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Home > Biology > Options > Genetics: the code broken? > Genetics: The Code Broken: 3. Inheritance of genes

9.7 Option – Genetics: The Code Broken? : 3. Inheritance of genes

Syllabus reference (October 2002 version)
3. Studies of offspring reflect the inheritance of genes on different chromosomes and genes on the same chromosomes	Students learn to: use the terms ‘diploid’ and ‘haploid’ to describe somatic and gametic cells   describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations   predict the difference in inheritance patterns if two genes are linked   explain how cross-breeding experiments can identify the relative position of linked genes   discuss the role of chromosome mapping in identifying relationships between species	Students: process information from secondary sources to analyse the outcome of dihybrid crosses when both traits are inherited independently and when they are linked   perform a first-hand investigation to model linkage

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

[Edit:12Jun09]

Prior learning: Stages 4 and 5 Science, 5.8.2 c) and d)

HSC 9.3.3 and 4

use the terms ‘diploid’ and ‘haploid’ to describe somatic and gametic cells

• Normal body cells are called somatic cells. In eukaryotic cells, the nucleus of somatic cells will contain the diploid number of chromosomes (2N).
   
• Gametic cells or gametes such as sperm and ova contain half the diploid number of chromosomes, or haploid number (N).
   
• In humans, the diploid number is 46 (found in such cells as muscles, bone, skin etc.) and the haploid number is 23 (found in gametic cells such as sperm and ova). On fertilisation, two haploid cells (sperm + ovum) unite to form a zygote with the diploid number of 46 chromosomes.

process information from secondary sources to analyse the outcome of dihybrid crosses when both traits are inherited independently and when they are linked

• Your teacher will give you information on dihybrid crosses when both traits are inherited independently. Before you start this activity it would be good to go back to your work on the core topic, Blueprint of Life and revise monohybrid crosses.

You could practise doing some dihybrid crosses when both traits are independent at Dihybrid Cross Problem Set The Biology Project, Mendelian Genetics, University of Arizona

• Once you have a good understanding of dihybrid crosses when traits are inherited independently (that is the genes for these traits are located on different chromosomes), you should then investigate inheritance when both traits are linked, that is, the genes for the traits are both located on the same chromosome.

A web site that could be useful on linkage, Dr Paul’s Virtual Biology Show, Johnson County Community College, Overland Park, Kansas, USA

• When you have enough data from both situations analyse the results for each. Are they both equally predictable? Have you taken into account crossing over with the linked characteristics? Will this make a difference?

perform a first-hand investigation to model linkage

• To perform the investigation you need to decide how to model linkage. It will probably be most effective if you model chromosomes that are going through meiosis to produce gametes (egg or sperm). You may decide to use material such as pipe cleaners, strips of coloured cardboard or wool to represent the chromosomes that carry the genes that are linked. Decide what material to use, what colours should be used, how you will join the ‘chromosomes’ at the centromere and what material to use to represent the matching genes or alleles on the chromosomes.
   
• You could have one group model linkage and another group modelling meiotic division when there is only one gene on the chromosome that is being considered. In this case you wouldn’t model crossing over but you might decide to do so with the linked genes.
   
• Once you have built your models, including the alleles, actually carry out the processes that occur. Drawing labelled diagrams at various stages will help cement the process in your memory. Decide how many drawings you want to do.
   
• When you have completed the processes and the labelled drawings, each group could demonstrate what they have done to the rest of the class and then could answer questions.
   
• Were there any problems with your model? If so what could you do to improve it so the other students will understand it better?

describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations

• Using Mendel’s examples of pea plants:
  Tall stem is dominant over short stem and round seed is dominant over wrinkled seed.
  T = tall stem, t = short stem; R = round seed, r = wrinkled seed.
  F₁ = first (filial) generation F₂ = second (filial) generation.

a) Pure breeding, double homozygous tall stem with round seed plant is crossed with double homozygous short stem with wrinkled seed:
Parents: TTRR x ttrr
Possible gametes: TR, TR, TR, TR and tr, tr, tr, tr
F₁ : genotypes: all TtRr
phenotypes: all tall stem, round seed

Breed two F₁ individuals together:
F₁: TtRr x TtRr
Possible gametes: TR, Tr, tR, tr and TR, Tr, tR, tr
F₂:

gametes	TR	Tr	tR	tr
TR	TTRR	TTRr	TtRR	TtRr
Tr	TTRr	TTrr	TtRr	Ttrr
tR	TtRR	TtRr	ttRR	ttRr
tr	TtRr	Ttrr	ttRr	ttrr

 

Genotypes	Number	Phenotypes
TTRR	1	Tall stem, round seed
TTRr	2	Tall stem, round seed
TtRR	2	Tall stem, round seed
TtRr	4	Tall stem, round seed
TTrr	1	Tall stem, wrinkled seed
Ttrr	2	Tall stem, wrinkled seed
ttRR	1	Short stem, round seed
ttRr	2	Short stem, round seed
ttrr	1	Short stem, wrinkled seed

