Mendelian Genetics 3

Question 1

Himalayan Phenotype in Rodents

Answer 1

• temperature sensitive gene product
• at low temperatures the gene product can form
• at higher temperatures the pigment is not produces
• this is why the dark colouring is only seen at the tips of the ears and paws

Question 2

Polydactyly

Answer 2

• having extra fingers or toes
• an autosomal dominant trait in humans
• 50-80% penetrant

Question 3

Penetrance

Answer 3

• not all individuals in a population with a particular genotype will show the expected phenotype
• it depends how penetrant the gene is
• if all individuals show the expected phenotype, the gene is said to have complete penetrance
• if 80% of individuals with that genotype show the phenotype the gene is said to be 80% penetrant
• most loci show 100% penetrance

Question 4

Expressivity

Answer 4

• a particular mutant gene may affect the phenotype of an organism to different degrees
• e.g. a mutation leads to a defective protein being produced, but under certain environmental or developmental conditions that defective protein can still function, so some cells will have a functional gene product others don’t
• the severity of a phenotype can be variable

Question 5

Calico Cats

Genes

Answer 5

• orange locus is on the X chromosome
• autosomal spotted locus
• the spotted locus is epistatic to the orange locus

Question 6

Calico Cats

Expressivity

Answer 6

• the spotted locus shows variable expressivity
• where the gene is expressed above a threshold level, the phenotype is seen, below that threshold the phenotype is not seen
• Sp inhibits the migration of melanocytes so patches are seen depending on the expression of the Sp allele

Question 7

Calico Cats

Epistasis

Answer 7

• the spotted locus is epistatic to the orange locus

- a dominant Sp allele at the spotted locus causes white spotting regardless of alleles at other loci

Question 8

X Inactivation

Answer 8

Males - XY
Females - XX
-so females carry twice as many X linked genes as males
-this problem is overcome by dosage compensation
-in each cell in a female one of the two chromosomes is inactivated appearing condensed in the nucleus, a Barr Body

Question 9

Klinefelter’s Syndrome

Answer 9

• humans with XXY chromosomes

- male but with female traits

Question 10

Calico Cats

X Inactivation

Answer 10

-in each cell one X chromosome is inactivated
-In a cat with XOXo chromosomes:
where the XO is inactivated the cell will be black
where the xo is inactivated the cell will be orange

Question 11

Calico Cats

Size of Spots

Answer 11

-depends on how late melanocytes migrates
AND how late X inactivation occurred
-the early these two things occurred, the larger the spots as the original cell will have divided more times to produce more cells
-in a cell where these processes happen later, the cell will divide fewer times so will produce fewer cells that look like it