Session 12 - Genotype, Phenotype and Inheritance

Question 1

Define genotype, phenotype, gene and allele

Answer 1

Genotype - The DNA sequence of an organism or individual, which determines the specific characteristics
Phenotype - An organism’s observable physical and biochemical characteristics directly influenced by the genotype
Gene - A stretch of DNA sequences at a specific chromosomal location that carries the code for a polypeptide or untranslated RNA (such as rRNA)
Allele - a version or variant of a gene

Question 2

What does hemizygous mean?

Answer 2

Only one allele of a gene on the X chromosome (such as a male is hemizygous for all the genes on the X chromosome)

Question 3

Define Dominant, Recessive and Co-dominant alleles

Answer 3

Dominant - the dominant allele in a heterozygote determines the phenotype
Recessive - the non-dominant allele in a heterozygote is called recessive
Co-dominant - neither allele in a heterozygote is dominant and the phenotype is new (different to just one)

Question 4

What are the human blood types and how are they linked?

Answer 4

The gene has 3 alleles A,B,O (IA, IB and IO)

• alleles A, B are dominant over O
• alleles A and B are co-dominant
• This results in 4 phenotypes and 6 genotypes (Group B can be BB or BO)

Question 5

How do you represent a heterozygous male, carrier female, unknown sex, deceased male, monozygotic twins (identical), dizygotic (non-identical) twins and a consanguineous marriage (related as 2nd cousins or closer)?

Answer 5

Heterozygous Male - Half shaded square
Carrier Female - Circle with a shaded dot in the middle
Unknown Sex - Diamond shape
Deceased Male - Square with a line through
Identical Twins - Triangle link with line
Twins (non-identical) - Triangle link without line
Consanguineous Marriage - Double Horizontal Line

Question 6

What are the key conventions for pedigrees?

Answer 6

1) Every generation on a separate line/level
2) Label every generation with roman numeral top to bottom
3) If possible, males left and females right in relationships
4) By convention, siblings in age order from left to right
5) label every individual of every generation with numbers, left to right
6) By convention, shading indicates phenotype

Question 7

What are the 4 main types of inheritance?

Answer 7

1) Mendelion or Monogenic (1 gene inheritance)
2) Mitochondrial (mitochondrial genetic material)
3) Polygenic (many genes involved in phenotypic expression)
4) Complex or multifactorial (more than 1 factor causes a trait or health problem)

Question 8

What are the key features of an autosomal recessive condition in a pedigree?

Answer 8

• Disease seem to skip generations and can “come out of nowhere” (i.e. there are carriers)
• Males and females are equally affected
• Both parents must be affected if offspring are

Question 9

What are the key features of an autosomal dominant condition on a pedigree?

Answer 9

• Males and females equally effected
• Every individual will have (at least) one affected parent
• Disease can’t skip a generation

Question 10

What are the key features of an X-linked recessive condition on a pedigree and give an example of a condition?

Answer 10

Haemophilia A:

• Males and unequally effected (males more)
• Every affected (hemizygous) male will have (at least) a heterozygous carrier mother.
• Every affected female will have an affected father and a carrier mother
• Affected males will have (at least) heterozygous daughters

Question 11

What are the key features of an X-linked dominant condition on a pedigree?

Answer 11

• Males and females equally effected
• Heterozygous (affected) females have 50% chance of having affected off spring
• Affected male will give trait to all daughters but can’t to son

Question 12

What are the key features of an Y-linked condition on a pedigree?

Answer 12

• Only males affected (linked to “maleness”)

- Trait given to all sons

Question 13

What are the key features of an mitochondrial condition on a pedigree?

Answer 13

All individuals will inherit from their mother

Question 14

What is polygenic inheritance and give an example?

Answer 14

• More than one gene can be involved in producing a phenotype
• This occurs in albinism which is inherited in a recessive manner.
• If you have two parents with albinism their offspring doesn’t necessarily have to have albinism. If each parent has albinism as a result of a different gene the offspring can be unaffected.

Question 15

What does it mean when genes are said to be linked and what effects recombination frequency?

Answer 15

Genes on the same chromosome are said to be linked. They don’t show independent assortment at meiosis unless chiasmas form. As such the recombination frequency between 2 linked genes is dependant on the distance between the genes.

• Linked genes close together are “tightly linked”
• Linked genes far apart on the same chromosome almost behave as unlinked genes (as so much recombination occurs)