Genetics Flashcards
Define genotype.
The genetic constitution of an organism.
Define phenotype.
The expression of the genotype and its interaction with the environment.
Alleles may be __________, ____________ or _______________.
Dominant, recessive or codominant.
What is a gene?
A sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic eg eye colour.
Define allele.
A different form of a gene. Only one occurs at the locus of a chromosome.
Define dominant.
The allele of the heterozygote that expresses itself in the phenotype, even if there’s only one copy.
Define recessive.
An allele who’s characteristic only appears in the phenotype of two copies are present.
Define codominant
Alleles that are both expressed in the phenotype - neither one is recessive.
What is a locus?
The fixed position of a gene on a chromosome.
Define homozygote.
An organism that carries two of the same alleles of genes at the same locus on homologous chromosomes. Eg BB.
Define heterozygote.
An organism that carries two different alleles of genes at the same locus on a homologous chromosome. Eg Bb.
What is a carrier?
A person carrying an allele which is not expressed in the phenotype but that can be passed onto offspring.
In what kind of organisms are homologous chromosomes found?
Diploid organisms.
What is monohybrid inheritance?
The inheritance of a single gene.
What are genetic diagrams used for?
Genetic diagrams are used to predict the genotype and phenotypes of the offspring produced if two parents crossed (bred).
What is a ratio?
A measure of the relative size of two groups that is expressed as a proportion.
What is dihybrid inheritance?
How 2 characters, determined by 2 different genes located on different chromosomes are inherited.
What is the theoretical phenotypic ratio at F2 in monohybrid inheritance?
3:1 (dominant:recessive)
What is the theoretical phenotypic ratio in F2 in dihybrid inheritance?
9:3:3:1
What is the theoretical phenotypic ratio in F2 of codominant inheritance?
1:2:1
Why is it that often you don’t get the expected phenotypic ratio?
Due to sex linkage, autosomal linkage or epistatis.
What does ‘multiple alleles mean?
There are more than two alleles, of which only two may be present at the loci of the homologous chromosome.
When does codominance occur?
When both alleles are equally dominant. Eg strawberry roan.
Give an example of ‘multiple alleles’.
The human ABO blood groups. (Allele IA, IB, IO).
Humans have 23 pairs of chromosomes. How many of these have homologous partners that are identical in appearance?
And why this number?
- The remaining pair are the sex chromosomes. In the male, there is XY, and female is XX.
What is a sex-linked gene?
Any gene that’s carried on either the X or Y chromosome.
Why are males more likely to show recessive phenotypes?
Because they only have one X chromosome, so often only have one allele for sex linked genes.
So, as they only have one copy, the express the characteristic of this aloe even if it’s recessive.
Give an example of an x-linked disorder.
Colour blindness and haemophilia.
Why is colour blindness much rarer in women than men?
Because females would need 2 copies of the recessive allele to be colour blind, whereas male sonny need one colour.
Why do males obtain haemophilia from their mothers?
Because males can only obtain a Y chromosome from their father, so it follows that their X chromosome comes from the mother.
Because the defective allel that does not code for the clotting protein is linked to the X chromosome, males always inherit the gene from their mother.
How can fathers pass on haemophilia to their offspring?
Fathers can pass their X chromosome onto their daughters (but only Y to sons), meaning the daughters can become carriers.
What are autosomes?
The remaining 22 chromosomes (excluding the sex chromosomes)
What is autosomal linkage?
The situation when two or more genes are carried on the same autosome.
What is epistasis?
When the allele of one gene affects / masks the expression of another in the phenotype.
How does epistasis work?
The allele of one gene masks (blocks) the expression of the alleles of other genes.
