MCBG Session 12 - Genotype, Phenotype & Inheritance

Question 1

Define genotype & phenotype.

Answer 1

Genotype = The DNA sequence of an organism that determines specific characteristics (phenotype)

Phenotype = An organisms observable physical and biochemical characteristics directly influenced by the genotype and/or environment.

Question 2

Define gene & allele

Answer 2

Gene = A stretch of DNA at a specific chromosome location that codes for a polypeptide

Allele = A varient of a gene.

Question 3

Do homologous chromosomes have the same genes and alleles?
Do sister chromatids have the same DNA and alleles?
Do non-sister chromatids have the same genes and alleles?

Answer 3

Homologous chromosomes = same genes, different alleles.
Sister chromatids = same DNA, same alleles
Non-sister chromatids = same genes, different alleles

Question 4

Define homozygous, heterozygous and hemizygous.

Answer 4

Homozygous = two alleles of the gene are the same
Heterozygous = two alleles of the gene are different
Hemizygous = one allele of a gene on the X-chromosome (men are hemizygous for all X-linked genes)

Question 5

Define “Co-dominant”

Answer 5

Neither allele in a heterozygote is dominant, and the phenotype is distinctly new (different from the phenotypes of the other alleles)

Question 6

In humans, there are 3 alleles for glycoproteins on the surface of RBC’s - A,B and O - A & B are dominant over O, and are also co-dominant.

How many possible phenotypes are there?
How many possible genotypes are there?

Answer 6

• 4 phenotypes - A, B, O or AB.

- 6 genotypes - AA & AO (type A), BB & BO (type B), AB (type AB) and OO (type O).

Question 7

What are the key symbols used for unaffected, carrier and affected males and females in a pedigree analysis, as well as unknown sex offspring and deceased family member?

Answer 7

Males = Square
Females = Circle
Affected = fully shaded
Carrier (heterozygote)= half shaded for males, dot in circle for females
Unknown sex = diamond
Deceased = line through square or circle

NB: generation number always on left (roman numeral)

Visit slide 13 in session 12 and understand how pedigrees look at this is likely to be examined.

Question 8

What are the 4 main types of inheritance?

Answer 8

1) Mandelian/Monogenic
2) Mitochondrial
3) Polygenic
4) Complex/Multifactorial

Question 9

Genotype controls phenotype - but what else has an impact on both?

Answer 9

Environment - can affect genotype and phenotype directly, e.g.: radiation, chemicals, mutagens, lifestyle and diet.

Question 10

Describe autosomal recessive inheritance.
What is the % chance 2 x carrier parents have an affected child?
What is the % chance 2 x affected parents have an affected child?
Are males and females equally affected?
Give an example of an autosomal recessive disease

Answer 10

• Both copies of the recessive allele required to be affected.
• 25%
• 100%
• Yes
• Cystic Fibrosis

Question 11

Describe autosomal dominant inheritance
Why do we assumed affected individuals are heterozygotes?
What is the chance of offspring being affected between an affected and non-affected parent?
Are males and females affected equally?
Give an example.

Answer 11

• Only 1 copy of dominant allele required to be affected
• 2 copies of dominant allele usually not viable for life
• 50% - cannot skip a generation, every affected individual has at least 1 affected parent
• Males and females affected equally
• Huntington’s disease

Question 12

Describe X-linked recessive inheritance.
What does every affected (hemizygous) male have?
What does every affected female have?
What will all affected males have at least in their offspring?
Are males and females equally affected?

Answer 12

• Every X chromosome has recessive allele
• At least a heterozygous (carrier) mother
• Affected father and a carrier mother (as they require 2 copies of recessive allele)
• They will have at least a carrier daughter
• Males affected more (only require 1 copy of recessive allele)

Question 13

Describe X-linked dominant inheritance.
What chance do affected females have of having affected offspring?
Can affected males give trait to their sons?
Will affected males give trait to all their daughters?

Answer 13

• Only 1 dominant allele on X-chromosome required (so males and females affected equally)
• 50%
• No - as X chromosome allele is from the mother
• Yes - as daughters get one X chromosome allele from their father

Question 14

Who are affected by Y-linked disorders?

What are these diseases linked to?

Answer 14

• Only males, they will give trait to all their sons

- Maleness - e.g.: spermatogenesis

Question 15

How does mitochondrial inheritance work?

Answer 15

An individual will inherit mitochondrial DNA from their mother as mitochondria comes from the egg cell.

Question 16

What is polygenic inheritance?
When are genes “linked”?
When are genes “not-linked”?
What is special about linked genes?

Answer 16

• When multiple genes are involved in inheritance of a trait.
• Genes on the same chromosome are linked
• Genes on different chromosomes are not linked
• Linked genes do not show independent assortment at meiosis

Question 17

What is recombination frequency between two linked genes dependent on?

Answer 17

The distance between the genes, linked genes far apart on the same chromosome almost behave as unlinked genes.