Modifications Of Mendel

Question 1

Define incomplete dominance and give example

Answer 1

Expression of heterozygote is intermediate between dominant homozygotes

• Four o’clock plant- red white pink
• Mendel’s peas- smooth/wrinkled
• Tay Sachs disease- homozygous recessive, hexoaminidase enzyme activity

Question 2

Define codominance and give an example

Answer 2

Phenotypic effects are fully and simultaneously expressed in heterozygote

• Red white cow- spotted
• Sickle cell haemoglobin

Question 3

Define multiple alleles

Answer 3

More than two alleles occurring at a locus

• Drosophila and human eyes
• ABO blood groups

Question 4

How does the ABO blood group system work? What is involved other than multiple alleles?

Answer 4

A and B each produce different antigens
O produces none
A n-acetyl galactosamine
B galactose
AB produces both

Complete and codominance

Question 5

Define pleiotropy

Answer 5

One gene affects many characteristics

• All blue eyed white cats are deaf
• Sickle cell haemoglobin is change in one base/amino acid- many detrimental effects- anaemia, heart failure, enlarged spleen

Question 6

Define gene interaction

Answer 6

Several genes affect one character. Alleles at one locus can alter the phenotype produced by an allele at another locus

• Autism
• Sex limitation effects in males- testosterone> secondary sex characteristics
• BRCA1 mutations- increase in breast cancer due to interaction btw cell division genes and oestrogen
• Mouse coat colours- at least 5 interacting loci

Question 7

Define epistasis

Answer 7

One allele preventing another at a different loci from expressing its effects

Question 8

What ratio results from a dihybrid cross involving epistasis?

Answer 8

9:3:4

Eg. Albino mice

Question 9

Define complementation and give an example

Answer 9

Two strains of an organism with different homozygous recessive mutations that produce the same phenotype

• Foxgloves:
w1w1 W2W2 =white
W1W1 w2w2 =white
cross-> F1 W1w1 W2w2 =purple
F1 cross F1 -> F2 9purple:7white

Question 10

What are complementation tests?

Answer 10

Show whether two alleles at a locus are involved or two/or more different loci

I.e. Cross two mutant homozygotes from different populations
If same locus, mutant F1 phenotypes shown
If different loci, parental phenotype shown- now het’s at both loci

Eg. Human deafness- many loci- outer ear, cilia of inner ear, nerve connections to brain. Two deaf parents can have hearing child.

Question 11

What is a test cross?

Answer 11

Dominant individual crossed with homozygous recessive to determine if individual is homozygous or heterozygous

Question 12

Define atavism

Answer 12

Phenotypic traits disappearing then reappearing later in the phylogenic tree

Question 13

What is a back cross?

Answer 13

Crossing a hybrid with one of it’s parents/genetically similar individual
-> offspring with genetic identity closer to parents

Question 14

Define lethal alleles and give an example

Answer 14

Homozygotes for certain alleles can be lethal

• Mice- Y allele dominant for yellow coat (y=wild-type), but recessive for viability-> Yy cross Yy -> 2yellow:1wild as YY dies
• Manx cat= no tail. Homozygous Manx allele= lethal
• Barchydactyly