Topic 2: Genetic inheritance

Question 1

Key Definitions: define gene

3 points

Answer 1

1. a section of DNA WITH A SPECIFIC SEQUENCE OF BASES
2. which codes for the sequence of amino acids in a {polypeptide/protein}
3. and occupies a particular locus on a chromosome

Question 2

Key Definitions: define allele

2 points

Answer 2

1. a {DIFFERENT/alternative} form of {THE SAME/one} gene

2. found at the same locus on a chromosome.

Question 3

Key Definitions: define locus

1 point

Answer 3

1. The POSITION of an allele on a chromosome.

Question 4

Define genotype
(1 point)

Answer 4

1. The alleles present in an organism

NOTE: represented by letters eg. TT or Tt

Question 5

Define phenotype
(2 points)

Answer 5

1. the observable appearance or characteristic of an organism,
2. as a result of the expression of the {GENOTYPE/ALLELES} present and also sometimes the ENVIRONMENT

Question 6

Define dominant allele

2 points

Answer 6

1. A different form of the same gene,

2. which is expressed in the phenotype of an organism whichever other allele (recessive or dominant) is present.

Question 7

Define recessive allele (2 points)

How can the recessive phenotype be expressed? (1 point)

Answer 7

1. A different form of the same gene,
2. which is only expressed in the phenotype of an organism if the dominant allele is not present
3. 2 recessive alleles need to be present, the homozygous recessive genotype, for the recessive phenotype to be expressed.

Question 8

Define homozygote
(1 point)

Answer 8

1. An ORGANISM with both alleles at a gene locus the same (e.g. TT or tt)

Question 9

Define heterozygote
(1 point)

Answer 9

1. An ORGANISM with both alleles present at a gene locus being different, one recessive and one dominant. (e.g. Tt)

Question 10

Define homozygous recessive

1 point

Answer 10

1. Genotype where both alleles present at a gene locus are the SAME AND RECESSIVE. (e.g. tt)

Question 11

Define homozygous dominant

1 point

Answer 11

1. Genotype where both alleles present at a gene locus are the SAME AND DOMINANT. (e.g. TT)

Question 12

Define heterozygous (1 point)

Answer 12

1. Genotype where there are 2 different alleles (e.g. Tt)

Question 13

Define incomplete dominance

3 points

Answer 13

1. When there are 2 different alleles in the genotype (heterozygous)
2. and both alleles are expressed in the phenotype of the organism.
3. So one allele does not completely dominate another allele.

Question 14

Cystic fibrosis is an ________ _______ condition.

Answer 14

autosomal recessive

NOTE: Recessive because 2 mutated copies of the allele are needed to be present for a person to have CF/
Autosomal because the alleles are not on a sex chromosome.

Question 15

What is an autosome?

1 point

Answer 15

1. Any chromosome which is not a sex chromosome.

Question 16

The cystic fibrosis gene has 2 alleles (two different forms of the same gene), one which is ________ (f) and one which is _________ (F).

Every cell (except sex cells) in a person contains ___ _____ of the CFTR gene, one which has come from the mother (in the egg) and one from the father (in sperm).

Answer 16

recessive

dominant

2 copies

Question 17

Every human cell nucleus contains ___ homologous pairs of chromosomes.

In each homologous pair of chromosomes, one chromosome has come from the _______ (in sperm) and the other from the _______ (in egg)

Answer 17

23

father

mother

Question 18

What are the 3 properties of homologous chromosomes? (3 points)

Answer 18

1. are the SAME LENGTH
2. their CENTROMERES ARE IN THE SAME POSITION
3. their GENES ARE IN THE SAME LOCUS (position)

Question 19

What are chromosomes?

Answer 19

1. Chromosomes are DNA molecules made up of sections of DNA called genes, which code for proteins,
2. and other sections of DNA which do not code for proteins.

Question 20

Which cystic fibrosis allele codes for a functioning CFTR protein and which one does not? (2 points)

Answer 20

1. F (dominant) - functional allele which codes for a functioning CFTR protein.
2. f (recessive) - mutated allele which codes for a non-functional CFTR protein.

Question 21

What are the 3 possible genotypes and phenotypes of CF alleles? (3 points)
(e.g bb = blue eyes, Bb = brown eyes)

Answer 21

1. FF (homozygous dominant) = unaffected (no CF)
2. ff (homozygous recessive) = CF sufferer
3. Ff (heterozygous) = {unaffected (no CF)/carrier}

Question 22

Define monohybrid inheritance (1 point)

Answer 22

1. the inheritance of CHARACTERISTICS controlled by ONE GENE with two (or more) alleles.

Question 23

Key Summary: How would you get all the marks on a genetic cross question?
(8 points)

Answer 23

1. Genotypes and phenotypes of parents stated e.g. {carrier/healthy/unaffected/sufferer}
2. Gametes clearly shown as individual alleles (single letters) - can usually be awarded from a Punnett square.
3. Offspring genotypes stated - can usually be awarded from a Punnett square.
4. Phenotypes correctly matched to genotypes.
5. Use the CORRECT FORMAT when writing out genetic crosses
6. If you need to choose letters to represent alleles, make sure you choose ones where CAPITAL AND LOWER CASE LETTERS ARE VERY DIFFERENT
7. Read the question - DOMINANT OR RECESSIVE CONDITION?
8. Use % when giving probabilities

Question 24

Key Summary: How would you get all the marks on a pedigree diagram question?
(4 steps)

Answer 24

1. Remember that each parent only passes on ONE allele (gene) to a child – so one allele from each parent makes up the genotype of the child.
2. Use {numbers/names} from the diagram in your explanations.
3. If not many people on the diagram have the disease, it is caused by a recessive allele (and vice versa for dominant) – quote the number out of the total.
4. Use key terms in your explanations – carrier, homozygous, heterozygous, dominant, recessive etc.

Question 25

Name 3 genetic diseases caused by a RECESSIVE ALLELE

Answer 25

1. Albinism
2. Sickle cell anaemia
3. Cystic fibrosis