered. Ways in which individual scientists are most likely

to be able to convince the scientific community could be considered, and also the need always to consider the evidence

rather than the views of individual scientists, however distinguished.

All chromosome which are NOT sex chromosomes are called Autosomes

Humans have 22 pairs of autosomes and one pair of sex chromosomes

Some genes can be linked (found on the same chromosome).

Groups of inherited genes are called linkage groups. Applies to autosomes & sex chromosomes.

Linked genes are the exception to the law of independent assortment. Linked groups can give a much wider variety of ratios - not just 9:3:3:1