18: Genetics & Inheritance

Question 1

Define:
- Genotype
- Phenotype
- Gene
- Allele
- Locus
- Homozygous
- Heterozygous
- Dominant Allele
- Recessive Allele

Answer 1

1. genetic constitution of an organism
2. the expression of the genotype and its interaction with the environment
3. section of DNA that codes for a polypeptide
4. different variants of a gene
5. the position of a gene on a chromosome/DNA molecule
6. both alleles of a certain gene are identical
7. both alleles of a certain gene are different
8. always expressed in the phenotype if present in genotype
9. only expressed in phenotype when organism is homozygous

Question 2

How can you tell between if an organism has the homozygous dominant trait or heterozygous trait?

Answer 2

• complete a backcross/testcross
• this is done by crossing the organism with the recessive phenotype, (homozygous recessive), to work out the genotype of the original organism

Question 3

What is Codominance?
What is the Codominance order for blood groups?

Answer 3

• alleles that are both expressed in the phenotype of a heterozygous organism, which gives rise to a 3rd phenotype even though the gene has only 2 different alleles
• iA = iB > iO

Question 4

What is Sex-Linkage?
Why are males more likely to have recessive versions of the alleles to females?

Answer 4

• many genes carried on the sex chromosomes are unrelated to sexual characteristics but are inherited in different ways in males + females
• for genes found only on the X chromosomes, males are more likely to have recessive versions of the phenotype than females, as males on have one X chromosome and so will only have one allele of that gene

Question 5

How to find evidence for alleles being Dominant/Recessive?
How to find evidence for Sex-Linkage in pedigree diagrams?
How to find evidence against Sex-Linkage in pedigree diagrams?

Answer 5

1. look for where an offspring has a different phenotype to the parent
2. look for much greater number of affected males than females
3. look for where a female (XX) with the recessive phenotype has a son or father (XY) with the dominant phenotype. This is not possible if the gene is on the X chromosome

Question 6

What is Epistasis?

Answer 6

• arises in a dihybrid cross when there is some form of interaction between the 2 genes, such that one gene suppresses the expression of the other in the phenotype