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It is important to remember that all body cells (in situations that you are likely to come across) will be diploid. In humans (except in red blood cells) there are 46 chromosomes in all body cells - 23 pairs. Each pair of chromosomes is numbered and has its own particular genes.

In gametogenesis, (the production of sperm and eggs) this number is reduced to 23.

Only one chromosome of a pair can be inherited. Gametes are haploid. Which chromosome of the pair is inherited is random (see Independent Assortment in Meiosis). When working out the chances of an offspring inheriting a particular genotype, this fact must be remembered.

When studying genetics, the following conventions are used:

P is used as shorthand for the parent generation.
F1 is used for their offspring.
F2 is used if the offspring (F1) are crossed.

Capital letters are used to denote a dominant allele.

Lower case letters are used to denote a recessive allele.

For example:


Drosophila (fruit flies) can be either straight-winged or curved-winged, this characteristic is controlled by one pair of genes. When straight- and curved-winged are bred together, all the offspring are straight-winged.

This means that straight-winged is dominant and curved-winged is recessive.


'straight' allele = S
'curved' allele = s

Since the allele for curved wings is recessive, if a fly has curved wings, it must have 2 alleles for curved wings = ss

Since the allele for straight wings is dominant, a straight winged fly will have either SS alleles or Ss alleles.

Question 1:

What would be the result in the F1 generation of crossing a homozygous straight-winged fly with a curved-wing fly?

The easiest way to show what the offspring will look like, is to work through this sequence:

Question 1

All F1 offspring are Ss.

This means that for each offspring there is a 100% probability that they will be Ss and therefore straight-winged.

Question 2:

What would be the result in the F2 generation of crossing 2 of the F1 flies?

Question 2

This means that for each offspring there is a 75% probability that they will be straight-winged and 25% probability that they will be curved-winged.

Another way of saying this is ratio of straight : curved is 3 : 1.

It does not necessarily mean that out of 4 offspring, 3 would definitely be straight-winged and 1 would be curved-winged. It is possible, though unlikely, that all offspring could be curved or straight-winged.

Question 3:

How would you determine the genotype of any unknown straight-winged fly?

To find out whether a genotype is homozygous dominant (SS) or heterozygous (Ss), a test-cross needs to be done since you cannot tell the genotype by looking at the fly.

The unknown is bred with a known. The only phenotype that gives a known genotype is homozygous recessive (ss).

If the fly was SS:

Question 3

All offspring, no matter how many, would be straight-winged.

If the fly was Ss:

Question 3

Some offspring (it should be 50%), will have curved wings.

In the previous case, there were only 2 alleles for one gene. In the case of the ABO blood grouping, there are 3 alleles for one gene and in this situation they are written a little differently:

i : protein is produced but it is not antigenic - this allele is recessive

IA : protein with antigen A made - this allele is co-dominant

IB : protein with antigen B made - this allele is co-dominant

Blood group (phenotype): Possible genotype:
A IA IA or IA i
B IB IB or IB i
O i i

This is where the inheritance of two characteristics is studied.

In this case we will look at a case where the genes are on separate chromosomes, the alleles are not linked (they are not necessarily inherited together).

In a certain variety of rabbits, grey coat colour is dominant over white, and short hair is dominant over long.


A breeder has homozygous long-haired white rabbits and homozygous short-haired grey rabbits.

Short = S
Long = s
Grey = G
White = g

1. What would be the ratio of offspring in the F1 generation?


Offspring genotypes: all are SsGg.

Offspring pheotypes: all are short-haired grey rabbits.

2. If 2 offspring were bred together, what would the ratio of offspring be in the F2 generation?


F2 Punnett Square:

  SG Sg sG sg
Sg SSGg SSgg SsGg SSgg
sG SsGG SsGg ssGG ssGg
sg SsGg Ssgg ssGg ssgg

Ratio of genotypes: Short grey Long grey Short white Long white
Ratio of phenotypes: 9 3 3 1