The phenotypic ratio of F₂ individuals is:

9 tall round : 3 tall wrinkled : 3 short round : 1 short wrinkled

According to Mendel, because chromosomes segregate independently and all genes are inherited independently, there is an equal chance of the four different types of possible gametes being produced to result in this outcome.

b) Parents: TtRr (tall round) x ttrr (short wrinkled)
Possible gametes: TR, Tr, tR, tr tr

F₁:

gametes	TR	Tr	tR	tr
tr	TtRr	Ttrr	ttRr	ttrr

25% TtRr : 25% Ttrr : 25% ttRr : 25% ttrr

predict the difference in inheritance patterns if two genes are linked

• Genes found on the same chromosome are usually inherited together and are said to be linked. Linked genes may be separated during meiosis if the process of crossing over occurs.
   
• Linked genes do not show the same pattern of inheritance as those that sort independently.
   
• If genes E and F are not linked and E is dominant over e and F is dominant over f:
  Parents: EeFf x EeFf
  Possible gametes: EF, Ef, eF, ef
  F₁: 9EF : 3Ef : 3eF : 1ef
   
• If genes E and F are linked and E is dominant over e and F is dominant over f:
  Parents: EeFf x EeFf
  Possible gametes: EF, ef EF, ef
  F₁:

	EF	ef
EF	EEFF	EeFf
ef	EeFf	eeff

producing a ratio of 3 EF : 1ef

All of Mendel’s results were for genes that were inherited independently of each other. This led to his law of independent assortment. Sutton and Boveri later explained this when they observed that chromosomes separate independently during meiosis. Mendel’s results would have been different if any of the traits that he studied had been linked.

Some results from crossing linked genes do not show the exact predicted results as some genes undergo recombination. This recombining of certain genes occurs during meiosis when homologous chromosomes have undergone DNA replication and pair up. At this time, crossing over may occur where sections of chromosomes break off at a point and then recombine with the “wrong” piece of chromosome or swap.

A pair of homologous chromosomes contains the following linked genes A, B, C and D:

____AB_____CD_____
_________X_________
        AB           CD              A homologous pair of chromosomes after DNA replication. Crossing over occurs at X

____ab______cd________
________X____________
         ab         cd

If crossing over did not occur, the four potential chromosomes would be: ABCD, ABCD, abcd and abcd.

If crossing over of the two inner chromatids occurs at X, then the resulting chromosomes would be: ABCD, ABcd, abCD, and abcd. Crossing over only occurs between homologous chromosomes and is only an important factor in linked genes.

____AB_____CD________
______________________
        AB           cd

____ab_____CD_______
________X___________
        ab          cd

explain how cross-breeding experiments can identify the relative position of linked genes

• Cross-breeding experiments are used to study the relative positions of linked genes on homologous chromosomes.
   
• In 1911, Sturtevant, a student of Morgan, studied recombination of genes in fruit flies and concluded that when the distance between genes on chromosomes was large, there was a greater chance of crossing over occurring between these genes. From this, chromosome mapping was developed.
   
• A chromosome map shows the relative positions of genes on a chromosome. Alleles are genes situated on corresponding loci (positions) on homologous chromosomes. Crossing over results in the exact swapping of alleles from one of the chromosome pairs to another.
   
• If crossing over or recombination occurs 5% of the time, then the two genes are said to be 5 units apart on a chromosome. The greater the percentage of recombination, the larger the distance that the genes are apart.

Example:
Genes A, B and C are linked. In a cross breeding experiment, ABC was crossed with abc and the percentages of recombinations were observed.

AB x ab	Results: AB and ab 85% Ab and aB 15% (recombined through crossing over)
BC x bc	Results: BC and bc 95% Bc and bC 5% (recombinations)
AC x ac	Results: AC and ac 90% Ac and aC 10% (recombinations)
A and B are 15 units apart	A ________________15 _______________ B
B and C are 5 units apart	B ____5___C
A and C are 10 units apart	A ________10 _________C
The resulting map is:	A __________10__________C______5_____B

discuss the role of chromosome mapping in identifying relationships between species

• Once chromosome or genetic maps have been produced for two different species, degrees of genetic similarity can be established by using various computer programs. If the two species are very closely related, then their chromosome maps will be very similar.
   
• Chimpanzees and humans have DNA similarities of more than 98%. This suggests that humans and chimpanzees had a common ancestor in quite recent times.
   
• A chromosome map that has been produced by percentages of recombinations of genes during crossing over does not completely represent a whole chromosome. Chromosome maps based on recombinations can only be used to produce the relative positions of genes, not the absolute ones.