DNA and Inheritance

Question 1

Describe genes

Answer 1

1. DNA is a long list of instruction of how to put an organism together and make it work
2. Each separate gene in a DNA molecule is a chemical instruction that codes for a particular protein
3. Proteins are important because they control most processes in the body and they also determine inherited characteristics
4. By controlling the production of proteins, genes also control our inherited characteristics
5. There can be different versions of the same gene, which give different versions of a characteristic like blue or brown eyes. The different versions of the same gene are called alleles

Question 2

Describe a DNA molecule

Answer 2

1. DNA is a double helix
2. A DNA molecule is two strands coiled together to form a double helix, and the strands are linked by a series of paired bases
3. DNA is a type of nucleic acid

Question 3

What are the two strands held together by?

Answer 3

Four nitrogenous bases: 
Adenine (A)
Cytosine (C)
Guanine (G)
Thymine (T)

Question 4

What must Adenine be paired to? (complimentary base pairing)

Answer 4

Thymine

Question 5

What must Guanine be paired to? (complimentary base pairing)

Answer 5

Cytosine

Question 6

What is a gene?

Answer 6

A section of a molecule of DNA that codes for a specific protein

Question 7

What is an allele?

Answer 7

• An alternate version of the same gene

- These give rise to differences in inherited characteristics

Question 8

What does the nucleus of a cell contain?

Answer 8

Chromosomes on which genes are located

Question 9

What does the term dominant mean?

Answer 9

A dominant allele is when the characteristic will show up in the offspring even if only one of the alleles is inherited

Question 10

What does the term recessive mean?

Answer 10

A recessive allele, the characteristic will show up in the offspring only if both alleles are inherited

Question 11

What does it mean if an organism is homozygous for a characteristic?

Answer 11

• Both alleles are the same

- Two of the same allele for a specific gene e.g. AA or aa

Question 12

What does it mean if an organism is heterozygous for a characteristic?

Answer 12

• It has two different alleles

- Two different alleles for a specific gene e.g. Aa or aA

Question 13

What does the phenotype describe?

Answer 13

The physical characteristics of an organism, with respect to a particular pair of alleles (the genotype)

Question 14

What does the genotype describe?

Answer 14

The alleles that a cell or organism has for a particular feature

Question 15

When does codominance occur?

Answer 15

• When neither allele is dominant and both contribute to the appearance of the offspring
• E.G not blood group A or B, but blood group AB

Question 16

Describe genetic diagrams

Answer 16

1. In genetic diagrams, letters are used to represent genes
   - Dominant alleles are always shown with a capital letter (e.g. C)
   - Recessive alleles are always shown with a lower case letter (e.g. c)
2. If you are homozygous for a characteristic ypu have two alleles the same for that particular gene e.g. CC or cc
3. If you are heterozygous for a characteristic you have two different alleles for that particular gene, e.g. Cc
4. Your genotype is the alleles you have
5. Your phenotype is the characteristic the alleles produce

Question 17

Why can you not necessarily tell the difference between a homozygous and heterozygous plant?

Answer 17

They both have the same dominant allele and so they both have the same phenotype even though one may be AA when the other Aa

Question 18

Describe the patterns of monohybrid inhertitance using a genetic diagram

Answer 18

• Recessive gene b
• Dominant gene B
  1. For characteristic x the person must have genotype bb (so it must be homozygous for this characteristic)
  2. However a normal person could have two possible genotypes, BB (homozygous) and Bb (heterozygous), because the dominant allele B overrules the recessive on b
• In a diagram the lines show all the possible way the heterozygous parent’s alleles could combine, however only one of these possibilities would actually happen for any one offspring
  3. Therefore there is a 75% chance of having a normal person and a 25% chance of having a person with characteristic x, so 3:1 ratio of normal people : x people
  4. This ratio is called a phenotypic ration (because it is a ratio of different phenotypes)

Question 19

What does a monohybrid cross mean?

Answer 19

When you breed two organisms together to look at one characteristic

Question 20

What happens if you breed two homozygous hamsters together?

Answer 20

There is only one possible offspring you can end up with, for example breeding BB and bb hamsters can only give offspring with a Bb genotype and they all have a normal (the same phenotype)

Question 21

Describe how you do a genetic diagram with a Punnett square

Answer 21

1. Draw a 2x2 square
2. Put the possible gamete from one parent down the side and those from the other parent along the top
3. In each middle square, fill in the letter from the top and side that line up with that square
4. The pairs of letter in the middle show the possible combination of the gametes (ADD NOT TIMES)

Question 22

What is an example of a Punnett square

Answer 22

1. Huntington’s is a genetic disorder of the nervous system
2. The disorder is caused by a dominant allele ‘N’ and so can be inherited if just one parent carries the defective gene
3. The parent who carries the gene will be a sufferer too since the allele is dominant, but the symptoms do not start to appear until after the person is about 40
4. As the Punnett square shows, a person carrying the N allele has a 50% chance of passing it on to each of their children
5. There is also a 1:1 phenotypic ration in the children of the carrier: unaffected child

Question 23

How do you do a genetic diagram for codominance inheritance?

Answer 23

• Example:
  1. Your blood type is determined by two codominant alleles (A and B) and one recessive one (O (always written with capital)
  2. Blood can be type A (AA or AO genotype), type B (BB or BO genotype) or type O (OO genotype)
  3. As the Punnett square shows, for two people with type AB blood there is a 50% chance their children will be type AB, a 25% chance they will be type A and a 25% chance they will be type B

Question 24

Describe family pedigrees

Answer 24

1. The allele which causes cystic fibrosis is a recessive allele ‘f’ carried by about 1 person in 30
2. Because it s recessive, people with only one copy of the allele will not have the disorder, they are known as carriers
3. For a child to have the chance of inheriting the disorder, both parents must be either carriers or sufferers
4. As the diagram shows there is a one in four chance of a child having the disorder if both parents are carriers

Question 25

What is the sex of a person controlled by? What is different in males and females?

Answer 25

• The sex of a person is controlled by one pair of chromosomes
• Female: XX, XX combination causes female characteristics
• Male: XY, Y chromosome causes male characteristics
• With genetic diagram, 50:50 ration, but this is only a probability because if you have four kids they could all be boys