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Home > Biology > Options > Genetics: the code broken? > Genetics: The Code Broken: 2. Variability within a trait

9.7 Option – Genetics: The Code Broken? : 2. Variability within a trait

Syllabus reference (October 2002 version)
2. Multiple alleles and polygenic inheritance provide further variability within a trait	Students learn to: give examples of characteristics determined by multiple alleles in an organism other than humans   compare the inheritance of the ABO and Rhesus blood groups   define what is meant by polygenic inheritance and describe one example of polygenic inheritance in humans or another organism.   outline the use of highly variable genes for DNA fingerprinting of forensic samples, for paternity testing and for determining the pedigree of animals	Students: solve problems to predict the inheritance patterns of ABO blood groups and the Rhesus factor   process information from secondary sources to identify and describe one example of polygenic inheritance

Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

Prior learning: Stage 4-5 Syllabus, 4.8.3(b) 5.8.2(a, b, c and d).
H.S.C. module 9.3 (subsections 3 and 5).

Background information: In preparation for this module it would be advisable to revise 3. and 4. of Blueprint of Life.

give examples of characteristics determined by multiple alleles in an organism other than humans

• Alleles are genes for the same characteristic found in corresponding loci (positions) on homologous (matching) chromosomes. These genes can be the same, resulting in a homozygous genotype (eg. TT) or different, resulting in a heterozygous genotype (eg. Tt).
   
• In the case of multiple alleles, there are more than two possible alleles in a population, however, only two ever occur in any one individual.
  Examples of multiple alleles in organisms other than humans include coat colour in mice, eye colour in fruit flies, leaf patterns in white clover plants.

 

solve problems to predict the inheritance patterns of ABO blood groups and the Rhesus factor

compare the inheritance of the ABO and Rhesus blood groups

These two dot points are covered in the information below.

• In humans an example of multiple alleles are those of the ABO blood groups.

There are four ABO blood groups in humans: A, B, AB and O. The genes A and B are both dominant over the gene o, but A and B are co-dominant when both present in the same individual resulting in blood group AB. The genes for the ABO blood groups are not sex-linked.

The ABO blood groups are named from the antigens that they carry on red blood cells.

Phenotype or Blood Group	Genotype	Antigens present
A	AA	A
A	Ao	A
B	BB	B
B	Bo	B
AB	AB	A and B
O	oo	none

Examples of problems to predict inheritance of ABO blood groups:

1. If two parents, both with blood group O, have children, what are the possible blood groups of the children?

2. gametes	o	o
   o	oo	oo
   o	oo	oo

   All children will have O type blood.

3. If two parents of blood group O and AB have children, what is the possibility of having an AB type child?

4. gametes	A	B
   o	Ao	Bo
   o	Ao	Bo

   There is a 50% chance of their children having A type blood and a 50% chance of them having B type blood. The possibility of having an AB type child is 0%.

5. If one parent has A type blood and the other has B type blood, what are the possible blood types of their children?

1. If the parents are both homozygous for the blood groups A and B:

   gametes	A	A
   B	AB	AB
   B	AB	AB

   100% of their children will have AB blood type.

2. If either parent is heterozygous for the blood group A:

   gametes	A	o
   B	AB	Bo
   B	AB	Bo

   There is a 50% chance of their children having AB blood and 50% chance of B type blood.

3. If either parent is heterozygous for the blood group B:

   gametes	A	A
   B	AB	AB
   o	Ao	Ao

   There is a 50% chance of their children having AB blood and 50% chance of A type blood.

4. If one parent is heterozygous for blood group A and one for blood group B :

   gametes	A	o
   B	AB	Bo
   o	Ao	oo

   There is a 25% chance of their children having each of the blood groups AB, A, B and O.

• A separate blood group classification system in humans is the rhesus or Rh factor, first found in Rhesus monkeys. The Rh blood groups are independent to the ABO blood groups. There are at least eight alleles controlling Rh antigens found on the surface of red blood cells. When the Rh factor is present, the blood group is Rh+. When the Rh factor is absent, the blood group is Rh-.

1. If one parent is B Rh+ and the other is B Rh- and their children have B Rh+, and O Rh+ blood, what are the genotypes of the parents?

   gametes	B-	o-
   B+	B+B-	B+o-
   o+	B-o+	o+o-

   The parents must have Bo Rh+ and Bo Rh- genotypes.

2. Two newborn babies were accidentally mixed up in hospital. To determine who were the parents, the blood types of the babies and the parents were taken:
   Baby 1 – O,
   Baby 2 –A,
   Mrs Brown – B,
   Mr Brown – AB,
   Mrs Smith – B,
   Mr Smith – B.

3. From this information, it can be concluded that:

   1. the babies do not seem to belong to either family
   2. Baby 1 may be the Brown's
   3. Baby 2 may be the Smith's or the Brown's
   4. Baby 1 is a Smith and cannot be a Brown.

   Answer: Baby 1 – oo, Baby 2 – Ao or AA, Mrs Smith – Bo or BB, Mr Smith – Bo or BB

   Mrs Brown BB or Bo, Mr Brown – AB

   Smiths :	Browns:
   (i) BB x BB = only BB (B)	(i) BB x AB = BB (B) or AB
   (ii) BB x Bo = BB or Bo (B)	(ii) Bo x AB = AB or Ao (A)
   or (iii)Bo x Bo = BB, Bo (B)	or (iii) oo (O) BB, Bo (B)

   Therefore,
   Baby 1 could be a Smith but definitely not a Brown;
   Baby 2 could be a Brown but definitely not a Smith.

   Answer: D

process information from secondary sources to identify and describe one example of polygenic inheritance

• Use the information below and/or look in a biology or genetics textbook and look for the words ‘'polygenic inheritance' in the index. You could also use the Internet and go to a search engine like ‘'Google’'.
• Compare the information you have obtained and use the material that is clearest and defines the term best for you.
• Use the information to identify and describe one example of polygenic inheritance.

define what is meant by polygenic inheritance and describe one example of polygenic inheritance in humans or another organism

• Wherever graduations in phenotype occur for a characteristic (rather than one or two definite phenotypes), the characteristic is polygenic and is controlled by more than one pair of genes.
   
• One example is height in humans – there are a large number of phenotypes, each differing slightly from the next and forming a graduated series. Some variation in height in humans is due to environmental factors such as diet, exercise and disease. However, if the environmental factors were constant, there would still be continuous variation in height due to such things as bone formation and hormone levels. The more genes that control a characteristic, the more possible gene combinations exist and the more phenotypes.
   
• Other examples of polygenic inheritance include: skin colour in humans, colour in wheat kernels, egg weight in poultry, fleece weight in sheep.

outline the use of highly variable genes for DNA fingerprinting of forensic samples, for paternity testing and for determining the pedigree of animals

• DNA fingerprinting involves comparing DNA patterns of different fragments from different individuals. Identical patterns of fragment size suggest identical sources of DNA. If enough identical DNA sequences are found, then identification can be determined. Skin, hair or blood cells are frequently used. This is used in forensic science to match blood from suspects at a crime scene, to determine the paternity of a baby, and to determine the pedigree or lineage of individual animals where more than one mating is made.
   
• By selecting desirable genes rather than phenotypes, animals and plants can be produced with the preferred characteristics. Examples of these characteristics include disease resistance in certain crops and fish, tolerance of extreme environmental conditions, meat from cattle with low fat.