Genetics 2

Question 1

Explain epistasis

Answer 1

• Another reason why pattern of inheritance is difficult to see
• About controlling genes
• A gene at one locus interferes with the expression of a gene at a different locus
• Human skin/eye color (polygenic inheritance)
• Polygenic inheritance occurs when one characteristic is controlled by two or more genes
• Flower colour in sweet peas/lamas

Question 2

Explain an example of epistasis

Answer 2

• Labradors
• One gene is responsible for the production of the pigment giving colour, melanin, another gene determines whether the melanin is deposited in the hair or not.
• These dogs typically can have any of three colours, i.e.
  black, brown/chocolate or yellow.
• Black (B) is dominant over brown (b) which determines the production of melanin.
• Thus, the allele pairs BB or Bb indicate the normal production of melanin, and bb results in the reduction of melanin production.
• The gene (E) will lead to melanin being deposited in the hairs (EE or Ee), whereas ee reduces the deposition.
• Black Labradors will have the following allele pairs: BB EE, Bb EE, BB Ee, or Bb Ee.
  Brown Labradors will be: bb EE or bb Ee.
  Yellow Labradors will be: BB ee, Bb ee or bb ee.

Question 3

Explain sex-linked genes having unique inheritance patterns

Answer 3

• Each egg cell produced during oögenesis contains one X chromosome, and half the sperm cells produced during spermatogenesis contain one X-chromosome and the other half one Y-chromosome.
• When an egg with an X-chromosome is fertilized by an X-bearing sperm the resultant individual will develop into a female.
• If the sperm carried a Y-chromosome the resultant individual will develop into a male. The Y-chromosome must thus carry the information for a male to develop.

Question 4

Explain X and Y chromosomes carrying sex linked genes

Answer 4

• SRY gene, it is present on the Y chromosome
• Any offspring with the Y chromosome will be male because this gene switches on other genes that produce the male traits and switches off genes that produce female traits
• Y chromosome has less than 100 genes while the X chromosome has over 1000 genes
• If a trait is X linked – it means that the X chromosome controls it.
• X linked recessive disorders affect males more than females

Question 5

Explain X-linked alleles

Answer 5

• Genes carried on autosomes are said to be autosomally linked
• Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked)
• X-linked genes have a different pattern of inheritance than autosomal genes have
  The Y chromosome is blank for these genes
• Recessive alleles on X chromosome:
  Follow familiar dominant/recessive rules in females (XX)
• Are always expressed in males (XY),
  Males said to be monozygous for X-linked genes

Question 6

Explain human x-linked disorders ( colour vision )

Answer 6

• Colour vision in humans:
• Depends on three different classes of cone cells in the retina
• Only one type of pigment is present in each class of cone cell
• The gene for blue-sensitive is autosomal
• The red-sensitive and green-sensitive genes are on the X chromosome
• Mutations in X-linked genes cause RG colour blindness:
  
  • All males with mutation (XbY) are colourblind
• Only homozygous females (XbXb) are colourblind
• Heterozygous females (XBXb) are asymptomatic carriers

Question 7

Explain human x-linked disorders ( hemophilia )

Answer 7

• “Bleeder’s Disease”
• Blood of affected person either refuses to clot or clots too slowly
• Hemophilia A – due to lack of clotting factor IX
• Hemophilia B – due to lack of clotting factor VIII
• Most victims male, receiving the defective allele from carrier mother
• Bleed to death from simple bruises, etc.
• Factor VIII now available via biotechnology
• X-linked recessive disease

Question 8

Explain genetic disorders

Answer 8

• Autosome - Any chromosome other than a sex chromosome
• Genetic disorders caused by genes on autosomes are called autosomal disorders
• Some genetic disorders are autosomal dominant :

An individual with AA has the disorder
An individual with Aa has the disorder
An individual with aa does NOT have disorder

• Other genetic disorders are autosomal recessive :

An individual with AA does NOT have disorder
An individual with Aa does NOT have disorder, but is a carrier
An individual with aa DOES have the disorder

Question 9

Explain humans being analysed for genetic patterns with pedigree charts

Answer 9

• Due to their rapid reproductive abilities it is easy to follow specific genetic traits through many generations of organisms like pea plants, fruit flies and mice.
• The human reproductive cycle, small family sizes, etc. makes similar analyses impossible.
• It is thus not possible to manipulate humans as in the case with experimental organisms. Geneticists thus have to rely on pedigree charts to determine the inheritance traits of humans, and these pedigree charts often lack in valuable information.

Question 10

What are autosomal recessive ( aa ) disorder examples

Answer 10

• Tay-Sachs Disease = progressive deterioration of psychomotor functions through deterioration of nerve cells
• Cystic Fibrosis = mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous
• Phenylketonuria (PKU) = link to Pleiotropy
  Mutation of gene leads to lack of enzyme for normal metabolism of phenylalanine

    - Phenylalanine builds up to toxic levels, can affect brain function and affect other body systems

Question 11

Explain autosomal dominant ( AA ) disorder examples

Answer 11

• Neurofibromatosis = tan or dark spots develop on skin and darken

   - Small, benign tumors may arise from fibrous nerve coverings or can be more severe and affect the brain
  
   - Several types

• Huntington Disease = neurological disorder (neurodegenerative)

   - Progressive degeneration of brain cells, mental ability and behaviour
  - Severe muscle spasms
  - Personality disorders
  - TheHTTgene provides instructions for making a protein called huntingtin.  CAG repeat increases from 36-39 times to 40 or more

Question 12

Explain Dentinogenesis imperfecta

Answer 12

• Autosomal dominant disorder
• Teeth that are translucent and discolored (most often blue-grey or yellow-brown in color)
• Teeth are weaker, which leads to wear, breakage, and loss of teeth.
• Mutations in the DSPP gene cause dentinogenesis imperfecta.
• The DSPP gene provides instructions for making three proteins that are essential for normal tooth development.
• These proteins are involved in the formation of dentin, which is a bone-like substance that makes up the protective middle layer of each tooth.
• Mutations inDSPPchange the proteins made from the gene, leading to the production of abnormally soft dentin.
• Teeth with defective dentin are discolored, weak, and more likely to decay and break.

Question 13

Explain the environment and multiple gene influence on traits

Answer 13

• Evidence shows that the environment has definite effects on expressed phenotypes.
• The environment can affect gene expressions, this means that a gene can be more active or less active depending on the situation.
• Some plants that are found in running streams show definite differences between the leaves on the surface and that under the water, i.e. the leaves on the surface will expose large surface areas whereas the underwater parts are more featherlike to allow water to move freely without breaking off the leaves.
• In humans, fetal alcohol syndrome is an example of where the environment affects the phenotype. A foetus exposed to alcohol during pregnancy will develop facial abnormalities or epilepsy

Question 14

Explain genotype + environement = phenotype

Answer 14

• For example temperature influences :

1. The quantity of pigment molecules - Colour of fur of Himalayan rabbits are darker on body parts exposed to colder temperatures.
2. Incubation temperature influences the sex of crocodiles as different combinations of genes are activated at different temperatures.

• Polygenic traits - Body weight and intelligence in humans are controlled by multiple genes and the environment.