8. LOCATING GENES ALONG CHROMOSOMES

Question 1

1. What are Mendel’s “Hereditary Factors” known as nowadays?

Answer 1

• genes
• Mendel did not know about the existence of genes or
  chromosomes when he conducted his experiments

Question 2

2. What is a gene locus?

Answer 2

• this is the location of a particular gene on a
  chromosome

Question 3

3. What technique can we use to see the specific locus of a gene on a chromosome?

Answer 3

• we can tag the isolated chromosomes
• they would be tagged with a fluorescent dye
• this dye highlights the gene

Question 4

4. Provide 2 fundamental characteristics of genes?

Answer 4

1. Each gene is a long DNA molecule
   
   • it consists of several nucleotides
2. Each gene carrier information
   
   • for the synthesis of a specific protein
   • it carries this information in its nucleotide sequence

Question 5

5. How many genes does one chromosome have?

Answer 5

• every chromosome has over 1000 genes

Question 6

6. What are Homologous Chromosomes (Homologs)?

Answer 6

• these are the chromosome pairs found during
  Meiosis I
• they carry the same genes
• they may carry different alleles of those genes

Question 7

7. Where are the chromosomes that make up a Homologous Pair inherited from?

Answer 7

IN EACH PAIR:
- one homologous chromosome is inherited from the
father
(this is known as the paternal homolog)
- the other homologous chromosome is inherited from
the mother
(this is known as the maternal homolog)

THIS MEANS THAT:
- one allele is inherited from the father
- one allele is inherited from the mother

Question 8

8. What does the Chromosomal Theory of Inheritance state?

Answer 8

1. Genes have specific positions on chromosomes
   
   • these are known as their Loci
2. Chromosomes undergo segregation
   
   • this is also known as Chromosomal Separation
   • this separation during meiosis accounts for Mendel’s
     Laws of Segregation and Independent Assortment
3. Chromosomes undergo Independent Assortment

Question 9

9. What does Mendel’s Law of Segregation state?

Answer 9

• the two alleles for each gene separate during gamete
  formation

Question 10

10. What does Mendel’s law of Independent Assortment state?

Answer 10

• the alleles of genes on Non-Homologous
  chromosomes will assort independently
• this happens during gamete formation

NB:
- this law does not apply to linked genes
- it does not apply to genes on the same chromosome
- these genes are usually positioned very close to each
other

Question 11

11. Are genes inherited?

Answer 11

• no
• the chromosomes that carry the genes are inherited

Question 12

12. Who was Thomas Hunt Morgan?

Answer 12

• he came up with the Chromosomal Theory of
  Inheritance
• this states that genes are located on chromosomes
• these chromosomes are then inherited

Question 13

13. What did Thomas Hunt Morgan run his experiments on?

Answer 13

• fruit flies
• these experiments provided evidence that genes are
  located on chromosomes
• genes can also be referred to as:
  “Mendel’s Heritable Characteristics”

Question 14

14. Why are Fruit Flies a convenient organism for Genetic Studies?

Answer 14

• they breed at a high rate
• a generation can be bred every 2 weeks
• they have only 4 pairs of chromosomes
  (this makes the genetics very easy to follow)

Question 15

15. What exactly did Thomas Hunt Morgan observe with relation to the fruit flies?

Answer 15

• Morgan observed wild type phenotypes that were
  common in the fruit fly populations
• these were considered the normal phenotypes
• Morgan also observed alternative traits
• these were considered mutant phenotypes

Question 16

16. Thomas Hunt Morgan mated white-eyed male flies with red-eyed female flies.
    What were his results?

Answer 16

• White eyes are considered a mutant trait
• Red eyes are considered the normal, wild trait
• the Fist Filial (F1) Generation all had red eyes
• the Second Filial (F2) Generation showed a 3:1 red eye
  to white eye ratio
• only the males had white eyes

Question 17

17. What did Thomas Hunt Morgan determine through his experiment that focused on fly eye colour?

Answer 17

• the allele for Red eyes was the dominant allele
• the White-eyes mutant allele must be located on the X
  Chromosome
• this means that eye colour for these flies is a X-linked
  condition

NB:
- this experiment supported the Chromosomal Theory
of Inheritance

Question 18

18. What can be found in humans and other mammals with regards to genetics?

Answer 18

• there is a Chromosomal Basis of Sex Determination
• there are 2 different types of sex chromosomes
• there is a larger X chromosome
• there is another smaller Y chromosome

Question 19

19. Are X and Y chromosomes the same?

Answer 19

• NO
• they are different
• they are non-homologous
• ONLY THE ENDS OF THE Y CHROMOSOME:
  
  • have regions that are homologous with the X
    chromosome

Question 20

20. How many genes do the X and the Y chromosomes have in common?

Answer 20

• 18 genes

Question 21

21. What is the genotype for females?

Answer 21

• XX

Question 22

22. What is the genotype for males?

Answer 22

• XY

Question 23

23. What does each ovum contain?

Answer 23

• each ovum contains only an X chromosome

Question 24

24. What does each sperm contain?

Answer 24

• a sperm can contain either an X or a Y chromosome
• 50% of sperm carry an X chromosome
• 50% of sperm carry a Y chromosome

Question 25

25. What can be said about the genes found on the sex chromosomes?

Answer 25

- sex chromosomes have genes for many characters - these characters are unrelated to sex

Question 26

26. What is a sex-linked gene?

Answer 26

- this is a gene located on either X chromosome

Question 27

27. What does the term "Sex-Linked" refer to in humans?

Answer 27

- it refers to a gene on the X Chromosome - this is known as an X-Linked gene - this is because the Y Chromosome carrier very few genes - this means that there are very few Y linked disorders present

Question 28

28. How many genes does an X chromosome roughly have?

Answer 28

- 1 100 genes

Question 29

29. How many genes does a Y chromosome have?

Answer 29

- 78 genes

Question 30

30. What do Sex-Linked genes follow?

Answer 30

- they follow specific patterns of inheritance

Question 31

31. What genotype is necessary for a recessive Sex-Linked Trait to be expressed?

Answer 31

A FEMALE: - needs 2 copies of the recessive allele A MALE: - needs only 1 copy of the allele - this is because the Y Chromosome cannot counteract the mutant gene

Question 32

32. In which gender are sex-linked recessive disorders much more common?

Answer 32

- males

Question 33

33. Link 3 examples of X-Linked Recessive Disorders?

Answer 33

1. Haemophilia 2. Colour Blindness 3. Duchenne Muscular Dystrophy

Question 34

34. What three types of Sex-Linked Disorders can an individual have?

Answer 34

1. X-Linked Recessive Disorders 2. X-Linked Dominant Disorders 3. Y-Linked Disorders